DE19715229A1 - Melt production and delivery system for e.g. plastic pressure injection mouldings - Google Patents

Abstract

This new process manufactures mouldings in batches. A material, e.g. plastic, plastic mixture or metal is melted or plasticised in a plasticiser (1), to be delivered into a mould cavity. The new feature of the process is an intermediate melt storage stage, in an interposed storage/transfer unit (2), totally decoupling plasticising or melting, from the delivery stage (3). Also claimed is the corresponding equipment.

Description

field of use

The invention relates to a method and a device according to the preamble of Claim 1. The preferred field of application is plastics processing, in particular the injection molding of plastics, plastic mixtures or metal art mixtures of substances. The invention is however also for other molding compositions Metal alloys and other manufacturing processes such as injection molding and. a. applicable.

State of the art, disadvantages of the prior art

In plastics processing, complex products are often injection molded produced. In general, Plastic granules through dissipation and heat conduction melted and then injected into a cavity under high pressure. Of the The overall cycle for the production of molded parts is characterized by individual phases with special Conditions. The individual phases run in succession and can partially overlap in time. The workflow relates to the phases and the Overall process is a discontinuous process. The economics of the process is largely determined by the cycle time. They are different devices known with which this discontinuous process is operated. Put the fixtures consists of a clamping unit and a plasticizing unit. The clamping unit has to open the task of carrying the tool with the corresponding cavity close, hold and eject the molded part with the tool open. The The plasticizing unit should melt the plastic, homogenize it and put it in the closed Insert tool. Due to various developments, the requirements for Injection molding plastic unit increased significantly and can be used with the conventional Solutions no longer perform satisfactorily.  

The increased demands on the injection molding plasticizer go to different Developments back. Performance increases achieved through reduced cycle times (20-40% are not uncommon), are today particularly due to the improved tool and Machine technology possible. On the other hand, the increasing use of Multiple tools or floor technology for larger shot weights or dosing volumes, often with almost the same cooling time. It can also often be optimized Reduce tool cooling. One consequence of these developments is increased Mass throughputs, which is why the plasticizing flows would have to be increased significantly. At the same time, however, high-quality melt quality is always expected / 1 /.

The most significant machine technology solution for Injection plasticizing units is a system in which a rotating screw free-flowing molding compounds i.a. directly from the machine hopper and Screw tip promotes. The plastic granules become through dissipation and heat conduction melted. Due to the rotation of the screw, the Plastic to produce a largely homogeneous melt. The snail builds through the conveying process a pressure before its tip. This pushes the snail backwards. / 2 / In order to achieve better thermal and mechanical homogeneity of the melt, the movement will return through an opposing, applied force difficult, so that a back pressure builds up in the melt, which the mixing processes in the Screw flights improved, but the plasticizing performance reduced. Once one for one too part sufficient supply of melt in the space between the screw tip and Nozzle stores, stops the screw rotation. With the nozzle attached and the nozzle closed The tool advances the screw and pumps the melt into the mold cavity / 2 /. For Shrinkage compensation often squeezes the screw after injecting it longer period, additional material in the holding phase in the cavity.

Since the funnel is fixed to the plasticizing unit, the Plasticize the effective screw length permanently. Because the melting process is essential more from the disspation processes in the screw flights than from the heat conduction is significantly reduced due to the decrease in the effective screw length Plasticizing performance depending on the dosing route. Add to that that first plasticized material goes through more worm gears and at the same time lasts longer in the barrel lingers. This results in a different melt quality in the axial direction, which the Process and product quality can have a very negative influence / 3 /.  

Due to the already mentioned reduction in plasticizing performance due to the The effective screw length and effective dynamic pressure can be shortened with this Machine technology for certain molded parts not for economic reasons to provide the necessary quantities and qualities of melt within the required times. For thin-walled articles with a relatively large molded part weight, such as packaging, CD's, housing covers etc. remain of the screw, which is due to the design during the Injection and hold pressure time can not plasticize plastic, only those Applications low cooling time and the time of tool movements, if one Parallel operation is made possible by the drive technology in order to melt the plastic and homogenize. To achieve the necessary plasticizing services, either the screw speed is significantly increased or the screw geometry be changed accordingly. Both measures inevitably have a negative impact the melt quality. Note the recommended maximum Circumferential speeds for certain plastics / 2, 4 /. Another disadvantage is in it justifies that the worm always accelerates from the rest position into the rotation and then must be braked again, this not only has negative effects energy consumption and the necessary installed drive line, but leads also with regard to the reproducibility of further negative process fluctuations.

