METHOD AND INJECTION-MOLDING NOZZLE FOR PRODUCING INJECTION-MOLDED PARTS FROM PLASTIC

Abstract

In a method and an injection-molding nozzle for producing injection-molded parts from plastic with an injection mold, the plastic melt in the form of at least one ribbon-like strand of melt is injected through a nozzle slit into a cavity of the injection mold before the injection-molded part is demolded once the plastic melt has solidified. The plastic melt is exposed to heat in the region of the sprue during solidifying in the cavity and the sprue is torn off during demolding of the injection-molded part along the nozzle slit in the region of the sprue as a result of the temperature gradient between the solidified injection-molded part and the plastic melt.

Claims

1. A method for producing injection-molded parts from plastic with an injection mold, wherein the plastic melt in the form of at least one ribbon-like strand of melt is injected through a nozzle slit (2) into a cavity (15) of the injection mold-before the injection-molded part is demolded once the plastic melt has solidified, wherein the plastic melt is exposed to heat in the region of the sprue during solidifying in the cavity (15) and wherein the sprue is torn off during demolding of the injection-molded part along the nozzle slit (2) in the region of the sprue as a result of the temperature gradient between the solidified injection-molded part and the plastic melt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Other objects and features of the invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention.

[0025] In the drawings,

[0026] FIG. 1 shows a partly exposed schematic view of an injection-molding nozzle in accordance with the invention;

[0027] FIG. 2 shows this injection-molding nozzle in a cross-sectional view which is perpendicular to the nozzle slit;

[0028] FIG. 3 shows a sectional view along the line III-III of FIG. 2;

[0029] FIG. 4 shows the nozzle core in a side view;

[0030] FIG. 5 shows the nozzle core according to FIG. 4 in a front view;

[0031] FIG. 6 shows in a schematic cross-sectional view an embodiment of an injection-molding nozzle which is inserted into an injection mold;

[0032] FIG. 7 shows a partly exposed view of a constructional variant of a nozzle core in the longitudinal direction of the nozzle slit;

[0033] FIG. 8 shows the nozzle core according to FIG. 7 in a top view;

[0034] FIG. 9 shows a constructional variant of a nozzle core in a simplified schematic view;

[0035] FIG. 10 shows this nozzle core in a top view on an enlarged scale;

[0036] FIG. 11 shows the housing for the nozzle core according to FIGS. 9 and 10 in a sectional schematic view; and

[0037] FIG. 12 shows an injection mold in the region of an injection-molding nozzle extending over two cavities in a longitudinal sectional view through the injection-molding nozzle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0038] The injection-molding nozzle according to FIGS. 1 to 5 comprises a housing 1, which forms a nozzle slit 2, as well as a nozzle core 3 which is accommodated by the housing 1, between which and the housing 1 a tapering nozzle channel 4 is obtained which tapers in the direction of flow and preferably fully encloses the nozzle core 3. For the purpose of supplying said nozzle channel 4 with a plastic melt, the nozzle core 3 comprises a central feed channel 5, which is adjoined by distributor channels 6 on the two longitudinal sides of the nozzle core 3. It would also be possible to supply the two distributor channels 6 not via a branch 7 of a common feed channel 5, but in a separate manner, e.g. in order to enable the injection of different plastic materials in layers.

[0039] The distributor channels 6 which originate from the branch 7 of the central feed channel 5 each form two symmetrically formed channel branches which taper in the direction of flow and which are flow-connected at their ends to the respective channel branches of the opposite distributor channel 6, so that the constructive prerequisites for the formation of a flow of the plastic melt which advantageously meets the rheological requirements can be provided over the entire region of extension of the nozzle slit 2. According to the embodiment, the distributor channels 6 are formed in form of a recess which is open towards the housing 1, which not only entails simple production conditions, but also ensures good heat transfer from the heated nozzle core 3 to the plastic melt in the region of the distributor channels 6 as a result of the surface of the nozzle core 3 which is increased by the recesses.

