Method of operation for an apparatus for layer-by-layer manufacture of 3D objects

Abstract

A method of operation for an apparatus for the layer-by-layer manufacture of a three dimensional (3D) object from particulate material, comprising: carrying out a thermal calibration process comprisin

Description

METHOD OF OPERATION FOR AN APPARATUS FOR LAYER-BY-LAYER MANUFACTURE OF 3D OBJECTS FIELD OF THE INVENTION The present disclosure relates to a method of operation for an apparatus for the manufacture of three-dimensional (3D) objects using a thermal sensor to control the thermal cycle of the process. The method might find particular benefit in a powder bed fusion apparatus in which 3D objects are built layer-by-layer from particulate material. A controller and an apparatus for applying the method are also disclosed. BACKGROUND In applications for forming 3D objects from particulate material, such as powder bed fusion applications like “print and sinter” and laser sintering, an object is formed layer-by-layer from particulate material spread in successive layers across a support. Each successive layer of the object is melted or partially melted to fuse or sinter the particulate material over defined regions and in so doing to consolidate it, in order to form a cross section of the 3D object. In the context of particulate polymer materials, for example, the process of melting achieves fusion of particles. Typically, several heating devices are operated in a print and sinter apparatus to heat the particulate material during each layer cycle. For example, one or more infrared bar heaters may be moved across each layer to heat the layer surface (the build bed surface) sufficiently to achieve fusion over selectively modified regions. The thermal processes of the layer cycle require accurate control to achieve high-quality, uniform objects with well-defined properties in a reliable, reproducible manner. Typically, a thermal sensor is also present, such as a pyrometer or thermal camera, that detects the temperature of the surface. The thermal camera may be used as part of feedback control in the operation of the one or more heating devices. In addition, the thermal camera may be used to calibrate itself against for example thermal events defined by the characteristics of the particulate material, such as the onset of melting. In addition, the various heating devices may also be calibrated. Despite efforts to develop thermal calibration processes for these components, such known processes do not provide the desired reliability and reproducibility. This is exacerbated when control across a fleet of printers, which may be located in different environments, is necessary, for example, for process transfer and support. Therefore, improvements to the thermal calibration processes are needed. SUMMARY Aspects of the invention are set out in the appended independent claims, while particular embodiments of the invention are set out in the appended dependent claims. The following disclosure describes, in one aspect, a method of operation for an apparatus for the layer-by-layer manufacture of a 3D object from particulate material, the apparatus comprising a first heat source configured to be moveable across the layer and a thermal sensor and configured to measure the temperature of the layer; the method comprising: carrying out a thermal calibration process comprising a calibration routine for a thermal control component of the layer, and subsequently carrying out a build process to manufacture a 3D object; wherein the calibration process and the build process each comprise a layer cycle comprising the steps of: (i) distributing a layer of particulate material over a build area, the layer providing a build bed surface of the build area; (ii) depositing absorption modifier over at least one of a layer-specific region within the build bed surface and a surrounding area surrounding the layer-specific region; and (iii) heating the layer-specific region over a period of time by moving the first heat source over the layer while operating it; (iv) measuring the temperature of the build bed surface using the thermal sensor at least once during one or more of steps (i) to (iii); wherein the layer cycle of steps (i) to (iv) is repeated a number of times until the respective calibration process and the build process are complete; wherein an outcome of the thermal calibration routine is based on the measured temperatures and is applied to the thermal control component for the subsequent layer cycle; wherein, for the layer cycle of the build process, the first heat source in step (iii) is operated to cause the layer-specific region to reach the melting temperature of the particulate material and cause it to melt such that the melted layer-specific region constitutes a cross section of the object for that layer; and wherein for both the thermal calibration process and the build process, the layer cycle step (i) of distributing each layer and (iii) of passing the first heat source over each layer are carried out in a first direction. In a second aspect, a controller is disclosed for an apparatus for the layer-by-layer manufacture of a 3D object from particulate material and configured to carry out the method of the first aspect. In a third aspect, disclosed is an apparatus for the layer-by-layer manufacture of a 3D object from particulate material comprising the controller of the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS Reference is now directed to the drawings, in which: Fig. 1A is a schematic cross-section of a side view of an apparatus configured to apply the method of the invention; Fig. 1B is a schematic plan view of the build bed surface of Fig. 1A; Fig. 2 is a flow chart of the method of the invention; Fig. 3 illustrates an implementation of the flow chart of Fig. 2; Fig. 4 illustrates time intervals between the method steps of Fig. 2; Fig. 5 is a flow chart of a build process layer cycle following the layer cycle of Fig. 2; Fig. 6 is a flow chart of a build process layer sequence following the method of Fig. 2; and Fig. 7 is a schematic cross-section of a variant of the apparatus of Fig. 1A. In the drawings, like elements are indicated by like reference numerals throughout. DETAILED DESCRIPTION Fig. 1A schematically illustrates a detail of a cross section of a powder bed fusion type apparatus 1, as an example of a 3D printing apparatus configured to carry out the method and its variants that will now be described with reference to Figs. 1A to 7. In a typical process for the layer-by-layer formation of a 3D object from particulate material, successive layers of particulate material are distributed, each to form a build bed surface 12 which is processed to form successive cross-sections of an object 2. In this context, the reference to the ‘build bed surface’ is to the surface of the top-most layer of particulate material. In other words, each newly distributed layer forms a new build bed surface 12 that is the build bed surface of the layer to be processed in that particular layer cycle. As indicated in Fig. 1A, the apparatus 1 comprises a distribution module 32 for distributing each layer of particulate material across a build bed 14, a deposition module 38 for selectively depositing absorption modifier over a layer-specific region 50 defined within the build bed surface 12, and a first heating module comprising a first heat source L1 to heat the layer-specific region 50 following deposition of the absorption modifier. The layer-specific region 50 may represent a cross section of an object 2, or a test region in a calibration process. These modules may be provided on one or more carriages moveable across the layer. In the example of Fig. 1A, an implementation with two carriages 30_1 and 30_2 is shown. The carriages are arranged on one or more rails 34 that allow them to be passed back and forth above the build bed surface 12 along a first direction along x and along a second direction opposite the first direction. The first carriage 30_1 in this variant comprises the deposition module 38, such as a droplet deposition module configured to deposit the absorption modifier in the form of fluid droplets. Mounted behind the deposition module 38, with respect to the first direction, is the first heat source L1. The second carriage 30_2 comprises the distribution module 32, for example comprising a roller as shown. Where the distribution module is a roller, a fresh portion of powder is supplied to the surface to the right of the roller and to the left of the build bed surface 12, and the portion is spread over the build bed as the roller is moved over the build bed surface 12. Alternatively, the distribution module may contain particulate material that it gradually releases and spreads over the build bed surface 12 as it moves along the first direction. The build bed 14 is contained between walls 10 and supported on a platform 16, which is arranged to move vertically within the container walls 10 to lower or raise the build bed surface 12; for example, by a piston located beneath the platform 16. The apparatus further comprises, without specifically showing, a reservoir to supply particulate material to a dosing module that doses an amount of fresh particulate material to be distributed across the build bed 14, thus forming a new build bed surface 12. Fig. 1B shows a plan view of the build bed surface 12 of Fig. 1A with the layer-specific region 50 and the carriages 30_1 and 30_2 with the distribution, deposition and heating modules spanning the width of the build bed surface 12 (along y). As indicated before, each carriage 30_1, 30_2 is moveable back and forth along the x-axis, which herein is also referred to as the length of the build bed surface 12, the length being perpendicular to the width, however reference to length and width is not intended to indicate relative extent of the two directions but to merely help reference directions of the process. The absorption modifier may be radiation absorber deposited over the layer-specific region 50. Selectivity of preferentially heating the layer-specific region 50 versus the surrounding area is achieved by providing a heat source L1 with a spectrum of radiation that is absorbed to a higher degree by the radiation absorber compared to the surrounding area. If the combination of absorber and power input to the heat source L1 (causing a certain energy input to the region 50) is sufficient, the particulate material of region 50 melts, or sinters, to fuse or consolidate and form a region of consolidated particulate material. Thus, during a build process of an object, the radiation absorber may be deposited over layer-specific regions 50 of the build bed surface 12 so as to define the cross sections of the object 2 over successive layers. The term “layer-specific region” indicates that its position, shape and pattern will be layer dependent within a specific process or routine, and might vary between different processes and routines. In a typical build process, the build bed surface 12 is maintained at or close to a predefined target temperature that is below the melting temperature of the particulate material and above the solidification temperature. This means it may for example be maintained within a temperature range of 10-20°C below the melting temperature. The fresh particulate material is generally at a significantly lower temperature, such that the distributed layer has a significant cooling effect on the build bed surface 12 of the previous layer. Such large temperature differentials can cause warping of the fused parts, and it is desirable to increase the temperature of the distributed layer to, or closer to, the target temperature of the build bed surface 12 without unnecessary delay. Therefore, a preheat source, such as a second heat source L2, as indicated in the apparatus 1 of Figs. 1A and 1B, may be provided behind the distribution module 32 on the second carriage 30_2 to immediately preheat the freshly distributed particulate material. The wavelength spectrum of the second heat source L2 is such that, over a preheat period of time, it is capable of sufficiently preheating the layer- specific region 50 and the surrounding area, both being void of radiation absorber, up to or towards the target temperature. The target temperature may be achieved in combination with, for example, operating a stationary overhead heater 20 provided above the build bed surface 12 as shown in Fig. 1. As for the heat source L1, the period