METHOD FOR PRODUCE A MOLDED PART

Description

[0001] The invention relates to a method for producing a molded part from a foamed plastic in a mold cavity bounded by walls of a mold tool that can be opened and closed in at least two parts. Such a method, usually also referred to as a particle foaming process, typically comprises the following steps: a. Closing the mold to provide the mold cavity; b. Pouring particulate granules of the plastic into the mold cavity; c. Heating the granules by introducing steam into the filled mold cavity at a temperature and pressure sufficient to fuse the granules into a molded part; d. Cooling the molded part in the mold cavity; e. Opening the mold and removing the molded part. wherein the steam introduced into the mold cavity is passed through steam chambers which are arranged on the side of the walls of the molding tool opposite the mold cavity and communicate with the mold cavity via openings passing through the walls.

[0002] For prior art, reference can be made, for example, to EP 0 259 597 A2 and EP 2 227 366 B1. US 4 627 946 A discloses a method for manufacturing a molded part from a foamed plastic.

[0003] To heat the walls of the mold cavity and simultaneously or subsequently heat the granules, the prior art typically involves introducing steam into the mold cavity after closing the mold and filling it with particulate granules. This steam heats the walls to the desired temperature and causes the particulate granules to fuse within the mold cavity. However, this steam undesirably leads to the formation of condensate, which settles on the walls of the mold cavity, impairing the molding process and / or negatively affecting the surface quality of the resulting molded part. Furthermore, the steam chambers must be relatively large, requiring correspondingly large molds, which in turn necessitates a correspondingly large overall molding machine design.

[0004] EP 2 227 366 B1 therefore proposes removing this water vapor from the mold cavity by applying a vacuum. However, this additional step increases the cycle time for producing a molded part.

[0005] Moreover, providing the large quantities of steam required is energy-intensive and requires large quantities of water, and the steam removed from the mold cavity leads to undesirably high levels of moisture outside the mold.

[0006] Furthermore, EP 2 875 928 A1 and EP 3 088 153 A1 disclose molding tools for producing molded parts using the particle foaming process, which, due to their production from metallic materials in an additive process, for example by means of selective laser welding, can also have fine hollow structures with integrated channels and openings, which cannot be produced using conventional machining and casting processes.

[0007] The object of the invention is to propose a method of the type mentioned above which overcomes the disadvantages of the prior art by means of particularly low cycle times and particularly low energy and water consumption as well as compact dimensions of the molding tools used.

[0008] To solve the problem posed, a method according to the features of claim 1 is proposed according to the invention.

[0009] Advantageous embodiments and further developments of the invention are the subject of the dependent claims.

[0010] The solution according to the invention provides that a temperature control medium is introduced within the walls of the mold in a cavity that does not communicate with the steam chambers and the mold cavity, and that the walls of the mold are preheated to a temperature suitable for fusing the granules to form the molded part. According to the invention, this preheating can take place even before the mold is closed. Thus, according to the invention, the preheating of the walls can be initiated before and during the closing of the mold (which consists of at least two parts) and / or during the filling process of the mold cavity before the introduction of the steam, thereby saving considerable process time compared to the prior art.With the prior art, it was necessary to wait for the mold to be completely closed and filled, and for the mold cavity to form, before a burst of steam could be released from the steam chambers into the mold cavity for heating. This, in turn, led to a large amount of condensate being produced, which then ended up in the molded part and caused the well-known problems.

[0011] The cavities provided in the walls of the mold according to the invention, in which a tempering medium can be guided in such a way that it does not communicate with the steam chambers and the mold cavity, are formed by manufacturing the mold in an additive process.

[0012] According to the invention, the molding tool used in the process thus has a total of three independently temperature-controlled levels, namely, in accordance with the prior art, on the one hand the steam chambers and on the other hand the mold cavity, and, in addition to the prior art, the cavity within the walls of the molding tool which does not communicate with the steam chambers and the mold cavity as a third temperature-controlled level.

[0013] In the process according to the invention, water vapor is used in particular as the temperature control medium; however, other suitable temperature control media, for example water or oil or mixtures thereof, are also possible.

[0014] It is intended to preheat the walls to a suitable temperature, which according to a proposal of the invention is approximately 100 to 200°C, e.g. when processing expanded polypropylene (EPP) is approximately between 110°C and 150°C, preferably 140°C, wherein the temperature is freely adjustable and is selected by the person skilled in the art according to the plastics to be processed.