[0040] Although the distribution of the plastic melt over the region of extension of the nozzle slit 2 is necessary, it is not adequate so as to ensure the desired flow distribution over the longitudinal extension of the nozzle slit 2. This is only achieved when the supply of the nozzle channel 4 with the plastic melt supplied via the distributor channels 6 occurs via a throttle zone 8, via which the distributor channels 6 are connected to the nozzle channel 4. The throttle zone 8 is generally determined by constrictions of the flow cross-section, which each extend over the length of the section of the nozzle slit 2 associated with the distributor channel 6, so that the plastic melt is subjected to pressure conditions which are predetermined over the entire region of extension of the nozzle slit 2. The throttle effect can differ in this respect for influencing the flow distribution over the flow cross-section.

[0041] In order to improve the flow conditions for the plastic melt emitted from the nozzle slit 2, the nozzle core 3 can form an inlet section 9 in the region of the nozzle channel 4 which adjoins the throttle zone 8 and a downstream outlet section 10, which in relation to the nozzle outflow direction has a smaller angle of inclination than the inlet section 9, as is shown in particular in FIGS. 1 and 5. As a result of the smaller angle of inclination of the outlet section 10, the plastic melt is subjected to an additional deflection in the direction of the nozzle slit 2.

[0042] The precondition for tearing off the sprue during the demolding of an injection-molded part is that the plastic melt does not solidify in the nozzle channel 4. The nozzle core 3 must therefore be heated accordingly in order to also supply the plastic melt with heat in the region of the nozzle channel 4. Although heating of the nozzle core is also possible via the housing 1, more advantageous heating conditions are achieved if the nozzle core 3 is heated directly. For this purpose, electrical heating cartridges 11 are inserted according to the illustrated embodiment into the nozzle core 3, which heating cartridges ensure a controlled heating of the nozzle core 3. According to the embodiment, the heating cartridges 11 extend perpendicularly to the nozzle slit 2, because the introduction of heat into the tapering end section of the nozzle core 3 is thus facilitated as a result of the spatial conditions. This arrangement of the heating cartridges 11 is not mandatory. FIG. 6 shows a nozzle core 3 with heating cartridges 11 extending parallel to the nozzle slit 2. It is generally understood that the electrical heating can also be replaced by a heating by means of a heat carrier flowing through the nozzle core 3.

[0043] In order to reduce the heat losses by heat transfer from the plastic melt to the housing 1, the housing 1 can be shielded against the nozzle core by a heat insulation 12, which advantageously forms the wall on the housing side of the distributor channels 6 at least in some sections. This heat insulation, which encloses the nozzle core 3 in the form of a jacket, need not be produced itself from a heat-insulating material. It is certainly possible to obstruct the heat transfer from the plastic melt, which generally forms an adverse heat conductor, to the housing 1 by an air gap in sections between the heat insulation 12 and the housing 1, which occurs in such a way for example that the exterior surface of the heat insulation 12 is provided with a corrugation.

[0044] In contrast to the embodiment according to FIGS. 1 to 5, the nozzle core 3 is displaceably mounted in the housing 1 for closing the nozzle slit 2 according to the embodiment in accordance with FIG. 6. An actuator 13 is used for adjusting of the nozzle core 3 to the closed position shown in FIG. 6, which actuator is formed in the embodiment in form of a wedge gear 14. Furthermore, the housing 1 is formed by a mold plate 16 which delimits the cavity 15 of an injection mold, so that a separate housing for the injection-molding nozzle which is to be inserted into such a mold plate 16 can be avoided.

