Impregnation process

Abstract

Impregnation process comprising at least the following steps: - Providing a workpiece to be impregnated (20), - Placing the workpiece (20) in a bath (7) of impregnating agent within a container (2) in order to impregnate the workpiece (20) with the impregnating agent during an impregnation period, - oscillating movement of a vibrating body (12) within the container (2) during the impregnation period, wherein the vibrating body (12) generates oscillating pressure changes within the bath (7) by acting upon the bath (7), as well as - after the impregnation period, removal of the workpiece (20) from the bath (7), characterized in that the currently prevailing pressure (p) is measured by a pressure sensor (8) which is in contact with the bath (7) and a pressure amplitude within the bath (7) is determined from its oscillating temporal course, a control unit (10) changes a movement frequency of the vibrating body (12) in order to increase the pressure amplitude, and ends the impregnation period when the pressure amplitude has reached a predetermined threshold value.

Description

[0001] The invention relates to an impregnation process with the features of the preamble of claim 1.

[0002] Today, various methods exist for producing three-dimensional models from amorphous or shape-neutral materials such as powders or liquids, based on design data. These additive manufacturing processes are also known collectively as "rapid prototyping." Often, a primary forming step is involved, in which the starting material is either liquid from the outset or is temporarily liquefied and hardens at the designated location. A well-known method is fused deposition modeling (FDM), in which a workpiece is built up layer by layer from thermoplastic material. The material is supplied, for example, in powder or strand form, melted, and applied in its molten state by a print head, which sequentially deposits individual, usually horizontal, layers of the object to be manufactured.In addition, processes are known in which a powdered material, e.g., a plastic, is applied layer by layer and selectively cured by means of a locally applied or printed binder. In other processes, such as selective laser sintering (SLS), a powder is applied layer by layer to a base plate, e.g., using a doctor blade. The powder is selectively heated and sintered by means of suitable focused radiation, e.g., a laser beam. After a layer has been built up, the base plate is lowered slightly and a new layer is applied. Plastics, ceramics, or metals can be used as powder in these processes. In a similar process, selective laser melting (SLM), the amount of energy introduced by the radiation is so high that the powder is melted in certain areas and solidifies into a cohesive solid.

[0003] A particular advantage of additive manufacturing processes is their high flexibility; that is, components with virtually any three-dimensional geometry can be produced within a relatively short time, which can be advantageous, for example, for the production of prototypes or small batches. However, due to the process, the additively manufactured component often has a porous structure, meaning it is not gas-tight or even liquid-tight. If such properties are necessary, for example, for testing a prototype, the component can be subsequently impregnated to achieve the required tightness.

[0004] In known impregnation processes, the air is typically first removed from the pores within the workpiece by creating a (partial) vacuum to facilitate the penetration of the impregnating agent. In so-called dry vacuum processes, evacuation takes place before immersion in the impregnating agent, while in wet vacuum processes, it occurs after immersion. Optionally, the impregnating agent and the immersed workpiece can be pressurized to accelerate the impregnation process. These various methods generally work well, but they are energy-intensive and require significant technical effort to implement. The transition from vacuum to overpressure, in particular, makes these known methods complex. Furthermore, the time required for successful impregnation can, in some cases, amount to several hours.

[0005] The article “Ultrasonic impregnation of porous sintered components” (in “Soviet Powder Metallurgy and Metal Ceramics”, September 1964, Volume 1, Issue 5, pages 375-378) reveals the assistance of an impregnation process through the application of ultrasound. In this process, porous parts are impregnated with, among other things, machine oil or molten metal.

[0006] US Patent 5,288,521 A discloses an impregnation process for workpieces made of porous material. The workpiece is placed in an autoclave, which is then sealed and evacuated. An impregnating agent is then introduced into the autoclave so that the workpiece is covered. During impregnation, the autoclave is vibrated by means of a vibrator, which is intended to reduce the impregnation time.

[0007] US Patent 2005 / 0065260A1 discloses a method for manufacturing an object for ornamental or architectural use. In this process, a base body made of porous stone is placed in a container and covered with an impregnating agent. The impregnating agent is then cured, and the impregnated object is mechanically processed. During the impregnation process, the base body can be evacuated by placing the container in a vacuum chamber. Optionally, the container can also be subjected to slight vibration, which is advantageous in some cases.