[0015] The preheating of the walls proposed according to the invention by means of the tempering medium guided within the walls and which does not communicate with either the mold cavity or the steam chambers offers the additional advantage that the mold cavity does not come into contact with steam during preheating, which therefore does not condense and need to be laboriously removed from the mold cavity.

[0016] The steaming times in step c) for heating the granules, possibly expanding them and fusing them into a molded part can be extremely short according to the inventive method, since only the energy for plasticizing the particles of the molded part to be produced and for heating the cavity between the individual particles by introducing the steam into the filled mold cavity has to be applied, so that almost no condensation occurs and molded parts with excellent surfaces are produced.

[0017] Due to the small amounts of steam required in the steam chambers, which must be introduced to heat the granules in the mold cavity, the steam chambers themselves can also be designed to be correspondingly compact, so that the dimensions of the molding tool used in the process according to the invention can be significantly reduced compared to the prior art.

[0018] According to a further proposal of the invention, the guidance of the temperature control medium through the cavity can also be maintained during the filling of the granules in step c) and / or during the heating of the granules in step d).

[0019] Furthermore, the temperature control medium can be circulated through the cavity in a closed loop to further optimize energy and water consumption.

[0020] According to the invention, after heating in step c), the filled mold cavity is subjected to a vacuum. Thus, immediately after the introduction of steam into the filled mold cavity to heat the granules and fuse them into the molded part, this steam is removed from the mold cavity as quickly as possible after the desired energy input into the particles within the mold cavity. This allows the resulting sintered molded part to be stabilized by utilizing the residual heat within the molded part and the walls of the mold. For this purpose, the vacuum is preferably maintained at a wall temperature of at least 70°C, or preferably around 110°C for EPP, for a predetermined period necessary to stabilize the fused particles in the mold cavity. Alternatively, for example,The foam pressure of the formed part is measured via a probe inserted into a wall bordering the mold cavity. As soon as this foam pressure falls below a predetermined limit, the successful stabilization of the fused particles within the mold cavity can be confirmed. Any remaining condensate in the mold cavity is also converted back into the vapor phase by the applied negative pressure and drained from the mold cavity through the openings into the vapor chambers, from where it is removed.

[0021] The negative pressure can be provided, for example, by means of vacuum pumps or by means of a vacuum tank, e.g. as described in EP 2 227 366 A1.

[0022] According to a further aspect of the invention, in step d) a cooling medium can then be passed through the cavity within the walls of the mold tool, which does not communicate with the steam chambers and the mold cavity, and through which the tempering medium was also passed in the preceding preheating step of the walls. In this way, the walls can be rapidly cooled to a demolding temperature without the molded part coming into contact with the cooling medium.

[0023] According to a further suggestion of the invention, the mold cavity can continue to be subjected to negative pressure during and / or after the passage of the cooling medium and the cooling of the walls.

[0024] According to another suggestion of the invention, the cooling medium can also be guided in a closed circuit.

[0025] Due to the preheating of the mold cavity walls by a tempering medium guided within a cavity of the walls, as provided for in the invention, no condensate is transported into the particle foam to be formed in the molded part, and the amount of energy introduced into the molded part is so low that only a reduced tendency for shrinkage of the manufactured part is observed. The stabilization times in the mold are therefore extremely short, and tempering times, which are unavoidable in the prior art, can potentially even be dispensed with.

[0026] Since both the heating and cooling of the walls of the molding tool are carried out by temperature control and cooling media guided in the enclosed cavity of the walls, individual process steps in the inventive method can run in parallel, which significantly reduces cycle times and makes enormous energy savings possible.

[0027] In a further embodiment of the method according to the invention, a very hot tempering medium with a raw material-dependent temperature of approximately 150 to 200 °C, preferably approximately 170 to 180 °C, is passed through the cavity before, during, and / or after heating the granules in step c) and before cooling the molded part. The resulting intense heating of the walls bounding the mold cavity allows a continuous plastic skin to form on the molded part, which is particularly desirable in certain applications. The molten volume fraction of the granules is compensated for by a second filling process or by compressing the component through so-called "gap compaction."

