模具類畢業(yè)設(shè)計(jì)外文資料翻譯--塑料產(chǎn)品故障由于設(shè)計(jì)材料或處理問(wèn)題-模具設(shè)計(jì)-全文預(yù)覽
【正文】 ies are in control, which was not the case. Inadequate antioxidant and regrind use were the main causes of molecular weight being out of control. This case illustrates a failure to realize how readily certain polymers, in particular PP, degrade during processing and that a small reduction in molecular weight (MW) may be sufficient to cause failure. The design played a part in that the fracture initiation is at the gate which is inherently effect of processing on the material can be monitored by how much melt index or melt flow rate increases in processing. Generally an increase of more than 1020% in most cases may be too much, unless the part experiences very little stress in service. The corresponding decrease in MW may be only about 5%, yet that may be more than the design and the service stresses will tolerate. Fortunately, melt index is a convenient and sensitive test which takes advantage of the fact that melt viscosity is a function of the power of MW above about 20,000 MW (231。 for good parts. Figure 1 is a crosssectional view of a welded junction obtained by sanding down a welded unit. The failure is a fracture of the outer wall of the molded part, which occurred only with out of round parts. Figure 2 is a sketch of how good and bad parts fit together with the insert. Fracture was due to flash pushing the edge of the part outwards as the ring insert was forced down. In good welds all the flash moved downward inside the part. In this case the human failure was not to check if parts or the mold cavity were perfectly round. 4. Material A glassfilled PBT (polybutylene terephthalate) part had a hole in the center in which a threaded metal part moved freely back and forth. In oven aging at 160EC to simulate under the hood automotive service the metal part lost its ability to move freely in the PBT part, which had shrunken slightly. Shrinkage was due mainly to further crystallization in service beyond the degree of crystallinity as molded. DSC showed that the heat of fusion increased approximately 20%, corresponding to a like increase in degree of crystallinity. The crystallinity developed on aging at 160EC is seen as a new peak at approximately 200EC. Shrinkage would not occur if the part was fully crystallized. It would not be a problem if the fit or tolerance between metal and plastic was not so tight. Possibly a nucleating agent in the PBT would give plete crystallization as molded, so that shrinkage as molded would not occur in service. What was not realized was that crystalline polymers may shrink in service if not fully crystallized. A prototype part was machined from a block of plastic believed to be acetal homopolymer. It performed in trial runs in service below expectations. Consideration was being given to redesign or to a change in PLASTIC PRODUCT FAILURE DUE TO DESIGN, MATERIAL OR PROCESSING PROBLEMS by Myer Ezrin, Gary Lavigne and John Helwig material. A check of the material by infrared spectroscopy and DSC showed that it was HDPE, not acetal. The trial run results were consistent with what would be expected of HDPE. The failure was in assuming incorrectly what the type of material was. 5. Processing As indicated in the Introduction, a mon failure is not to realize that the most severe and potentially damaging stage in a p
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