模具方面的外文资料翻译

外文资料翻译：PLASTIC PRODUCT FAILURE DUE TO DESIGN,MATERIAL OR PROCESSING PROBLEMSBy Myer Ezrin, Gary Lavigne and John Helwig University of Connecticut, Institute of Materials ScienceAbstractSeveral examples are given in which design, processing, or an aspect of the material were primary contributors to failure of plastic products.A common pattern is failure to realize the consequences of seemingly inconsequential practices or decisions. Mold design was a factor in some cases. Material factors and processing were involved in other cases. Frequently design, material and processing are so closely related that failure cannot be ascribed solely to one of the three (8).1. IntroductionIn many cases of failure the cause is at least partly due to failure to know or realize the potential consequences of seemingly safe practices or decisions. In many of the cases cited failure occurs at the manufacturing stage, either in primary processing, such as injection molding, or in secondary operations. All failures can be traced to the design, the material, or processing, assuming service conditions are not unusually severe. The interdependence of the three main causes of failure is such that often all are contributors. Material and processing are particularly strongly linked.The material contribution to failure may be in the polymer itself or in an additive. Processing imposes on plastics thermal and mechanical stresses that frequently are the most severe a part will experience in its entire lifetime. Failure is often due to lack of realization of how severe the stresses in processing are and of the effect on the material. Examples are given of failures due to part design, mold design, material selection and processing.2. Part Design2.1 Polypropylene (PP) caps for a packaging application required that the top of the cap be flexed substantially due to direct contact with a round ball at the top of the container. Fracture occurred with some caps from the high flexural load and deformation. The gate was at the center of the top of the cap where stress was greatest in service. The design and material can withstand the service stresses only if the material properties 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 weak.The 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 10-20% 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 3.4 power of MW above about 20,000 MW (ç = KM3.4). Another relatively simple test that provides a measure, in effect, of antioxidant content is oxidative induction time by differential scanning calorimetry (DSC) (ASTM D3895). This test is particularly applicable to polyolefins (PE, PP). Without adequate antioxidant, PP and PE are very susceptible to oxidative degradation during processing. While this case is cited as an example of part design, it also illustrates how material and processing considerations are also involved.Presented at National Manufacturing Week, Design for Manufacturability of Plastic Parts, March 16, 1999, Chicago.PLASTIC PRODUCT FAILURE DUE TO DESIGN, MATERIAL OR PROCESSING PROBLEMS by Myer Ezrin, Gary Lavigne and John Helwig2.2 An O ring made of plasticized PVC was in contact with a polycarbonate part in an assembly that requiredthat the PC move freely when the O ring pressure was removed. In service there was sticking, i.e., separation did not occur readily as designed. Plasticizer at the surface transferred to the PC, which is not completely impervious to plasticizer. In effect, the plasticizer became an adhesive between PC and PVC. This failure was probably also due in part to the fact that plasticizers are less compatible in PVC under pressure. In this case the effect of plasticizer on PC, an amorphous polymer, was not realized, as well as the pressure effect on compatibility. ABS is also adversely affected by contact with plasticizer from PVC.2.3 Bottle caps were spray painted for color and scratch resistance. The bottom of the caps were to be bonded to another part of the cap with silicone adhesive. The adhesive failed to bond to the plastic in some cases. The cause was that some spray paint contacted the bottom surface. Waxy ingredients in the paint, for scratch resistance, interfered with the bond that normally would have been made to the silicone. The design and processing did not take into account the need to protect the bottom surface while the cap above was spray painted. It should have been realized that even traces of contaminant on a surface can reduce bond strength very strongly.3. Mold Design3.1 An ABS injection molded part of a syringe needle holder (4) consistedof two flats on the inside 180Eapart. A metal eyelet and tubing inserted after molding were held in place by stress at the flats. The design called for the flats, which are high stress points, to be 90E removed from the part’s t wo weld lines. In some mold cavities the flats were not located as intended, so that the flats were at the weld lines, contributing to failure.3.2 A hollow ABS injection molded part had a top ring of ABS ultrasonically welded into the insidediameter of the part. Some welds had a protrusion at one point in the circumference, which was thought to be flash from the