In addition to the plasticizing performance, the plasticizing and injection unit is one Injection molding machine characterized by how large the amount of melt that is inside one cycle can be injected into the tool cavity. With these machine systems 65% of the installed stroke volume should be regarded as a practicable maximum value. Of the Dosing path used (travel of the screw during plastication) should be 3D (D = Screw diameter) / 5 /. Larger dosing routes are, besides those already Disadvantages described with regard to axial quality differences, not useful, since major routes there is a risk of air pockets. Because after injecting successive solid islands and air-precipitated in the rear screw section Vacancies that move forward during plasticizing / 1, 4 /. The air pockets are dependent on the dynamic pressure, the friction of the solid bed and the Speed of the main bed and the solid islands, carried along and only in at best pushed through the solid bed to the funnel. When the air stops can escape, this leads to considerable problems during the injection process.  

Burns and intolerable product qualities are the result, like different ones Evidence shows.

So it turns out that the linking of different tasks to one relative simple machine technology, but hardly for the increased requirements has sufficient potential to optimize the system. This is especially true this is because the possible measures to meet the individual requirements for this System always have multiple effects on different requirements, often are opposed. With this system, one must always try to get one Resolve target size conflict.

The plasticizing process was used for the so-called piston and screw pre-plastification separated from the injection process. In a piston, the plastic is by heat conduction or melted in a screw by heat conduction and dissipation and to one Injection pistons passed on. The injection piston then injects the material into the cavity. This results in more freedom for optimization.

This is also the reason why numerous devices have been used in the past respective advantages and disadvantages were developed. With the so-called Piston pre-plastification was in the foreground of considerations, such as the plastic can be melted as effectively as possible, since almost with a piston plasticizing unit only the heat conduction causes the melting process. A continuous way of working and this principle basically excludes the tasks to be solved.

Piston plastification itself can only be found today on the smallest machines with one Piston diameter up to about 20 mm / 2 /.

The introduction of the screw for plasticizing led to further solutions based on the Principle of the separation of plasticizing and injection processes. The numerous Developments are described in / 2, 6, 7, 8, 9, 10, 11 /.

In the case of pre-plasticizing screws, two systems essentially have a wider one Market acceptance achieved and kept to this day. In one system, Battenfeld GmbH, the material is melted by a rotating screw that does not move axially and conveyed directly into an injection cylinder with piston. There is enough melt in the injection cylinder available for the molded part, is via an external closure or via a Check valve of the plasticizing cylinder separated from the injection cylinder. The snail will stopped and the injection cylinder presses the material into the cavity. In the injection and No further plastic can be melted by the screw during the holding pressure phase.  

The increase in plasticizing performance can therefore only be achieved as a result of the separately possible Both stages can be optimized. Since the screw does not move axially backwards and if the effective screw length is not shortened, the result is a more even one Melt quality. However, since the screw accelerates in rotation and then must be braked again, is here with quality losses in terms of Reproducibility to be expected. In addition, the filling time of the Injection cylinder directly depends on the plasticizing performance and thus the cycle time extend or the high plasticizing flow required can damage the melt.

In the second basic solution with screw pre-plastification, the screw rotates during melting and moves axially backwards. This system, the husky Injection Molding Systems Ltd. / 12 /, thus consists of a conventional one Injection molding plasticizing unit with a downstream injection plunger. Is enough material melted, the screw functions as a piston and conveys through an axial Movement forward the generated melt in the plunger. It can therefore be short Transfer times of the melt in the injection pistons can be realized. After The plasticizing screw can be transferred by a closure from the melt Injection unit are separated so that they are already back in the mold can be plasticized. However, there is a serious disadvantage when this design is compared with an extruder with continuous operation. By driving back the effective length of the screw is continuously reduced during the melting process with corresponding negative consequences for the melt quality, which in principle unwanted axial quality profile. There are also downtimes, and Accelerations that have a negative impact on the process.

