Control of injection moulding process

Abstract

A method of controlling an injection moulding process in an injection moulding machine (10), the injection moulding machine (10) comprising - a mould (20) comprising one or more of mould cavities (30), and - an injection unit (40) configured for injecting a molten polymer material (50) into the one or more mould cavities (30), wherein the method comprises the steps of G. injection molding a plurality of moulded articles (A) in a plurality of sequential injection cycles; H. at a predetermined interval of a first number (n1) of injection cycles, selecting the moulded articles (A) produced in an injection cycle; I. conveying the selected moulded articles (A) to a weighing device; J. measuring, by the weighing device, said selected moulded articles (A) to obtain a measured mass (Wm) of the selected moulded articles (A); K. comparing the measured mass (Wm) of the selected moulded articles (A) with a target mass (Wt) for the selected moulded articles (A); and L. controlling, based on the comparison between the measured mass (Wm) and the target mass (Wt), the injection moulding machine by adjusting a moulding parameter of the injection moulding machine, to adjust the mass of melted polymer material in subsequent injection cycles, wherein the steps B-F is performed by collecting moulded articles (A) from a second number (n2) of injection cycles, and where the collected moulded articles are measured to obtain a measured mass (Wm) of the moulded articles (A) collected over the second number (n2) of injection cycles; and wherein, when the measured mass (Wm) is determined to be lower than or higher than the target mass (Wt), the interval of a first number (n1) of injection cycles is reduced to a predetermined third number (n3) of injection cycles.

Description

[0001] CONTROL OF INJECTION MOULDING PROCESS

[0002] The present invention relates to control of an injection moulding machine. More particularly, the invention concerns a method of controlling an injection moulding process.

[0003] Background of the invention

[0004] Injection moulding is a key process for manufacturing plastic parts. The parts are used in a broad range of sectors such as automotive, medical, packaging and the toy industry. The demand for plastic parts are growing, and the broad use cases increases the requirement for both quality, tolerances and competitive pricing.

[0005] The sustainable agenda in the public domain comes on top of the increased plastic consumption in the world, resulting in larger demands for both virgin and reused plastic material. Resilience in the supply chain has further put focus on the use of multiple vendors to ensure continued production under all situations, including situation where supply of various goods are impeded, such as experienced under the so-called covid-19 crises. This challenge manufactures to find methods to produce with plastic material from multiple vendors or plastic material with increased variance between batches. The increased variance of raw material needs to be handled to ensure a uniform high quality of the manufactured plastic parts.

[0006] Injection moulding of articles made from a polymer is done in injection moulding machines comprising a mould having one or more mould cavities and an injection unit configured for injecting a polymer material into the one or more mould cavities of the mould. When the one or more mould cavities have been filled with molten polymer material by the injection unit, after a brief cooling time, the one or more moulded articles is / are removed from the mould, and normally collected for after treatment, decoration, packaging, etc. When injection-moulding polymer objects it is important to secure a uniform quality of the moulded objects, such that the injection moulded articles obtain the same size, finish, etc. This requires control over at least some of a host of moulding parameters. Such include the size of the cavity, properties of the polymer material, injection pressure, injection time, injection temperature (of the melted polymer material), mould temperature, etc. In order to control these, various types of sensors may be provided in the injection moulding machine, such that control of a moulding parameter may be based on sensor information about the injection process. For example, especially for producing large quantities of identical moulded articles over long operating periods (very large numbers of injection cycles, such as way larger than 1000 injection cycles), it is known to control the injection pressure based on sensor input regarding the viscosity of the melted polymer material. In this case sensors may be provided in for example the injection nozzle. During such elongated production periods, once an initial calibration of the injection moulding process has been done, often the injection moulding process is stable over long periods of time (injection cycles).

[0007] Many such control mechanisms are either inaccurate or technically cumbersome and requires very expensive equipment to carry out with sufficient precision in order to secure a uniform quality of the moulded articles.

