This invention relates to methods for constructing molded composite articles, and more particularly to a method for constructing molded composite articles comprising a shaped foam cushion, such as automotive interior trim components.
Historically poor foam formation in the construction of automotive interior trim components such as instrument pads, door panels, arm rests, glove boxes, center consoles and close out panels has been the foremost cause of scrap and repair related problems in production.
Urethane foam defects of underfills, voids, rattyfoam cell structure, and poor mix is a major manufacturing problem plaguing the entire industry that produce automotive interior trim components, that use a composite construction of a thermoplastic cover, metal or plastic insert and urethane foam to fill the space between the cover and the insert.
In addition to the manufacturing defects, poor urethane foam formation can and does cause field failures of the assembled products. Field failures are exhibited in a number of forms.
"Tiger Striping" that is color changes or staining can be caused by inadequate mixing of the urethane components; isocyanate and polyol.
"Measles" are generally associated with ratty cell formation or small multiple voids which cause an undesired rough irregular surface to the cover.
"Sinks" are another defect caused by large voids in the urethane foam. In this case the void is large enough for the cover stock to sink into the void causing a depression in the cover.
While poor urethane foam quality has been an ongoing problem in the manufacturing plants for years, it has been only in the past few years that major field performance problems have occurred. The reason for the field failures is directly attributable to the increased interior temperatures found in today's aerodynamically designed automobiles.
The increased use of glass and the reduced angle of the windshield has dramatically increased the internal temperatures of the modern automobile. This is commonly referred to today as "the greenhouse effect". Ten years ago the highest test temperature required by one automobile manufacturer for product environmental temperature stability testing was 212.degree. F. Most automotive specifications today require extended exposure to temperatures of 250.degree. F. (121.1.degree. C.).
A good example of the greenhouse effect is found in the results of a test conducted at South Florida Test Services in Miami, Florida in November of 1988. An automotive instrument pad was instrumented with fifteen thermocouple probes to monitor the temperature of various sections of the pad over a twenty-four hour period.
The test fixture was a closed box that positioned the instrument pad under automotive glass at a 45.degree. angle to the sun. At 11:15 in the morning the surface temperature of the pad registered 242.degree. F.
Additional environmental testing supports the Florida study and also indicates that in the peak summer months internal car temperatures can exceed 260.degree. F. These temperatures are sufficient to soften the cover materials so that they conform to the structure of the underlaying urethane foam.
Traditionally a system called "perimeter open pour" has been used to deliver the urethane chemicals to the back side of the cover. The insert is mounted to the lid of a closeable mold or foaming tool. After the mechanically mixed urethane is applied to the back side of the cover, the lid is closed, sealing the mold cavity.
The urethane is applied in a bead strip approximately 2 inches wide around the periphery of the cover. After the mold is closed the urethane has to fill in the space between the cover and the insert. In order to produce a defect free composite article the urethane chemicals must be mixed correctly, chemical temperatures and ratios must be correct, the mold must be at the right temperature, and the mold must be sealed and have a uniform clamping pressure.
Now the urethane has to travel and fill the space between the cover and the insert. The urethane is required to travel up hill, go around corners, go thin and then go thick. In some configurations the urethane has to move laterally as much as 12 to 14 inches. While all this is happening the urethane polymerization reaction is going on to create the cell structure of the foam.
An alternative to the open pour process is the "CMIP" or closed mold injection pour process. The CMIP process injects the urethane into a closed mold through a single entry port. Unfortunately, this process has the same type of problems that open pour has. Additionally, dimensional accuracy of the mold and uniform clamping pressures appear to be even more critical than what is required for the open pour process.
As indicated above, no one has been totally successful in developing the urethane chemistry and tooling to consistently produce a void free urethane foam in automotive interior trim components.