METHOD FOR PRODUCING A MOULDED PART BY STRUCTURAL FOAM MOULDING, MOULDED PART OF AN EXPANDED THERMOPLASTIC MATERIAL AND USES THEREFOR

Abstract

The invention relates to a method for producing a moulded part (50) by structural foam moulding, in which a polymer melt (18) is provided by melting a thermoplastic material, in which the polymer melt (18) is charged with a foaming agent (22) and in which the polymer melt (18) charged with the foaming agent (22) is injected under pressure into a cavity (26) of a mould (28), and so the polymer melt (18) fills the cavity (26) behind a melt front (34) running through the cavity (26), wherein the rate of injection at which the polymer melt (18) is injected into the cavity (26) of the mould (28) is set such that the internal pressure of the polymer melt (18) in the cavity (26), in a region (40) that follows a portion of the melt front (34) with a time delay of at most 0.15 seconds, is greater than the critical pressure of the foaming agent (22), at least at one point in time during the injection-moulding operation. The invention also relates to a moulded part (50) of an expanded thermoplastic material, wherein the moulded part (50) has a surface region with visual structuring formed by the expanded thermoplastic material of which the average ratio of the degrees of gloss measured in the direction of flow in relation to the degrees of gloss measured transversely to the direction of flow is below 1.9, preferably below 1.5, in particular below 1.2. The invention also relates to uses of such a moulded part.

Claims

1.-18. (canceled)

19. A process for the production of a molding by structural foam molding, comprising providing a plastics melt by melting of a thermoplastic, loading the plastics melt with a blowing agent, and injecting the plastics melt loaded with the blowing agent under pressure into a cavity of a mold in such a way that the plastics melt fills the cavity behind a melt front proceeding through the cavity, wherein the injection velocity at which the plastics melt is injected into the cavity of the mold is adjusted in such a way that, in a region that follows a section of the melt front with a chronological separation of at most 0.15 s, at least at one juncture during the injection procedure, the internal pressure of the plastics melt in the cavity is greater than the critical pressure of the blowing agent.

20. The process as claimed in claim 19, wherein the region in which the internal pressure of the plastics melt is, at least at one juncture during the injection procedure, greater than the critical pressure of the blowing agent follows the section of the melt front with a chronological separation of at most 0.1 s.

21. The process as claimed in claim 19, wherein the thermoplastic comprises a transparent plastic selected from the group consisting of polycarbonates (PC), polystyrenes (PS), polymethyl methacrylates (PMMA), styrene-acrylonitriles (SAN), cycloolefin copolymers (COC), transparent polyamides (PA), transparent polyesters, polyester made of terephthalic acid with cyclohexanedimethanol and tetrarnethylcyclobutanediol, and mixtures of these polymers.

22. The process as claimed in claim 19, wherein the thermoplastic is selected from the group consisting of polycarbonates (PC), polystyrenes (PS), polymethylmethacrylates (PMMA), cycloolefin copolymers (COC), styrene-acrylonitrile (SAN), transparent polyamides (PA), polyvinyl chlorides (PVC), polyphenylene ethers (PPE), and mixtures thereof.

23. The process as claimed in claim 19, wherein the plastics melt is loaded with a blowing agent via introduction of a gas into the plastics melt.

24. The process as claimed in claim 19, wherein the concentration of the blowing agent in the blowing-agent-loaded plastics melt before injection into the cavity is from 0.5 to 3% by weight for chemical blowing agents and from 0.2 to 1.0% by weight for physical blowing agents.

25. The process as claimed in claim 19, wherein the design of the mold is such that, in the direction of flow of the plastics melt, the cross section of the cavity does not narrow by more than 10%.

26. The process as claimed in claim 19, wherein the mold has been designed for a film gate or for a direct gate.

27. A molding made of a foamed thermoplastic, wherein the molding has a surface region with optical structuring which is formed by the foamed thermoplastic and for which the averaged ratio of the gloss levels measured in the direction of flow to the gloss levels measured perpendicularly to the direction of flow is below 1.9.

28. The molding as claimed in claim 27, wherein the molding has been produced by structural foam molding.

29. The molding as claimed in claim 27, wherein the molding is produced by the process as claimed in claim 19.

30. The molding as claimed in claim 27, wherein the molding has a surface region with optical structuring which is formed by the foamed thermoplastic and for which the averaged ratio of the gloss levels measured in the direction of flow to the gloss levels measured perpendicularly to the direction of flow is below 1.5.