[0008] US 6,019,917 discloses a method for controlling an injection moulding press, wherein the injection process is controlled by controlling the injection force, is disclosed. According to the method, the mass (m) of a moulded product is correlated with the viscosity (n) and either the pressure (Pm) of the material in the mould or the injection pressure (Ph), the resulting correlations are processed to define an analytical model expressing the mass (m) of the moulded product in relation to the operating parameters, an injection force control law is derived from the analytical model on the basis either of the pressure (Pm) of the material in the mould or of the injection pressure (Ph), and the press injection force is controlled on the basis of the analytical model. Thus, US 6,019,917 discloses a method which is cumbersome, and requires measurements through multiple sensors in the mould. There is thus a need for a simple, robust, and cost effective way of securing control of uniform quality of moulded articles, formed in an injection moulding process, over prolonged periods of constant production using an injection moulding machine.

[0009] Summary of the invention

[0010] Thus, it is an object of the invention to provide simple, robust, and cost effective way of securing control of uniform quality of moulded articles, formed in an injection moulding process, over prolonged periods of constant production using an injection moulding machine.

[0011] This is achieved by a method of controlling an injection moulding process in an injection moulding machine, the injection moulding machine comprising

[0012] - a mould comprising one or more of mould cavities, and

[0013] - an injection unit configured for injecting a molten polymer material into the one or more of mould cavities, wherein the method comprises the steps of

[0014] A. injection molding a plurality of moulded articles in a plurality of sequential injection cycles;

[0015] B. at a predetermined interval of a first number of injection cycles, selecting the moulded articles produced in an injection cycle;

[0016] C. conveying the selected moulded articles to a weighing device;

[0017] D. measuring, by the weighing device, said selected moulded articles to obtain a measured mass of the selected moulded articles;

[0018] E. comparing the measured mass of the selected moulded articles with a target mass for the selected moulded articles; and

[0019] F. controlling, based on the comparison between the measured mass and the target mass, the injection moulding machine by adjusting a moulding parameter of the injection moulding machine, to adjust the mass of melted polymer material in subsequent injection cycles. The target mass is adjusted according to at least the expected number of moulded articles collected.

[0020] Experiments has shown that there is a good correlation between the quality of moulded articles (as defined by it dimensions) and the mass of the moulded articles. Thus, the method according to the invention provides an approach for simple quality control that can be adapted to any injection moulding machine with access to the machine controller.

[0021] In an embodiment, the mould is of a type comrprising a plurality of mould cavities, whereby a plurality of moulded articles may be moulded simultaneously in each injection moulding cycle. The mass of the plurality of moulded articles of at least one “shot” (articles moulded in one injection moulding cycle) is determined rather than the mass of single moulded article of the shot.

[0022] This approach does not rely on any material data or sensors within the mold, making it suitable for retrofitting in existing factories.

[0023] Thereby, a control of the injection moulding procedure is provided, which secures an improved quality of the moulded articles, which is very simple to implement, and very robust, and which may be obtained by very simple means, potentially replacing very expensive measuring and control equipment. Further, experiments has shown, that the method of controlling is very robust over a wide range of plastic / polymer material types, such that the simple control may be obtained in a simple and inexpensive way, also when new types of plastic granulate is used in existing injection moulding machines.

[0024] In the method, the moulded articles of the plurality of moulded article in the cavities of the mould are preferably all of the same shape.

[0025] In an embodiment the step of controlling is performed exclusively based on the comparison between the measured mass and the target mass. In principle, in an embodiment the steps B-F may be performed continuously every injection cycle (n 1 = 1 ). Thus, in this embodiment, a sample of molded articles is collected every injection cycle, which sample is measured (weighed), compared against the target mass, and the machine is controlled, based on the comparison from cycle to cycle.