[0008] US Patent 3,842,796 A discloses a device for treating wood, textiles, and other materials, in which a treatment chamber is divided into a lower part for receiving a treatment agent and an upper part. A carrier for the material to be treated can be moved between the upper and lower parts of the treatment chamber by means of lifting devices in order to immerse the material contained in the carrier into a treatment agent located in the lower part and then return it to the upper part of the treatment chamber. The pressure in the treatment chamber can be varied relative to the ambient pressure, and in particular, it can be provided that a partial vacuum is created before immersion into the treatment agent. According to one embodiment, this partial vacuum can be oscillating.

[0009] US Patent 2005 / 0244586A1 discloses an impregnation method for wooden workpieces in which the surface of the workpiece is first covered with a thin layer of impregnating agent. A sound generator is then positioned so that it is immersed in the layer but does not touch the workpiece, with a specific angle between the direction of sound propagation and the grain direction of the wood being preferred. In a comparative test, the workpiece is completely immersed in impregnating agent and exposed to ultrasound.

[0010] EP 0 172 304 A2 discloses a device for impregnating porous parts, e.g., cast or sintered metal parts, with a liquid impregnating agent. The parts to be impregnated are placed in a basket with a permeable wall, which is lowered into a rotatably supported centrifuge in the upper part of a tightly sealable container. A basin for the impregnating agent is located in the lower part of the container and can be moved upwards so that the basket is immersed in the impregnating agent.

[0011] US Patent 4,196,231 A discloses an impregnation device, for example, for electronic components or porous castings. This device has an evacuable container for impregnating agent, at the top of which a bearing for a centrifugal basket is arranged. The bearing consists of a single pendulum bearing, which is held by a support that can be clamped to the upper edge of the container by means of spreading devices.

[0012] US Patent 7,503,976 B2 discloses a system for impregnating components, comprising an impregnation device and a centrifugation station arranged below it. The impregnation device has a closable container with an access opening in one side wall. Inside the container, a lifting device is arranged on which a parts holder containing the parts to be impregnated is positioned before being immersed in the impregnating liquid in the lower part of the container by lowering the lifting device. The centrifugation station has a centrifuge carrier for receiving impregnated parts, which is rotatably mounted via a first bearing arranged above and a second bearing arranged below.

[0013] EP 0 100 474 A1 discloses a method for impregnating or coating electrical or electronic components with thixotropic, solvent-free reactive resins. The method involves subjecting the reactive resin to ultrasound during immersion. In particular, an ultrasound source can be introduced into the resin, the effect of which reduces the resin's viscosity. The component is then lowered into the immersion bath and, after a suitable immersion time, re-emerged. The absence of air bubbles on the surface of the immersion bath is specified as the criterion for the end of the immersion time.

[0014] DE 18 09 825 A relates to a method for impregnating windings of insulated electrical conductors and / or laminated cores of electrical devices. For this purpose, these are immersed in a bath of curable synthetic resins, whereby it is provided that they are exposed to ultrasound during their immersion in the resin bath. An ultrasound generator can be immersed in the bath. Regarding the immersion time, it is only mentioned that it can be "extremely short".

[0015] Given the current state of the art, the efficient impregnation of components, especially additively manufactured components, still offers room for improvement. In particular, it would be desirable to develop a process that is fast and cost-effective.

[0016] The invention is based on the objective of providing a method for the efficient impregnation of components.

[0017] According to the invention, the problem is solved by an impregnation process with the features of claim 1.

[0018] An impregnation process is shown, comprising at least the following steps: - Providing a workpiece to be impregnated, - Immersion of the workpiece in a bath of impregnating agent within a container in order to impregnate the workpiece with the impregnating agent during an impregnation period, - oscillating movement of a vibrating body within the container during the impregnation period, wherein the vibrating body generates oscillating pressure changes within the bath by acting upon the bath, as well as - after the impregnation period, remove the workpiece from the bath.