31. The molding as claimed in claim 27, wherein the molding has a surface region with optical structuring which is formed by the foamed thermoplastic and for which the averaged ratio of the gloss levels measured in the direction of flow to the gloss levels measured perpendicularly to the direction of flow is below 1.2.

32. The molding as claimed in claim 27, wherein the thermoplastic is a transparent plastic.

33. The molding as claimed in claim 27, wherein the thermoplastic comprises a compound selected from the group consisting of polycarbonates (PC), polystyrenes (PS), polymethyl methacrylates (PMMA), styrene-acrylonitrile (SAN), transparent polyamides (PA), polyvinyl chlorides (PVC), polyphenylene ethers (PPE), and mixtures thereof.

34. The molding as claimed in claim 27, wherein the surface region with the optical structuring which is formed via the foamed thermoplastic comprises at least 30% the entire surface of the molding.

35. The molding as claimed in claim 27, wherein the thickness of the molding is in the range from 1 and 20 mm.

36. An article comprising the molding as claimed in claim 27, wherein the article is selected from the group consisting of an item of furniture or lighting elements, product casings, cups, bowls, protective covers, coolboxes, cladding parts for coolboxes, and multiple-use containers for refrigerated and fresh products.

Description

[0101] Other features and advantages of the process, of the molding, and of its use are apparent from the description below of a plurality of embodiments, with reference to the attached drawing.

[0102] The drawing shows,

[0103] in FIG. 1, a diagram of an apparatus for carrying out a process in one embodiment of the process of the invention,

[0104] in FIG. 2a, a mold of the apparatus from FIG. 1, depicted during the injection of a plastics melt during conduct of an embodiment of the process of the invention,

[0105] in FIG. 2b, an enlarged detail from FIG. 2a,

[0106] in FIG. 3a-b, a depiction of a mold cavity for a sheet molding in plan view and in cross section,

[0107] in FIG. 4a-b, a depiction of a mold cavity for a bowl-shaped molding, in plan view and in cross section,

[0108] in FIG. 5, a graph showing simulated internal pressure profiles in the mold from FIG. 3a-b for various injection velocities,

[0109] in FIG. 6, a graph showing simulated internal pressure profiles in the mold from FIG. 4a-b for various injection velocities, and

[0110] in FIG. 7a-b, an image of a molding surface of a molding of the invention, and also of a comparative molding.

[0111] FIG. 1 shows a structural foam molding apparatus of the type that by way of example can be used to carry out a process in an embodiment of the process of the invention.

[0112] The apparatus 2 comprises a screw conveyor 4 with a conveyor tube 6 designed as hollow cylinder, and with a driven transport screw 8 mounted rotatably in the conveyor tube 6. The apparatus 2 moreover has a feed neck 10 for input of plastics pellets 12. The pellets 12 are transported by the transport screw 8 from the feed region into a melt region 14 which has heating elements 16, in order to heat the plastic in the conveyor tube 6 to a temperature above its melting point and thus produce a plastics melt 18. The plastics melt 18 is further transported in the conveyor tube 6 in a region which is in front of the transport screw 8 and in which there is a blowing agent inlet 20 arranged, through which a blowing agent 22 (by way of example carbon dioxide or nitrogen) can be introduced into the plastics melt 18 in the conveyor tube 6. For the conduct of the injection procedure, the transport screw 8 is translated in the direction of the injection aperture in such a way that the plastics melt 18 loaded with the blowing agent 22 is injected through an injection aperture 24 into the cavity 26 of a mold 28. The plastics melt 18 then spreads behind a melt front proceeding through the cavity 26, and thus fills the cavity 26. During this injection procedure, the plastics melt 18 loaded with the blowing agent foams by virtue of the blowing agent.

[0113] FIGS. 2a and 2b show the mold 28 of the apparatus 2 from FIG. 1 in cross section from the side during the injection of a plastics melt during conduct of an embodiment of the process of the invention. FIG. 2b here shows an enlarged detail from FIG. 2a.