[0026] It is also conceivable that the steps B-F may be performed every first number of injection cycles (n1 >1 ), such that a sample of moulded articles of one injection cycle is collected at regular intervals, which sample is measured (weighed), compared against the target mass, and the machine is controlled, based on the comparison.

[0027] However, in an alternative embodiment of the method, the collection of moulded articles for obtaining the measured mass, comparing the measured mass with the target mass, and controlling based on the comparison, involves collecting moulded articles from a second number of injection cycles. Then, the collected moulded articles are measured (weighed) to obtain a measured mass of the moulded articles collected over the second number of injection cycles.

[0028] Thus, for every first number, n1 , of injection cycles, a sample of moulded articles from a second number, n2, of injection cycles are collected, weighed, the measured mass is compared to the target mass, and the control of the injection machine is based on the comparison.

[0029] The target mass is adjusted according to at least the expected number of moulded articles collected from the second number of injection cycles.

[0030] The second number of injection cycles, where moulded articles are collected for mass measurement, may be consecutive injection cycles, or they may collected regular intervals, for example every second injection cycle, every third injection cycle, every fourth injection cycle, etc., until the collection over the second number of injection cycles has been obtained.

[0031] In these embodiments, the second number of injection cycles is greater than one. In some embodiments hereof, the second number of injection cycles is much smaller than the number of injection cycles between collecting samples of the moulded articles, first number, n1. Thus, 1<n2«n1.

[0032] It will be appreciated that in some embodiments, the moulded articles collected over the second number of injection cycles, may be collected and measured, using the weighing device, as one batch.

[0033] Alternatively, the moulded articles collected over the second number of injection cycles, may be collected, transported and measured, using the weighing device, separately for each injection cycle. In this case, the measured mass is registered and summed for the second number of injection cycles, before the comparison with the target mass.

[0034] In a further embodiment, when the measured mass is determined to be lower than or higher than the target mass, the interval of a first number, n1 , of injection cycles is reduced to a predetermined third number, n3, of injection cycles.

[0035] This can also be formulated such that the interval between the steps of collecting (over one or the second number, n2, injection cycles) the molded articles and measuring (weighing) the mass of the collected moulded articles is reduced from the first number, n1, to a predetermined third number, n3, of injection cycles.

[0036] Thereby, if a the moulding parameter is controlled to adjust the volume of melted polymer material in the subsequent injection cycles, the time until the next sample is reduced such that it can be checked if the desired adjustment has been achieved.

[0037] Any selected moulded articles deviating (for example with more than a threshold value) from the target mass may be discarded. Discarded moulded articles may be grinded and the plastic reused.

[0038] In some embodiment the step of measuring, by the weighing device, said selected moulded articles to obtain a measured mass of the selected moulded articles is performed separately for each of the second number, n2, of selections of moulded articles. In further embodiments, the second number, n2, of measurements are registered and saved.

[0039] In embodiments where the second number, n2, of measurements are registered and saved, then, in the step of comparing the measured mass of the selected moulded articles with a target mass for the selected moulded articles, the measurements are given individual weights.

[0040] In one embodiment, the last measurements are weighed higher than the earlier measurements.

[0041] Thereby, the later measurements of the mass of the selected moulded articles are given higher emphasis in the comparison of the measured mass and the target mass. Thereby, in a simple way, it may be avoided that the control of the injection moulding machine is influenced by singular errors in measurements or singular errors in an injection cycle (e.g. the filling of one or more cavities), while still having an improved ability to detect and correct a long term drifting in the quality of the produced moulded articles.

[0042] In an embodiment of the last described embodiment, the last measurement is given the highest weight, the second last measurement, a weight lower the last, and so on.

