1. Field of the Invention
The invention relates to a method for the discontinuous production of foam plastic parts, in particular of rigid polyurethane foam blocks, and to a specially designed mold for the execution of this method which is adapted to facilitate the production of homogeneous foamed plastic parts having substantially uniform properties throughout to include uniform compressive strength in the marginal regions.
2. Description of the Prior Art
Various methods and associated molds are known in the art which attempt to achieve the desired characteristics of fixed shape with little scrap, homogeneity of structure, and uniformity of, among other things, marginal strength. The now commonly used methods for the discontinuous production of polyurethane foam in blocks provide either for closed foaming molds or molds with what are conventionally referred to as floating lids. Both methods, however, entail considerable disadvantages with respect to the obtainable foam quality.
The foam produced in closed molds has internal tensions and, due to the friction occurring at the mold walls during the foaming process, shows in the vertical marginal zones a drop in volumetric weight and in quality. Small deviations from the desired production values lead to uncompletely foamed upper edges or to tearing of the foam blocks, resulting in a high scrap rate. Due to the occurring foam pressures, closed block molds are relatively costly to produce.
While the block foam made in molds with floating lid is produced without substantial pressure and shows no substantial tensions, the friction between the developing foam and the wall occurs fully in the same manner as in a closed mold. Here too, inhomogeneous qualities are caused in the vertical marginal zones due to the fact that the frictional forces increase with increasing foam level. The upper edges of the blocks are not square, thereby producing more scrap when the blocks are cut into panels.