[0114] The cavity 26 of the mold 28 comprises a feed channel 30, in the present case designed for a film gate, and also a molding region 32 which corresponds to the exterior shape of the required molding.

[0115] During the conduct of the process, the blowing-agent-loaded plastics melt 18 is injected through the injection aperture 24 into the cavity 26. The plastics melt then fills the cavity 26 behind a melt front 34 proceeding through the cavity 26, and specifically this occurs initially in the region of the feed channel 30 and then in the molding region 32 of the cavity. The injection of the plastics melt 18 into the cavity 26 takes place under pressure, whereupon a pressure gradient becomes established from the location of the injection aperture 24 extending to the melt front 34. In that region of the plastics melt 18 where the internal pressure is above the critical pressure of the blowing agent, the blowing agent is in supercritical solution with the plastics melt 18. Since the internal pressure decreases from the injection aperture 24 to the melt front 34, the internal pressure of the plastics melt 18 becomes less than the critical pressure of the blowing agent at a point 36, and blowing agent between this point 36 and the melt front 34 is therefore no longer in supercritical solution with the plastics melt 18, and therefore evolves gas bubbles 38.

[0116] The injection velocity, i.e. the velocity at which the transport screw 8 is moved in the direction of the injection aperture 24 in order to inject the plastics melt 18 into the injection aperture 24, is adjusted in such a way that the chronological separation between the melt front 34 and a region 40 in which the plastics melt retains an internal pressure that is greater than the critical pressure of the blowing agent, and in which therefore the blowing agent is in supercritical state, is at most 0.15 s. This chronological separation corresponds to a spatial distance that is traveled within 0.15 s by the melt front 34.

[0117] Experiments have revealed that this type of small separation between the region 40 with supercritical blowing agent and the melt front 34 leads to turbulent flow in the region of the melt front 34, in such a way that the resultant molding has an esthetic visually structured surface with the appearance of ice.

[0118] FIGS. 3a-b show a diagram of a mold cavity 50 for a sheet molding in plan view (FIG. 3a) and in cross section (FIG. 3b). The cavity 50 comprises a molding region 52 and a feed channel 54. The feed channel 54 extends from an injection aperture 56 through which, during the injection procedure, the plastics melt is injected into the mold, initially in a tubular section 58 and then in a continuously widening, flat section 60 extending as far as the molding region 52. The width of the cross section of the feed channel 54 increases considerably in the flat section 60 extending as far as the molding region 52, in such a way that when the plastics melt is injected a uniform, broad melt front is formed. A gate produced with this type of feed channel is also termed film gate. After a small cross-sectional narrowing 62, which serves inter alia to permit easier removal of the solidified sprue region of the plastics melt from the actual molding, the cross section increases abruptly at the transition from the feed channel 54 to the molding region 52. This is in particular assisted via a relatively small width of the feed channel 54 in comparison with the molding region 52 at the transition, and also via the brief cross-sectional narrowing 62. The prior art generally uses the term gate mark 64 for the transition of the feed channel 54 to the molding region 52 in the molding produced by the cavity 50 (even when the transition in the present case corresponds to a rectangular area with large side-to-side ratio).

[0119] Table 1 states the dimensions of the cavity 50 depicted as example in FIG. 3a-b:

TABLE-US-00001 TABLE 1 Geometry of cavity for the sheet molding Variable Dimension Height of molding region (wall thickness)  3.5 mm Width of molding region (perpendicularly to the 150.0 mm direction of flow of the plastics melt in the gate) Length of molding region (in the direction of 200.0 mm flow of the plastics melt in the gate) Width of feed channel at the transition to the 120.0 mm molding region Cross-sectional enlargement at the transition to 75% the molding region

[0120] During the injection procedure, the plastics melt injected into the injection aperture 56 fills the cavity 50 behind a melt front 66 progressing through the cavity 50. The melt front 66 here proceeds initially through the tubular section 58 and then through the flat section 60 of the feed channel 54, before it then proceeds through the molding region 52. FIGS. 3a-b indicate the position of the melt front 66 by way of example for a juncture at which the melt front 66 has already entered the molding region 52.