[0019] According to the invention, the prevailing pressure is measured by a pressure sensor that is in contact with the bath, and a pressure amplitude within the bath is determined from its oscillating temporal profile, wherein a control unit changes a movement frequency of the vibrating body to increase the pressure amplitude and ends the impregnation period when the pressure amplitude has reached a predetermined threshold.

[0020] The dependent claims relate to advantageous embodiments of the invention.

[0021] It should be noted that the features and measures listed individually in the following description can be combined in any technically feasible manner and demonstrate further embodiments of the invention. The description further characterizes and specifies the invention, particularly in conjunction with the figures.

[0022] The invention provides an impregnation process. In a first step, a workpiece to be impregnated is provided. This can be, in particular, a workpiece that has been manufactured at least partially by an additive manufacturing process. Various manufacturing processes can be used, e.g., FDM or SLS. The manufacturing process can be based on a particulate material, e.g., a powder or granules, which can consist of, for example, plastic, ceramic, or metal. Optionally, a post-processing step, e.g., machining of the surface, can be performed before the impregnation process begins. In any case, the workpiece has a porous structure, at least in sections, which enables impregnation and is necessary for certain purposes, e.g., when a seal against liquids or gases is required.The workpiece may be a prototype, but the impregnation process according to the invention is expressly also suitable for series production.

[0023] In the next step, the workpiece is placed in a bath of impregnating agent within a container. Embodiments are conceivable in which the workpiece is only partially immersed in the bath, for example, when only a portion of the workpiece needs to be impregnated. Normally, the workpiece is fully immersed. This immersion is preferably at least partially automated. It is understood that the workpiece can be held by a holding device, such as a gripper, hook, basket, or the like. Optionally, such a holding device can also hold several workpieces simultaneously. To place the workpiece in the impregnating agent bath, the holding device can be moved into the container, or the container can be moved towards the holding device. The impregnating agent serves to penetrate the porous structure of the workpiece and at least partially seal the cavities.The impregnating agent is initially liquid, but normally hardens after the workpiece has been impregnated with it. This hardening process can be initiated or accelerated by factors such as contact with air, moisture, radiation, or other influences.

[0024] After the workpiece is placed in the bath, a vibrating body is moved oscillatingly within the container during an impregnation period. The vibrating body generates oscillating pressure changes within the bath by acting upon it. This explicitly includes the possibility that the vibrating body is also moved oscillatingly before and / or during the placement of the workpiece. Conversely, it is possible that the oscillating movement does not occur throughout the entire impregnation period, but is interrupted, for example, at certain times.

[0025] The motion of the vibrating body is oscillatory, meaning it undergoes repeated changes of direction. This motion can be linear, but circular or elliptical motions are also conceivable, which can be considered a composite motion of two oscillating linear motions. The temporal progression of the motion can correspond, at least in intervals, to a (sinusoidal) fundamental oscillation; however, such a fundamental oscillation can also be superimposed with harmonics. In particular, it is also possible for the frequency of the fundamental oscillation to change over time. The same applies to the amplitude of the vibrating body's motion. The term "vibrating body" indicates that its motion can also be described as an oscillation.There may be a certain time interval between placing the workpiece in the bath and the start of the oscillating movements; however, a longer delay is not desirable for the sake of a swift process. The vibrating body acts on the bath, meaning that at least part of its surface is in contact with the bath. The primary effect is the application of force or pressure to the bath in the contact area. The vibrating body could, for example, be a type of piston that moves within a cylindrical section of the container, with one end face of the piston in contact with the bath. Naturally, a drive mechanism is necessary to move the vibrating body, such as an electric drive.

[0026] After the impregnation period, the workpiece is removed from the bath. This can be done by moving a holding device with the workpiece out of the container, or by moving the container downwards relative to the holding device. Optionally, further steps to remove excess impregnating agent can follow, such as centrifugal cleaning. Additionally, process steps to cure the impregnating agent within the pores of the workpiece can be carried out. For example, the workpiece can be exposed to radiation or heated, depending on the conditions that promote the curing of the impregnating agent.

