Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Patent
1995-08-30
1997-08-19
Tentoni, Leo B.
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
425547, 425548, 425573, 425592, 425593, B29C 4526, B29C 4566
Patent
active
056585225
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention concerns a process and an apparatus for the production of a plastic article of a large area and a small wall thickness, wherein the plastic material is injected in the viscous condition through at least one inlet which opens into a mold cavity. The mold cavity which corresponds to the plastic article is defined by a bottom mold portion and a top mold portion and the bottom and top mold portions are moved away from each other after the plastic material has set for removal of the fully molded plastic article, wherein the top mold portion with the bottom mold portion forms a premolding cavity during the operation of injection of the viscous plastic material and immediately after termination of the injection operation the bottom mold portion and the top mold portion are moved relatively towards each other forming the definitive mold cavity, so that the viscous plastic material is distributed in the diminishing mold cavity.
BACKGROUND OF THE INVENTION
A process and an apparatus for the production of a plastic article having high-precision surface structures and/or high-luster surfaces is known from the present applicants' DE 41 08 236 C1. That known apparatus involves a not inconsiderable level of machine expenditure, and that has an effect on the production costs of the plastic articles produced with that apparatus or by means of that process. The specified plastic articles are for example Fresnel lenses of a highly transparent plastic material. The plastic material used is preferably PMMA.
A process and an apparatus of the kind set forth in the opening part of this specification are known from the German journal: "Kunststoffe" 80 (1990), issue 5, pages 583 to 587.
Therein the reduction of the mold cavity from the pre-molding cavity to the definitive mold cavity which corresponds to the plastic article to be produced is effected by a linear movement of the mold portions relative to each other in such a way that the mold cavity between the top mold portion and the bottom mold portion diminishes in parallel relationship. In that situation the viscous plastic material introduced into the pre-molding cavity is uniformly distributed from the centrally disposed inlet on all sides, that is to say in all directions, perpendicularly to the injection direction. That requires a certain amount of force and energy to be applied, as well as involving a considerable level of expenditure in terms of machinery or apparatus.
The object of the present invention is to provide a process and an apparatus of the kind set forth in the opening part of this specification, wherein it is possible, at a comparatively low level of machine expenditure, inexpensively to produce plastic articles of large area and small wall thickness, enjoying good optical properties.
SUMMARY OF THE INVENTION
In terms of the process, for the production of a flat plastic article of large area, the plastic material is introduced through a lateral inlet into a pre-molding cavity which is enlarged in a wedge shape starting from the inlet, the wedge shape of the premolding cavity being abolished in order to form the definitive mold cavity. In this case therefore that closing movement of the top mold portion and the bottom mold portion can take place in term of time with or after the beginning of the injection operation and can be concluded with or shortly after the termination of the injection operation. The best times for the commencement and the conclusion of the closing movement are dependent on the flow path and the wall thickness of the plastic material and can be ascertained by tests.
In the process according to the invention therefore the viscous plastic material is introduced into a pre-molding cavity which is of larger volume in comparison with the definitive mold cavity that corresponds in terms of shape to the plastic article and which is enlarged in a wedge shape from the inlet, the plastic material being introduced into the pre-molding cavity from the edge region which is of small internal height, so that th
REFERENCES:
patent: 4751029 (1988-06-01), Swanson
Friedrichs, et al., "Spritzpragen dunnwandiger thermoplastischer Formteile", Kunststoffe, May 1990, pp. 583-587.
Abstract of Japan 59-42,933 (Published Mar. 9, 1984).
Abstract of Japan 61-95,917 (Published May 14, 1986).
Mobius & Ruppert
Tentoni Leo B.
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