The invention relates generally to a method of thermoforming an article from thermoplastic panels having textured or grained surfaces and more specifically to a method of thermoforming an article in a mold having textured or grained surfaces from thermoplastic panels having textured or grained surfaces which provides improved surface appearance.
The suitability of relatively large thermoplastic panels for use in various vehicular and automotive products is a direct result of both the ruggedness and stability of the material from which the panel is formed and the final characteristics of the product which include both structural and aesthetic considerations. Stated somewhat differently, the utilization of thermoplastic panels in vehicular and automotive applications has increased as the stability, strength and appearance of such products has increased.
Frequently such products are utilized as covers for hatches, storage containers and pickup truck beds. As such, a thermoformed product must typically span a relatively large region and must therefore be strong and relatively immune to creep or dimensional instabilities resulting from temperature swings or short or long term loads impressed thereupon. If, as is often the case, such thermoplastic products are utilized as hatches or covers on a vehicle where they are readily visible, their appearance is critical both from a large dimension standpoint, i.e., warps and ripples, and a small dimension standpoint, i.e., surface uniformity, whether the surface is smooth or contains grain or texture.
For product surfaces requiring texture or grain, a typical prior art manufacturing process begins with a smooth thermoplastic panel which is placed into a mold wherein one or both mold surfaces are textured or grained. The mold is closed and vacuum and/or pressure are applied to appropriate faces of the thermoplastic sheet to drive it into conformance with the mold and simultaneously grain or texture the surface in contact with the textured or grained mold surface.
An aesthetic problem has been detected with regard to such processes wherein the grained or textured surface appears to have a ring or halo-like imperfection or variation of the texturing or graining which is typically centered about the center of the panel. The visible halo is the result of changes in reflectivity of the textured or grained surface. While the mechanism of formation of the halo is not fully understood, it is believed to result from non-uniform stretching of the thermoplastic panel along radial paths extending from the center of the panel to the edges. That is, the very center of the panel which typically contacts the mold first, and thus typically cools somewhat more quickly, undergoes very little stretching whereas points at increasing distances from the center and particularly those proximate the edges of the mold cool more slowly and may undergo significant stretching. This stretching or radial movement which occurs parallel to the surface of the mold is believed to interfere with full formation of the grain or texture as the graining or texturing surface irregularities of the mold are not filled because the movement vectors of the thermoplastic material thereacross have significant horizontal components and relatively small vertical components. By way of contrast, the vector in the center of the mold has essentially only a vertical component directed toward the surface of the mold. Because the sliding and stretching motion occurs in a radial direction substantially uniformly about the center of the mold, a visible halo-like imperfection appears on the finished product. The halo is, as noted, a result of varying reflectivity of the surface from the center of the molded panel outward to its edges.
The invention disclosed and claimed herein is directed to eliminating the problem of halos and other irregularities appearing on textured or grained panels of thermoformed products.