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know-how materials mike sepe Dimensional stability after Molding—Part 2 My first exposure to the prolonged shrinkage process exhibited by some materials came when i was molding large parts in acetal homopolymer. Most of my experience with molding acetal parts to close tolerances had come from producing small gears with diameters of no more than half an inch. However, our company landed a project that involved housings and quadrant gears with critical dimensions in the range of 3.5 to 4 in. and print tolerances of ±0.010 in. on parts with nominal wall thicknesses of 0.110 in. One specifc dimension governing the spacing of alignment holes called for a specifcation of 4.046 ±0.010 in. During initial sampling we produced parts with measurements that ranged from 4.038 to 4.042-in. in a 30-piece capability study. Statistical process control (SPC) was a relatively new concept in U.S. manufacturing at that point, so the fact that we were operating at one end of the tolerance range did not particularly concern us. We were quite satisfed that the dimensional range was tight and all the parts were to print. But the parts had been measured about 90 minutes to two hours after they were produced. The parts were room temperature to the touch and based on our experience with other semi-crystalline materials we were satisfed that everything was fne. The next day the quality assurance people pulled us into the lab to show us that half of the parts we had produced the day before were too small to meet the print. A review of all 30 parts showed that they had all continued to shrink and were now 0.004 in. smaller than the previous afternoon. In another day they moved an additional 0.001 in. and then things seemed to settle down. Later I observed similar problems with large parts produced in polypropylene, even in flled PP. In order to understand what was happening it is important to appreciate the relationship between mold shrinkage and crystallization in semi-crystalline materials. The more the material crystallizes the more it shrinks. Optimal levels of crystallinity are desirable. Semi-crystalline materials offer improved levels of fatigue and wear resistance compared with amorphous polymers and they generally provide improved creep resistance at elevated temperatures. But if the material is molded in a manner that prevents the development of crystallinity, these properties are not realized at the intended levels. The opportunity for crystallization exists in a temperature window below the melting point of the polymer and above the After molding, acetal parts can continue to shrink at room temperature and even in the cold. (Photo: Ticona). glass-transition temperature (Tg). Processes like injection molding involve rapid cooling of the polymer as it enters the mold. Even when running a material such as PEEK, where the mold temperature may be 375 F (190 C), this represents a thermal shock to the fowing polymer that enters the mold at 700 F (371 C). This rapid reduction in temperature is needed to solidify the material so that the part can assume its intended shape. But as long as the polymer remains at a temperature above its glass transition, about 295 F (146 C) for PEEK, there will be suffcient mobility at a molecular level to allow the crystal structure to develop. Once the temperature of the material drops below this point no more crystals can form. So what happens with acetal and PP? I often hear people state that these materials do not follow the rules. The rule they are referring to is that if the part feels like it is at room temperature then it is dimensionally stable. In fact, these materials follow the rules precisely. The problem is that the Tg of these materials is below room temperature. This is also true of polyethylene and ethylene copolymers such as EVA. The Tg of PP can vary from about -10 C to +15 C (14 to 59 F) depending upon the grade. The Tg of acetals is -78 C (-109 F), or the temperature of dry ice. This is a temperature only rarely experienced anywhere on our planet. So the chances are quite good that wherever you might be using an acetal part, it is above its Tg. Room temperature is 100° C above the Tg of acetal. So as our molded parts sat in the quality lab overnight, they had more than enough freedom at a molecular level to continue crystallizing. A properly packed out acetal part produced at the correct mold temperature will exhibit continued shrinkage of about 0.001 in./in. between the time the part reaches room temperature and the time that it is truly stable. This value may increase if the nominal wall is very thick. For small parts this change will be diffcult to detect unless very precise measurements are made. Plastics technology february 2013 17