WO2011009745A1

WO2011009745A1 - Method for producing a plastics molded part having a foamed core and a non-foamed shell

Info

Publication number: WO2011009745A1
Application number: PCT/EP2010/059899
Authority: WO
Inventors: Bernd Klotz
Original assignee: Kraussmaffei Technologies Gmbh
Priority date: 2009-07-21
Filing date: 2010-07-09
Publication date: 2011-01-27
Also published as: DE102009034265A1

Abstract

The invention relates to a method for producing a plastics molded part (1) having a foamed core (3) and a non-foamed shell (2), having the following essential steps: adjusting a cavity (10) reduced in size relative to the finished plastics molded part, filling said cavity with the first, unfoamed plastics material (M1); injecting the second, foamable plastics material (M2) into the interior of the first, non-foamed plastics material, the cavity being expanded by injecting the second, foamable plastics material, the pressure in the second, foamable plastics material in the cavity being maintained at a value at which the second plastics material cannot foam; actively enlarging the cavity by movement apart of the components of the mold tool forming the cavity until a cavity of a third size corresponding to the finished plastics molded part is present, the pressure in the second, foamable plastics material dropping below a value at which the second plastics material can foam.

Description

Process for producing a plastic molding with a foamed

Core and a non-foamed shell

description

The invention relates to a method for producing a plastic molded part, wherein the plastic molded part comprises a skin material or a shell made of a first, unfoamed plastic material and a core of a second, foamed plastic material.

The production of plastic moldings having a core which is completely enclosed by a skin material is known from the prior art (Johannaber / Michaeli "Handbuch Spritzgießen", Hanser-Verlag, 2001, Chapter 6.5.7.1) The basic procedure is as follows: First, a partial filling of the mold with the skin material is carried out, where the skin material is applied to the wall of the mold, resulting in the formation of a skin Plastic material of the core can only displace the molten skin material inside, the skin material is forced outwards by the inflowing plastic material of the core in the cavity and is pressed completely against the walls of the molding tool subsequent so-called In the rinsing phase, the sprue system is cleaned of the core material by rinsing with the skin material so that clean skin material can be re-injected into the mold at the next cycle. The core material may also be a frothable Bares plastic material, which has the advantage that a good Nachdruckwirkung can be achieved even in remote areas.

Sandwich injection molding allows the use of relatively inexpensive plastic materials for the core, which is not visible anyway. especially the Use of plastic waste or of regranulate proves both economically and ecologically advantageous.

The problem with sandwich injection molding is that the viscosities of the skin material on the one hand and the core material on the other must be properly matched to one another in order to avoid the processes mentioned as the finger effect (see Figure 6.101 in the prior art mentioned at the beginning). In the worst case, it may even lead to a breakthrough of core material to the molding surface (see Figure 6.103 in the above-mentioned prior art). For this reason, one is not free in the selection of the core material, i. In particular, it is not possible to use any particularly inexpensive plastic material, for example of plastic waste.

Proceeding from this, the present invention seeks to provide a method for producing a plastic molding of a core material and a skin material enveloping this core material, being free in the selection of the core material and almost no coordination with the skin material is required, and wherein nonetheless the so-called finger effect is largely avoided or does not occur at all.

The solution of this object is achieved by a method having the features of claim 1. Advantageous embodiments and further developments can be found in the subclaims.

With the method according to the invention, it is ensured that the second, foamable plastic material impinges on a "wall" of first, unfoamed plastic material and is held by this pressure immediately below the wall under a pressure at which foaming of the second plastic material can not occur.

Another advantage of the method according to the invention is that after the end of step (2), ie after the end of the first process phase (1st step), if the cavity of the first size has been filled with the first, unfoamed plastic material, the surface quality of the later plastic molded part has been generated. In the case of a substantially parallelepiped-shaped or sheet-like plastic molded part, the surface quality on the upper side, on the underside and on the side surfaces is thus generated at the end of step (2).

With the following steps (3) and (4), i. Injecting a certain amount of second, foamable plastic material and foaming the same, the thickness of the plastic molding can be determined very precisely. This can be done depending on the closing path, for example via suitable displacement sensors on the closing unit or on the molding tool. However, it can also be carried out by means of a screw path, i. over a certain stroke of the screw a defined amount of the second, foamable plastic material is injected. About the amount of material to second, foamable plastic material and the way of foaming the thickness of the plastic molding can be largely independent of the molding design and additionally determined independently of viscosity.

