A MOLD TOOL FOR INJECTION MOLDING

An injection-molding tool has at least two separate mold parts forming a set of mold cavities, and a set of runner channels extending between the mold inlet and leading melt flow to the mold cavities. The set of runner channels comprises a number of junctions, each junction dividing a downstream end of a primary runner channel extending upstream from the junction into two or more separate secondary runner channels extending downstream from the junction. A stretch of the downstream end of the primary runner has a cross section area being smaller than the cross section area of the primary runner further upstream from the junction.

The present application claims priority to Danish Patent Application No. PA 201970805 dated Dec. 20, 2019, and is a U.S. national stage application under 35 U.S.C. 371 of co-pending International Application No. PCT/EP2020/086924 filed on Dec. 18, 2020, the content of each are incorporated by reference herein in their entirety for all purposes.

FIELD OF THE DISCLOSURE

The subject disclosure relates to an injection-molding tool, and more particularly to an injection-molding tool for being mounted in an injection-molding apparatus for automated molding of work pieces in plastics.

BACKGROUND

When designing injection-molding tools of the above mentioned kind it is a recurring challenge to ensure even supply of molten material to an increasing number of separate mold cavities. This is primarily due to the problem that even though the all the melt supplied to the injection-molding tool has the same temperature, then some of the material in the flowing through the runner system is exposed to a higher shear and thereby having a higher temperature and a lower viscosity than other parts of the material, and that the geometry of the runner systems, especially at runner junctions where a single primary runner is divided into two or more secondary runners or branches, may lead more of the melt having a higher temperature and lower viscosity to one mold cavity than to another mold cavity.

Therefore several different constructions of runner channels such as especially cold runner channels are suggested in order to ensure even distribution of the molten material often referred to as balancing the runner. In the prior art a lot of different examples of such runner systems are suggested comprising different embodiments of melt flippers and melt mixers.

SUMMARY

Based on this, it is the object of the subject disclosure to provide an injection-molding tool with runners, such as cold runners, being well balanced, and allowing on the one hand that the cold runners or injection runners convey molten material to all mold cavities, but without using complex runner geometry.

In one embodiment of the subject disclosure, the downstream end of the primary runner has a cross section area being smaller than the cross section area of the primary runner further upstream from the junction, and smaller than the largest cross section of the secondary runners.

The baling effect is obtained due to the fact that the reduced area of the runner at the junction thereby locally creates a high shear of more of the melt flowing through the junction and at the same time it provides a mixing effect so that the high shear material and the low shear material is more mixed after the junction than it was before the junction.

According to a preferred embodiment of the molding tool, the cross section area of the downstream end of the primary runner gradually decreases in the flow direction.

Furthermore the cross section area of the upstream end of each of the secondary runners may advantageously be smaller than the cross section area of the secondary runner further downstream from the junction.

In this relation, the cross section area of the upstream end of the secondary runner may furthermore gradually increase in the flow direction.

Preferably the smallest cross section area of at least one primary or secondary runner connected via a junction is less than 75% and preferably less than 50% of the cross section area of the same runner at a distance from the junction. The selected optimal reduction depends on e.g. the characteristics of the plastic material supplied though the runners, and the aim is to increase the shear rate in the supplied plastic material significantly at least before the junction.

In an especially simple embodiment of the subject disclosure, one of or both the primary and the secondary runners are formed by groves arranged in the abutting side face of either the inlet mold part or the secondary mold part, or each of the primary and the secondary runners at least at a distance downstream and upstream from the junction are formed by groves arranged in only one of the abutting side faces of either the inlet mold part or the secondary mold part.

DETAILED DESCRIPTION

FIG.1illustrates a conventional injection molding tool with an inlet mold part1and a second mold part shown with dotted lines. The inlet mold part1and the second mold part2have abutting surfaces forming a separation plane5also shown in dotted lines. The inlet mold part has an inlet4connected to multiple mold cavities3via a sprue14and a set of runner channels8,9and a set of runner junctions6,7. InFIG.1the design of the mold cavities3, the sprue14, the runner channels8,9and the runner junctions6,7are illustrated as the shape of the molded component, including the sprue, the runners and the molded products/work pieces, that are produced in such an injection molding tool.

An embodiment of the subject disclosure will, in the following, be explained in principle with reference to the embodiment of an injection molding tool as shown inFIG.1, but it will be evident to one having ordinary skill in the art that the subject disclosure may also be implemented in various different types of injection molding tools, such as molding tools having an intermediate mold part between the inlet mold part1and the second mold part2.

In this relation,FIG.2shows an enlarged section20of the set of runners as shown inFIG.1, where runner junctions6,7divides primary runner channels8,9into secondary runner channels9,10respectively, so that the primary runners8extending from the sprue14are divided into secondary runners9by the runner junction6, and the secondary runners9, when looked at from the runner junctions7are now primary runners9, being divided into secondary runner10via the runner junctions7. In this way the set of runners may be further subdivided several times into further secondary runners that the most downstream end of the runners is connected to the mold cavities3via runner gates11.

FIGS.3and4disclose two junction inserts21,22each forming a runner junction6,7as shown inFIG.2for dividing the most downstream end of a primary runner channel8,9(partly shown in dotted lines), into the most upstream end of the secondary runner channels9,10respectively (partly shown with dotted lines). The junction inserts are made as blocks being adapted for insertion into a correspondingly shaped socket in the second mold part2, and a screw hole23is arranged for the purpose of securing the junction inserts in the mold part2. In this way it is possible to change the junction inserts with other junction inserts having different geometries, e.g. when the injection molding tool is to be used with other plastic materials, or to work under different conditions.

With the purpose of ensuring more even filling of the mold cavities3, the downstream end of the primary runner8as shown inFIG.3has a cross section area being significantly reduced with respect to the cross section of the same runner8at a position upstream. In the embodiment shown inFIG.3, a stretch12of the downstream end of the primary runner8is gradually decreasing in the flow direction in the runner8, and it has its smallest cross section just before the junction6where the primary runner channel is divided into the two secondary runner channels9.

In the same way the secondary runners9inFIG.3becomes the primary runner9in the embodiment shown inFIG.4where the primary runner9in the same way has a significantly reduced cross section at its most downstream position just before the junction7, where the primary runner9is divided into two secondary runners10. In this embodiment, however, each of the secondary runner channels10at their most upstream end has the smallest cross section area and the cross section at a stretch13of the upstream end of each of the secondary runner channels10are gradually increasing in the flow direction.

From the description above it will be apparent to one having ordinary skill in the art that the subject disclosure may be implemented in many different embodiments apart from the embodiment shown in the figures. As mentioned above, the subject disclosure may also be used e.g. with molding tools having an intermediate mold part between the inlet mold part1and the second mold part2, or molding tools equipped with a hot runner system, or even a combination of hot and cold runners. Furthermore, it will also be apparent to one having ordinary skill in the art that the runner system may comprise more or less mold cavities requiring more or fewer runner channels and junctions for distribution of the plastic material to the mold cavities.