of preheat time may be determined by the speed at which the preheat source L2 traverses the layer-specific region 50 and transfers heat to the particulate matter of the layer-specific region 50. To adequately control the temperature of the build bed surface 12 during the layer cycle, the build bed surface 12 may generally be monitored by a thermal sensor 72 provided above the build bed surface 12. The thermal sensor 72 may be centrally mounted in the form of a thermal camera or a pyrometer within the area of the overhead heater 20, or it may be provided on one or both of the carriages in the form of a thermal line scan sensor. The measurements from the thermal sensor 72 may be used to apply feedback control to the one or more heating devices, for example to the overhead heater 20, to achieve thermal uniformity on the build bed surface 12. If adequately controlled, local differences in temperature can be avoided, which improves object quality, for example mechanical and visual properties, across the build bed surface 12. In order to adequately control thermal components that are operated during the layer cycle to control the temperature of each layer, it is desirable to carry out a calibration process of one or more calibration routines before a build process is carried out. For example, for the overhead heater 20 to be adequately feedback controlled, it is desirable that the thermal sensor 72 is calibrated with respect to a thermal characteristic of the particulate material, such as the melting point of the particulate material. In such a process the layer-specific region 50 for each layer is processed according to a layer cycle specific to a calibration routine for the thermal sensor 72. Similarly, it may be desirable to calibrate the one or more heat sources such as the first and / or second heat source L1, L2 before starting an object build process. However, it has been found that known calibration routines for such thermal components do not provide adequate calibration outcomes that ensure the required mechanical and aesthetic qualities of an object. The inventor has discovered that an improved method of operation for an apparatus for the layer-by-layer manufacture of a 3D object from particulate material may be provided, the apparatus comprising a first heat source L1 moveable across a build bed surface 12 of the layer and a thermal sensor 72 configured to measure the temperature of the build bed surface 12. The method comprises carrying out a thermal calibration process comprising a calibration routine for a thermal control component of the layer, and subsequently carrying out a build process to manufacture a 3D object 2, wherein the calibration process and the build process each comprise a layer cycle comprising the steps of: (i) distributing a layer of particulate material over a build area, the layer providing a build bed surface 12 of the build area, (ii) depositing absorption modifier over at least one of a layer-specific region 50 within the build bed surface 12 and a surrounding area surrounding the layer-specific region 50; and (iii) heating the layer-specific region 50 over a period of time by moving / passing the first heat source L1 over the layer while operating it; and (iv) measuring the temperature of the build bed surface 12 using the thermal sensor 72 at least once during one or more of steps (i) to (iil). The layer cycle of steps (i) to (iv) is repeated a number of times until the respective calibration process and the build process are complete. The total number of layers for the calibration process may be different to that of the build process. The outcome of the thermal calibration routine is based on the measured temperatures and is applied to the thermal control component for a subsequent layer cycle. Furthermore, during the layer cycle of the build process, the first heat source L1 in step (iii) is operated to cause the layer-specific region 50 to reach the melting temperature of the particulate material and cause it to melt, such that the melted layer-specific region contributes to a cross section of the object 2 for that layer. For both the thermal calibration process and the build process, the layer cycle step (i) of distributing each layer and (iii) of passing the first heat source L1 over each layer are carried out in a first direction. The calibration routine may be one carried out to calibrate any one of the thermal control components controlling the temperature of the build bed surface 12 of each layer, such as a thermal sensor 72, or one or more heat sources configured to heat each layer, such as the first and / or second heat source L1, L2. The method according to the invention was found to provide improved reliability in object quality by at least some of the layer cycle conditions of the calibration process being substantially, or at least predominantly, the same as those of the build process layer cycle. This may in some cases increase the number of layers that are processed compared to known calibration processes, which is not generally considered desirable, since more particulate material may be consolidated that is lost to recycling after completing an operation. Known calibration processes with comparatively fewer processed layers in their calibration routines may vary the conditions of the layer cycle from layer to layer, for example, by adjusting the input power of a heat source within the same layer, by using multiple heat sources one by one over a number of layers while in the subsequent build process they are used in combination over each layer, or by applying a static heating process in which a heat source that would normally travel back and forth across the build bed surface 12 is instead positioned in a static position to apply heat. Another calibration process may apply a very different layer cycle to that of a build process, such as uni-directional heating in a calibration routine and bi-directional heating in a subsequent build process. The method and its variants will now be described first with reference to the flow diagrams of Figs. 2 and 3 and with respect to use of a radiation absorber as absorption modifier. However, equivalent effects may be achieved by use of absorption inhibitor instead of, or in combination with, radiation absorber. The use of inhibitor on its own or in combination with absorber is well known in the art. Turning first to Fig. 2, a flow diagram is shown for a calibration process 100 followed by a build process 200. The calibration process for illustrative purposes comprises a calibration routine for the thermal sensor 72 of the apparatus, however the calibration routine may instead, or in addition, be a calibration routine for at least the first heat source L1. The calibration routine of the calibration process 100 starts with the distribution of a fresh layer at block 104 to form the build bed surface 12. The direction of distribution of the fresh layer is along a first direction, for example along x from left to right as indicated in Fig. 1. Optionally, at block 106, as indicated by the dashed outline, the build bed surface 12 is preheated by a second heat source L2. Any suitable heat source may be provided to preheat each newly distributed layer. Where a moveable heat source L2 is used, the step of heating may comprise passing the moveable heat source L2 across the layer along the first direction while operating the heat source L2 to heat the layer-specific region 50. Alternatively, the overhead heater 20 may be arranged to provide the heating function of block 106 either alone, or in combination with the moveable heat source L2. At block 110, radiation absorber is deposited over the layer specific region 50, which is then heated at block 112 with the first heat source L1 by moving the first heat source L1 along the first direction while operating it to heat the layer-specific region 50. Depending on the calibration routine, the particulate material of the layer-specific region 50 may be heated to melt or sinter, or it may be heated to a temperature below the onset of melting or sintering. For example, in a typical calibration routine for the thermal sensor 72, the layer-specific region 50 may be progressively heated over a number of layers, the layer-specific region 50 of each further layer reaching a higher temperature than that of the preceding layer until the particulate material of the layer-specific region 50 starts to melt or sinter. Thus, not all layer- specific regions 50 may melt during the calibration routine. At block 114, which may apply after any one or more blocks of the layer cycle, the temperature profile of the build bed surface 12 is measured at least once during one or more of blocks 104 to 112 using the thermal sensor 72. Irrespective of whether the calibration routine is for the thermal sensor 72 itself of for one or more of the heat sources, the thermal sensor 72 is used to measure the temperature of the build bed surface 12 at least once during one or more of blocks 104 to 112, for example following at least one of the steps of the layer cycle, such as following heating with the second heat source and / or following heating with the first heat source. At decision point 120, the calibration routine may be repeated for a number of times along repeat loop 124 to process further layers until the calibration routine is complete, after which it moves on to a further calibration routine, or on to the build process 200. The layer cycle of the calibration routine thus comprises blocks 104 to 114. Further calibration routines will be described below with reference to Fig. 5.The layer cycle of the object build process 200 comprises the same steps as the calibration routine of the calibration process 100, and is carried out by applying the calibration outcome of the calibration process. In this example, the calibration outcome of the calibration routine for the thermal sensor in calibration process 100 may be a set point for the temperature scale of the thermal sensor with respect to the melting point of the particulate material, and the temperature measurements taken during the build process 200 are calibrated temperature measurements by applying the set point. As illustrated in Fig. 2, the layer cycle of the build process 200 begins at block 204 of distributing a fresh layer of material, and optionally heating the fresh layer at block 206. During the build process, the second heat source is applied to preheat the distributed layer to a temperature at or close to the target temperature. Where a moveable second heat source L2 is used, the step at block 206 is carried out while moving the second heat source L2 in the first direction over the build bed surface 12. Next at block 210, radiation absorber is deposited over the layer-specific region 50, and at block 212 the layer-specific region 50 is heated by passing the first heat source L1 in the first direction over the build bed surface 12 so as to melt or sinter the layer-specific region to form a cross section of the object. Atblock 214, which may apply after any one or more blocks of the build process layer cycle, the temperature profile of the build bed surface 12 is measured at least once using the thermal sensor 72, for example following at least heating with the second heat source L2 and / or following heating with the first heat source L1. At decision point 220, the process continues for the total number of layers along repeat loop 224 until the object is complete, after which the build process ends. The layer cycle of the build process routine is described by the sequence of blocks 204 to 214. The layer cycle is thus similar, or substantially identical, to that of the calibration process. It may for example only vary from certain calibration routines in the pattern and shape of the deposited absorption modifier, which may vary layer-by-layer. Fig. 2 further illustrates by way of respective blocks 112 and 212 and identical respective blocks 104 and 204 how, for both the calibration process and the build process, the steps of distributing each layer and the step of heating the layer-specific region are both carried out in the same, first, direction. It should be noted that it is not necessary to immediately proceed from one layer of the layer cycle described herein to the next layer of either the calibration process 100 or the build process 200. Instead, between adjacent layers, additional layers having a different layer cycle may be carried out, for example, a number of unprocessed, blank layers