[0028] The method according to the invention is carried out with molding tools that have cavities or channels in the walls defining the mold cavity, through which the temperature control medium and, optionally, also the cooling medium can be guided without contact or transfer into the mold cavity and the steam chamber. According to the invention, molding tools are used that are manufactured from metallic materials using an additive manufacturing process, for example, by selective laser welding. With this process, molding tool walls can be produced that ideally have a continuous cavity between the surfaces adjacent to the mold cavity on the one hand and the steam chamber on the other. The openings connecting the steam chamber and the mold cavity for the steam inlet and outlet are designed as tubular sections that are delimited from the cavity and penetrate it.Due to the additive manufacturing process, the surfaces defining the cavity, as well as the openings arranged within it, are integrally produced from the metallic material. The arrangement and orientation of the openings can be largely freely selected according to the requirements of the molded part. The molds are equipped with suitable connection ports for conveying the temperature control medium and, if applicable, the cooling medium through the cavity.

[0029] Further embodiments and details of the invention are explained below with reference to the drawing illustrating an exemplary embodiment. The drawing shows: Figure 1 shows a section through a partial area of ​​a molding tool; Figures 2 to 6 show, in a highly simplified, partially cut-away side view through the molding tool, the process of the inventive method in successive steps for the production of a molded part.

[0030] From theFigures 2 to 6 The production of a molded part 6 in a two-part molding tool with a first upper molding tool part 1 and an associated second lower molding tool part 2 is shown in a sequential sequence, each of which is attached to carrier plates 3, 4 of a molding machine not shown in detail, e.g. in monoblock construction.

[0031] By means of drives not shown in detail, the forming tools 1, 2 are inserted into the dies from the Figure 2 The visible open state can be moved and, by reducing the distance A, into a closed state according to Figure 2 movable, which will be explained in more detail below.

[0032] The two forming tools 1, 2 have a contour of the forming tool wall 100, 200 designed according to the specifications of the part to be produced, which in the closed state according to Figure 2 define a mold cavity 5 in which the molded part 6 is produced in the manner explained below.

[0033] On the respective side of the wall 100, 200 opposite the mold cavity 5, a steam chamber 30, 40 is defined between the receiving plate 3, 4 and the mold tool 1, 2 attached to it, which is equipped in a manner not shown in detail with connection openings for the supply and discharge of steam or for applying a vacuum.

[0034] A key feature of the two forming tools 1, 2 is that they are manufactured using an additive process from a metallic material, such as stainless steel (V4A), for example by selective laser welding, and the resulting Figure 1 exhibit a visible structure.

[0035] Using the example of the upper part forming tool 1, a hollow structure of the wall 100 can be seen, which is also formed in the wall 200 of the lower part forming tool 2, so that the following explanations also apply to the forming tool 2.

[0036] Accordingly, the wall 100 is bounded by a surface 10 facing and limiting the mold cavity 5, and by a second surface 12 adjacent to the steam chamber 30 and opposed by the surface 10, between which a continuous cavity 11 is formed, which is penetrated by tubular or column-shaped openings 13 that extend from the surface 12 to the surface 10 and widen in a funnel shape or conically with a circular cross-section towards the surface 10. The openings 13 are thus separated from the cavity 11, but allow the passage of material between the steam chamber 30 and the mold cavity 5 adjoining the surface 10. The cross-sectional design of the walls 100, 200 with two spaced-apart surfaces 10, 12 with an intermediate cavity 11 and tubular openings 13 penetrating the cavity 11 results in an extremely lightweight and rigid construction of the mold tools 1, 2.

[0037] On the other hand, the cavity 11 in the mold tool 1 is provided as an additional functional level with at least two connection nozzles 110, so that a medium can be guided through the cavity 11 via the connection nozzles 110 without it being able to enter the steam chamber 30 or the mold cavity 5 from the cavity 11, since the openings 13 are separated or sealed off from the cavity 11.

[0038] The same applies to the construction of the wall 200 in the mold 2, where the connecting nozzles are designated with reference numeral 210.

[0039] To produce a molded part from a foamed plastic within the mold cavity 5, the following steps are taken starting from the open state in the Figure 2In a first step, with the mold 1, 2 still open, a tempering medium, for example steam at a temperature of about 130 °C and a regulated pressure of about 3 bar, is directed through the connection nozzles 110, 210 and the cavities 11 in the walls 100, 200 of the mold 1, 2 via the connection nozzles 110, 210 in order to preheat the mold 1, 2 in the area of ​​their walls 100, 200 and the surfaces 10, 12 bounding these walls to the temperature required for fusing the particulate granulate made of plastic into a molded part, for example to a temperature greater than or equal to 120 °C.