[0027] The oscillating motion of the vibrating body, which in turn leads to an oscillating pressure change within the bath, also subjects a near-surface area of ​​the workpiece to oscillating pressure. This results in alternating periods of negative and positive pressure, whereby during the negative pressure phase any remaining gas is drawn out of the pores, while during the positive pressure phase impregnating agent is forced into the pores. The negative and positive pressure phases alternate in an oscillating manner, with each phase potentially being short. One could say that the effect of the inventive method is based on the sum of the negative pressure and positive pressure phases. It has been shown that this method achieves effective impregnation that is qualitatively in no way inferior to that achieved with conventional methods that operate with prolonged vacuum and / or prolonged positive pressure.Furthermore, it has been shown that a shorter impregnation period is necessary compared to classical methods, i.e., the method according to the invention is very time-efficient.

[0028] Furthermore, energy consumption can be reduced compared to conventional methods. This is due, on the one hand, to the shorter duration, and on the other hand, to the fact that oscillating, potentially minor pressure changes can be generated with less energy than a continuous vacuum or overpressure. Additionally, the equipment requirements can be reduced, as no high-performance pumps are needed to generate the vacuum and / or overpressure, nor are particularly pressure-resistant containers required.

[0029] As already mentioned, the method according to the invention does not require a continuous application of vacuum or overpressure. Preferably, a gas pressure prevails above the bath before, during, and / or after the workpiece is introduced, which differs from atmospheric pressure by no more than 10% and is preferably equal to atmospheric pressure. Here, atmospheric pressure refers to the ambient air pressure, which is typically between 0.9 bar and 1 bar. The area above the bath, i.e., at its surface, can contain either air or another gas, e.g., an inert gas, depending on the requirements of the impregnating agent. Similar to wet vacuum processes in the prior art, the workpiece can be introduced into the bath without prior removal of air or gas from the pores of the workpiece by means of a vacuum. Therefore, atmospheric pressure can prevail before and during the introduction.A pressure that differs slightly from atmospheric pressure. Even after the workpiece is placed in the bath, atmospheric pressure (or a pressure differing slightly from it) may prevail outside or above the bath. This contrasts, for example, with the wet vacuum process and with processes in which overpressure is created above the impregnating bath to force the impregnating agent into the pores of the workpiece. The statements regarding the gas pressure above the bath refer to a time-averaged value, disregarding any spatially and temporally limited pressure fluctuations, e.g., due to sound waves.

[0030] To better assess the effectiveness of the impregnation process, a pressure amplitude within the bath is determined, as mentioned above. Although the pressure amplitude at the measurement point may differ from the pressure amplitude at the surface of the workpiece, it is generally possible to draw at least qualitative conclusions about the prevailing pressure conditions. A pressure sensor in contact with the bath measures the current pressure, and the amplitude can be determined from its oscillating time course. Typically, a minimum and a maximum value are measured and stored, and half the difference between these two values ​​is defined as the amplitude.

[0031] Preferably, the impregnation period is determined based on the measured pressure amplitude. That is, the length of the impregnation period, or its end, depends on the measured pressure amplitude. Qualitatively, this usually means that a larger pressure amplitude leads to a shorter impregnation period. However, different process configurations are conceivable. Typically, the impregnation period is less than 30 minutes.

[0032] For energy efficiency reasons, it is preferable to stop the vibrating body after the impregnation period has elapsed. This can be done before the workpiece is removed from the bath, or afterwards. Alternatively, for example in series production, it would be conceivable to operate the vibrating body continuously.

[0033] The vibrating body can be considered the exciter of a vibrating system, which includes the bath, the workpiece, and, if applicable, the container. This system has a resonant frequency at which a maximum vibration amplitude results, which manifests itself as a maximum pressure amplitude in relation to the bath. Generally, it can be assumed that the impregnation process is optimized by excitation occurring approximately at the resonant frequency. To utilize this, as mentioned above, a control unit modifies the vibration frequency of the vibrating body to increase the pressure amplitude. The control unit can, for example, include a microprocessor that receives signals from the pressure sensor and, in turn, controls the vibrating body's drive. The control unit can, for example, drive the vibrating body with a specific initial frequency and determine the pressure amplitude. It can then increase the frequency or...The frequency is reduced, and the pressure amplitude is then determined. If the pressure amplitude decreases due to the frequency change, the control unit changes the frequency in the opposite direction. If this leads to an increase in the pressure amplitude, the frequency can be changed again in the same direction. If not, a frequency change with a smaller step size can be made. This is, of course, only one possible approach, and other strategies for finding the resonant frequency are conceivable. In some cases, it may not even be necessary to reach the resonant frequency; it may be sufficient to change the frequency in the direction of the resonant frequency.