distributed without the steps of depositing absorption modifier and heating with the first heat source L1. Thus, one or more intermediate layer cycles may be applied that are different to the layer cycle of the disclosed calibration method without substantially affecting the desired outcome of the method disclosed herein. Furthermore, due to the cyclical nature of the processes described, the steps of heating preferably comprise heating the build bed surface 12 comprising the layer-specific region 50. This means that the thermal processes are applied across the entirety of the build bed surface 12. During both the calibration process 100 and the build process 200, the thermal sensor 72 is operated intermittently or continuously to measure the temperature of at least certain areas of the build bed surface 12. The areas may represent the entire build bed surface, or they may represent portions of the build bed surface 12. The thermal sensor may, for example, be used to measure the temperature of the build bed surface 12 following the layer cycle steps of blocks 106, 206 of heating with the second heat source L2 and / or following layer cycle steps of blocks 112, 212 of heating with the first heat source L1. During the calibration process 100, the measurements may be used to analyse the temperature of the layer-specific region 50, while during the build process 200, the measurements may be used to analyse the temperature of, or temperature profile over, zones of the build bed surface 12 not comprising the layer-specific regions 50. During a calibration routine the thermal control component may be the thermal sensor 72, and from the temperature of the layer-specific regions 50 a set point may be determined for the thermal sensor scale. For example, during at least one of the layers of the calibration routine for the thermal sensor 72, the first heat source L1 may be operated at a fusing power input so as to cause the particulate material of the layer- specific region 50 to melt or sinter; and from the layer-by-layer evolution of the measured temperatures of the layer-specific region 50 following block 112 of heating with the first heat source L1, and optionally from the layer-by-layer evolution of the measured temperatures of the layer-specific region 50 following block 106 of heating with the second heat source L2, a set point for the temperature scale of the thermal sensor 72 may be determined based on a characteristic of the particulate material, such as the latent heat of fusion. The outcome of the calibration routine in this case is a set point for the temperature measurements of the thermal sensor, and is used in a subsequent routine or process. During the subsequent build process 200, the calibrated temperature measurements for example of the area surrounding the layer-specific region 50 may be used to maintain the build bed surface 12, or at least the surrounding area surrounding the layer-specific region 50, at a uniform target temperature. The step of heating the layer at block 106 and block 206 may be carried out by operating the second heat source between the respective steps of distributing the layer and depositing radiation absorber; preferably, the second heat source is a moveable heat source, such as heat source L2, that is moved over the build bed surface 12 in the first direction whilst being operated. The calibration routine for the thermal sensor may be carried out regularly, for example before each build process 200. Different materials may have different melting points; similarly, the same material treated or aged differently may display a shift in melting point. Furthermore, in “print and sinter” processes, typically the unfused particulate material is recycled and reused. The material properties may change as a result of thermal cycling, potentially causing a shift in the melting conditions and necessitating an adjustment in the set point. For a fleet of apparatus 1, some of which may be located in different environments, the calibration process disclosed herein may significantly improve the alignment between such apparatus and assist in service support. Fig. 3 further illustrates the steps of Fig. 2 using the specific carriage layout of Fig. 1. The first carriage 30 1 is arranged to the right of the second carriage 30 2 with respect to the first direction, here along x. The layer cycle starts with the carriages in “Carriage Position 1” at step S301 ready to carry out the steps of blocks 110, 210 of depositing absorber over the layer-specific region 50 (not shown in Fig. 3). The step of distributing the first layer and optionally preheating it is not shown in Fig. 3. In other words, the physical state of the layer at “Carriage Position 1” may be that after block 106 and 206 respectively, where the preheat source 1.2 is used. At step S302, the first carriage 30_1 starts to move from left to right along the first direction to deposit radiation absorber over the layer-specific region 50. The first carriage 30_1 further comprises the first heat source L1, which is operated as the first carriage 30_1 moves along the first direction to heat the layer-specific region 50. At step S303, the second carriage 30_2 starts to move along the first direction, following the first carriage 30_1, to distribute a fresh layer of particulate material using distribution module 32. Where the second source is a moveable heat source L2, mounted downstream of the distribution module 32 with respect to the first direction, the second heat source L2 is operated to heat the particulate material after it has been distributed. After steps S302 and S303 of heating and distributing in the first direction, both carriages reach “Carriage Position 2” beyond the far side of (or to the right of) the build bed surface 12 with respect to the first direction. The build platform 16 may be lowered at this point by a layer thickness, and at step S305 both carriages return to the starting position to the left- hand side of the build bed surface 12, moving from right to left in the second direction, to arrive at “Carriage Position 1” ready for a next layer cycle at S301. In variants of the methods disclosed, each layer cycle may comprise a set of sublayers, wherein each sublayer is processed according to the same steps of distributing (and optionally of preheating), depositing absorption modifier and heating of that layer. For each layer, the measured temperatures may be an average temperature determined from the measurements of one or more of the sublayers of that layer. Over, for example, the first few sublayers of a calibration routine layer cycle, thermal stability may be achieved and for the remaining sublayers, a respective average temperature value is determined for that layer. Furthermore, in variants the layer-specific region may comprise a plurality of subregions distributed over the build bed surface 12. The layer cycle steps may be provided by different carriage layouts of one or more carriages and different order of the modules. Comparing the starting points of the layer cycle in Figs. 2 and 3 and Fig. 7, as will next be described, the layer cycle may start at any suitable block illustrated in Fig. 2, as determined by the starting position for a specific layout of the modules and the order of the carriages, without being substantially changed in any other way. Turning to Fig. 7, a schematic cross section of the apparatus 1 is illustrated having a variant carriage layout to that in Fig. 1A. In Fig. 7, a single carriage 30 comprises the distribution module 32, which is located at the leading edge of the carriage with respect to the first direction, followed by the second heat source L2, the deposition module 38, and the heat source L1 with respect to the first direction. With this variant, the layer cycle illustrated in Fig. 3 is adapted so that from “Carriage Position 1”, the carriage is operated to carry out steps S303 comprising the blocks 104, 204 of distributing (and, preferably, 106, 206 of preheating) and S304 comprising the blocks 110, 210 of depositing radiation absorber and blocks 112, 212 of heating the layer-specific region 50 in a single pass from left to right. The pass ends at “Carriage Position 2” with a build bed surface 12 heated by the first heat source L1 before returning to “Carriage Position 1” to start the layer cycle again at block 104, 204 of distributing a fresh layer. In other words, the physical state of the layer at “Carriage Position 1” is that after block 112 and 212 respectively of Fig. 2. Thus, the layer cycle may be defined from different starting points, depending on how it conveniently fits with the carriage layout of the apparatus. The directionality of the process steps is not affected by the carriage layout. Considerations with respect to timings between steps It was found that a preferable layer cycle of the calibration process 100 and the build process 200 comprises steps that have a significant thermal impact on the build bed surface 12 are being carried out along the first direction only, and not along a second direction opposite, or perpendicular to, the first direction. Significant thermal events are considered to comprise heating (by the first heat source L1 and / or the second heat source L2) and cooling (by the fresh layer distributed by distribution module 32) of the build bed surface 12. During movement along the second direction therefore, the heat source(s) and the distribution module are not operated to heat or cool the build bed surface 12. Optionally, the step of depositing radiation absorber may also occur in the same (first) direction, for example, when moving the deposition module in the first direction, however on most processes this is not considered a significant thermal event. It is furthermore preferable that the layer cycle of the calibration process 100 adopts similar process conditions of the layer cycle of the build process 200, so that as many as possible of the significant thermal events of the calibration process layer cycle are substantially, or at least predominantly, the same as those of the build process layer cycle. For example, the respective power inputs during each heating and / or preheating step by the first and / or second heat source L1, L2 for each layer may be substantially the same or at least as similar as possible for the calibration process and the build process. Where the first heat source L1 and / or the second source L2 are a moving heat source, the respective speed profiles of passing the first heat source L1 and optionally the second heat source L2 may preferably be the same in the layer cycle of the build process and the calibration process. The speed profile for distributing each layer at blocks 104, 204 of the layer cycle is preferably also the same as the speed profile of passing the first heat source L1 over the layer at blocks 112, 212 of the layer cycle. The speed profile of heating each layer by operating the second heat source L2 at blocks 106 and 116 may be the same as the speed profile of passing the first heat source L1 over the layer at blocks 112 and 212. Furthermore, the various speed profiles may preferably be constant speed profiles, i.e., the one or more carriages 30_1, 30_2 preferably move at a substantially constant, non-variable speed over the build bed surface 12. The speeds of passing the first and second heat source L1, L2 over each layer and the speed of distributing each layer at step (i) may be equal and constant for the layer cycle of the calibration process 100 and of the build process 200. This means that over the entire method of operation, regardless of whether the process is for a calibration process or for a build process, the period of time over which a location on the build bed surface 12 is heated by one of the first heat source, or by the first and second heat source, is the same as that at any other location on the build bed surface 12. In addition, the duration of time over which the distribution module moves over a location on the build bed surface is the same as that at any other location on the build bed surface 12. Another consideration is the timing between the different steps of the layer cycle. Preferably, the time interval between initiating adjacent steps is constant from layer to layer, and is the same for both the calibration process layer cycle and the build process layer cycle. The calibration process layer cycle and the build process layer cycle may thus comprise the same of at least one, and preferably all, of the