[0121] FIGS. 4a-b are diagrams of a mold cavity 70 for a bowl-shaped molding in plan view (FIG. 4a) and in cross section (FIG. 4b). The cavity 70 comprises a molding region 72 and a feed channel 74. The feed channel 74 extends from an injection aperture 76, through which the plastics melt is injected into the mold during the injection procedure, in a tubular section 78 that widens slightly in the manner of a cone as far as the molding region 72. A gate resulting from this type of feed channel 74 is also termed direct gate. The transition of the feed channel 74 to the molding region 72 takes place in essence perpendicularly to a wall section of the molding region 72, i.e. perpendicularly with respect to a region that forms a wall of the injected molding. After a slight increase of cross section in the conical section 78, therefore, the cross section increases abruptly during the transition from the feed channel 74 to the molding region 72. The transition of the feed channel 74 to the molding region 72 is termed gate mark 80 for the molding produced by the cavity 70. The molding region 72 comprises a base region 82 and an edge region 84, and these respectively form the base and the edge of the bowl that can be produced by the cavity 70.

[0122] Table 2 states the dimensions of the cavity 70 depicted as example in FIG. 4a-b:

TABLE-US-00002 TABLE 2 Geometry of cavity for the bowl-shaped molding Variable Dimension Diameter of base region 112 mm Largest diameter of edge region 170 mm Height of edge region (from base region to  70 mm upper bowl edge) Wall thickness of base region (at gate mark)  4.8 mm Smallest wall thickness of edge region  3 mm Diameter of feed channel at gate mark  9 mm Cross-sectional enlargement during transition 113% to molding region

[0123] During the injection procedure, the plastics melt injected into the injection aperture 76 fills the cavity 70 behind a melt front 86 progressing through the cavity 70. The melt front 86 here initially proceeds through the feed channel 74, and then proceeds through the molding region 72. FIGS. 4a-b indicate the position of the melt front 86 by way of example for a juncture at which the melt front 86 has already entered the molding region 72.

[0124] For each of the mold cavities 50 and 70 depicted in FIGS. 3a-b and 4a-b, rheological simulations were carried out for injection procedures at various injection velocities, in order to determine the internal pressure profile of the plastics melt during the injection procedure.

[0125] The rheological simulations here were in each case carried out as follows:

[0126] The “Autodesk® Simulation Moldflow® Insight 2013 FCS—lantanum_fcs” program was used for the injection simulation calculations. The mold cavities 50 and 70 depicted in FIGS. 3a-b and 4a-b, with the stated dimensions, were first replicated in the computer program.

[0127] The following parameters were then moreover defined for the simulation of the injection procedures:

[0128] The plastic selected for use for the simulation was the polycarbonate Makrolon AL2647, obtainable from Bayer MaterialScience AG, Leverkusen, Germany. The material parameters used for the simulation of this plastic were those from the material database file for Makrolon AL2647 provided by Bayer MaterialScience AG, Leverkusen, Germany, for the users in particular of said computer program.

[0129] In particular, the simulation of the viscosity η (in Pa.Math.s) of the plastics melt used the Cross-WLF viscosity model with the formula

[00002]$\begin{array}{cc}\eta =\frac{{\eta }_0}{1+{\left(\frac{{\eta }_0\stackrel{.}{\gamma }}{{\tau }^{*}}\right)}^{1-n}},\mathrm{where}& \left(1\right)\\ {\eta }_0=D_1\exp \left(-\frac{A_1\left(T-T^{*}\right)}{A_2+\left(T-T^{*}\right)}\right),& \left(2\right)\end{array}$

[0130] and T is the temperature (in K), T*=D.sub.2+D.sub.3p is the glass transition temperature (in K), A.sub.2=A.sub.3+D.sub.3p, p is the pressure (in Pa) and {dot over (γ)} is the shear rate (in s.sup.−1), and where the individual parameters were selected in accordance with table 3:

TABLE-US-00003 TABLE 3 Parameters for the cross-WLF viscosity model Parameter Value n 0.1555 τ* 740 472 Pa D.sub.1 5.45517e+11 Pa .Math. s D.sub.2 417.15 K D.sub.3 0 K/Pa A.sub.1 28.056 A.sub.2 51.6 K

[0131] For the thermodynamic behavior of the plastics melt, i.e. for the dependency of the specific volume v of the plastics melt of the temperature T (in K) and on the pressure p (in Pa) a 2-domain Tait pvT model was used with the formula