literature

[1] Bürkle E. Mahlke M .: Developments in melt processing. Field of activity injection molding machine, VDI publishing house 1995
[2] Johannaber F .: plastic machine operator. Munich: Vienna: Carl Hanser Verlag, 3rd edition 1992
[3] Elbe W .: Studies on the plasticizing behavior of screw injection molding machines. RWTH Aachen, dissertation 1973
[4] Bayer AG: Processing data for the injection molder. Issue 01/94
[5] Johannaber F .: Dispensing path in injection molding machines. Kunststoffe 79 (1989) 1 pp. 25-28
[6] Beck H .: injection molding. Munich: Carl Hanser Verlag, 2nd extended edition 1963
[7] Munns MG: presses for plastics processing. Mainz; Krausskopf publishing house, volume 2 1965
[8] Laeis M .: The injection molding of thermoplastic materials. Munich; Carl Hanser Verlag, 2nd revised edition 1959
[9] Mink W .: Basic principles of injection molding technology. VDI-Verlag, 4th edition 1971
[10] NN: injection molding technology. VDI publishing house 1980
[11] Sonntag R .: Development of injection molding technology, plastics 75 (1985)
[12] Husky: injection molding machine with pre-plastification. Kunststoffe 76 (1986) 10 p. 882

Object of the invention

The object of the invention is to provide a method and an apparatus in which entire cycle time plastic is melted under constant boundary conditions but is only carried out within a certain period of time in the cycle. The Plastic must be plasticized very gently to ensure a consistently good quality for all parts To achieve melt quality.

Solution of the task

The new device and the method for plasticizing presented here and the Injection is based on the well-known separation of plasticization and injection. she represents a multi-stage system just like piston and screw pre-plastification A complete decoupling of both processes is only possible with one intermediate storage / conveyor module. This module has the task of creating the Buffer until the injection cylinder, the processes of injection and finished pressing. Then the storage / conveyor module should the material handed over gently and quickly. The time available for this depends on the Cooling time and tool times. More extruded material will be added over time Storage / conveyor module directly conveyed from the plasticizing unit into the injection cylinder, so that the plasticizing unit can be operated continuously and the transfer supported.

To compensate for unavoidable fluctuations in the process times, the Storage / conveyor module should be dimensioned so that it is able to store more material than is theoretically required by the cycle sequence for continuous extrusion operation.  

These fluctuations can be compensated by adjusting the extruder speed respective control criteria only in small steps, but over a longer period, is adjusted. This allows strong acceleration and braking in favor of one high process quality can be avoided.

It is important to ensure that the solutions in the filling processes in and out of the Storage / conveyor module no disturbing repercussions on the plasticizing process occur.

In the different versions, this is generally achieved in that Pressure fluctuations due to the transfer mechanisms are kept within narrow limits.

The solution thus enables a simultaneous comparison to the known solutions continuous plasticizing with high plasticizing performance and optimal melt quality by avoiding an axially moving screw and thus the transmission of the entire extrusion knowledge.

Advantages of the invention

The main advantage of this invention is that the total cycle time for the Plasticizing is available. On the one hand, this can be used to meet high demand Generate melt amounts and still treat the material gently. The Plasticizing unit may be due to the longer time available for plasticizing stands, dimensioned smaller and thus cheaper to manufacture.

Because the screw does not move axially backwards, the effective remains Screw length constant, so that a thermally and mechanically constant High quality melt is produced.

A quality advantage also results from the fact that strong accelerations and braking can be avoided when plasticizing, this also has a positive effect on the Energy consumption.

Since the plasticizing unit is completely decoupled from the injection unit, everyone can Extrusion techniques and existing knowledge are extensively integrated.

For example, melt filtration behind the extruder is very easy integrate, while this is a big problem today because of a filtration between Tool and the injection unit has large flushing pressures in the filter, the very small Effect downtimes. In addition, the filter acts as a pressure consumer and can Complete filling of the molded part is difficult due to the lack of injection power.  