following: - initiating the step of heating the layer comprising the layer-specific region 50 with the first heat source at blocks 112 and 212 after a predefined first time interval tA; after initiating the step of distributing the layer at block 104 or 204, or, where applied, after initiating the step of heating the layer with the second heat source L2 at optional blocks 106, 206; and - initiating the step of distributing each new layer at block 104 and 204 after a predefined second time interval At; following initiating the step at blocks 112 and 212 of heating the previous layer with the first heat source L1; - initiating the optional step of heating the layer with the second heat source at block 106, 206 after a predefined third time interval At; after initiating the step (i) of distributing the layer; - optionally, initiating the step of depositing absorption modifier after a predefined fourth time interval Ats after initiating the step of distributing each new layer at block 104 and 204, or, where the step at block 106 and 206 of heating with the second heat source is present, after a predefined fourth time interval Ats following initiating the step of heating the layer with the second heat source at blocks 106 or 206; wherein the predefined first, second, and optionally the third and fourth time interval remains the same respective constant time interval across the build bed surface 12 for each layer. It should be noted that these conditions are relevant to the thermal events on the build bed surface, and it is over the build bed surface 12 where the speed and time intervals require to be controlled. Furthermore, preferably all of first, second, third and fourth time intervals are the same time interval so that the layer cycle occurs over a constant period of time for each layer. This may be achieved by moving the first heat source L1, the distribution module 32, the second heat source L2 and optionally the deposition module 38 over the build bed surface 12 at a constant speed profile. Such a preferred layer cycle is illustrated in Fig. 4, which is a schematic illustration of a preferred variant of the layer cycle using, for example, the two-carriage layout of Fig. 1A and Fig. 3. For simplicity, only the first heat source L1 and the distribution module 32 are shown to represent significant thermal events of heating (by first heat source L1) and of cooling (distribution module 32). The direction of traverse of the distribution module 32 and the first heat source L1 is along the first direction, indicated along x. Two locations Al and A2 are indicated on the build bed surface 12, A2 towards the near end of the build bed surface 12 with respect to “Carriage Position 1” to the left of the build bed surface 12, and Al to the far end of the build bed surface 12 with respect to the near end (and closer to “Carriage Position 27). In Fig. 4A, the distribution module 32 is in the “Carriage Position 1” with the first heat source just in front of the distribution module 32, with respect to the first direction along x. The first heat source L1 (in analogy to Fig. 3, the first carriage 30_1) starts to traverse the build bed surface 12 at a constant speed v1=const to carry out the step of heating at block 106 of Fig. 2. In Fig. 4B, after a time interval corresponding to the above described second time interval At, the distribution module 32 is initiated to follow the heat source L1 in the first direction (or in analogy to Fig. 3, the second carriage 30_2 starts to follow the first carriage 30_1) to carry out the step of distributing a new layer at block 104 of Fig. 2. Ahead of the distribution module 32, in analogy to Fig. 1A, the first heat source L1 has reached position x1 and moves at the same speed v1 as the distribution module 32, and the second time interval At: is determined by the speed v1 and the distance x1 between the distribution module 32 and the first heat source L1. In Figs. 4C and 4D, the first heat source L1 and the distribution module 32 are illustrated in different positions during their traverse of the build bed surface 12. In Fig. 4C, the first heat source L1 is in position x2 and has passed location A2, and the distribution module 32 at position x1 is just reaching the near edge of the build bed surface 12. The distance between the first heat source L1 and the distribution module 32 remains the same since both move at the same speed v1, and thus the distance between them and the second time interval At; remains constant. In Fig. 4D, the first heat source L1 is at position x4 and has now also passed location Al, and the distribution module 32, at position x3, has passed location A2. In Fig. 4E, the first heat source L1 has reached position x5 at the end of its traverse and stops. It may be turned off once it passes the far end of the build bed surface 12. At this time in the layer cycle, the distribution module 32 has reached position x4 at the far end of the build bed surface 12 and has also passed location A1 and has distributed a fresh layer over it. Both the first heat source and the distribution module, or the first and second carriage, reach “Carriage Position 2” at step S304 of Fig. 3, not shown, after which both the distribution module 32 and the first heat source L1 return to the starting position at a return speed or respective speeds that may be faster than v1 (also not shown, but analogous to step S305 in Fig. 3). The return speed / speeds of traverse of, and the distance between, the distribution module 32 and the first heat source L1 (or in analogy return speed / speeds of traverse of, and the distance between the first and second carriage) to the starting position determine the first time interval At) before the layer distributed is heated by the first heat source L1. Thus, the first time interval At; may be longer than the second time interval Ata. The same considerations apply for the third time interval At; where a second heat source L2 is provided, for example fixed to the second carriage 30_2 behind the distribution module 32, such that the third time interval is determined by the speed of the second carriage 30_2 and the distance between the second heat source L2 and the distribution module 32. The fourth time interval Ats between the steps of initiating distributing each new layer at block 104 and 204 and depositing absorption modifier at block 110, or, where the step at block 106 and 206 of heating with the second heat source is present, between the steps of initiating the step of heating the layer with the second heat source at blocks 106 or 206 and depositing absorption modifier at block 110, depends, like the second time interval, on the distance between the carriages and the speed at which the carriages move. Thus, the second time interval between the layer cycle steps of distributing each layer and heating each layer with the first heat source may preferably be substantially constant. Furthermore, the time interval between the layer steps of distributing each layer and heating each layer with the second heat source may be substantially constant. In addition, the time interval between the layer steps of heating each layer with the first heat source L1 and the step of distributing each further layer may be substantially constant. This means that the sequence of the respective first, second, third and fourth constant time intervals may be the same for each layer cycle, such that the duration of time of each layer cycle is constant. It has been found that this provides for an improved stable thermal cycle and for a more reliable build process when these layer cycle conditions are equally applied to the calibration process. In variants of the build process and of the calibration routine for the thermal sensor 72, the first heat source L1 may further be operated at substantially the same, constant power input for each layer. Additionally, or instead, the step of preheating may be carried out at the same power profile along the first direction by the second heat source L2. Calibration routines A calibration routine for one or more of the components of the apparatus operated to control the temperature of the build bed surface 12 and the layer-specific region may be provided in such a way that it follows the layer cycle of the method and its variants described herein. In an example of such a calibration routine, the thermal sensor 72 may be calibrated with respect to the onset of melting, which represents the start of a phase change for which a change in the thermal behaviour of the particulate material may be expected. A set point for the temperature scale of the thermal sensor 72 may relate to a characteristic material property identified from, for example, a change in the rate of increase of the measured temperature of the layer-specific region 50 due to the onset of melting. To achieve melting of the layer- specific region 50, the layer cycle of a calibration routine for the thermal sensor 72 may be repeated at least until the particulate material within the layer-specific region 50 starts to melt, wherein the step of depositing absorption modifier comprises depositing for each further layer a different amount of absorption modifier compared to the previous layer so as to cause the particulate material of the layer-specific region 50 to absorb more energy from the first heat source L1 than that of the preceding layer. From the layer-to-layer evolution of the temperature measurements of the layer-specific region 50 made during the layer cycle, for example, following the step of block 112 of heating with the first heat source L1 and optionally following the step of block 106 of heating with the second heat source L2, a set point for the thermal sensor 72 may be determined based on a characteristic of the material as identified from a characteristic in the evolution of the measured temperature following the step of block 112 of heating with the first heat source L1 and optionally following the step of block 106 of heating with the second heat source L2. It should be noted that it is not necessary to correct the scale of the thermal sensor 72 to an absolute temperature scale. Instead, it is sufficient to apply a set point relative to the particulate material properties to subsequent measurements of the primary thermal sensor 72, and to control subsequent layers based on the thermal sensor readings relative to this set point. Another example of a calibration routine of a thermal component may be a calibration routine for the first heat source L1 and / or the second heat source L2. A further thermal control component may thus be the first heat source, wherein the calibrated first heat source is used during step (iii) of the build process layer cycle. Where the further thermal control component comprises the first and second heat source L1, L2, the layer cycle of a calibration routine for the first and second heat source L1, L2 may further comprise operating the first heat source L1 during the heating step at block 112 at a first input power profile and operating the second heat source L2 during the heating step at block 106 at a second input power profile, wherein for each layer at least one of the first and second input power profiles is different to that of a preceding layer. The “input power profile” indicates an input power applied along at least one dimension of the build bed surface, for example in direction of movement of the first and / or second heat source. The input power profile may be a constant input power profile, or an input power profile that varies along the direction of movement, i.e. with distance over the build bed surface 12. Block 114 further comprises measuring the temperature of the layer-specific region 50 after the step of heating with the first heat source L1 at block 112 and after the step of heating with the second heat source L2 at block 106. The calibration routine for the first and second heat source further comprises receiving a first target temperature that the layer-specific region 50 should reach after heating with the first heat source L1 and a second target temperature that the layer-specific region 50 should reach after heating with the second heat source L2, and determining, from the measured first and second set of temperatures and based on the first target temperature and the second target temperature, a corrected first input power profile along the first direction for the first heat source L1 and / or a corrected second input power profile along the first direction for the second