[0034] As mentioned above, the length of the impregnation period can depend on the pressure amplitude. As stated above, the control unit terminates the impregnation period when the pressure amplitude reaches a predetermined threshold. This threshold can depend, among other things, on the size and material of the workpiece or the type of any upstream additive manufacturing process. It can also depend on the type of impregnating agent. In any case, the threshold serves as an indicator that sufficient evacuation of the pores initially present in the workpiece has occurred and that these pores are adequately filled with impregnating agent. Once the threshold is reached, the control unit determines the end of the impregnation period, either immediately or after a certain delay.If the threshold value were exceeded for a longer period of time and / or significantly, this could potentially lead to damage to the workpiece or the impregnation device.

[0035] As explained above, the creation of a continuous vacuum or overpressure is unnecessary in the method according to the invention. For this reason, the container generally does not need to be sealed pressure-tight. However, it is optional to seal the container after the workpiece has been inserted. This sealing can be achieved, for example, by means of an access flap that closes an opening through which the workpiece is inserted into the container. The container can be sealed to be liquid-tight or even gas-tight.

[0036] According to one embodiment of the method, the workpiece is placed in a first chamber of the container, which is connected via a connecting channel to a second chamber in which the vibrating element is located. As indicated above, the second chamber can form a kind of cylinder for a piston-like vibrating element. The connecting channel is a region whose cross-section is narrower than that of the first and second chambers.

[0037] It is preferred that the pressure amplitude be determined with respect to the first chamber. This means that the pressure measurement on which the determination of the pressure amplitude is based is carried out in the first chamber, or that a pressure amplitude prevailing in the first chamber is determined. This is advantageous because the measurement of the pressure amplitude in the first chamber has a stronger correlation with the pressure amplitude prevailing at the surface of the workpiece than that in the second chamber. In particular, it is conceivable that, due to damping effects, the amplitude in the second chamber, near the vibrating body, is significantly larger than at a greater distance from it. In this case, a measurement in the second chamber would greatly distort the result.

[0038] Further advantageous details and effects of the invention are explained in more detail below with reference to an embodiment illustrated in the figures. It shows Fig. 1 a schematic representation of a device for carrying out the method according to the invention; and Fig. 2. A time course of a measured pressure.

[0039] In the different figures, identical parts are always provided with the same reference symbols, which is why they are usually only described once.

[0040] Fig. Figure 1 shows a schematic representation of an impregnation system 1 with which the method according to the invention can be carried out. The impregnation system 1 has a container 2 which can be closed by a movable lid 3. Inside the container 2 are a larger, first chamber 4 and a smaller, second chamber 6, which is connected to the first chamber 4 by a connecting channel 5. Both chambers 4 and 6, as well as the connecting channel 5, are filled with a bath 7 of impregnating agent. The impregnating agent can be, for example, a resin. Adjacent to the first chamber 4 are a pressure sensor 8 and a resonance sensor 9, which are connected to a control unit 10, hereinafter also referred to as CPU 10. The CPU 10 receives measured values ​​from the sensors 8 and 9. It, in turn, controls an electric drive 11, which drives a piston 12 in the second chamber 6. The resonance sensor 9 is optional and can be used if necessary.can also be omitted.

[0041] The system 1 is designed for impregnating additively manufactured workpieces 20, which may be prototypes, for example. After the workpiece 20 has been manufactured, for example, by FDM or SLS, it still has a large number of small pores, which means it is neither gas-tight nor liquid-tight. For applications where a corresponding tightness is required, the workpiece 20 is impregnated as follows: First, the workpiece 20 is placed on or picked up by a holding device 13 shown schematically here.

[0042] The workpiece 20 is then placed in bath 7 by lowering the holding device 13 into container 2. The workpiece 20 is not evacuated; atmospheric pressure prevails above bath 7 before and during placement.