The step (3) of injecting the cavity by injecting a predetermined amount of the second foamable plastic material into the interior of the first unfoamed plastic material, wherein the pressure in the second foamable plastic material in the cavity is maintained at a value in which the second plastic material can not foam, moreover offers the advantage that the distribution of the second plastic material in the interior of the first plastic material is almost independent of the filling characteristic of the plastic molded part. The distribution of material in the interior of the first plastic material, that is, in the interior of the plastic molding takes place virtually automatically in an optimal manner. The complete space which is "cleared" in the interior of the first plastic material in step (3) is filled up by the second, foamable plastic material in the non-foamed state, whereby the volume shrinkage of the first plastic material during its cooling can also be disregarded, because immediately after the filling the cavity with the first plastic material is injected with the second plastic material. The volume shrinkage of the same starting with the cooling of the first plastic material is easily compensated by the second plastic material.

A further advantage of the method according to the invention lies in the fact that the entire process sequence can be clearly determined in terms of process technology, namely via the course of the closing force generated by the closing unit. As a result, definable internal pressures can be generated and maintained in the cavity. The course of the internal pressure over the entire cycle can thus be optimally designed in the desired manner according to the molding geometry and the plastic materials used. In particular, in step (3), when injecting the second foamable plastic material, the internal pressure can be kept at a value at which foaming of this plastic material certainly does not occur. In step (4), the foaming phase can be introduced quickly, so that the second plastic material can foam up immediately after completion of its filling. In this step (4), the process of foaming can advantageously be set and influenced in a targeted manner via a defined closing force curve. This has a particularly favorable effect on the cycle time and the quality of the molded part. Optionally, towards the end of the foaming process, when the foam cells formed generate a foaming pressure, a slight increase in the closing force may be provided to achieve some compression of the foam cells depending on the molded part and material.

The invention is based on embodiments and below

Reference to the figures will be explained in more detail. Show it:

Fig.1 top view of a table top;

Fig.2 internal pressure curve;

3a-3d process sequence without insert

4a-4d process flow with insert According to FIG. 1, a rectangular table top 1 has an outer layer 2 of a non-foamed plastic matehal and a core 3 made of a foamed plastic material located therein. In the corner areas or in edge regions of the table top 1 (see the mark X) fasteners, for example in the form of sockets, in the manufacture of the table top 1 can be incorporated.

FIG. 2 shows a typical course of the internal pressure in the interior of the plastic molded part during its production. Further details on this course are given below after the description of the following two variants of the method, as shown in FIGS. 3a-3d and FIGS. 4a-4d.

In FIGS. 3a-3d, a mold consisting of a plurality of components is shown in a partial cross-section. This mold comprises a fixed mold half 4 and a movable mold half 5. The movable mold half 5 has a support plate 9 with a mold insert 6, a circumferential dipping edge strip 7 and biasing means 8, with which the dipping edge strip 7 can be pressed close to the fixed mold half 4. As a biasing means, for example, piston-cylinder units or spring assemblies may be provided. Such a dipping edge tool is described and patented, for example, in DE19809720C2. However, it is also possible to provide dipping edge tools of a different design or other molding tools suitable for an embossing process. In the closed state, a cavity 10 is formed in the mold into which plastic melt can be introduced via a sprue channel 11. Further, a pressure transducer 12 is provided to measure the internal pressure in the cavity. Not shown is the closing unit for receiving such a mold. In principle, each closing unit can be considered here in order to open and close the stationary and movable mold halves and to build up suitable closing forces. In order to determine the wall thickness of the plastic molding, suitable displacement transducers can be provided on the closing unit and / or on the molding tool be provided so that the paths when pressing or driving the cavity are clearly determined. In the figure 3d, a displacement transducer 20a / 20b is shown purely schematically on the mold.

Below, the method according to the invention with the steps (1) to (7) will be explained according to claim 1.

Step 1 )

FIG. 3a shows the state of the molding tool at the beginning of a cycle. The mold is closed and a cavity 10 which is smaller than the finished plastic molded part 1 is formed, the cavity thus formed having a first size G1 corresponding to a predetermined or desired amount of first, unfoamed plastic material M1. This step corresponds to phase I in FIG. 2.

Step 2)

In a subsequent step (phase II in FIG. 2), an unfoamed plastic material M1, for example unfoamed polypropylene (PP), is introduced into the cavity 10 via the sprue 11 until it is completely filled with the PP. Optionally, a reprint can be applied, for example, as completely as possible to fill and mold more complicated part geometries. This condition is shown in FIG. 3b. The first unfoamed plastic material M1 forms a firm skin on the walls of the cavity 10 and initially remains liquid in the core. Due to the cooling, a certain volume shrinkage occurs.