heat source L2, wherein the outcome of the calibration routine is the corrected first and / or second input power profile. During the layer cycle of the build process 200, the corrected first input power profile to the first heat source is applied during the step at block 212 and / or the corrected second power input profile to the second heat source L2 is applied during the step at block 206 of heating each distributed layer. One of the input power profiles may for example vary along the first direction; for example, the input power profile for the second heat source may be a decreasing input power profile along the first direction. Turning to Figure 5, which is a flow chart of two calibration routines 100_1 and 100_2 of a calibration process 100, it can be seen that in a preferred variant, the layer cycle of each routine comprises the same steps: the layer cycle steps at blocks 104_1 to 114_ 1, which are analogous to those in Figure 2, are the same as those of the second calibration routine, namely the same as those of blocks 104_2 to 114_ 2. Calibration routine 100_1 may be the calibration routine for the thermal sensor 72 and calibration routine 100 2 may be applied subsequently to calibrate the power input to the first and / or the second heat source L1, L2. The second calibration routine 100_2 may thus use the calibrated temperature measurements of the layer-specific region 50 to evaluate the heating effects of the one or more heat sources L1,L2. In some variants of the method, a pre-calibration routine for the first and / or second heat source L1, L2 may be carried out, for example before the calibration routine for the thermal sensor 72. In the case of a calibration routine for the first heat source L1 only, for example, operated as a preheat source and a heat source during the layer cycle, the power input to the first heat source L1 may be corrected with respect to itself when operated in the two different modes. In both cases, the corrections are made with respect to the uncalibrated thermal sensor measurements. The layer cycle of such a ‘pre-calibration’ routine for a single heat source, to be calibrated against different modes of operation of preheating and heating to fuse, may further comprise operating the first heat source L1 during the heating step at block 112 at a power input different to a preceding power input of the preceding layer so as to heat the layer-specific region 50 to a temperature different to that of the layer-specific region 50 of the preceding layer. At block 114, the method comprises measuring the temperature of the layer-specific region 50 after the step of heating the layer-specific region 50 with the first heat source L1 at blocks 106 and 112, and determining a corrected power input for the first heat source L1 based on the temperatures measured for each layer. The outcome of the pre-calibration routine is a correction of the input power profile to the first heat source with respect to its heating modes at blocks 106 and 112. A subsequent calibration routine for calibrating the thermal sensor 72, and / or a subsequent build process 200, may thus comprise applying the correction to the power input of the first heat source L1 during the step of heating with the first heat source L1 at block 112. Where a first and second heat source L1, L2 are used in the apparatus for steps at block 106 and 112, the layer cycle of the pre-calibration routine may further comprise: - operating the first heat source L1, during the heating step at block 112, and the second heat source L2, during the heating step at block 106 following distributing each layer, at respective power inputs different to those of a preceding layer so as to heat the layer- specific region 50 (and preferably so as to heat the entire build bed surface 12, for thermal uniformity); - block 114 comprises measuring the temperature of the layer-specific region 50 after each step of heating with the first heat source L1 at block 112 and measuring the temperatures of the layer-specific region 50 after the step at block 106 of heating with the second heat source L2; and - determining from the measured temperatures at block 114 a calibration to at least one of the power input profiles of the first and second heat source L1, L2 so as to operate the first and second heat sources as relative calibrated first and second heat sources; wherein the calibration to the at least one of the power input profiles is the outcome of the pre-calibration routine. The subsequent calibration routine for calibrating the thermal sensor 72, and / or the subsequent build process 200, may thus comprise applying the outcome of the pre- calibration routine to at least one of the power inputs of the first and / or second heat source L1,L2. In another example, a calibration routine for a third heat source (overhead heater 20) may be carried out after a calibration routine for the thermal sensor 72, wherein the calibration routine for the third heat source is based on measurements using the calibrated thermal sensor 72, and wherein the calibrated third heat source is used during the layer cycle of any subsequent calibration routines and during the layer cycle of the build process. Fig. 6 is a flowchart illustrating a calibration process 100 with three calibration routines 100_1, 100_2 and 100_3. Routine 100_1 may be an optional pre-calibration routine for at least one of the first and second heat sources L1, L2, and is indicated as optional in dashed outline in Fig. 6. An outcome of the calibration routine 100_1 may be a corrected input power profile determined for the at least one of the first and second heat sources L1, L2 based on measurements of the temperature of the layer specific region 50 using the thermal sensor 72. For example, the input power of the first heat source L1 may be adjusted so that its radiated energy is similar to that of the second heat source L2. Following the optional calibration routine 100_1 for the pre-calibration of the heat source(s), a calibration routine 100_2 for the thermal sensor 72 is carried out, in which the pre- corrected power input(s) to at least one of the first and second heat sources L1, L2 are applied during the layer cycle. A set point is determined for the temperature scale of the thermal sensor 72 with respect to a thermal characteristic of the particulate material, wherein the correction is based on the one or more measured temperature profiles during the layer cycle. Following calibration of the set point of the measurement scale of the thermal sensor 72, a further, “final”, heat source calibration routine 100_3 is carried out, wherein during the layer cycle of the final heat source calibration routine 100_3, the calibrated thermal sensor 72 is used to measure the temperature of the build bed surface 12 comprising the layer-specific region 50. During the final heat source calibration routine 100_3, a respective corrected power input profile along the first direction to at least one of the first and second heat sources L1, L2 is determined. In other words, a correction to the power input to the first and / or second heat sources is determined with respect to the calibrated temperature measurements by the thermal sensor 72. Following this, the method of operation as illustrated proceeds to the build process 200, which further comprises applying the respective corrected input power profile along the first direction to the at least one of the first and second heat sources L1, L2, and wherein the temperature measurements taken by the thermal sensor 72 of the build bed surface 12 are calibrated temperature measurements, applying the outcome of the calibration routine 100_2 for the thermal sensor. Thus, the layer cycle of each layer of the subsequent build process comprises the steps of’: - heating (preheating) at block 206 with the second heat source L2 operated at the corrected first input power profile determined during routine 100_3 of Fig. 6 to the input power of the second heat source L2; - heating at block 212 with the first heat source L1 operated at the corrected second input power profile determined during routine 100 3 of Fig. 6; - measuring the temperature of the build bed surface 12 using the calibrated thermal sensor 72 at least once during the layer cycle, wherein the calibrated thermal sensor 72 is provided by applying the outcome determined during calibration routine 100 2 of Fig. 6 to the measurements of the thermal sensor 72. Each of the calibration routines described above comprises the layer cycle of the build process, and such that the steps of distributing each layer, and of passing the second heat source L2 and the first heat source L1 over each layer, are carried out in the same (e.g, first) direction. The layer cycle is repeated a respective number of times for each calibration routine until the respective calibration routine is complete. Preferably, the speed of passing each of the modules over the build bed surface 12 along the first direction is a constant speed that is the same for each module. Furthermore, the same considerations for the layer cycle described with reference to the variant of Fig. 4 and as applied by the carriage layouts of Figs. 1A and 7, or suitable variants, are preferably applied to each of the calibration routines. Thermal sensor and thermal feedback control The thermal sensor 72 may be a pyrometer, an array of pyrometers, an infrared line scan sensor array, or a thermal camera with a high-resolution two-dimensional pixel array able to monitor the entire build bed surface 12. The pixel array may, for example, be arranged such that different groups of pixels monitor corresponding different zones of the build bed surface 12. As described above, a respective one or more heater elements of the overhead heater 20 may be controlled to affect the temperature of a corresponding zone of the build bed surface 12 based on the temperature measurements of the groups of pixels of the thermal sensor 72, thus achieving zonal temperature feedback control. During the layer cycle of the build process and / or the calibration process, the temperature of each layer may be measured more than once using the thermal sensor 72. For example, measurements may be made following each of the layer cycle steps. This allows assessment of the thermal state of the layer between the different steps of the layer cycle. Such measurements may be used as follows. When applied during either or both of the build process 200 and the calibration process 100, a third heat source such as the overhead heater 20 may be operated in response to the measured temperature profiles measured by the thermal sensor 72 during the layer cycle. Preferably, the thermal measurements comprise a plurality of temperature measurements of some, and preferably of a majority, of the build bed surface 12. The third heat source may comprise an array of individually operable heating elements positioned above the build bed surface 12, and the thermal sensor 72 may comprise an array of individual sensor pixels, wherein measuring the temperature at block 114 / 214 of the layer cycle may further comprise: determining a zonal temperature for each of a plurality of zones of the build bed surface 12 as measured by a subset of the sensor pixels; and determining a zonal temperature difference between each zonal temperature and a target temperature for the build bed surface 12 that is lower than a melting temperature of the particulate material. The step of heating each layer by the third heat source may furthermore comprise heating each zone by operating one or more corresponding heating elements in response to the determined zonal temperature difference to adjust the temperature of the build bed surface 12 towards a uniform target temperature. As described above, the third heat source may be operated continuously, such that the step of heating by the third heat source comprises heating each layer continuously during the steps of the layer cycle by operating the third heat source. In preferred methods of operation, the overhead heater 20 (the third heat source) is operated during both the calibration process 100, during each calibration routine, and the build process 200. Furthermore, the overhead heater 20 may be operated continuously during each layer cycle so as to assist in returning the temperature profile of the newly distributed layer to the target temperature and / or to maintain the temperature profile of the build bed surface 12 at or near the target temperature, for