Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Positioning of a mold part to form a cavity or controlling...
Patent
1991-08-01
1993-09-28
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Positioning of a mold part to form a cavity or controlling...
2643281, 2643288, 425562, 425567, B29C 4557
Patent
active
052484602
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to an improved process and apparatus for injection molding of plastic materials, particularly preferable in injection molding of super-precision products, which requires an extremely reduced variation from an average weight in every shot.
2. Background Art
According to a prior art, an apparatus for injection molding of plastic material comprises an injection machine of a screw plunger type and a mold arrangement defining a mold cavity where a molded article is formed. The mold arrangement may be a single mold for a molded product or consist of a primary mold for a molded product and a cold runner mold for a runner. In place of the cold runner mold, the apparatus may be provided with a hot runner mold incorporated with a manifold between the single mold and the injection machine. A nozzle passage is formed between the mold arrangement and a body of the injection machine, by a nozzle, in the most simple one cavity case or a hot runner mold arrangement and the nozzle.
With such an apparatus, a conventional injection molding process comprises steps of:
having a plastic material, in every shot cycle, plasticized and metered while being heated within the machine body;
having the hot plasticized material injected under pressure for the mold cavity through the nozzle passage;
having the hot injected material held at least partially within the entire mold cavity under pressure while the mold arrangement is being cooled to thereby provide and freeze a molded article therein; and
having the frozen molded article removed from the mold cavity after the mold arrangement is opened.
Among the important factors influencing the quality of a molded product, it has been recognized that the material pressure-holding step is one of the most critical steps. If this step is not carried out in a suitable manner, the molded products have undesired shrinkages due to short shot and/or flashes due to over-packing.
The time of the material pressure-holding step depends on the time needed for cooling a molded article in the mold cavity.
With a fixed mold cooling capability and a fixed volume of the mold cavity, a thinner molded article takes a shorter time than a thicker molded article to freeze it enough for removal from the mold cavity. In this connection, the thicker molded article requires more time in the material pressure-holding step, although the injecting step and the plasticizing and metering step each take the same time as for the thinner molded article.
According to the prior art, the material pressure-holding step is carried out using the injection machine with the plunger exerting an external holding pressure, subsequent to an injection pressure, on the injected material in a combination of the mold cavity and the nozzle passage against the mold cavity. The conventional technology involves an external holding pressure exerted by the injection machine being controlled to form a multi-stepped pressure, rather than a non-stepped pressure, which is stepped at predetermined strokes of the plunger.
Since a function and effect of the material pressure-holding has not yet been precisely known, although there are some theories which have been developed, there have been various attempts to improve the material pressure-holding step for a plasticized material which is viscid and elastic in the multi-stepped external pressure approach. In the conventional approach, as matter of course, there is a serious difficulty in controlling such a multi-stepped pressure stepped in an accurate manner at predetermined precise stroke positions, since a time between the neighboring steps of the pressure is very short on the order of 0.01 second and with a very short distance between the strokes at the neighboring pressure steps being on the order of 0.1 mm, while an inner diameter of a barrel of the injection machine body is very large relative to such a small stroke difference. Under these circumstances the multi-stepped pressure control cannot rely on a manual operation. Therefore, most
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Product Pamphlet of Komatsu (related to U.S. Pat. No. 4,846,651).
Heitbrink Jill L.
Seiki Corporation
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