Another advantage, which is also valid for other screw pre-plastification, is to see that a separate optimization of the plasticizing and the injection unit is possible. The extrusion unit can be optimally adapted to the material and operating points will. The cross section of the piston can meet the requirements of the mold filling process be adjusted. For thick-walled molded parts with relatively small filling pressures use large cross-section and for products with large flow resistances and therefore very small cross sections are used for the high filling pressures required. With a big one Shot volume can become clear due to the elimination of the air intake problem of the stroke be raised. The injection system can also be adapted to the drives to be used will. A reduction in the cross section of the injection piston leads to a reduction in the required driving forces or torques. This aspect comes to the use of electro mechanical systems very good.

In addition, however, the storage / conveyor module offers the possibility of additional functions integrate. It offers space for adding and mixing additives and for Dehumidification and for the integration of control mechanisms, for example to assess the Melt quality. So there is a very high level of modularity and adaptability, so that with this concept different by quite simple and clear adjustments Requirements can be optimally met.

Description of exemplary embodiments

The decisive factor in such a system for high-performance applications is the interaction of the at least three components. The first stage of the system consists of a plasticizing unit, which is constructed in exactly the same way as in the continuous extrusion technology (see Fig. 1-5, No .: 1). It can be a conventional single-screw extruder, a grooved bush extruder, a degassing extruder, a twin-screw extruder, possibly with a downstream melt pump, a multi-screw system, a kneader, etc.

Since the melt produced is not continuously introduced into the tool, the melt must be stored temporarily. This takes place in the second module, the so-called storage / conveyor module (see Fig. 1-5, no .: 2). In high-performance applications, however, the storage process must not have a negative impact on the plasticization, i.e. the extrusion module, or the injection process, i.e. the discharge module.

The feeder / conveyor module can be characterized by a constant or an at the plasticized amount of melt adapted volume. In addition to saving the block have a fast and material-saving conveyor that can be used in a short time can store the stored melt in the third building block, the discharge building block.

In the case of a construction with a constant storage volume, the storage and conveyance can take place by means of a screw conveyor (see FIG. 1) that is adapted to this task in geometry or in a melt pump (see FIG. 2), for example in a gear pump. The speed of the screw or pump can be adjusted independently depending on the task. The element can also stand still when the memory is being filled. A very high speed can be selected for the delivery in the discharge module in order to achieve short transfer times. A separate drive can be provided to generate the rotational movement of the storage element, or the drive for the extruder can be integrated via a shaft, a corresponding transmission ratio and a clutch system or centrifugal force system. The design can include rotating and static mixing elements.

The system, with a volume adapted to the existing melt quantity, can be opened can be realized in different ways. One possibility is that the volume when saving due to its elasticity and the pressure rise due to the increase in storing amount of melt increased. In the transfer phase to the injection unit the volume of the storage / conveyor module by applying a surface force from the outside reduced again. The block can be designed as a bubble memory. The Operation is based on a membrane that is stretched and deflected in a container then by pressure or mechanical force application to the initial position is moved back. With this design, special measures for flushing the Membrane. The storage / conveyor module can also be used as an expandable hose be formed, which connects the plasticizing unit and the injection unit, so that overall, there are very short flow paths. After the hose is filling has expanded, it is compressed by a squeezing device to the extent that the material flows into the injection unit and further melt directly from the extruder into the Injection unit can be promoted. This version is a very good flush and to achieve a very short dwell time.

In a further embodiment, a piston or annular piston is moved in a cylinder in accordance with the task. The cylinder can be arranged at an angle between 0 ° and 90 ° to the flow direction (see Fig. 3).

In the first case, a cylinder with a piston is integrated at an angle to the extruder or flanged. If the piston is fed with melt close to the cylinder base, the piston becomes pushed up by the melt. For the shortest possible dwell time and good rinsing to achieve, however, the melt should be fed laterally above. In the cylinder is supposed to Melt distributors are provided so that the melt is evenly distributed around the piston flows down. The piston moves upward due to the melt pressure, but not across the feed and distribution, otherwise there is no sensible flushing of the piston is achieved. This also ensures that the plasticized material first the discharge module is passed on. The so-called fifo (first in first out) applies. Principle for this module. Furthermore, the dwell time in the system is very good check. When transferring, the piston moves down and is held there while further melt produced by the extruder then along the piston directly into the Discharge module flows.