example to within no more than + / -1°C deviation. During a build process 200, the heat source L2 may be predominately used to return the temperature profile of the freshly distributed layer towards the target temperature, and the third heat source may be used to maintain the temperature profile of the freshly distributed layer and of the unprocessed regions of the build bed surface 12 (the surrounding areas surrounding the layer-specific region 50 that are not selectively heated by the first heat source L1) at or close to the target temperature throughout the duration of the layer cycle. The build process may thus further comprise operating the second heat source L2 in response to the temperature profiles measured by the thermal sensor 72 so as to preheat each distributed layer to the target temperature between the melting temperature and the solidification temperature of the particulate material. In the case of the first heat source L1, the measured temperature profiles may be measured temperature profiles at least following the step at block 112 of heating with the first heat source L1 (“post-fuse temperature measurements”), and are compared against a pre-defined fusing temperature of the layer-specific region 50. Based on a determined deviation between a measured post-fuse temperature and a predefined fusing temperature, the build process may further comprise operating the first heat source L1 in response to the measured post- fuse temperature profiles to heat each further layer-specific region to the predefined fusing temperature. Deviations from the predefined fusing temperature may be due to local variations in thermal mass within the build bed, as a result of the shape of the object or the location of the object, for example. The first heat source and the second heat source may comprise one or more individually controllable heat sources. For example, a further first heat source may be arranged on the one or more carriages 30_1, 30_2 to provide a further step of heating the layer-specific region during each layer cycle between the step of heating with the first heat source and the step of distributing the further layer using the distribution module. The further first heat source may for example be mounted on the same carriage as the first heat source, and follows the first heat source L1 during movement along the first direction. Alternatively, the further first heat source may be mounted ahead of and on the same carriage as the distribution module 32. Similarly, the further second heat source may be arranged on the one or more carriages to provide a further step of heating the freshly distributed layer during each layer cycle between the step of heating with the second heat source and the step of depositing absorption modifier. The further second heat source may for example be mounted on the same carriage as the second heat source, and follows the second heat source L2 during movement along the first direction. Alternatively, the further second heat source may be mounted ahead of and on the same carriage as the deposition module 38. Absorption modifier variants and use of varying levels of absorption modifier. In the examples described herein, the radiation of the various heating devices (heat source L1, preheat source L2, overhead heater 20) may preferably be infrared radiation, and the absorption modifier in general terms causes the layer-specific region to heat more than the surrounding area during the step of heating with the heat source L1. The absorption modifier may be radiation absorber such as carbon black, or absorption inhibitor, such as water acting as a coolant, or a reflector of the radiation of the heat source. However, other wavelength spectra may be suitable in combination with respective absorption modifiers and particulate materials. For infrared radiation absorber, the layer cycle comprises depositing the radiation absorber over the layer-specific region 50. For absorption inhibitor, the layer cycle comprises depositing the absorption inhibitor at least over the surrounding area surrounding the layer-specific region 50. The entire layer may be provided with inhibitor such that the surrounding area is provided with an amount that prevents melting of the particulate material, and such that the layer-specific region 50 is provided with a comparatively lower amount of inhibitor, or no inhibitor. In this way, the layer-specific region 50 is capable of absorbing more heat from the first heat source L1 than the surrounding area, thus achieving selectivity. In some examples, both types of absorption modifier may be provided to achieve the desired selectivity. For some calibration routines requiring progressive heating of the layer-specific region 50, such as the calibration routine for the thermal sensor at when heating with the second heat source L2 at block 112, it was found that improved calibration outcomes with respect to a subsequent build process 200 may be obtained when the speed profile along the first direction of the first and / or second heat source is substantially the same during each layer cycle. Furthermore, it may be preferable that the period of time of heating by the first and optionally the second heat source is substantially constant and the same for each layer of the layer cycle. Progressive heating effects may be achieved by altering the absorptive properties of the layer-specific region 50, for example by changing the amount of absorption modifier applied to the layer-specific regions 50 of successive layers of build material. This reduces the number of, or avoids any, changes to the input power profile in successive layers. For example, the coverage of the absorption modifier per unit area over the layer-specific region 50 may be altered to achieve progressive heating. For each subsequent layer during the layer cycle of a calibration routine, the method may comprise depositing a further amount per unit area of absorption modifier over the layer-specific region 50, wherein each further amount per unit area of absorption modifier is different to the preceding amount per unit area of absorption modifier. In the case of radiation absorber, the amount of radiation absorber deposited per unit area over the layer-specific-region 50 for each further layer may thus be higher than that deposited over the layer-specific region 50 of the preceding layer. Varying the amount of absorption modifier for each subsequent layer may conveniently be achieved by providing the absorption modifier comprised within a fluid, and depositing the amount of absorption modifier using a droplet deposition module. The coverage may be defined by one or both of the print pattern, determining the spacing between drops deposited (as controlled by the printed image pattern and / or by a dither scheme), and the volume of each drop deposited at each location on the layer-specific region 50. For each further layer therefore, one or more of the following may be applied at block 110: - depositing a different number of fluid drops (for radiation absorber, a higher number; for inhibitor, a lower number) per unit area over the layer-specific region 50 compared to the number of drops deposited per unit area over the layer-specific region 50 of the preceding layer, preferable for a substantially constant volume per drop; and - depositing fluid drops of different volumes (for radiation absorber, a larger volume; for inhibitor, a smaller volume) per unit area over the layer-specific region 50 compared to the volume of each drop deposited per unit area for the preceding layer. For example, a radiation absorbing fluid of certain absorber pigment loading, for example of carbon black, may be provided. By depositing increasing absorber amounts per unit area of the layer-specific region 50 for each subsequent layer, the temperature of the layer- specific region 50 after heating by the first heat source L1 increases layer by layer. The absorption modifier may be deposited during one pass of the droplet deposition module or more over the build bed surface 12. Additionally, or instead, the type of absorption modifier may be altered to achieve progressive heating. Multiple absorption modifiers (e.g. radiation absorbers) may be provided to the apparatus such that for each further layer, a different absorption modifier is deposited over the layer-specific region 50, wherein each different absorption modifier is capable of causing the particulate material of the layer-specific region 50 to absorb a different amount of energy of radiation from the heat source compared to the absorption modifier deposited over the layer-specific region of the preceding layer. Each absorption modifier may comprise a different colour of absorption modifier capable of absorbing a different, larger, amount of energy of the radiation spectrum provided by the heat source compared to that of the preceding amount of absorption modifier (radiation absorber or inhibitor). Multiple fluids provided to the apparatus may be deposited in an overlapping multi-fluid pattern to vary the absorption of the energy of the radiation of the heat source. The radiation absorber (or absorption modifier) may be deposited in the form of a multi- fluid pattern, wherein the multi-fluid pattern deposited over the layer-specific region 50 of each further layer is different to that deposited over the layer-specific region 50 of the preceding layer, and such that the layer-specific region 50 is capable of absorbing a higher amount of energy of radiation provided by the heat source compared to the layer-specific region 50 of the preceding layer. For the calibration routine of the first and / or second heat source, a plurality of subregions may be measured by the thermal sensor after each heating step. At least two different absorption modifier amounts may be deposited over two respective ones of the plurality of regions so that two temperatures are measured after each step of heating. For example, the layer specific region 50 may comprise a first and a second layer-specific subregion, and the calibration routine may further comprise at block 106, heating a first layer-specific subregion with the second heat source L2; block 114 comprises measuring a first temperature of the first layer-specific subregion using the thermal sensor 72; block 110 comprises: depositing a first amount of absorption modifier in the form of radiation absorber over the first layer-specific subregion and a second amount of absorption modifier in the form of radiation absorber over a second layer-specific subregion; or, depositing a first amount of absorption modifier in the form of absorption inhibitor over a surrounding area surrounding the first layer-specific subregion and the second layer-specific subregion region, and depositing a second amount of absorption modifier in the form of inhibitor over the second layer-specific subregion. The first and second amounts are chosen so that the step of heating the first and second layer-specific subregions with the first heat source L1 at block 112 causes the following: where the absorption modifier is absorber, the first amount causes the first layer-specific subregion to heat to a higher temperature than the second layer- specific subregion; or where the absorption modifier inhibitor, the second amount causes the second layer-specific subregion to heat to a higher temperature than the surrounding area and to a lower temperature than the first layer-specific subregion. At block 114, measurements by the thermal sensor further comprise measuring a second temperature of the first layer-specific subregion and a third temperature of the second layer-specific subregion, and determining from the measured first, second and third temperatures for each pair of input power profiles a corrected input power profile for at least one of the first and second heat source. The mount of absorption modifier may be varied by any of the above described approaches. The method of operation and its variants as described may be carried out by a controller 70 of the apparatus 1 and configured to carry out any of the method or variants in part or fully. Corrections of the thermal sensor 72 based on the determined set point may be applied to the temperature measurements of each group of pixels or to each pixel of the thermal sensor 72. The set point may be determined again at any time to adjust for changes in material properties due to recycling rate and / or ageing of at least some of the components of the particulate material.

