Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Patent
1994-01-03
2000-06-06
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
264335, 425552, 4253871, B29C 4517
Patent
active
060714638
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for the injection moulding of thermoplastic polymers.
2. Discussion of Prior Art
In such a process, molten polymer material is injected into one or more cavities defined by separable mould parts, allowed to cool and solidify and extracted after the mould parts are separated. Thermoplastic polymers shrink on cooling and this causes sink marks apparent on the surface at regions where the article is thicker than in neighbouring regions.
Sink marks seriously detract from the appearance of moulded thermoplastic articles and efforts have been made to obviate them. One process injects high pressure air into the partly solidified melt and it is found that the air forms channels within the melt and permeates principally to relatively thick regions of the article. Here the air is locked under high pressure and exerts an outward force which inhibits external sinking as the melt cools.
A difficulty with the above described method is that air flow through the melt is restricted and often cut off at narrow regions of the melt since these usually solidify first. The method is therefore not suitable for some forms of article and in particular may not be suitable for multi-cavity moulds for moulding multiple parts since variation in air passage profile would cause inequality in part material.
An alternative method is shown in patent specification U.S. Pat. No. 3,345,687 where air under pressure is introduced between one mould surface and the semi-molten polymer of the part being moulded. This method uses low air pressure. For current commercial products and polymers very much greater air pressures would be required and the use of higher pressures introduces the difficulties that air can leak from between the part being moulded and the mould parting line and polymer can be forced back through the cavity gate point. The present invention seeks to provide an improved injection moulding process.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a method of injection moulding which consists in injecting molten thermoplastic polymer material into a cavity defined by separable mould parts, allowing the material to cool and solidify in a cooling phase, separating the mould parts and extracting the solidified material, wherein, during the cooling phase gas is applied under pressure via apertures in the cavity wall to form a pressurised gas layer between the cooling polymer and the cavity wall, characterised in that the gas is distributed to selected places in the cavity wall via micro-channels in the wall. Preferably the gas is air.
It is normal to make the cavity walls smooth but it is found that by roughening the walls slightly in selected areas the gas is provided with micro-channels along which it can flow to establish the required gas layer. Furthermore, the gas can be confined to the roughened regions by providing smooth boundaries, where the polymer will make a seal. In this way a very thin high-pressure gas layer can be established where desired. In practice a gas layer will be positioned to urge the solidifying polymer against the opposing wall of the cavity where sink marks would otherwise be experienced. Clearly, it is possible to apply gas where required in the mould and the invention is applicable to multi-cavity moulding as well as single cavity moulding.
According to another aspect of the invention there is provided a moulding tool for the injection moulding of thermoplastic polymer articles, the tool having at least one cavity defined by separable mould parts, gas ducts within at least one of the mould parts for introducing gas into the cavity via the cavity wall whereby where a gas layer may be introduced between the cavity wall and polymer cooling in the cavity; characterised in that in use the gas is distributed to selected places in the cavity wall via prepared surface portions which are roughened to provide micro-channels.
There are other disadvantages in conventional injection moulding
REFERENCES:
patent: 4101617 (1978-07-01), Friederick
patent: 4164523 (1979-08-01), Hanning
patent: 4177238 (1979-12-01), Allen
patent: 4474717 (1984-10-01), Hendry
patent: 4740150 (1988-04-01), Sayer
patent: 4824732 (1989-04-01), Hendry et al..
patent: 4913644 (1990-04-01), Kauer
patent: 4923666 (1990-05-01), Yamazaki et al.
patent: 4923667 (1990-05-01), Sayer
patent: 4944910 (1990-07-01), Hendry
patent: 5028377 (1991-07-01), Hendry
patent: 5069858 (1991-12-01), Hendry
patent: 5069859 (1991-12-01), Loren
patent: 5098637 (1992-03-01), Hendry
patent: 5110533 (1992-05-01), Hendry
patent: 5137680 (1992-08-01), Hendry
patent: 5225141 (1993-07-01), Hendry
patent: 5232711 (1993-08-01), Hendry
Heitbrink Jill L.
Kontor Moulding Systems Limited
LandOfFree
Method and apparatus for preventing sink marks in the injection does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for preventing sink marks in the injection , we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for preventing sink marks in the injection will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2210999