This invention relates to an automated method and apparatus for removing flash from molded flexible urethane parts and from flexible Polyvinyl Chloride (PVC) parts in an efficient manner and without causing cosmetic damage to the surface of the part.
Many of today's products utilize polyurethane parts that are produced by way of pressure injected molding processes including low pressure mixing, high pressure mixing, and reaction injection molding. These processes can be combined with a process whereby the polyurethane part is painted at the time of molding.
The industry also manufactures many flexible polyvinyl chloride (PVC) parts by an injection molding process or a blow molding process. Flexible PVC parts are also utilized as component parts in many ways, and particularly as interior parts utilized by the automotive industry.
The molds or dies utilized in the pressure injected molded processes are generally comprised of two or more pieces that are clamped or otherwise bound tightly together. As a result of the pressure utilized to inject the liquid polyurethane into these molds, or the design of the molds to allow for the escape of evacuating air from within the mold during the injection process, frequently a thin layer of polyurethane and/or paint is forced past the connecting seams of the pieces comprising the mold. As a result, a thin layer of material called flash adheres to the surface of the finished molded piece and disrupts the symmetry of the finished surface. The flash may be comprised of excess and unwanted polyurethane material which is referred to as flash. Also, flash may be comprised of excess and unwanted paint, which is referred to as paint flash.
Manufacturing companies that purchase polyurethane molded parts from suppliers are becoming ever more demanding as to the dimensional specifications and cosmetic criteria for the finished molded products. As a result, polyurethane molded parts producers are spending ever more time and money in order to meet the increasing demand for quality molded products. A primary area of interest to the purchasers of molded parts is the seam area where the molds fit together. The finished products may not have any flash. Further, molded parts manufacturers must produce products that are seamless, and are without any disruption or discontinuity of the textured surface of the part.
The difficulties faced by polyurethane molded parts manufacturers in meeting these finished product specifications have increased in recent years. Most polyurethane parts are manufactured utilize a process which forms a thick, bubble-free outer surface referred to as the skin. In the industry molded polyurethane parts manufactured by this process are said to use "self-skinning" polyurethane. The primary manufacturing technique employed in the production of self-skinning polyurethane parts utilized freon and was referred to as a freon blown process. In 1994 the federal government banned the freon blown process. An advantage associated with parts manufactured by way of the freon blown process was the formation of a very thick skin on the molded part. With the governmentally mandated changes in the manufacturing process, water blown urethanes are now employed. Polyurethane parts made with the water blown process have a substantially thinner skin. A thinner skin on the polyurethane part magnifies the problems for manufacturers attempting to meet the ever more rigid dimensional and quality requirements as it is much easier to break through or otherwise disrupt the surface continuity of the part during the flash removal process.
In order to meet the requirements of the industry, manufactures have attempted to employ mechanized devices in the removal of flash. Many of these have failed. The primary modes of failure of mechanized flash removal apparatus involve the inability to consistently remove all of the flash material, or the extensive removal of surface material of the part resulting in the a break in the skin or destruction of the texturing or other cosmetic features of the part in the area of the mold seam.
Polyurethane parts manufacturers have also attempted to utilize cryogenic methods to remove unwanted flash with very limited success. The thickness of the flash to be removed from polyurethane molded parts varies, but is generally in the range of 3/1000.sup.th to 5/1000.sup.th of an inch. As a result of the mechanized failures and limited successes with the cryogenic removal of this thin layer of flash, many manufactures of polyurethane molded parts have resorted to removing unwanted flash by a hand work process. This process employs the use of abrasives, such as very fine grade sandpaper, and/or knives. The hand work process has meet with limited success. As with the automated processes, the finished product specifications are very rigid, which results in the rejection of a substantial number of polyurethane molded parts as a result of a break or other discontinuity of the skin of the part, or the disruption or cosmetic flawing of the textured surface of the skin. Further, this process is very time consuming and labor intensive. As much as 40% of the cost associated with labor in the manufacture of a polyurethane molded part is consumed by this hand trimming process. Even though this process produces the best through-put of acceptable parts compared to other flash removal methods, there is still a high rate of nonreworkable rejected parts adding to the expense of the manufacturers. The present invention represents a significant improvement to the industry.