welding. These defects occurred with parts from one of a two cavity mold. Lowering the force of insertion of the top ring during welding did not eliminate all defects. Examination of molded parts for frozen-in stress by immersion in acetic acid (ASTM D1939) showed very little stress. A check for out of roundness showed that bad parts were out of round as much as ±0.0025", compared to ±0.0005" for g ood parts. Figure 1 is a cross-sectional 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. Material4.1 A glass-filled PBT (polybutylene terephthalate) part had a hole in the center in which a threaded metalpart 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 complete 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.4.2 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 inPLASTIC PRODUCT FAILURE DUE TO DESIGN, MATERIAL OR PROCESSING PROBLEMSby Myer Ezrin, Gary Lavigne and John Helwigmaterial. 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. ProcessingAs indicated in the Introduction, a common failure is not to realize that the most severe and potentially damaging stage in a plastic’s entire experience is the thermal and mechanical stresses of processing. This problem is particularly serious for condensation polymers (nylon, PET, PC, PUR) and for polyolefins, although it is a problem for all materials. In the former case, hydrolysis to lower MW can take place if water content is above about 0.01%. The requirement of practically complete dryness in the melt cannot be overemphasized. For polyolefins like PE and PP, oxygen is the enemy, together with free radicals (reactive carbon atoms lacking one hydrogen atom) (5). Without adequate and effective antioxidant, the stage for failure is set in the molding machine or extruder. Section 2.1 above refers to the 3.4 power relationship between melt viscosity and MW. For even a small increase in MW, the melt viscosity penalty is high, making processability even more difficult. At the same time, the curve of properties (strength, toughness, chemical resistance) vs. MW is leveling off (6), so that the gain in product performance may come at a high price in processability. The lesson seems to be to design so that properties can be achieved at the lowest MW possible (highest MI). The interdependence of design, material and processing is very high. This is noted in some cases above.5.1 In injection molding of a PC part requiring high resistance to outdoor exposure, a deposit formed onthe mold, requiring shutdown and cleaning more often than was usually experienced with PC. It also contaminated the surface of the part. Analysis identified the deposit as a UV absorber present at a relatively high concentration because of the light stability requirement. It was near its compatibility limit and the high temperature and pressure of molding deposited a small amount each time. Over a two week period, what was not noticeable at first became a real problem. lower temperature might help reduce the problem, but change in amount or type of UV absorber may also be needed.5.2 A brominated hydrocarbon flame retardant in nylon 6 is making it difficult to process and to retain thedesired properties. Such materials act as flame retardants by decomposing and releasing bromine (Br2) and hydrogen bromide (HBr), which act as a blanket to exclude air. But when the bromo compound does that during processing, even to a small extent, the HBr, in contact with any water present, becomes H+Br-. H+ acid is a strong catalyst for hydrolysis ofcondensation polymers; nylon 6 is a polyamide (-NHCO-). Hydrolysis occurs at the nitrogen-carbon bond. Processing in this case is a virtual tightrope walk.5.3 A polyurethane containing a metal carbonate for radiopacity gave porosity in extrusions. Analysis bythermal desorption gas chromatography/mass spectroscopy (7) identified carbon dioxide in the extruded material. This was traced to the carbonate, which contained CO2 as received, and decomposed further in processing. Elimination of the problem is focusing on removing the initially present CO2 in the carbonate and molding so as to cause very little to form during processing.6. SummarySuccessful manufacture of plastic parts is a far more complicated matter than may be realized. Thetechnology involved cannot be used most effectively without knowledge of the basic organic chemistry and physical chemistry that underlie the materials, including polymers and additives, and the rheology that is involved in melt flow processing. A major category of failure has not been dealt with in this paper, i.e., orientation and frozen-in stress and the consequences in environmental stress-cracking, warpage, etc. Orientation is affected very strongly by polymer molecular weight and composition, as well as processing; design also plays a role. Failure to understand the ways in.PLASTIC PRODUCT FAILURE DUE TO DESIGN, MATERIAL OR PROCESSING PROBLEMS by Myer Ezrin, Gary Lavigne and John Helwigwhich design and material affect processing and, in turn, how all three affect product performance is the underlyingcause of many plastic problems.译文：塑料产品故障由于设计，材料或处理问题由迈尔埃兹林，加里Lavigne和约翰埃尔格康涅狄格大学，材料科学研究所抽象给出几个例子中，设计，加工，或者是物质方面的主要贡献者在塑料产品的失败。