[0040] Due to the guidance of the temperature control medium, here water vapor, through the cavity 11 formed within the walls 100, 200, a homogeneous preheating of the walls 100, 200 is achieved without steam being able to enter the steam chamber 30 or the mold cavity 5 or the surfaces 10, 12 that bound them and condense.

[0041] Simultaneously with the preheating of the walls 100, 200, the forming tools 1, 2 together with the associated support plates 3, 4 are moved towards each other by reducing their distance A until they reach the closed position according to Figure 3 to reach a point where they interlock and define the closed mold cavity 5 between them.

[0042] The preheating of the walls 100, 200 can be maintained throughout the entire closing process, resulting in a significant time saving.

[0043] Once the mold cavity 5 between the two mold tools 1, 2 has been formed, particulate granules of the plastic are filled into the mold cavity 5 via corresponding filling openings in the walls 100, 200, whereby the preheating of the walls 100, 200 can be maintained further if necessary by the tempering medium guided in the cavities 11.

[0044] Once the filling of the mold cavity 5 with the granules is complete, steam is introduced into the steam chamber 30, 40 in a manner not shown in detail, but known per se, and from there emerges according to arrows H into Figure 3 The hot steam passes through the openings 13 formed in the walls 100, 200 into the mold cavity 5. There, it encounters the granules and heats their surfaces as well as the air between them, causing the particles to fuse together to form a cohesive molded part 6 according to the contour of the mold cavity 5. Due to the circular, conically widening cross-section of the openings 13 towards the mold cavity 5, they are subject to a self-cleaning effect.

[0045] Since, as described above, the walls 100, 200 of the molding tool 1, 2 have already been preheated to the required temperatures from steam chambers 30, 40 into the mold cavity 5, the steaming times—i.e., the time interval in which steam must be introduced from steam chambers 30, 40 into the mold cavity 5 according to arrows H to fuse the granule particles into the molded part 6—can be extremely short. This is because only the amount of energy required to fuse and, if necessary, foam the inserted particles needs to be introduced into the mold cavity 5 via the steam from steam chambers 30, 40. Due to the small amount of steam introduced into the mold cavity 5, virtually no condensate forms there that could negatively affect the surface of the molded part 6.The manufactured molded parts 6 therefore exhibit outstanding surface properties. Furthermore, the volume of the steam chamber 30, 40 can be very small, allowing the mold tool 1, 2 to be designed very compactly. This not only reduces the space requirement but also significantly reduces the mass to be heated. Specifically, compared to a conventional tool, the heated mass and the steam chamber volume can be reduced by up to 95%.

[0046] Immediately following the introduction of the required amount of steam from steam chambers 30 and 40 into the mold cavity 5 according to arrows H in Figure 3The steam chamber 30, 40 is subjected to a negative pressure or vacuum, so that the water vapor located in the mold cavity 5 is returned in the opposite direction from the mold cavity 5 via the openings 13 through the walls 100, 200 into the steam chamber 30, 40 and is discharged from there in a manner not shown in detail.

[0047] Due to the application of negative pressure or vacuum to the steam chamber 30, 40, the pressure in the mold cavity 5 also drops accordingly, and any condensate that may have formed in the mold cavity 5 and / or the molded part 6 is immediately converted back into the vapor phase as a result of the residual heat contained in the molded part 6 and the walls 100, 200 of the molding tool 1, 2 and is discharged via the steam chamber 30, 40.

[0048] The application of negative pressure or vacuum to the steam chamber 30, 40 is maintained for a predetermined period and / or until the foam pressure drops below a predetermined limit value, so that the molded part 6 is dried in the mold cavity 5 and stabilized using the residual heat exclusively with the applied negative pressure.

[0049] Shortly before the demolding of the molded part 6 located in the mold cavity 5, a cooling medium, for example water, is then guided through the connection openings 110, 210 via the cavity 11 in the walls 100, 200, if necessary while maintaining the negative pressure in the steam chambers 30, 40, in order to cool the molding tool 1, 2 and the molded part 6 contained therein to the demolding temperature. This results in a particularly high-quality surface finish of the molded part 6.