[00003]$\begin{array}{cc}v\left(T,p\right)=v_0\left(T\right)\left(1-C\ln \left(1+\frac{p}{B\left(T\right)}\right)\right)+v_t\left(T,p\right),& \left(3\right)\end{array}$

[0132] where ν.sub.0=b.sub.1m+b.sub.2m (T−b.sub.5)B(T)=b.sub.3m exp(−b.sub.4m (T−b.sub.5))ν.sub.1(T, p)=0 for T>T.sub.t, and ν.sub.0=b.sub.1s+b.sub.2s(T−b.sub.5)B(T)=b.sub.3sexp(−b.sub.4s(T−b.sub.5))ν.sub.1(T, p)=b.sub.7 exp(b.sub.8(T−b.sub.5)−b.sub.9p) for T≤T.sub.t, where T.sub.t(p)=b.sub.5+b.sub.6p, where C=0.0894, and where the individual parameters were selected in accordance with table 4:

TABLE-US-00004 TABLE 4 Parameters for the 2-domain Tait pvT model Parameter Value b.sub.5  427.97 K b.sub.6  2.487e−7 K/Pa b.sub.1m 0.008738 m.sup.3/kg b.sub.2m 6.497e−7 m.sup.3/(kg K) b.sub.3m 8.86889e+7 Pa b.sub.4m 0.003935 K.sup.−1 b.sub.1s 0.0008738 m.sup.3/kg b.sub.2s 2.927e−7 m.sup.3/(kg K) b.sub.3s 1.00166e+8 Pa b.sub.4s 0.001681 K.sup.−1 b.sub.7  0 m.sup.3/kg b.sub.8  0 K.sup.−1 b.sub.9  0 Pa.sup.−1

[0133] The Makrolon AL2647 density values provided for the simulations were moreover 1.0329 g/cm.sup.3 for the melt, and 1.1965 g/cm.sup.3 for the solid-state density.

[0134] The blowing agent (in this case nitrogen) was ignored for the sake of simplicity in the simulations, because experiments have shown that even when the blowing agent is ignored the simulations provide useful and comparable internal pressure distribution results.

[0135] Table 5 below states the other parameters used for the simulations:

TABLE-US-00005 TABLE 5 Other simulation parameters Process parameter Value Mold surface temperature 100° C. Minimal mold surface temperature  80° C. Maximal mold surface temperature 120° C. Melting point of plastic 300° C. Minimal melting range temperature 280° C. Maximal melting range temperature 320° C. Absolute maximum of melting point 360° C. Injection temperature 130° C. Maximal transverse stress 0.5 MPa Maximal shear rate 40 000 s.sup.−1

[0136] The simulations simulated an injection procedure using a screw conveyor with screw diameter 50 mm. The quantity of the plastics melt injected via translation of the screw into the respective cavity was adjusted in each case to be appropriate to the corresponding volume of the cavity 50 and, respectively, 70.

[0137] For each of the two cavities 50 and 70, the injection procedure was simulated respectively with an injection velocity of 20, 40, 60, 80, and 100 mm/s. The injection velocity here corresponds in each case to the velocity at which the screw is translated during the injection procedure. The volume per second injected into the cavity is thus calculated from the product of the injection velocity and the cross section of the screw conveyor (=πD.sup.2/4 where D=50 mm).

[0138] In each case, the injection procedure was simulated from its start (i.e. when the location of the melt front is at the injection aperture 56 and, respectively, 76) as far as the position depicted in FIGS. 3a-b and, respectively, 4a-b for the melt front 66 and 86 (i.e. when the respective melt front 66 and, respectively, 86 has entered the molding region 52 and, respectively, 72).

[0139] The simulations were in each case used to determine internal pressure profiles of the plastics melt at the juncture depicted in FIGS. 3a-b and 4a-b, i.e. after entry of the respective melt front into the molding region. The internal pressure profiles for the various injection velocities are depicted in the graph in FIG. 5 for the cavity from FIG. 3a-b and in the graph in FIG. 6 for the cavity from FIG. 4a-b.