[0043] After the workpiece 20 has been positioned in a designated location within the container 2 and the lid 3 closed, the CPU 10 controls the drive 11 to move the piston 12 in an oscillating motion at a predetermined starting frequency. This motion can be sinusoidal. The piston 12 pressurizes the bath 7 with an oscillating pressure, resulting in an overall oscillating pressure change within the bath. This oscillating pressure change propagates through the second chamber 6 and the connecting channel 5 into the first chamber 4. This results in oscillating pressure changes in the area of ​​the workpiece 20's surface; that is, there is an alternation between vacuum, which leads to the evacuation of the pores within the workpiece 20, and overpressure, which leads to the penetration of impregnating agent into the pores.

[0044] Furthermore, these pressure changes are registered via the pressure sensor 8. The optional resonance sensor 9 can detect the frequency of the pressure change. The CPU 10 thus receives feedback on the pressure changes caused by the movement of the piston 12. In particular, it can determine a pressure amplitude by comparing the minimum and maximum values. This pressure amplitude depends on the one hand on the movement amplitude of the piston 12, but also and especially on its movement frequency. Starting from the initial frequency, the CPU 10 begins to change the movement frequency in order to increase the pressure amplitude. As the movement frequency approaches a resonance frequency of the system, the determined pressure amplitude increases sharply, as shown in simplified form in Fig.Figure 2 shows the measured pressure p within the bath 7 as a function of time t. When the movement frequency of the piston 12 reaches or approaches the resonance frequency sufficiently, the pressure amplitude exceeds a predetermined threshold. This threshold is interpreted by the CPU as an indication that sufficient impregnation of the workpiece 20 has been achieved through the alternating vacuum and overpressure. Once this occurs, the CPU stops the drive 11, and the impregnated workpiece 20 can be removed from the container 2 using the holding device 13. Stopping the drive 11 prevents potential damage to the workpiece 20 and / or the impregnation system 1.

[0045] Optionally, excess impregnating agent can then be spun off the surface of the workpiece 20. Depending on the type of impregnating agent, various methods (radiation, heat, etc.) can be used to initiate or accelerate the curing of the impregnating agent.

[0046] The invention also includes the use of the described impregnation system 1 for impregnating additively manufactured workpieces 20. Reference symbol list: 1 impregnation plant 2 containers 3 lids 4 first chamber 5 connection channel 6 second chamber 7 bathrooms 8 Pressure sensor 9 Resonance sensor 10 CPU 11 Drive 12 pistons 13 Holding device 20 workpieces p print t time

Claims

An impregnation method comprising at least the steps of: - providing a workpiece (20) to be impregnated, - placing the workpiece (20) into a bath (7) of impregnating agent within a container (2) in order to impregnate the workpiece (20) with the impregnating agent during an impregnation period, - oscillating movement of a vibrating body (12) within the container (2) during the impregnation period, wherein the vibrating body (12) generates oscillating pressure changes within the bath (7) by acting upon the bath (7), and - after the impregnation period, removing the workpiece (20) from the bath (7), characterized in that the currently prevailing pressure (p) is measured by a pressure sensor (8) which is in contact with the bath (7) and a pressure amplitude within the bath (7) is determined from its oscillating temporal profile, a control unit (10) changes the motion frequency of the vibrating body (12),to increase the pressure amplitude, and ends the impregnation period when the pressure amplitude reaches a predetermined threshold. Impregnation method according to claim 1, characterized in that before, during and / or after the insertion of the workpiece (20) a gas pressure prevails above the bath (7) which differs from atmospheric pressure by at most 10% and is preferably equal to atmospheric pressure. Impregnation method according to one of the preceding claims, characterized in that the vibrating body (12) is stopped after the impregnation period has elapsed. Impregnation method according to one of the preceding claims, characterized in that the container (2) is closed after the workpiece (20) has been inserted. Impregnation method according to one of the preceding claims, characterized in that the workpiece (20) is placed in a first chamber (4) of the container (2), which is connected via a connecting channel (5) to a second chamber (6) in which the vibrating body (12) is arranged. Impregnation method according to claim 5, characterized in that a pressure measurement is carried out to determine the pressure amplitude in the first chamber (4).

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