[0050] Then, as in Figure 4As shown, the mold is opened by moving the support plates 3, 4 with the attached mold tools 1, 2 apart and opening the mold cavity 5. The finished molded part 6 can be ejected by means of an ejector as shown. Figure 5 removed and falls out of the tool ( Figure 6 The process can then be carried out at Figure 2 start again.

[0051] The temperature control medium guided within the cavity 11 in the walls 100, 200 for preheating the walls 100, 200 and the cooling medium guided within the cavity 11 are preferably guided in closed circuits, so that the required quantities of temperature control and cooling medium are kept low and the energy required for heating or cooling these media is minimized.

[0052] Since the guidance of the temperature control medium and / or the cooling medium in the third temperature control level of the mold 1, 2 within the enclosed cavity 11 prevents any transfer into the steam chamber 30, 40 and / or the mold cavity 5, the process steps described above can be carried out in quick succession and, in some cases, even simultaneously, so that the cycle times of the entire process sequence are reduced. Figures 2 to 5 become extremely short.

Claims

1. Method for producing a molded part (6) from a foamed plastic in a mold cavity (5) of an at least two-part molding tool (1, 2) that can be opened and closed, which mold cavity is delimited by walls (100, 200) along a surface thereof, comprising the steps: a. closing the molding tool (1, 2) to provide the mold cavity (5); b. filling the mold cavity (5) with a particulate granulate of the plastic; c. heating the granulate by introducing water vapor into the filled mold cavity (5) at such a temperature and such a vapor pressure that the granulate is melted to form a molded part (6); d. cooling the molded part (6) in the mold cavity (5); e. opening the molding tool (1, 2) and removing the molded part (6), wherein the water vapor introduced into the mold cavity (5) is passed through vapor chambers (30, 40) which are arranged on the surface of the walls (100, 200) of the molding tool (1, 2) opposite the mold cavity (5) and communicate with the mold cavity (5) via openings (13) passing through the walls (100, 200), characterized in that molding tools (1, 2) with walls (100, 200) produced in an additive method and a cavity (11) extending inside the walls (100, 200) are used, wherein the openings (13) pass through the cavity (11) as tubular sections delimited from the cavity (11) between the surfaces of the walls (100, 200), and the walls (100, 200) with the cavity (11) formed in between and the openings (13) passing through them are integrally formed from a metallic material and, before and / or during the closing of the molding tool (1, 2), inside the walls (100, 200) of the molding tool (1, 2), a temperature control medium is introduced into the cavity (11) in the walls (100, 200), which does not communicate with the vapor chambers (30, 40) and the mold cavity (5), in order to preheat the walls (100, 200), and the walls (100, 200) of the molding tool are preheated to a temperature suitable for melting the granulate into the molded part (6), wherein, after the heating in step c), the filled mold cavity (5) is subjected to negative pressure.

2. Method according to claim 1, characterized in that the walls (100, 200) are preheated to a temperature of approximately 100 °C to 200 °C.

3. Method according to claim 1 or 2, characterized in that water vapor, water or oil is used as the temperature control medium.

4. Method according to any one of claims 1 to 3, characterized in that the flow of the temperature control medium through the cavity (11) is maintained even when the granulate is being filled in step b).

5. Method according to any one of claims 1 to 4, characterized in that the temperature control medium is guided through the cavity (11) in a closed circuit.

6. Method according to any one of claims 1 to 5, characterized in that the filled mold cavity (5) is subjected to negative pressure at a temperature of the walls (100, 200) of above 70 °C for a predetermined period of time such that the molten granulate in the mold cavity (5) is stabilized.

7. Method according to any one of claims 1 to 6, characterized in that in order to cool the molded part (6) in the mold cavity (5) of the molding tool (1, 2) in step d) a cooling medium is passed through the cavity (11) inside the walls (100, 200) of the molding tool (1, 2), which cavity does not communicate with the vapor chambers (30, 40), in order to cool the walls (100, 200) to a demolding temperature.

8. Method according to claim 7, characterized in that during and / or after the passage of the cooling medium and cooling of the walls (100, 200), the mold cavity (5) continues to be subjected to negative pressure.

9. Method according to any one of claims 7 or 8, characterized in that the cooling medium is passed through the cavity (11) in a closed circuit.

10. Method according to any one of claims 1 to 9, characterized in that during and / or after heating the granulate in step c) and before cooling the molded part in step d), a hot temperature control medium with a temperature of approximately 150 to 200 °C, preferably approximately 170 to 180 °C, is passed through the cavity (11).