Another embodiment which has very good properties in terms of size, residence time behavior and flushing behavior is based on the use of an annular piston which can also be arranged directly in the direction of extrusion (see FIG. 4.5). The fifo principle for this module also applies here. Such a system can also be flanged (see FIG. 4) or integrated into the screw cylinder (see FIG. 5). will. In the first case, additional degrees of freedom for dimensioning the memory are obtained. Large storage volumes can be achieved by a large ratio of the annular piston cross sections with a then small stroke, or with relatively small ratios of the annular piston cross sections with a larger stroke. In such a system, the storage is shifted during storage due to the pressure built up by the melt quantity. For this purpose, as with the other versions, it is necessary to provide a closure for the discharge module. If the melt is to be conveyed into the discharge module, the closure between the storage / conveyor module and the discharge module is opened. The melt can now flow into the discharge module. In order to achieve favorable transfer times and relatively low pressures in the storage / conveyor module, the transfer channel should be as short as possible and with a large flow channel cross section. When it is transferred to the injection cylinder, the closure or the check valve is opened. As a result of switching on the storage drive or the energy stored in the spring, the ring piston is now moved back and displaces the melt into the discharge module. Simultaneously and afterwards, the melt newly generated by the extruder is passed directly through the ring piston into the discharge module, so that very short transfer times can be achieved by adding both effects.

The respective piston or ring piston can have its own drive. Depending on Boundary conditions can be electrical direct drives (linear motors), electro-mechanical, pneumatic or hydraulic drives can be used. The use is particularly cheap of a preloaded spring system that is further tensioned when the melt is stored and releases its energy when the closure system is opened. In addition, the translatory movement of the accumulator piston also mechanically with the return movement of the Injection pistons are coupled.

The design of the capacity and the drive of the storage / conveyor module must ensure that no cycle time extension occurs due to the transfer time. The various exemplary embodiments of the storage / conveyor module can be seen in FIG. 6. In the worst case, only the tool times are available. In addition, the maximum intermediate storage volume should make it possible to compensate for process fluctuations, ie a certain amount of buffer should be able to be absorbed, which is then reduced by means of a lower extruder speed to be set.

In order to achieve as constant conditions as possible when plasticizing, the back pressure should be in the extruder, which shapes the pressure flow and thus significantly mixes and Exchange processes are influenced in a certain controllable range. With it you'll get basically two options. On the one hand, there can be the counterforce that goes through the snail or the pump or the accumulator piston is introduced into the system, depending on Filling volume can be changed. In the case of the screw or the pump, these can be used a correspondingly adjusted speed profile are operated while the Retraction movement of the piston is varied by a correspondingly variable counterforce becomes. If the storage piston is driven by a spring, the design is thereby favors that with increasing amounts of melt and consequently increasing Compressibilities the spring force increases. In addition, the preload is too choose that conditions are as constant as possible.

The second option is to use a customizable plasticizing block Decouple the pressure consumer (throttle) from the accumulator / conveyor module. The combination both options can be used.

The third component is the injection plunger (see Fig. 1-5, no .: 3), which presses the melt into the tool. The structural separation of the processes of plasticizing, intermediate storage and discharge into a tool can be used to achieve more favorable boundary conditions for the electro-mechanical drive and transmission technology available on the market.

Through a now possible reduction in the cross section of the injection piston at the same time Raising the stroke, the forces can be reduced so that the use of Direct drives in the form of linear motors is possible with smaller machines that Forces can be transmitted via racks, or the transmission through one Cable system with the use of a pulley can be used. This also has far-reaching consequences for the electric drives used. These are in In contrast to hydraulic drives, they are capable of very high speeds (high Speeds), but they are also available at comparable prices and sizes unable to realize similarly high forces or torques. Besides these new ones A conventional hydraulic drive for the can also be used Use the injection process.

