Thermal Wave Molding Corporation

Other

Frank J Baresich

Robert F Thompson

An injection mold and process are arranged to produce very thin articles, such as data discs. Molten plastic is injected into mold parts forming a thin cavity (), in a cyclic molding process wherein the mold parts are subjected to a substantially constant temperature stimulus and rise and fall in temperature during injection and cooling of the molten plastic. Temperature boosting thermal insulation layers () are placed along at least certain parts of the molding cavity surface. This elevates the temperature of the molted melt material for a time during injection. According to a calculated relationship, this thermal insulation is sized to permit the thin mold cavity to fill before heat transfer to the mold parts solidifies the molding material and blocks further flow. The temperature boosters can be contoured in thickness.

Mold and process for making a very thin wall article

A nozzle for injection molding is made of at thermally conductive metal material but is thermally conductively blocked or interrupted. An abrupt reduction in wall thickness and/or a gap in the continuity of thermally conductive material occur along the nozzle material passageway. This provides a localized restriction to thermal conduction, causing a reduction in heat flow from the mold to the material supply while cooling the melt material near the end of the nozzle. This material, which is relatively cooled to near the mold temperature, becomes the last injected material when filling of the mold is completed, occupying the area of the sprue. As a result, the sprue is relatively cool, thus reducing stringing and shortening mold cycle time.

Injection molding nozzle structured for cooling at extreme tip

Mold cycle time is accelerated by employing thermally insulating surface temperature boosters, which are of a minimum thickness to promote cooling by heat transfer through the boosters. According to the thermal transfer properties of the insulating boosters and the respective temperatures of the molten material and the dies, the temperature of the cavity surface is raised by contact with the molten material to equal or exceed the temperature required to produce a molded article, preferably just until the time that the mold is fully filled. Heat transfer through the boosters to the dies then cools and solidifies the molded article until it can be removed from the mold. The temperature boosters result in increased cavity surface temperatures, such that the mold dies can be kept at substantially lower temperatures. The overall result is a reduction in mold cooling time and therefore acceleration of mold cycling. When the molded article is an optical disc, where the digital information is transfered to at least a part of a surface of the optical disc from a stamper that forms at least a part of the cavity surfaces, a stamper heating means can be used to improve the quality of optical performance. Typically, the stamper may contact high thermal conductivity materials at or beyond the outside diameter of the mold cavity, creating a path for the heat to flow from and cool the outer edge of the optical disc excessively. Transfer of the pits from the stamper is more difficult in the cooler material at the outer edge of the disc, reducing pit quality, and stresses are also created at the outer edge of the disc which cause birefringence, resulting in reduced optical performance. The stamper heating means increases the temperature of the stamper at the area of contact enough to reduce, stop, or even reverse the direction of the heat flow.

Mold for optimizing cooling time to form molded article

Mold cycle time is accelerated by employing thermally insulating surface temperature boosters, which are of a minimum thickness to promote cooling by heat transfer through the boosters. According to the thermal transfer properties of the insulating boosters and the respective temperatures of the molten material and the dies, the temperature of the cavity surface is raised by contact with the molten material to equal or exceed the temperature required to produce a molded article, preferably just until the time that the mold is fully filled. Heat transfer through the boosters to the dies then cools and solidifies the molded article until it can be removed from the mold. The temperature boosters result in increased cavity surface temperatures, such that the mold dies can be kept at substantially lower temperatures. The overall result is a reduction in mold cooling time and therefore acceleration of mold cycling.

Process for forming a molten material into molded article

Methods and apparatus for molding a thermoplastic article bring the surface of the mold cavity above the flow stress relaxation temperature of the molten plastic during molding of a part sufficiently long enough to relax the resin at the surface of the molded part and produce negligible residual stress. The surface of the mold cavity is maintained above the flow stress relaxation temperature of the molten plastic as the center of molten plastic in the mold cavity cools from a temperature above the solidification temperature toward the solidification temperature, thereby reducing flow-induced and temperature gradient induced stresses in the solidified mold material of the molded part. The heat of the molten plastic introduced into the mold is used as the source of the heat for heating the surface of the mold cavity. A layer of thermal flow control material about the mold cavity has a mathematical product of thermal conductivity, specific heat and density and appropriate thickness, so that heat from the molten plastic introduced into the mold cavity raises the temperature of the surface of the mold cavity above the flow stress relaxation temperature of the plastic being molded. The methods and apparatus are particularly useful for the molding of amorphous and crystalline resin parts and for rewritable optical data storage discs and optical lenses with minimized birefringence.

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Centerville, GA, United States of America

Frank J Baresich