Claims

CLAIMS 1. A method of operation for an apparatus for the layer-by-layer manufacture of a 3D object from particulate material, the apparatus comprising a first heat source configured to be moveable across the layer and a thermal sensor and configured to measure the temperature of the layer; the method comprising: carrying out a thermal calibration process comprising a calibration routine for a thermal control component of the layer, and subsequently carrying out a build process to manufacture a 3D object; wherein the calibration process and the build process each comprise a layer cycle comprising the steps of’ (1) distributing a layer of particulate material over a build area, the layer providing a build bed surface of the build area; (11) depositing absorption modifier over at least one of a layer-specific region within the build bed surface and a surrounding area surrounding the layer-specific region; and (iii) heating the layer-specific region over a period of time by moving the first heat source over the layer while operating it; (iv) measuring the temperature of the build bed surface using the thermal sensor at least once during one or more of steps (1) to (iii); wherein the layer cycle of steps (1) to (iv) is repeated a number of times until the respective calibration process and the build process are complete; wherein an outcome of the thermal calibration routine is based on the measured temperatures and is applied to the thermal control component for the subsequent layer cycle; wherein, for the layer cycle of the build process, the first heat source in step (iii) is operated to cause the layer-specific region to reach the melting temperature of the particulate material and cause it to melt such that the melted layer-specific region constitutes a cross section of the object for that layer; and wherein for both the thermal calibration process and the build process, the layer cycle step (1) of distributing each layer and (iii) of passing the first heat source over each layer are carried out in a first direction.

2. The method of claim 1, wherein step (ii) comprises moving a deposition module over the build bed surface while operating the deposition module to deposit absorption modifier.

3. The method of claim 2, wherein step (ii) comprises moving the droplet deposition module over the build bed surface in the first direction.

4. The method of claim 2 or claim 3, wherein the absorption modifier is comprised within a fluid, wherein the deposition module is a droplet deposition module, and wherein step (ii) comprises operating the droplet deposition module to deposit absorption modifier in the form of droplets.

5. The method of any preceding claim, wherein the layer cycle further comprises the step of heating the build bed surface by operating a second heat source between the steps of distributing the layer and depositing absorption modifier.

6. The method of claim 5, further comprising passing the second heat source over the build bed surface in the first direction while operating the second heat source.

7. The method of any preceding claim, wherein step (iv) of the layer cycle further comprises measuring the temperature of each build bed surface more than once.

8. The method of claim 7 further comprising, during the step (iv) of the layer cycle of the build process: measuring a post-fuse temperature of the layer-specific region after the step (iii) of heating with the first heat source; and heating a subsequent layer- specific region by operating the first heat source in response to the measured post-fuse temperature.

9. The method of any one of claims 7 or 8 when dependent on claim 5 or claim 6, further comprising during the step (iv) of the layer cycle of the build process: measuring a preheat temperature of build bed surface after the step of heating with the second heat source, and operating the second heat source over a subsequent layer cycle in response to the measured preheat temperature.

10. The method of any preceding claim, further comprising heating each layer by operating a third heat source in response to the one or more measured temperatures.

11. The method of claim 10, wherein the third heat source comprises an array of individually operable heating elements positioned above the build bed surface, and wherein the thermal sensor comprises an array of individual sensor pixels, wherein measuring the temperature at step (iv) of the layer cycle comprises: determining a zonal temperature for each of a plurality of zones of the build bed surface as measured by a subset of the sensor pixels; and determining a zonal temperature difference between each zonal temperature and a target temperature, wherein the target temperature is lower than a melting temperature of the particulate material; wherein the step of heating each layer by the third heat source comprises heating each zone by operating one or more corresponding heating elements in response to the determined zonal temperature difference.

12. The method of claim 10 or claim 11, wherein the third heat source is operated continuously during the layer cycle.

13. The method of any preceding claim, wherein the thermal control component is the thermal sensor, and wherein step (iii), when carried out during at least one of the layer cycles of the calibration routine for the thermal sensor, comprises: operating the first heat source at a fusing power input so as to cause the particulate material of the layer- specific region to melt; and wherein the one or more measured temperatures of step (iv) of the layer cycles are used to determine a set point for the temperature scale of the thermal sensor based on a thermal characteristic of the particulate material, and to calibrate the thermal sensor to the set point, and wherein the measurements by the thermal sensor in step (iv) of the build process layer cycle are calibrated temperature measurements.

14. The method of any preceding claim, wherein the or a further thermal control component is the first heat source, and wherein the temperatures measured in step (1v) of the calibration layer cycle are used to calibrate the first heat source during step (iii) of the build process layer cycle.

15. The method of claim 5, or of any one of claims 6 to 13 when dependent on claim 5, wherein the or a further thermal control component is the first heat source and the second heat source, wherein the layer cycle of the calibration routine further COMPprises: operating the first heat source at a first input power profile during the step (iii); operating the second heat source L2 at a second input power profile; wherein for each layer each pair of first and second input power profiles is different to a preceding pair of a preceding layer by at least one of the first and second input power profiles; wherein step (iv) further comprises measuring a first set of temperatures of each layer- specific region after step (iii) of heating with the first heat source and measuring a second set of temperatures of each layer-specific region after the step of heating with the second heat source; and receiving a first target temperature for the layer-specific region after heating with the first heat source and a second target temperature of the layer-specific region after heating with the second heat source; determining, from the measured first and second set of temperatures, and based on the first target temperature and the second target temperature, a corrected first input power profile along the first direction for the first heat source and / or a corrected second input power profile along the first direction for the second heat source, wherein the outcome of the calibration routine is the corrected first and / or second input power profile; and wherein the layer cycle of the build process comprises applying the corrected first input power profile to the first heat source during the step (iii) and / or applying the corrected second power input profile to the second heat source during the step of heating each distributed layer.

16. The method of claim 12, or of claim 13 or of claim 14 when dependent on claim 12, wherein the calibration process comprises a pre-calibration routine for the first heat source carried out before the calibration routine for the thermal sensor, wherein successive layer cycles of the pre-calibration routine comprise: operating the first heat source during step (iii) at a power input different to a preceding power input of a preceding layer so as to heat the layer-specific region to a temperature different to that of the layer-specific region of the preceding layer; wherein step (iv) comprises measuring the temperature of the layer specific region following the step of heating with the first heat source at step (iii); wherein the outcome of the pre-calibration routine is a corrected power input for the first heat source based on the measured temperatures of the layer-specific region of each layer; and wherein the calibration routine for the thermal sensor comprises, during step (iii) of heating with the first heat source, applying the corrected the power input of the first heat source.

17. The method of any preceding claim, wherein the layer-specific region for the or the further calibration routine comprises a plurality of layer-specific regions.

18. The method of any preceding claim, wherein the speed profile for distributing each layer in step (1) is the same as the speed profile for passing the first heat source over the layer in step (iii).

19. The method of claim 6, or of any one of claims 7 to 17 when dependent on claim 6, wherein the speed profile for heating each layer by operating the second heat source is a constant speed and substantially equal to the speed profile for passing the first heat source over the layer in step (iii) of each layer.

20. The method of any preceding claim, wherein respective time periods between the initiation of each step and the initiation of the previous step in each layer cycle are constant for each corresponding step in each successive layer cycle.

21. The method of any preceding claim, wherein for each layer the layer cycle further COMPprises: initiating the step (iii) of heating each layer with the first heat source after a predefined first time interval after initiating the step (i) of distributing the layer; and initiating the step (i) of distributing each layer after a predefined second time interval after the step (iii) of heating the previous layer with the first heat source; and wherein the period of time of the layer cycle is the same for each layer.

22. The method of claim 5, or of any one of claims 6 to 20 when dependent on claim 5, wherein for each layer, the layer cycle further comprises initiating the step of heating the layer with the second heat source after a predefined third time interval after initiating the step (i) of distributing the layer.

23. The method of any preceding claim, wherein each heat source is an infrared heat source, and wherein the absorption modifier is one of’ an infrared radiation absorber, wherein step (ii) comprises depositing the radiation absorber over the layer-specific region within the build bed surface, and wherein during step (iii) the radiation of the first heat source causes the particulate material of the layer-specific region to absorb more energy than the surrounding area; and an infrared radiation inhibitor, wherein step (ii) comprises depositing the radiation absorber over the surrounding area surrounding the layer-specific region within the build bed surface, and wherein during step (ii1) the radiation of the first heat source causes the particulate material of the layer-specific region to absorb more energy than the surrounding area.