Step 3)

Immediately following step (2), in the next step (3) (phase III in FIG. 2), a foamable plastic material M2, for example PP + foaming agent, is injected into the molten core of the plastic material M1, for example via the same sprue channel 11. Instead of a chemical foaming agent, physical foaming may also be provided be, for example, according to the so-called Mucell® method. By injecting the second, foamable plastic material M2, the cavity 10 is pressed onto a reduced compared to the finished plastic molding 1 cavity second size G2, this size G2 from a predetermined amount of foamable plastic material M2 or, taking into account temperature and Pressure results from a predetermined volume of foamable plastic material in the non-foamed state. The amount or the volume of the second, foamable plastic material M2 can be determined and set depending on the screw path. This means that the amount of material or the volume is determined by a specific stroke of the screw, by which a certain amount of material or a certain volume of the second plastic material M2 from the Schneckenvorraum is displaced into the interior of the first plastic material M1. The amount of material or the volume of the second, foamable plastic material M2 can also be determined depending on the path or depending on the tool position bwz. be set. This means that the amount of material or the volume is determined by a certain distance by which the mold is allowed to start, ie by which the closing unit (in short the buckle) may start. The movable mold half may thus be moved away by a predetermined distance from the fixed mold half. This distance can be measured by means of the displacement sensor 20a / 20b. The last-mentioned method variant (depending on the closing path or tool position-dependent) has the advantage that the volume shrinkage of the first plastic material M1 can be disregarded when injecting the second plastic material M2. It is simply as much material, ie, such an amount injected at the second plastic material M2 until the desired tool position is reached, that is, until the cavity has reached the desired second size G2. The closing force counteracts the pressing of the cavity 10 and thus determines the internal pressure in the cavity 10. Thus, the internal pressure in the cavity 10 can be selectively maintained at a value at which the second plastic material M2 does not foam can. At the end of this process of injecting the second plastic material M2, the mold assumes a position as shown in Figure 3c.

Step (4)

Subsequently, the cavity 10 is actively enlarged by retracting the support plate 9 with the mold insert 6 and that extent until a third-size cavity G3 corresponding to the finished plastic molded part 1 is present (phase IV in Figure 2). The dipping edge strip 7 is held by means of the biasing means 8 to stop on the fixed mold half 4. This state of the molding tool is shown in FIG. 3d. By actively enlarging the cavity, the internal pressure in the cavity and thus the pressure in the second, foamable plastic material M2 drops below a value at which the second plastic material M2 can foam.

Step (5) - Step (7)

After a certain cooling phase, the mold can be raised and the finished plastic molded part, for example, the table top 1, are removed from the mold. This plastic molded part 1 has a core 3 of a foamed plastic material M2 with a surrounding shell or skin 2 made of a non-foamed plastic material M1. Depending on the thickness of the shell or skin 2, one can also speak of an outer layer 2.

Between step (3) and step (4), material can be injected from the first, unfoamed plastic material M1 in the region of the gate, so that the surface of the finished plastic molding in the region of the gate at the end is completely formed by the first, unfoamed plastic material M1 and the core material M2 is completely enveloped by the skin material M1 before the start of the fourth step (4th step).

FIGS. 4a to 4d show the same method sequence, but with the difference that in a corner area or an edge area of the table top 1 there is a socket 13 for receiving a table leg into the table top 1 is incorporated. For this purpose, a rod 14 in the dipping edge strip 7 is provided laterally of the mold insert 6, which protrudes into the cavity 10 and on which the bushings 13 can be mounted. For the sake of clarity, here is the pressure transducer 12 - although actually present - not shown. The method is analogous to the embodiment described above. First, as in FIG. 3a, a pre-reduced cavity 10 of first size G1 is set (step (1)), which is subsequently filled with the unfoamed plastic material M1. In contrast to FIGS. 3a-3d, a somewhat thicker edge is formed from the plastic material M1 into which the bushing 13 is embedded. The sleeve 13 is thus completely encapsulated by the plastic material M1 except for the voltage applied to the dipping edge strip 7 surface. The subsequent steps correspond to those as in the previous embodiment. At the end of step (3), the foamable second plastic material M2 is still in the unfoamed state in the interior of the first plastic material M1, wherein the cavity 10 has assumed the second size G2. Subsequently, an active enlargement of the cavity 10 to an end size G3 by driving away the support plate 9 together with the mold insert 6, whereas the dipping edge strip 7 and the rod 14 are held in their forward position. In this case, there is a pressure drop and foaming of the second plastic material M2. After a cooling phase, the mold can be raised and the table top 1 with integrated sockets 13 are removed from the mold.