[0140] The graphs in FIGS. 5 and 6 show the local internal pressure in the plastics melt as a function of the position within the cavity. The position in the cavity is shown on the abscissa here as time in seconds. The juncture at 0 s corresponds in each case to that position in the cavity at which the internal pressure of the plastics melt falls below 33.9 bar, which is the critical pressure of the nitrogen blowing agent used for the plastics melt in the present case, this pressure being depicted by the horizontal line in FIGS. 5 and 6. A particular time t>0 then in each case corresponds to the position of a small volume of the plastics melt in the cavity, where the location of said volume before a period of duration t was still at the position t=0 s. If by way of example the flow velocity of said volume is constant at v, the spatial separation s from the position t=0 along the direction of flow is calculated from the product s=v.Math.t.

[0141] The (chronological) position of the melt front in FIGS. 5 and 6 corresponds to that point at which the respective curve falls in essence to a pressure of 0 bar (or to ambient pressure/atmospheric pressure), i.e. intersects with the abscissa. In the region of the melt front the plastics melt is in essence not subject to any significant counterpressure from the as yet unfilled region of the cavity, and the pressure at the melt front therefore in essence falls abruptly to 0 bar (or to ambient pressure/atmospheric pressure).

[0142] Since the time axis in FIGS. 5 and 6 has been standardized in such a way that t=0 s corresponds to the position where the pressure falls below the critical pressure, the chronological separation between the melt front (or a section thereof) and a region in which the internal pressure of the plastics melt is greater than the critical pressure of the blowing agent can be read directly from FIGS. 5 and 6; this chronological separation namely corresponds precisely to the chronological position of the melt front.

[0143] In the present invention this chronological separation is permitted to be at most 0.15 s, preferably at most 0.1 s, and more preferably at most 0.05 s. These limits are emphasized by vertical lines in FIGS. 5 and 6. The region between 0.15 s and 0.1 s here can be termed transition region, since with these chronological separations the desired surface structuring with the appearance of ice is very generally obtained, but sometimes not quite uniformly across the entire molding. For chronological separations below 0.1 s, and certainly below 0.05 s, the desired surface structuring with the appearance of ice could be achieved over an entire surface.

[0144] It is then possible to read from FIGS. 5 and 6 the injection velocities for which the abovementioned condition is met.

[0145] For the sheet component, FIG. 5 shows that an injection velocity of 20 mm/s is too small, since the chronological separation between the critical internal pressure region and the melt front here is almost 30 s. The value at 40 mm/s is in the transition region, and injection velocities of 60, 80, or 100 mm/s comply with the required criterion in such a way that the desired surface structuring with the appearance of ice can be achieved reliably and in essence over an entire surface with these injection velocity values.

[0146] For the bowl-shaped component, FIG. 6 shows that an injection velocity of 20 mm/s reaches the transition region, while injection velocities of at least 40 mm/s comply with the required criterion in such a way that the desired surface structuring with the appearance of ice can be achieved reliably and in essence over an entire surface with these injection velocity values.

[0147] The injection procedures described above were not only simulated but also carried out in practice.

[0148] For this, two molds were used, corresponding to FIGS. 3a-b and 4a-b, attached to an ARBURG Allrounder 570C screw conveyor obtainable from ARBURG GmbH & CoKG, Lossburg, Germany, with screw diameter 50 mm.

[0149] Makrolon AL2647 polycarbonate pellets, obtainable from Bayer MaterialScience AG, Leverkusen, Germany, were charged to the screw conveyor, where they were heated to a temperature of 300° C. to form a plastics melt. The screw of the screw conveyor was then in each case used to transport, in the hollow cylinder surrounding the screw, a volume adapted to be appropriate to the volume of the respective mold cavity to a position in front of the mold-facing end of the screw.

[0150] During the injection procedure, the screw was then in each case translated forward, i.e. in the direction of the mold, with the appropriate injection velocity, in such a way as to inject the plastics melt from the screw conveyor through the injection aperture 56 and, respectively, 76 into the cavity of the corresponding mold 50 and, respectively, 70. Shortly before injection into the respective injection aperture, the plastics melt was moreover loaded with 0.60% by weight of nitrogen, in order to foam the plastics melt in the cavity.

[0151] Findings on the sheet moldings produced in the manner described above were that the desired surface structuring with the appearance of ice was not achieved at injection velocities of 20 mm/s, was achieved over part of a surface at injection velocities of 40 mm/s, and was achieved over an entire surface at injection velocities of 60 mm/s and above.

