Injection compression moulding

Abstract

A mould for injection compression moulding of an article includes a female mould part and a core for mounting in use on the platens of an injection moulding machine for movement towards and away from one another between an open and a closed position. A sealing ring surrounds the core for effecting a seal between the female mould part and the core. The core, the female mould part, and the sealing ring together define a closed mould cavity as the core approaches the closed position. The sealing ring has tapering sections, each tapering section having a tapering surface that seals against a tapering surface on the core only after the core has reached the closed position, leaving a venting clearance between the tapering surfaces of tapering sections of the sealing ring and the tapering surface of the core as the core approaches the closed position.

Description

RELATED APPLICATION [0001] This application is based upon and claims priority from prior British Patent Application No. 0400936.1, filed on Jan. 16, 2004 and PCT Patent Application No. PCT / GB04 / 005422 filed Dec. 30, 2004 the entire disclosures of which are hereby incorporated by reference in their entirety.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to injection compression moulding and is concerned in particular with injecting a plastics material into a mould cavity. [0004] 2. Description of the Related Art [0005] In the injection compression moulding technique to which the present invention relates, an accurately measured quantity of plastics material is injected into a mould cavity before it has been fully closed. As the parts of the mould are brought together, the injected plastics material is compressed and made to fill the cavity by the force applied to close the mould rather than by the pressure applied to inject the plast...

AI Technical Summary

Benefits of technology

[0005] In the injection compression moulding technique to which the present invention relates, an accurately measured quantity of plastics material is injected into a mould cavity before it has been fully closed. As the parts of the mould are brought together, the injected plastics material is compressed and made to fill the cavity by the force applied to close the mould rather than by the pressure applied to inject the plastics material into the mould. As a consequence, it is possible to achieve much higher length to thickness ratios than achievable by conventional injection moulding, even when using lower cost plastics materials having high viscosity. This enables the technique to be used in the manufacture of such items as cups and margarine tubs which have hitherto needed to be manufactured by other methods, such as by vacuum or pressure forming of a heat softened sheet material. A further advantage is the greatly reduced cooling times due to lower processing temperatures and improved packing allowing for faster heat loss through the cavity (conventionally 70% of the heat loss is expected through the core due to shrinkage away from the cavity wall).

Problems solved by technology

It has been found in practice, however, that it is difficult to form a rim closure ring with a cylindrical bore that effectively seals around the core and yet allows the core to pass freely through it.

The clearance required to permit reliable and free movement of the core relative to the rim closure ring does not permit creation of a perfect seal and results in an unacceptable witness line around the rim of the moulded article.

The above technique however places a limitation on the length to thickness ratio of the moulded article.

Injection compression moulding does however present certain problems that are not encountered in conventional injection moulding.

The first of the problems is concerned with the alignment of the mould parts.

However, before the mould is fully closed, the mould parts may not be fully aligned, that is to say they may not be concentric or they may not be co-axial.

The guiding that is achieved by the tie bars, or other guiding systems used by machine manufacturers of the injection moulding machine, may not guarantee alignment to the required accuracy, especially when it is noted that the main purpose of using injection compression moulding is to achieve very large flow length to thickness ratios in articles such as cups, margarine tubs or dustbins.

A further problem in injection compression moulding is that of containing the plastics material within the cavity as its volume is being reduced.

This is not a satisfactory solution because it is difficult to form a rim closure ring with a cylindrical bore that effectively seals around the core and yet allows the core to pass freely through it.

The clearance required to permit reliable and free movement of the core relative to the rim closure ring does not permit creation of a perfect seal and results in an unacceptable witness line around the rim of the moulded article.

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