Plastic and nonmetallic article shaping or treating: processes – With step of making mold or mold shaping – per se – Utilizing surface to be reproduced as an impression pattern
Reexamination Certificate
1998-09-10
2001-01-16
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
With step of making mold or mold shaping, per se
Utilizing surface to be reproduced as an impression pattern
C264S220000
Reexamination Certificate
active
06174481
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an improved tool for short to intermediate production run use.
Complex-shaped plastic articles are typically formed by either injection molding or compression molding techniques. Typically, the tool or mold utilized in the molding operation is produced from a metal such as steel. The utilization of a tool made from steel is generally appropriate when the tooling will be used to produce in excess of 50,000 components from the tool. Because of the high cost of producing a tool made from steel, a steel tool is generally only selected when large numbers of components will be produced from the tool as the cost can be more easily amortized over a large number of components. However, when the desired quantities of a plastic article or component only range from hundreds to tens of thousands, the cost of producing a machined steel tool can often be prohibitive.
Since aluminum can be machined more rapidly than steel, aluminum has been a somewhat less expensive alternative to steel. Additionally, aluminum tooling is not generally suitable for filled plastics and/or intermediate to high volume use due to its poor abrasion resistance.
In addition to the expense associated with steel and aluminum tooling, it is necessary in the plastics industry to have the capability to rapidly prototype plastic components by utilizing techniques which are as close as possible to those techniques which will be used in the actual production process. For example, in the automobile industry, an automobile manufacturer may wish to field a small test fleet of vehicles. It is often the goal of these test fleets to closely mimic the form, fit, and the function of the production vehicle. For this use, steel or aluminum tooling could be too expensive.
Cast tooling provides the ability for short run, rapidly fabricated tooling. Cast tooling is commonly formed of castable materials such as epoxy, silicone or urethane resins, with or without ceramic or metal fillers. In the process of fabricating a cast tooling, a pattern is made from a suitable material, such as plastic, wood, steel, and/or aluminum, and the pattern is placed in a cavity. A castable material such as epoxy is then poured around the pattern. Then the epoxy cures and the pattern is removed. The resultant tool or mold is suitable for producing parts in quantities of generally less than 100 pieces.
While cast tooling is much less expensive than traditional steel tooling it is subject to several drawbacks. First, the cast tool has a reduced strength and elastic modulus compared to a metal tool and distorts under the pressure exerted under typical injection molding and compression molding applications. Second, the cast tooling has a lower thermal conductivity than a metal tool which can result in longer mold cycle times and can contribute to difficulty in solidifying plastics in articles having thicker cross-sections. Third, the cast tool is mechanically weak as compared with metallic tools. This problem manifests itself in the breakage of thin cross-section mold areas during use. Finally, the cast tools can typically only be used in a temperature range of approximately 300 to 350° F. (150 to 180° C.) which can limit the types of plastics or filled plastics that can be utilized in combination with the cast tool for injection molding.
In an attempt to improve the thermal conductivity of polymer based cast tools, such as the epoxy castings, additives such as aluminum or silicon carbide powder or fillers have been added to the castable material. These types of additives are dispersed within the castable material in a discontinuous manner. These fillers (e.g., aluminum) also provide a mechanism to channel the heat given off (exotherm) produced during resin curing and impart additional abrasion resistance to the cast tool (e.g., silicon carbide). However, in general, it was found that the addition of these powders or fillers to cast systems reduce the mechanical strength of the cast tool relative to its unfilled resin by producing defects in the cured structure. This use of discontinuous fillers results in marginal cast tooling; limiting the total number of parts that can be made.
Accordingly, it would be desirable and advantageous to have a method for rapidly forming tooling for plastic molding which is suitable for use in short to intermediate length production runs, which is less expensive than metallic tooling, such as steel, and which provides the ability to prototype parts using a “near production” process so that all aspects of both the prototype part and the production thereof can be analyzed and observed. This includes the ability to affect heat transfer rates during molding that mimic the heat transfer rates of production steel tooling.
SUMMARY OF THE INVENTION
The present invention is directed to providing a method for rapidly forming a tool adapted for plastic molding which is more convenient and more durable than the prior art. In one main aspect of this invention, the material utilized for the tool is a castable material. The cast tool can be formed by preparing a mixture of stable material, providing a pattern for a desired object to be formed by molding, applying the castable mixture to the pattern, casting a co-continuous structure with the castable material, and then curing the mold to form the tool.
According to the method, a cast tool can be rapidly formed which does not require complex machining, is abrasion resistant, mechanically strong, is highly thermally stable, and, therefore, is suitable for producing hundreds to thousands of molded components.
In another feature of the present invention, the continuous structure has higher thermal conductivity than the castable, which makes the tooling more suitable for plastic molding operations. This is true since the cycle times of the molding operation can be reduced as more heat is transferred away from the plastic article, enabling it to solidify faster than prior art tooling.
In another aspect of the present invention, the co-continuous reinforcing structure incorporated into the castable material comprises a continuous matrix of structure rather than a discontinuous filler. The continuous reinforcement provides a continuous pathway for heat to be efficiently removed from the surface of the tool and away from the molded part. The castable material including the continuous reinforcement provides a continuous matrix which itself can provide increased mechanical strength to the cast tool. The continuous structure is a co-continuous composite structure wherein the continuous phase is continuous and penetrable by the cast material, the cast material also being continuously disposed.
The use of a continuous structure cast within a castable material can also provide enhanced tool stiffness, enhanced abrasion resistance, and enhanced compressive strength. Additionally, the method of forming the cast tooling can be accomplished utilizing patterns made from stereolithographic polymer, machined aluminum, steel, plastic, or other suitable materials and methods known to those skilled in the art.
The present invention is also directed to providing a method for rapidly forming a tool adapted for plastic molding which is more convenient and more durable than the prior art. In one main aspect of this invention, the material utilized for the tool is a ceramic material. The ceramic tool can be formed by preparing a uniform mixture of ceramic particles in a vehicle, providing a pattern for a desired object to be formed by molding, applying the ceramic mixture to the pattern, solidifying/interlocking the ceramic particles together to form a mold, and then heat treating the mold.
According to the method, a ceramic tool can be rapidly formed which does not require complex machining, is abrasion resistant, mechanically strong, is highly thermally stable, and, therefore, is suitable for producing hundreds to thousands of molded components.
In another feature of the present invention, the ceramic materials are thermally conductive which makes the tooling more suita
Bak Joseph V.
Holowczak John E.
Schmidt Wayde R.
Souder Blair V.
Fiorilla Christopher A.
Lear Automotive Dearborn Inc.
MacMillan Sobanski & Todd LLC
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