The present invention relates to the manufacture of fabric reinforced ceramic tools or molds (collectively molds) or molds made from a fabric reinforced ceramic intermediate(s) for use in high temperature organic polymer fabrication. Although the molds of the present invention may be suitable for fabrication of a wide variety of components at temperatures from ambient to 1000.degree. C., the molds have particular applicability and suitability in the field of rotational molding (sometimes referred to herein as "rotomolding").
Molding is a well-known technique for the fabrication of parts and various molding technologies have, over the years, employed various constituents materials for the molds. To a large extent the selection of which materials can be used is determined by the nature of the molding technique and the environment to which the mold will be subjected, tempered with an evaluation of the cost of materials which have acceptable characteristics. The present invention is directed to the suitability of a category of materials often referred to as "Continuous Fiber Ceramic Composites" (CFCC) and an intermediary product in the creation of CFCC, to wit, a modified silicone filled material, both of which have been found to be highly suitable as the principal constituent for molds, particularly of a type which are likely to be subjected to relatively elevated temperatures during processing, such as in rotational molds. A series of fabrication steps, in concert with applied physical, kinetic, and mechanical properties are disclosed which are preferably employed to successfully fabricate a mold.
Rotational molding is a plastics processing technique which permits the fabrication of hollow seamless parts with relatively thin walls and excellent structural integrity. Although technically known since the early 1900's, it became more popular in the late 1950's with the advent of micronized polyethylene.
In general, in the molding process, a predetermined amount of plastic material in liquid or powder form is placed in a mold cavity and then the mold is closed. The mold is then heated to bring the plastic to a molding temperature, if the plastic is a thermoplastic, such as high density polyethylene (HDPE). As the mold is heated, it is rotated continuously both vertically and horizontally and the plastic material flows along the interior surface of the mold to form a smooth evenly distributed fused plastic shaped part. The rotation is continued through the cooling process with the plastic hardening and with the retention of the shape of the mold. The mold is then opened and the finished part removed.
Numerous materials have been used for the mold. For example, cast aluminum molds are commonly used. Cast aluminum can be relatively easily shaped to replicate intricate detail; has good thermoconductivity so that the mold can be heated and cooled relatively quickly; and the inside surfaces (which act as the molding surface) can be made very smooth through polishing, so that the outside surface of the article that is ultimately molded will, in turn, have a smooth surface. However, cast aluminum molds are relatively expensive. Moreover, aluminum being relatively soft, reinforcement or strengthening is generally required in certain locations, particularly the parting line where the mold sections meet. As well, the polishing of the inside surface is often time-consuming, expensive and requires a repolishing at relatively frequent intervals.
Sheet metal molds are also often employed for rotomolding, with the sheet metal molds fabricated from a number of metals such as steel, aluminum or stainless steel. While less expensive than cast molds, sheet metal molds do not always lend themselves well to the production of freeform shapes.
Other somewhat more exotic materials have also been employed as molds for rotomolding, such as electroformed nickel, but they are difficult to fabricate due to size limitations of the tanks necessary to plate the nickel. Some workers have even suggested using plastic molds, but such mold would be generally restricted to the use of liquid thermosetting polyester, urethane and epoxy materials which are formed and cured at room temperature, since the plastic molds do not easily withstand elevated temperatures. (Plastic molds are not commonly used, primarily due to their inability to eliminate voids.)
Ideally, molds should be made from a material which can be preformed, preferably layer-by-layer, from a material which can be built-up in layers and shaped. This would require somewhat self-adhering and malleable material so that successive layers can be integrated into a shaped body. As well, a layering process would permit the embedding of sensors within the mold, an extremely beneficial attribute given the criticality of temperature to the molding process. However, up to the present, there has been no suggestion of any material, or group of materials, which would satisfactorily provide such benefits particularly in molding where relatively high temperatures are experienced in the processing.
In recent years, continuous fiber ceramic composites (CFCC) have been developed for a number of uses, particularly in end products which are subjected to extreme heat, such as parts for rockets and space vehicles. CFCC's are composite materials which bind a continuous reinforcement fabric within a matrix, which upon firing converts to a ceramic matrix. The matrix generally consists of articles of heat resistant, generally inorganic materials combined with binders, some of which are organic and which are used to provide certain handling characteristics. A fabric is then coated with the matrix material. The fabric is generally one having high heat resistance which can withstand, (that is, not to be sacrificed or catastrophically lose its physical reinforcement properties) during the ceramic forming process. Once suitably combined the composite material is cured either at room temperature, by mixing in a "part B" catalyst before fabrication or more commonly at elevated temperatures. After curing, the composite is then fired at significantly greater elevated temperatures, causing the organic materials to be driven off, and accomplishing the necessary conversion chemistry with the resultant structure being a continuous fiber ceramic composite.
Although CFCC's are in various stages of development and prototype production, for examples as jet engine parts, heat exchangers, waste incineration components and the like, the types of CFCC's suggested for such end uses do not include the combination of physical, kinetic, viscoelastic, and mechanical properties needed to produce a successful CFCC tool for rotomolding.
Indeed, CFCC's have heretofore held little interest to those in the field of molding and in particular rotomolding, for a number of reasons. First, the fibers or fabrics used in CFCC's are exotic and high-priced and thus do not lend themselves readily to use in molds for mass produced items. Second, in driving off the organic materials during the firing stage, the space left by the removed organic materials result in micropores in the CFCC which are often undesirable for a mold, as the pores yield a less than finely smooth molding surface. If the surface of the mold does not have a sufficiently smooth finish, that surface will be imparted to the molded part and may result in an unacceptable end product, where for example a smooth, sleek surface is considered highly desirable. Given the heating and cooling stages that are present in most molding operations, and in particular rotational molding, with a porous mold air can pass from the outer surface of the mold to the inner surface of the mold with a possible resultant stippled or bubbled effect on the surface of the molded article. Moreover, present technology has not provided economical alternatives to the sacrificial organic and organic portions of inorganic/organic binders driven off by firing. Since it is these binders that are required to provide the tack, drape and lay-up capabilities to the CFCC material to permit it to be shaped in the design of the end product, and enable low temperature oxide ceramic conversion to take place, total elimination of the use of organic binders is not practical. Although a 100% yield is a goal, as long as the practical processing characteristics are not sacrificed, the realties are such that a typical matrix yield is 75.fwdarw.88%.
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