[0043] In a further embodiment of any of the previous embodiments, a. if the measured mass (Wm) is lower than the target mass (Wt), the moulding parameter is adjusted so that the mass of molten material injected into the cavity is increased in the following injection cycles; b. if the measured mass (Wm) is higher than the target mass (Wt), the moulding parameter is adjusted so that the mass of molten material injected into the cavity is reduced in the following injection cycles; and c. if a difference between the measured mass (Wm) and the target mass (Wt) is below a predetermined threshold, the moulding parameter is maintained constant. In a further embodiment of any of the previous embodiments, the moulding parameter for controlling the injected volume of melted polymer material is one or more of melt temperature, mould temperature, injection pressure, holding pressure, injection flow rate, and melt pressure.

[0044] In a further embodiment of any of the previous embodiments, the injection moulding machine is controlled by a control system, wherein the control system is connected to a weighing device forming part of or located in the vicinity of the injection moulding machine, and wherein the measured mass is transferred automatically to the control system.

[0045] Alternatively, the measured mass, Wm, may be entered into the control system manually.

[0046] In a further embodiment of any of the previous embodiments, the step of selecting the moulded articles may be performed manually.

[0047] In a further embodiment of any of the previous embodiments, the step of measuring the mass (weighing) the moulded articles is performed manually.

[0048] As an alternative, to the last three mentioned embodiments, the injection moulding machine may form part of an injection moulding system, where the injection moulding system further comprises

[0049] - a selecting device for selecting moulded articles from the mould , the selecting device being controlled by the control system; and

[0050] - a transport device for conveying the selected articles to the weighing device, the transport device being controlled by the control system, where

[0051] - the step of selecting moulded articles is performed by the selecting device, and

[0052] - the step of conveying the selected moulded articles to the weighing device is performed by the transport device. In a further embodiment of any of the previously mentioned embodiments, if the raw material for the molten polymer material is changed during the injection moulding process (step A), the selecting of the moulded articles produced in the one or more injection cycles is commenced immediately after to the change from one the molten polymer material to another.

[0053] Such a change of molten plastic material could occur for a number reasons. For example the raw material could be replenished with grinded reused material from runners, or the plastic raw material was changed to a similar material from another vendor. There are often only minute differences in the material properties, but these minute differences can have large effects on the injection moulding process.

[0054] When filling in a new raw material in the hopper / barrel, it takes a while before the new material is melted in the injection unit of the injection moulding machine and reaches the mould. The exact timing of this cannot be determined.

[0055] The weights may be changed to put height emphasis to the latest measurements

[0056] It should be emphasized that the term "comprises / comprising / comprised of' when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0057] Brief description of the drawings

[0058] In the following, the invention will be described in greater detail with reference to embodiments shown by the enclosed figures. It should be emphasized that the embodiments shown are used for example purposes only and should not be used to limit the scope of the invention.

[0059] Fig. 1 , in a principle sketch, shows an injection moulding machine; Fig. 2, in a principle sketch, shows an injection moulding system comprising a an injection moulding machine, a weighing device and a controls system, for controlling at least the injection moulding machine; and

[0060] Fig. 3 shows a flowchart of a control flow for a production cycle of an injection molding machine in a method according to one embodiment of the invention.

[0061] Detailed description of the embodiments

[0062] Fig. 1 illustrates schematically an injection moulding machine 10 known in the art. The injection moulding machine 10 generally comprises an injection unit 40, shown in the left side of the figure, and a clamping unit 70, shown in the right side of the figure. The injection unit 40 handles injection of plastic material (polymer material) into a mould 20 arranged in the clamping unit 70 of the injection moulding machine 10. The injection unit 40 and the clamping unit 70 of the injection moulding machine 10 are attachable to a mount or frame 80.

[0063] Injection moulding machines 10 generally works in the following way: Plastic granules 90 are fed into a barrel 41 of a reciprocating screw conveyor 42 of the injection unit 40 via a hopper 43. The reciprocating screw conveyor 42 is driven by a drive mechanism 44, such as an electrical motor. The plastic granules 90 fed through the hopper 43 are thus transported towards the mould 20 provided in the clamping unit 70 by the reciprocal screw, while being compacted and heated by heating devices 45 surrounding the reciprocating screw conveyor 42, until the granules melt and reach a suitable viscosity at a nozzle 95 at the entrance to the clamping unit 70 with the mould 20.

