Thin-walled molded part

In this paper, proeessability and higher-order stmcture of thin-wall molded parts for polypropylene/hexagonal boron nitride (PP/h-BN) composites were investigated to produce new polymer composites with high heat conductivity. Effects of h-BN content and process parameters on processability, thermal property and internal morphology of molded products were discussed. 

Rotational molding creates a wide variety of plastic products that cannot be made effectively, efficiently, or economically by other means. What sets this method apart from others is that it can create thin-walled, hollow parts that exhibit no weld lines or scarring from ejector pins and from the process itself. It also has the advantage of having little scrap and minimal molded-in stresses, due to the low pressure and low shear rate characteristics of the process. Finally, it can be used to make parts that are very large which would be impossible to manufacture by other methods. 

Thermoplastics have non-Newtonian melt flow characteristics it means that their viscosity will change dependent on their velocity or the amount of shear that occurs in the melt (Chapter 1). This non-Newtonian characteristic is a key in thin wall molding. As in any molding setup one cannot just simply ram the melt into the cavity. It s flow characteristic, gate size as well as position, and venting have to be balanced in order to obtain the desired structural part and meet tight tolerance requirements. 

Other developments have not been as dramatic, but they have nevertheless contributed significantly to the process efficiency. The electric screw drives were replaced by hydraulic motors with a gear box on the reciprocating screw unit. In the future the hydraulic motor may in turn be replaced by a direct hydraulic drive (19). The Introduction of stack molds has also increased productivity. In a stack mold, two cavity plates are placed on top of each other so that the projected area is the same and twice as many parts can be formed with the same clamping tonnage. This technique works best with thin wall, flat parts with short cycle times. 

Uniform heat removal from the mold is critical for securing consistent dimensional tolerances and the best set of properties. Higher mold temperatures are required for thin walls, large parts, and parts that require dimensional stabUity and low level of molded-in stresses. Mold temperature has minimal effect on gloss level. 

A major development has been to provide easy release of parts from the mold. Special mold treatments and careful physical removal of parts were required, increasing the cost. PURs now incorporate an internal mold release (IMR) agent that improves part performance and process economics. However, thin-walled, complex parts may require the use of release agents on the mold. Material developments are under way to rectify this situation. 

RM is a simple, basic, four-step process that uses a thin-walled mold with good heat transfer characteristics. This closed mold requires an entrance for insertion of plastic and, most important, the capability to be opened so that cured parts can be removed. These requirements are no problem. Liquid or dry-powder plastic, equal to the weight of the final part, is put... 

For a thin-walled molding, the distance between the opposing cavity surfaces (i.e., part thickness) is typically orders of magnitude smaller than the length and width of the part. In this case, the traditional boundary integral formulation leads to... 

Thin-Wall Molding. This is identical in concept to normal injection molding but the parts produced have much thinner nominal walls notebook computer... 

The flow-induced stress plays an important role especially in thin-wall molding, for the incomplete relaxation of polymer molecules that freeze too fast and the less shrinkage that generates less the thermal-induced stress. Frozen layers on the surface of mold cavities act as poor thermal conductors. The molecular orientation in the hot core is allowed to relax. Yet the whole parts are cooled and frozen in a very short period and thus could not relax completely in thin-wall molding. Therefore, the flow-induced stress is a significant amount for thin-wall parts, while the thermal-induced stress dominates in thick-wall parts. 

PCTFE plastic is available in products that conform to ASTM 1430-89 Type 1 (Grades 1 and 2) and is suitable for processing into parts that meet MIL-P 46036 (Federal Specification LP-385C was canceled 1988). Standards for fabricated forms are available for compression molded heavy sections (AMS-3645 Class C), thin-walled tubing, rod, sheet, and molded shapes (AMS-3650). PCTFE plastics have been approved for use in contact with food by the FDA (55). 

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