24. A controller for an apparatus for the layer-by-layer manufacture of a 3D object from the particulate material, configured to carry out the method of any one of claims 1 to 23 25. An apparatus for the layer-by-layer manufacture of a 3D object from the particulate material configured to carry out the methods according to any one of claims 1 to 23 and comprising the controller of claim 24. CLAIMS 1. A method of operation for an apparatus for the layer-by-layer manufacture of a 3D object from particulate material, the apparatus comprising a first heat source configured to be moveable across the layer, an overhead heat source comprising an array of individually operable heating elements, and a thermal sensor comprising an array of individual sensor pixels and configured to measure the temperature of the layer; the method comprising: carrying out a thermal calibration process comprising a calibration routine for a thermal control component of the layer, and subsequently carrying out a build process to manufacture a 3D object; wherein the calibration process and the build process each comprise a layer cycle comprising the steps of: (1) distributing a layer of particulate material over a build area, the layer providing a build bed surface of the build area; (11) depositing absorption modifier over at least one of a layer-specific region within the build bed surface and a surrounding area surrounding the layer-specific region; and (iii) heating the layer-specific region over a period of time by moving the first heat source over the layer while operating it; (iv) measuring the temperature of the build bed surface using the thermal sensor at least once during one or more of steps (1) to (iii); wherein the layer cycle steps are repeated a number of times until the respective calibration process and the build process are complete; wherein the overhead heat source is positioned above the build bed surface and wherein the layer cycle comprises heating each layer by operating the overhead heat source in response to the one or more measured temperatures; wherein measuring the temperature at step (iv) of the layer cycle comprises: determining a zonal temperature for each of a plurality of zones of the build bed surface as measured by a subset of the individual sensor pixels; and determining a zonal temperature difference between each zonal temperature and a target temperature, wherein the target temperature is lower than a melting temperature of the particulate material; wherein heating each layer by the overhead heat source comprises heating each zone by operating one or more corresponding heating elements of the array of individually operable heating elements in response to the determined zonal temperature difference; wherein an outcome of the thermal calibration routine is based on the measured temperatures and is applied to the thermal control component for the subsequent layer cycle; wherein, for the layer cycle of the build process, the first heat source in step (iil) is operated to cause the layer-specific region to reach the melting temperature of the particulate material and cause it to melt such that the melted layer-specific region constitutes a cross section of the object for that layer; and wherein for both the thermal calibration process and the build process, the layer cycle step (1) of distributing each layer and (iii) of passing the first heat source over each layer are carried out in a first direction.

2. The method of claim 1, wherein step (ii) comprises moving a deposition module over the build bed surface while operating the deposition module to deposit absorption modifier.

3. The method of claim 2, wherein step (ii) comprises moving the droplet deposition module over the build bed surface in the first direction.

4. The method of claim 2 or claim 3, wherein the absorption modifier is comprised within a fluid, wherein the deposition module is a droplet deposition module, and wherein step (ii) comprises operating the droplet deposition module to deposit absorption modifier in the form of droplets.

5. The method of any preceding claim, wherein the layer cycle further comprises the step of heating the build bed surface by operating a second heat source between the steps of distributing the layer and depositing absorption modifier.

6. The method of claim 5, further comprising passing the second heat source over the build bed surface in the first direction while operating the second heat source.

7. The method of any preceding claim, wherein step (iv) of the layer cycle further comprises measuring the temperature of each build bed surface more than once.

8. The method of claim 7 further comprising, during the step (iv) of the layer cycle of the build process: measuring a post-fuse temperature of the layer-specific region after the step (iii) of heating with the first heat source; and heating a subsequent layer-specific region by operating the first heat source in response to the measured post-fuse temperature.

9. The method of any one of claims 7 or 8 when dependent on claim 5 or claim 6, further comprising during the step (iv) of the layer cycle of the build process: measuring a preheat temperature of build bed surface after the step of heating with the second heat source, and operating the second heat source over a subsequent layer cycle in response to the measured preheat temperature.

10. The method of any preceding claim, wherein the overhead heat source is operated continuously during the layer cycle.

11. The method of any preceding claim, wherein the thermal control component is the thermal sensor, and wherein step (iii), when carried out during at least one of the layer cycles of the calibration routine for the thermal sensor, comprises: operating the first heat source at a fusing power input so as to cause the particulate material of the layer-specific region to melt; and wherein the one or more measured temperatures of step (iv) of the layer cycles are used to determine a set point for the temperature scale of the thermal sensor based on a thermal characteristic of the particulate material, and to calibrate the thermal sensor to the set point, and wherein the measurements by the thermal sensor in step (iv) of the build process layer cycle are calibrated temperature measurements.

12. The method of any preceding claim, wherein the or a further thermal control component is the first heat source, and wherein the temperatures measured in step (iv) of the calibration layer cycle are used to calibrate the first heat source during step (iii) of the build process layer cycle.

13. The method of claim 5, or of any one of claims 6 to 11 when dependent on claim 5, wherein the or a further thermal control component is the first heat source and the second heat source, wherein the layer cycle of the calibration routine further comprises: operating the first heat source at a first input power profile during the step (iii); operating the second heat source at a second input power profile; wherein for each layer each pair of first and second input power profiles 1s different to a preceding pair of a preceding layer by at least one of the first and second input power profiles; wherein step (iv) further comprises measuring a first set of temperatures of each layer-specific region after step (iii) of heating with the first heat source and measuring a second set of temperatures of each layer-specific region after the step of heating with the second heat source; and receiving a first target temperature for the layer-specific region after heating with the first heat source and a second target temperature of the layer-specific region after heating with the second heat source; determining, from the measured first and second set of temperatures, and based on the first target temperature and the second target temperature, a corrected first input power profile along the first direction for the first heat source and / or a corrected second input power profile along the first direction for the second heat source, wherein the outcome of the calibration routine is the corrected first and / or second input power profile; and wherein the layer cycle of the build process comprises applying the corrected first input power profile to the first heat source during the step (iii) and / or applying the corrected second power input profile to the second heat source during the step of heating each distributed layer.

14. The method of claim 10, or of claim 11 or of claim 12 when dependent on claim 10, wherein the calibration process comprises a pre-calibration routine for the first heat source carried out before the calibration routine for the thermal sensor, wherein successive layer cycles of the pre-calibration routine comprise: operating the first heat source during step (iii) at a power input different to a preceding power input of a preceding layer so as to heat the layer-specific region to a temperature different to that of the layer-specific region of the preceding layer; wherein step (iv) comprises measuring the temperature of the layer specific region following the step of heating with the first heat source at step (iii); wherein the outcome of the pre-calibration routine is a corrected power input for the first heat source based on the measured temperatures of the layer-specific region of each layer; and wherein the calibration routine for the thermal sensor comprises, during step (iii) of heating with the first heat source, applying the corrected the power input of the first heat source.

15. The method of any preceding claim, wherein the layer-specific region for the further calibration routine comprises a plurality of layer-specific regions.

16. The method of any preceding claim, wherein the speed profile for distributing each layer in step (1) is the same as the speed profile for passing the first heat source over the layer in step (iii).

17. The method of claim 6, or of any one of claims 7 to 15 when dependent on claim 6, wherein the speed profile for heating each layer by operating the second heat source is a constant speed and substantially equal to the speed profile for passing the first heat source over the layer in step (iii) of each layer.

18. The method of any preceding claim, wherein respective time periods between the initiation of each step and the initiation of the previous step in each layer cycle are constant for each corresponding step in each successive layer cycle.

19. The method of any preceding claim, wherein for each layer the layer cycle further COMPprises: initiating the step (iii) of heating each layer with the first heat source after a predefined first time interval after initiating the step (i) of distributing the layer; and initiating the step (1) of distributing each layer after a predefined second time interval after the step (111) of heating the previous layer with the first heat source; and wherein the period of time of the layer cycle is the same for each layer.

20. The method of claim 5, or of any one of claims 6 to 18 when dependent on claim 5, wherein for each layer, the layer cycle further comprises initiating the step of heating the layer with the second heat source after a predefined third time interval after initiating the step (1) of distributing the layer.

21. The method of any preceding claim, wherein each heat source is an infrared heat source, and wherein the absorption modifier is one of’ an infrared radiation absorber, wherein step (ii) comprises depositing the radiation absorber over the layer-specific region within the build bed surface, and wherein during step (iii) the radiation of the first heat source causes the particulate material of the layer-specific region to absorb more energy than the surrounding area; and an infrared radiation inhibitor, wherein step (ii) comprises depositing the radiation absorber over the surrounding area surrounding the layer-specific region within the build bed surface, and wherein during step (iii) the radiation of the first heat source causes the particulate material of the layer-specific region to absorb more energy than the surrounding area.

22. A controller for an apparatus for the layer-by-layer manufacture of a 3D object from the particulate material, configured to carry out the method of any one of claims 1 to 21.

23. An apparatus for the layer-by-layer manufacture of a 3D object from the particulate material configured to carry out the methods according to any one of claims 1 to 21 and comprising the controller of claim 22.

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