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This invention relates to polymeric foams and more particularly to polymeric foams derived from polyolefins and having a characteristic quality which is referred to as “cloudiness” and which is believed to arise from the migration of nucleating agents, such as finely divided supercooled liquid, into the amorphous regions of the foam. The present invention relates also to a method for producing such foams, especially polypropylene foams.
Conventional manufacturing processes for blowing and foaming thermoplastic polymers generally employ a preblowing step at the surface of the polymer which is then expanded by the introduction of a heated gas to form a foam. Preblown foams are normally obtained by passing the polymeric material, such as polypropylene, through a preblowing tunnel at very high pressures. The preblown material is then expanded by introducing a heated gas, normally air, through ports located in the preblowing tunnel. A three-piece, two-stage mold which comprises a mold top part and a corresponding mold bottom part, is generally used to construct the foam mold. The mold bottom part, which is located on the foam molding table, includes apertures through which the expanding gas is directed into the mold bottom part to form the preblown foam.
In order to produce a desirable quality of foam, the mold bottom should be smooth and non-porous. For a high quality foam, the gas is first introduced into the mold bottom before the foam mold begins to expand. When the preblown foam enters the mold top part of the mold, the mold should be smooth and the resulting foam mold should be uniformly non-porous. Unfortunately, it is difficult to produce such mold configurations and mold sizes which are sufficiently large to produce thick molded foam in the order of 10 to 20 millimeters to provide a desirable quality in the finished foam product.
One approach which has been used to produce a molded foam having the desired quality characteristics described above has been to employ a preblown foam mold which includes at least two sides or sides at an angle with respect to one another. For example, in the first stage of the mold, the bottom part of the mold includes two sides which are parallel to one another and which are generally rectangular in shape. The mold is heated by