[0064] The fluid, melted plastic material is fed from the nozzle 95 through sprue channels 21 in at least one mould half, first mould plate 22 of the mould 20, and reaches a mould cavity 30 formed in the second mould of the mould 20. A second mould half, second mould plate 23, which may comprise a mould core and / or further portions of a mould cavity 30 is arranged moveably relative to the first mould plate 22, such that the mould 20 may be completely closed (clamped together) to allow injection of the melted plastic, and such that the mould 20 may be opened to extracted a moulded article A. The closing and clamping together of the first and second mould plates 22, 23, as well as the separation thereof is powered by a linear drive mechanism 50, such as a hydraulic or electronic mechanism.

[0065] When the molten plastic / polymer material has filled the mould cavity 30 completely, and has cooled sufficiently for the plastic to be in solid state, the first and second mould plates 22, 23 are separated, and the moulded article, A, is ejected from the mould 20. The ejection of the moulded article A (illustrated in Fig. 2) is typically done by ejector pins (not shown) formed in / through portions of the mould 20.

[0066] It will be appreciated that the mould 20 and the injection moulding machine 10 are described in a simple way, and that both can be much complex. For example the mould 20 may comprise more than two (half) plates, and the runner may be separated from the moulded articles when the mould is reopened after filling.

[0067] In the process according to the present invention, the mould 20 may be of the cold runner type.

[0068] Further, for simplicity, the mould 20 of the injection moulding machine 10 has been shown with a single mould cavity 30 formed in the mould 20. It will however be appreciated that the method of the invention is for controlling an injection moulding machine 10 having a mould 20 with one or more of mould cavities 30, such as a first number, ml, of mould cavities 30, and one or more corresponding mould cores for each mould cavities 30, such that a first number, ml, of moulded articles A may be formed in each moulding cycle. A moulding cycle is a filling of the mould, cooling of the moulded articles, A, and ejection / removal of the moulded articles, A, from the mould 20.

[0069] The first number, ml, of mould cavities and corresponding mould cores, may be identical in shape, in order to mould identical moulded articles. As is known in the art, moulded articles, A, ejected from the mould may be received on a conveyor (not shown) or the like, and transported for any kind of after- treatment.

[0070] Fig. 2, in a principle sketch, shows an embodiment of an injection moulding system 200 comprising an injection moulding machine 10, a weighing device 210 and a control system 300, for controlling at least the injection moulding machine 10.

[0071] The injection moulding machine 10 comprises a mould having a plurality, ml, of mould cavities, and corresponding cores, such that ml moulded articles may be produced (moulded) in every injection cycle (or simply cycle) of the injection moulding machine 10, when operating.

[0072] According to the invention, for every predetermined number, first number, n1, of injection cycles of injection moulding moulded articles, A, in the injection moulding machine 10, a plurality, ml, of moulded articles of one injection cycle may be selected and conveyed to a weighing device 210. Such a weighing device 210 may form part of the injection moulding system 200.

[0073] Selecting the one or more moulded articles, which have been ejected from the mould 20 of the injection moulding machine 10, may be done manually by an operator. Alternatively, and as indicated in Fig. 2, the selection of the one or more moulded articles could be performed by a selection device 220. Such a selection device 220 could be an articulated robot arm (not shown) with a suitable gripper, or as indicated in the figure a reception tray or the like, wherein the ejected moulded articles are dropped before being conveyed to after treatment (decoration, packaging, etc.).

[0074] Conveying the one or more selected moulded articles to the weighing device 210, may be done manually by an operator. Alternatively, and as indicated in Fig. 2, the conveying of the one or more selected moulded articles, A, could be performed by a transport device 230. Such a transport device 230 could be an articulated robot arm (not shown) with a suitable gripper possibly the same articulated robot arm (not shown) which exemplifies the selection device 220. Alternatively, the transport device could be a conveyor.