[0045] FIGS. 7 and 8 show an especially advantageous embodiment of a nozzle core 3, because the circular-cylindrical basic shape of this nozzle core 3 corresponds to that of a round nozzle. Simple sealing conditions are obtained from the circular-cylindrical basic shape. In order to ensure a nozzle channel 4 which opens into a nozzle slit 2, the cylindrical nozzle core 3 is provided in the region of the nozzle channel 4 with two roof surfaces 17 which are symmetrical with respect to the longitudinal axis of the nozzle slit 2 and which delimit the nozzle channel 4. The supply of melt occurs via a central feed channel 5 with a branch 7, to which the distributor channels 6 are connected. The length of the nozzle slit 2 is obviously limited to the diameter of the nozzle core 3 in the case of such a formation of the nozzle core 3.

[0046] In order to increase the melt throughput despite the spatial limitations provided by the housing 1, the nozzle slit 2 and the nozzle channel 4 opening into the nozzle slit 2 can comprise several branches 18 between the housing 1 and the nozzle core 3, as is illustrated in FIGS. 9 to 11. According to the embodiment of FIG. 11, the housing 1 forms the nozzle slit 2 in form of a cross slot with four branches 18 that originate from a center. In FIG. 10, the nozzle slit 2 with its four branches 18, which are connected to each other to form a cross slot, is indicated in its position in relation to the nozzle core 3 by the dot-dash line. The nozzle core 3, which is circular-cylindrical in its basic shape, is provided according to FIGS. 9 and 10 in the region of the nozzle channel 4 with roof surfaces 17 which are associated in pairs to each branch 18 of the nozzle slit 2 and which end in a cross-shaped edge corresponding to the cross shape of the nozzle slit 2. When the nozzle core 3 is inserted into the housing 1, the nozzle channel 4 which opens into the cross slot is obtained between the roof surfaces 17 of the nozzle core 3 and respective counter-surfaces 19 of the housing 1, which nozzle channel 4 is associated with distributor channels 6 which are formed in sections in the nozzle core 3 and which are connected to a feed channel 5 via a respective branch 7 on mutually opposite sides of the nozzle core 3. The supply of the nozzle channel 4 via a throttle zone is ensured via respective cross-sectional constrictions, which is not shown in closer detail in the drawing for reasons of clarity of the illustration. The heating of the nozzle core 3 occurs via a heating cartridge 11.

[0047] If an injection mold comprises several cavities 15 which are mutually separated by a separating wall 20, as is indicated in FIG. 9, the cavities 15 which are mutually separated by a separating wall 20 can be supplied by a common injection-molding nozzle whose nozzle slit 2 extends on both sides of the separating wall 20 according to FIG. 9. It is necessary to take the progression of the separating wall 20 into account concerning the distribution of the plastic melt over the region of extension of the nozzle slit 2.

[0048] As a result of the introduction of the plastic melt into the cavity 15 of an injection mold via a nozzle slit 2, the shearing stress of the plastic melt can be kept at a comparatively low level with respect to the potential melt throughput, which is a relevant precondition for injection of the plastic melt into the cavity 15 in a material-protecting manner. The tearing off of the sprue depends on the strength properties of the plastic in the region of the nozzle slit 2, which plastic is solid during demolding within the cavity 15 but is molten in the sprue region, so that in the transitional region from the cavity 15 to the nozzle channel 4 a high temperature gradient is obtained within a thin layer in the region of the nozzle slit 2, thus providing the preconditions for tearing off a sprue along the surface determined by the opening of the nozzle slit 2. For this purpose, it is recommended to cool the housing in the region of the nozzle slit 2. Cooling channels 21 are shown for this purpose in FIGS. 1, 2 and 6. It is therefore possible, with a respective selection of the influencing parameters, to move the tearing-off surface into the mold surface of the respective injection-molded part without requiring post-processing of the torn-off portion of the sprue. The sprue is thus moved into the region of the hot channel.

[0049] Especially advantageous demolding conditions are obtained in this connection according to FIG. 6 if the possibility is provided to close the nozzle slit 2 by means of the nozzle core 3.

[0050] Although only a few embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.