Depending on the geometry of the molded part, the injection of the second plastic material M2 can also begin even if the cavity is not completely filled with the first plastic material. The penetrating second material M2 then displaces the first material M1 into the not yet filled regions of the cavity. In this case, however, care must be taken that the minimum pressure for preventing the foaming of the second plastic material M2 is not undershot.

Now, a typical course of the internal pressure in the cavity 10 will be explained with reference to FIG. At the beginning of the procedure, the cavity is depressurized or at atmospheric pressure (phase I). When filling the cavity 10 with the unfoamed plastic material M1, the internal pressure rises rapidly, reaches the end of the filling process or at the end of the holding pressure its maximum (in this case, for example 250 bar) and drops after completion of the injection process of the unfoamed plastic material M1 initially slightly (Phase II). Next, the foamable plastic material M2 is injected into the molten core of the first plastic material M1, whereby the cavity is pressed (phase III). The closing force must be adjusted so that there is an internal pressure in the cavity at which the incoming foamable plastic material M2 can not yet foam. This minimum pressure P _min , which must not be _{exceeded in} order to avoid foaming, is material- _dependent and can be preset accordingly in the control of the injection molding machine. In the present example, a minimum pressure of 200 bar is given. After a predetermined amount or volume has been injected on the second plastic material M2, phase III is over and the next phase begins. In phase IV, the cavity is actively enlarged, which leads to a rapid pressure drop in the plastic material M2 and this foaming can. At the end of phase IV atmospheric pressure is present again. Optionally, a further increase in internal pressure after the foaming phase can be provided, so that the surface layers of the unfoamed plastic material M1 (here of the PP ^' ) forming under pressure are supplied with pressure by a closing force profile and thus also kept compacted. This makes it possible to ensure by means of small pressure increases that also at the end of the process before the tool opening a definable internal pressure in the molding stops. This is illustrated by a slight increase in the pressure curve in FIG. 2 at the end of the pressure curve in phase IV.

Claims

claims

1. A method for producing a plastic molding (1), wherein the plastic molding a sheath (2), in particular a skin or an outer layer, of a first, unfoamed plastic material (M1) and a core (3) of a second, foamed plastic material (M2), characterized by the following steps:

(1) closing a mold and setting one opposite the

finished plastic molded part (1) reduced cavity (10), wherein the cavity thus formed has a first size (G1);

(2) filling the reduced first size cavity (10) (G1) with the first unfoamed plastic material (M1);

(3) Injecting a certain amount of the second foamable plastic material (M2) into the interior of the first unfoamed one

Plastic material (M1), wherein the cavity (10) by the injection of the second, foamable plastic material (M2) corresponding to the determined amount of the second, foamable plastic material (M2) on a compared to the finished plastic molded part (1) reduced cavity ( 10) of second size (G2) is pressed, wherein the pressure in the second foamable plastic material (M2) in the cavity (10) is maintained at a value at which the second plastic material (M2) can not foam;

(4) actively enlarging the cavity (10) by moving apart from the cavity (10) forming components (6, 9) of the mold until a third size cavity (10) (G3) corresponding to the finished plastic molding (1) is present the pressure in the second foamable plastic material (M2) falls below a value at which the second plastic material (M2) can foam;

(5) allowing the plastic molding (1) to cool,

(6) opening the mold,

(7) removal or ejection of the finished plastic molding (1).

2. The method according to claim 1,

characterized in that

in step (3), the amount of second, foamable plastic material (M2) is determined as a function of the screw path.

3. The method according to claim 1,

characterized in that

in step (3), the amount of second foamable plastic material (M2) closing is determined depending on the distance or position of the tool.

4. The method according to any one of the preceding claims,

characterized in that

between step (3) and step (4) material from the first,

non-foamed plastic material (M2) in the region of the gate (11) is injected, wherein the surface of the plastic molding (1) in the region of the gate (11) is completely formed by the first, unfoamed plastic material (M1), so that the core ( 3) is completely surrounded by the sheath (2) before the start of step (4).

5. The method according to any one of the preceding claims,

characterized in that

the internal pressure in the second, foamable plastic material (M2) by means of a suitable internal pressure transducer (12) is measured, and that in step (3) the closing force is set to a value taking into account the projected area of the plastic molding (1) a Internal pressure at which the second plastic material (M2) can not foam.

6. The method according to claim 5,

characterized in that

the internal pressure is measured from outside through the first unfoamed plastic material (M1), using an internal pressure transducer (12) positioned on the surface of the unfoamed plastic material (M1). Method according to one of the preceding claims,

characterized in that

in step (2), the reduced first size cavity (10) (G1) is completely filled with the first unfoamed plastic material (M1).