[0075] Preferably, the selection device 220 and the transport device 230 (also in embodiments where they are one and the same) may be controlled (not shown) by the control system 300.

[0076] Further, in some embodiments, where the measuring of the moulded articles, A, to obtain a measured mass, Wm , i.e. weighing, is automated, the weighing device 210 may be communicatively connected to a controller 310 of the control system 300, by a suitable connection 321.

[0077] As described in further detail below, based on the measured mass, Wm, of a plurality of moulded articles A, and a target mass, Wt, of the selected moulded articles A, the control system may be configured to compare the measured mass, Wm and a target mass, Wt, of the selected moulded articles A, and to control the injection moulding machine 10 based on the comparison. For this purpose the controller 310 of the control system 300 is connected to the injection moulding machine 10 via a suitable connection 310.

[0078] Fig. 3 shows a diagram of the control process for controlling the injection moulding machine 10 as described above.

[0079] In one aspect, the invention concerns a method for controlling an injection moulding process in an injection moulding machine 10.

[0080] The method involves running the injection molding in a plurality of sequential injection cycles in order to mould a plurality of moulded articles, A. In each injection cycle ml moulded articles are produced / moulded.

[0081] Injection moulding machines are run several thousands of injections cycles, to continuously produce large quantities of injection moulded articles A. Once the injection moulding machine is started and running, normally the moulding process reach a stable condition, where the quality (size, shape) of the moulded articles is constant over many injection cycles, such as hundreds or thousands of injection cycles. However, over time the quality may slide away from the desired quality.

[0082] The method according to the present invention may provide a simple and cost efficient control mechanism for maintaining a uniform quality of the moulded articles over time. Further, the method according to the invention may be used in calibrating upstart of a production for example after changing a mould 20 from one injection moulding machine to another, etc.

[0083] Thus, in one aspect, a control loop is provided, where at a predetermined interval of a first number, n1, of injection cycles, one or more moulded articles, A, which have been ejected from the one or more mould cavities 30 of the mould 20 in one or more injection cycles are selected. As mentioned above, the selection may be performed manually by an operator or automatically and controlled by the control system by a selecting device 220, e.g. an articulated robot arm or other robot.

[0084] When the moulded articles, A, which have been selected, these are conveyed to the weighing device 210. As mentioned above, the conveying may be performed manually by an operator or automatically and controlled by the control system 300, by a transport device 230, which may be the same as the selecting device 220, e.g. an articulated robot arm or other robot.

[0085] At the weighing device 210, the selected moulded articles, A, are weighed. Thereby, a measured mass, Wm, of the selected moulded articles, A, is obtained.

[0086] An operator may manually enter the measured mass into the control system 300, Alternatively, the weighing device 210 may - as mentioned - be communicatively connected with the controls system 300, such that the measured mass, Wm, may be communicated to a controller 301 of the control system 300 electronically and automatically. In such embodiments, the control system 300 and the controller 310 thereof, is configured for receiving and processing communication from the weighing device 210, and the weighing device 210 is configured for communicating electronically a measured mass, Wm, to the controller 310 of the control system 300 via suitable connection 321 , wirelessly or via cable.

[0087] In the control system 300, the measured mass, Wm, of the one or more selected moulded articles is compared with a target mass Wt for the one or more moulded articles, A. The target weigh Wt is the sum of a desired weigh of each of the one or more moulded articles, A.

[0088] Then, based on the comparison between the measured mass Wm of the moulded article and the target mass Wt, the injection moulding machine 10 is controlled to adjust the volume of melted polymer material in the subsequent injection cycles, by adjusting a moulding parameter of the injection moulding machine 10.

[0089] If the measured mass Wm is lower than the target mass Wt, the moulding parameter is adjusted so that the mass of molten material injected into the cavity is increased in the following injection cycles.