Claims (25)

1. A method for discontinuous molding production with the steps of melting or plasticizing a material, such as a plastic, a plastic mixture, metal alloy and the like, in a melting or plasticizing unit and discharging the melted or plasticized material, in particular into a tool with a cavity, characterized by the writing of the intermediate storage of the melted material, in particular in an intermediate storage / conveying means, for completely decoupling the steps of melting or plasticizing and discharging. 2. The method according to claim 1, characterized by the step of regulating the pressure during the filling / unloading process of the storage / conveying means. 3. The method according to claim 1 or 2, characterized by the step of a variable Adapting the storage volume of the storage / conveying means, at least to that for discharge required minimum volume of melted material. 4. The method according to claim 3, characterized by the step of regulating the plasticizing or melting capacity of a plasticizing or melting unit for plasticizing or Melting according to control criteria with small steps and long periods. 5. Apparatus for discontinuous molding production, especially in the Plastic processing, with a plasticizing or melting unit for plasticizing or Melting of material such as a plastic, a plastic mixture, metal alloy and the like and with a discharge unit for discharging the melted material, in particular in a tool with a cavity, characterized in that between the Plasticizing or melting unit and the discharge unit an intermediate storage unit, in particular a storage / conveying element is arranged. 6. The device according to claim 5, characterized in that the plasticizing or Melting unit an extruder like a conventional single screw extruder Grooved sleeve extruder, a degassing extruder, a twin-screw extruder with or without downstream melt pump, a multi-screw system or a kneader.   7. The device according to claim 5 or 6, characterized in that the storage / conveyor element has a constant capacity, the promotion preferably via a screw integrated in a cylinder or via a melt pump. 8. The device according to claim 5 or 6, characterized in that the storage Conveyor element has a capacity that can be adapted to the volume to be stored. 9. The device according to claim 8, characterized in that the storage / conveying element has an elastic membrane that stretches as a result of the pressure increase and that through an external force is deferred in the handover phase. 10. The device according to claim 8, characterized in that the storage / conveying element is an elastic hose. 11. The device according to claim 8, characterized in that the storage / conveying element is a cylinder with an integrated piston. 12. The apparatus according to claim 11, characterized in that the storage / conveying element can be filled laterally from below or laterally from above, preferably in the cylinder or on a plasticizing material or melt distribution system for the piston to ensure a good one Flushing the piston is integrated. 13. The apparatus according to claim 8, characterized in that the storage / conveying element a cylinder with at least two different cross sections and an annular piston a corresponding number of different cross sections. 14. Device according to one of claims 5 to 13, characterized in that in Moving parts are provided for each storage element, which, in particular their own Drive like an electric direct drive, with pistons also a linear drive, one electromechanical, pneumatic, hydraulic and electromagnetic drive, exhibit.   15. The apparatus according to claim 14, characterized in that the drive for the Storage element is coupled to the drive for the discharge unit. 16. The apparatus according to claim 14 or 15, characterized in that the drive for the Storage element is coupled to the drive for the plasticizing or melting unit. 17. The device according to one of claims 8 to 16, characterized in that the drive of the storage / conveying element by means of an energy storage element such as a spring. 18. Device according to one of claims 5 to 17, characterized in that the Storage / conveyor element assigned a flow resistance for pressure decoupling, in particular is integrated into this. 19. Device according to one of claims 5 to 18, characterized in that between Plasticizing or melting unit and the storage / conveying element a melting filter is provided. 20. Device according to one of claims 5 to 19, characterized in that between the Plasticizing or melting unit and the discharge unit, a static mixer is provided. 21. Device according to one of claims 5 to 20, characterized in that the Discharge unit a syringe piston in particular with a linear motor, a rack or has a cable system as a drive. 22. The apparatus according to claim 21, characterized in that the diameter of the Spray plunger small compared to the pump or screw diameter of the melting unit is. 23. The method according to any one of claims 1 to 4, characterized by interim Switch off the plasticizing or melting unit when a permissible one is exceeded Pressure value in the storage / conveying element.   24. The method according to claim 2 or 4, characterized by varying the pressure ratios in plasticizing or melting by controlling the melting capacity. 25. The method according to any one of claims 1 to 4, 23 or 24, characterized by varying the pressure conditions in the storage / conveying element via a controllable counterforce.