[0152] Findings on the bowl-shaped moldings produced in the manner described above were that the desired surface structuring with the appearance of ice was achieved over part of a surface at injection velocities of 20 mm/s, and was achieved over an entire surface at injection velocities of 40 mm/s and above.

[0153] The overall findings in the experiments were that the desired surface structuring with the appearance of ice was achieved for those injection velocities at which compliance with the abovementioned criterion for the internal pressure profile of the plastics melt was achieved.

[0154] FIG. 7a shows the surface of the sheet molding (of the invention) produced with an injection velocity of 60 mm/s. FIG. 7b shows, for comparison, the surface of the sheet molding (not of the invention) produced with an injection velocity of 20 mm/s.

[0155] Whereas the surface of the molding not produced in the invention in FIG. 7b reveals a regular, streaky, laminar surface structure, the result for the molding produced in the invention in FIG. 7a is the desired turbulent surface structuring reminiscent of the appearance of ice.

[0156] Objective differentiation between inventive surface structures (e.g. FIG. 7a) and non-inventive surface structures (e.g. FIG. 7b) can be achieved by determining the averaged ratios of the gloss levels measured in the direction of flow to the gloss levels perpendicularly to the direction of flow on the basis of the measurement method described above.

[0157] Haze-gloss AG-4601 gloss level measurement equipment, obtainable from BYK-Gardner GmbH, Geretsried, Germany was used to determine the gloss level ratios in accordance with the test method described above on a series of moldings produced in the invention and on a series of comparative components, and in particular on the moldings from FIGS. 5 and 6.

[0158] Table 6 shows the results of the gloss level measurements parallel to and perpendicularly to the direction of flow at in each case six measurement points for three moldings (A-C) produced in the invention and three comparative moldings (D-F) not produced in the invention. The moldings produced in the invention here have turbulent surface structuring comparable with the surface structuring depicted in FIG. 7a, whereas the moldings not produced in the invention have in each case streaky, laminar surface structuring comparable with the surface structuring depicted in FIG. 7b.

[0159] The units of the gloss level measurements in table 7 correspond to the gloss level units used by the abovementioned equipment. They are not stated in the present case because in the final analysis the only important factor is the ratio of the gloss levels.

TABLE-US-00006 TABLE 6 Results of the gloss level measurements Measurement point Direction A B C D E F 1 perpendicular 11 7 8 13 13 13 2 perpendicular 13 8 5 15 13 13 3 perpendicular 7 8 6 13 14 15 4 perpendicular 7 8 7 14 14 15 5 perpendicular 11 11 12 19 12 11 6 perpendicular 13 13 9 19 14 10 1 parallel 10 8 9 34 30 34 2 parallel 22 7 5 29 29 33 3 parallel 9 9 8 37 37 51 4 parallel 7 16 7 34 34 28 5 parallel 18 13 14 31 27 20 6 parallel 24 16 9 31 30 21

[0160] The results from table 6 give the ratios listed in table 7 of the gloss levels parallel to the direction of flow to the gloss levels perpendicularly to the direction of flow.

TABLE-US-00007 TABLE 7 Gloss level ratios and averaged gloss level ratio Measurement point A B C D E F 1 0.9 1.1 1.1 2.6 2.3 2.6 2 1.7 0.9 1.0 1.9 2.2 2.5 3 1.3 1.1 1.3 2.8 2.6 3.4 4 1.0 2.0 1.0 2.4 2.4 2.5 5 1.6 1.2 1.2 1.6 2.3 1.8 6 1.8 1.2 1.0 1.6 2.1 2.1 Average 1.5 1.3 1.1 2.1 2.3 2.6

[0161] The gloss level ratio averaged over the individual measurement points is the decisive factor for objective differentiation between the inventive and noninventive surface structure. Said ratio is stated in the last row of table 7.

[0162] From table 7 it can be seen that the averaged gloss level ratio for the moldings A-C produced in the invention is less than 1.9, in particular less than 1.5, and to some extent indeed less than 1.2, whereas the averaged gloss level ratios for the moldings D-F not produced in the invention are greater than 2, indeed in the present case greater than 2.1.

[0163] The criterion using the averaged gloss level ratios therefore permits objective differentiation between moldings of the invention and moldings not of the invention.