[0090] If the measured mass Wm is higher than the target mass Wt, the moulding parameter is adjusted so that the mass of molten material injected into the cavity is reduced in the following injection cycles

[0091] If a difference between the measured mass Wm and the target mass Wt is below a predetermined threshold, the moulding parameter is maintained constant.

[0092] In principle, in an embodiment, the control loop of collecting, transporting and measuring (weighing) moulded articles A may be performed continuously every injection cycle, i.e. at an interval of a first number n 1 = 1. Thus, in this embodiment, a sample of ml molded articles is collected every injection cycle, which sample is measured (weighed), compared against the target mass, and the machine is controlled, based on the comparison from cycle to cycle.

[0093] Alternatively, a control loop of collecting, transporting and measuring (weighing) moulded articles A, and controlling based on the comparison between the measured mass and the target mass Wt, may be at regular intervals (n1 >1 ), contains only moulded articles A, from one injection cycle, i.e. ml moulded articles.

[0094] However, experiments has shown that an even more improved control may be obtained if the control loop involves collecting and measuring the mass of moulded articles A from several injection cycles.

[0095] Thus, in some embodiments, involves collecting moulded articles from a second number n2 of injection cycles. Then, the collected moulded articles are measured to obtain a measured mass of the moulded articles collected over the second number n2 of injection cycles.

[0096] Thus, for every first number, n1 , of injection cycles, a sample of moulded articles from a second number, n2, of injection cycles are collected, transported to the weighing device 210 and weighed to obtain a mass Wm of the n2xm1 moulded articles A in the sample. The target mass Wt is adjusted according to the expected number of moulded articles A in the sample collected from the second number n2 of injection cycles, i.e. n2xm1 moulded articles. The measured mass Wm is compared to the target mass Wt, and the control of the injection machine is based on the comparison as described above.

[0097] In some embodiment the step of measuring, by the weighing device (210), said selected moulded articles (A) to obtain a measured mass (Wm) of the selected moulded articles (A) is performed separately for each of the second number, n2, of selections of moulded articles. (A). Then, the second number, n2, of measurements are registered and saved. This may preferably be performed in an automated way by the control system 300, but in principle could be carried out manually by an operator.

[0098] In embodiments, where the second number, n2, of measurements are registered and saved, then, in the step of comparing the measured mass of the selected moulded articles with a target mass for the selected moulded articles, the last measurements are weighed higher than the earlier measurements. Fig. 3 is a flowchart illustrating embodiments of the method according to the invention. The flowchart shows a closed loop control cycle according to embodiments of the invention, where a preselected target value for the mass of injection moulded articles, A, the target mass, Wt, is first set. Then the preselected target value, target mass, Wt, is during a later production cycle compared by a comparator with at least one measured mass, Wm, of the moulded articles moulded in n2 cycles prior to a present injection cycle, as described above. Based upon the difference between the n2 stored values for the previously measured mass, Wt, and the preselected target value a new moulding parameter (or production parameter), such as mentioned above is calculated in order to reduce or eliminate the difference between the previously measured mass and the preselected target value, target mass, Wt, of the subsequent production cycles. The new moulding parameter is then set on the injection molding machine for the next cycle.

[0099] It is to be noted that the figures and the above description have shown the example embodiments in a simple and schematic manner. Many of the specific mechanical details have not been shown since the person skilled in the art should be familiar with these details and they would just unnecessarily complicate this description. For example, the specific materials used and the specific injection moulding procedure have not been described in detail since it is maintained that the person skilled in the art would be able to find suitable materials and suitable processes to manufacture the moulded article according to the current invention.

