Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By gas forming or expanding
Patent
1997-04-16
1999-11-16
Copenheaver, Blaine
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By gas forming or expanding
264327, 26432816, 425552, 425577, B29C 3502, B29C 3516, B29D 6720
Patent
active
059851910
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of forming an article via injection of plastics material into a mould.
BACKGROUND OF THE INVENTION
A recognised problem, in injection moulding from plastics materials of articles having thin walls in particular, is that at marked differences in cross-section, differential shrinkage causes blemishes in the finished surface of the article.
Despite this problem, many new designs of injection moulded article could be feasible if marked changes in cross-section were possible, without shrinkage blemishes.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an improved method based on injection moulding and facilitating the production of substantial changes in cross-section.
According to one aspect of our invention there is provided a method of forming an article via injection of plastics material into a mould, the finish formed article having thin wall portion(s) and thick wall portion(s), the thick wall portion(s) being at least partially foamed, the method consisting in the steps of: part and its core part, thin wall portion(s) whose mould part gap is to be substantially reproduced in the thin wall portion(s) of the article and thick wall portion(s) whose mould part gap is less than the thickness of the thick wall portion(s) of the finish formed article; producing additive into the mould tool; in the thin wall portions of the article; part before the plastics material mixture has at least substantially solidified in the thick wall portion(s) to allow the mixture to foam and form at least some of the thick wall portion(s); and
In one alternative of our invention, the article has thick wall portion(s) at which foaming is constrained on the withdrawal step until the ejection step and allowed to occur after ejection of the article from the mould.
In another alternative, the withdrawal step consists of withdrawing one or more portions of the one part of the mould tool from its closed position to increase locally the mould part gap for foaming of the plastics material mixture to form the thick wall portion(s) of the article in the locally increased gap; and of opening the mould tool after the thick wall portion(s) have substantially solidified to shape. Alternatively, an entire mould part can be partially withdrawn to allow foaming and subsequently the mould tool is fully opened for ejection of the solidified article.
The foaming can occur against and between the withdrawn portion(s) or part of the mould tool to fully fill the locally increased mould part gap; or the foaming can occur to an extent to not fully fill the locally increased mould part gap.
In accordance with one preferred feature of our invention the withdrawal step consists of opening the mould tool before the thick wall portion(s) have solidified to shape, foaming thereof then occurring in the ambient atmosphere to form the thick wall portion(s). The moulding can be left on/in either of the core or the cavity of the mould tool, according to whether the foaming is intended to expand the article outwards or inwards in the thick wall portions.
Mechanical or pneumatic ejection means can be used.
In another alternative, the withdrawal and ejection steps occur at the same time.
One possible advantageous feature is that the step of allowing the plastics material mixture to solidify in the thin wall portion(s) of the mould tool includes the step of cooling these portions of the mould tool to a greater extent than the thick wall portion(s) thereof.
Another such feature is that the step of allowing the plastics material mixture to solidify in the thin wall portion(s) of the mould tool includes the step of cooling these portions of the mould tool and heating the thick wall portion(s) thereof.
Preferably the step of withdrawing the one part of the mould tool, or its withdrawable portion, is delayed until the surface of the plastics material mixture has substantially solidified in the thick wall portion(s).
According to another aspect of the invention there is provided an artic
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Copenheaver Blaine
Coraltech Limited
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