Claims

Claims1. A method of controlling an injection moulding process in an injection moulding machine (10), the injection moulding machine (10) comprising- a mould (20) comprising one or more of mould cavities (30), and- an injection unit (40) configured for injecting a molten polymer material (50) into the one or more mould cavities (30), wherein the method comprises the steps ofA. injection molding a plurality of moulded articles (A) in a plurality of sequential injection cycles;B. at a predetermined interval of a first number (n 1 ) of injection cycles, selecting the moulded articles (A) produced in an injection cycle;C. conveying the selected moulded articles (A) to a weighing device;D. measuring, by the weighing device, said selected moulded articles (A) to obtain a measured mass (Wm) of the selected moulded articles (A);E. comparing the measured mass (Wm) of the selected moulded articles (A) with a target mass (Wt) for the selected moulded articles (A); andF. controlling, based on the comparison between the measured mass (Wm) and the target mass (Wt), the injection moulding machine by adjusting a moulding parameter of the injection moulding machine, to adjust the mass of melted polymer material in subsequent injection cycles, wherein the steps B-F is performed by collecting moulded articles (A) from a second number (n2) of injection cycles, and where the collected moulded articles are measured to obtain a measured mass (Wm) of the moulded articles (A) collected over the second number (n2) of injection cycles; and wherein, when the measured mass (Wm) is determined to be lower than or higher than the target mass (Wt), the interval of a first number (n1) of injection cycles is reduced to a predetermined third number (n3) of injection cycles.

2. The method according to claim 1, wherein the step of controlling a moulding parameter to adjust the mass of melted polymer material in the subsequent injection cycles is performed exclusively based on the comparison between the measured mass (Wm) and the target mass (Wt).

3. The method according to claim 1 or 2, wherein, the step of measuring, by the weighing device, said selected moulded articles (A) to obtain a measured mass (Wm) of the selected moulded articles (A) is performed separately for each of the second number (n2) of selections of moulded articles (A), wherein the second number (n2) of measurements are registered and stored, and wherein, in the step of comparing the measured mass (Wm) of the selected moulded articles (A) with a target mass (Wt) for the selected moulded articles (A), measurements are given individual weights.

4. The method according to any one of the claims 1-3, wherein, a. if the measured mass (Wm) is lower than the target mass (Wt), the moulding parameter is adjusted so that the mass of molten material injected into the cavity is increased in the following injection cycles; b. if the measured mass (Wm) is higher than the target mass (Wt), the moulding parameter is adjusted so that the mass of molten material injected into the cavity is reduced in the following injection cycles; and c. if a difference between the measured mass (Wm) and the target mass (Wt) is below a predetermined threshold, the moulding parameter is maintained constant.

5. The method according to any one of the claims 1-4, wherein the moulding parameter for controlling the injected volume of melted polymer material is one or more of melt temperature, mould temperature, injection pressure, holding pressure, injection flow rate, and melt pressure.

6. The method according to any one of the claims 1-5, wherein the injection moulding machine (10) is controlled by a control system (300), wherein the control system (200) is connected to a weighing device forming part of or located in the vicinity of the injection moulding machine (10), and wherein the measured mass (Wm) is transferred automatically to the control system (300).

7. The method according to any one of the claims 1-6, wherein the step of selecting the moulded articles (A) is performed manually.

8. The method according to any one of the claims 1-7, wherein the step of measuring the mass the moulded articles is performed manually.

9. The method according to any one of the claims 1-4, wherein the injection moulding machine (10) forms part of an injection moulding system (200), where the injection moulding system (200) further comprises- a selecting device for selecting moulded articles (A), the selecting device being controlled by the control system (300); and- a transport device for conveying the selected moulded articles (A) to the weighing device, the transport device being controlled by the control system (300), where- the step of selecting moulded articles (A) is performed by the selecting device, and- the step of conveying the selected moulded articles (A) to the weighing device is performed by the transport device.

10. The method according to any one of the claims 1-9, wherein, if the molten polymer material is changed during step A, selecting the moulded articles produced in the one or more injection cycles is commenced immediately after to the change from one the molten polymer material to another.

Images