High molding pressures

In order to obtain in practice the theoretical highest apparent density of the syntactic foam, the mixture has to be molded at the pressure at which the microspheres pack the closest. Several methods (mold vibration, high molding pressures, etc.) have been recommended as ways of obtaining a syntactic foam density close or equal to y by helping the microspheres pack more closely and thus approach the foam s maximum strength. It should be remembered that ideal packing is never actually achieved and there is always some disorder 80). 

Another factor in shrinkage is wall thickness. Thicker wall sections will shrink more than thin ones. Also high molding pressure areas will shrink less than low pressure areas. Shrinkage will also increase as mold temperature increases. 

In any case, due to the high molding pressures, as with injection-molded products, it is difficult to remove a finished part from the mold cavity despite the fact that the part shrinks upon cooling. Even with a mold taper and coating a release agent on the inside surfaces of the cavity, great surface tension forces and mechanical compression can adhere the part to... 

PPS resins are chiefly used for injection mol ding. The melt flow of the glass-fiUed resins is very stiff, and high injection pressures are required. Mold surface wear is heavier than for most other engineering plastics. Mol ding melt temperatures are near 330°C for optimum surface gloss and impact strength, mold temperatures of 130°C should be used. The resins are brown to brown-black. 

Blind hole In regard to molding products that include holes, it is important to ensure that sufficient material surrounds the holes and melt flows property. A core pin forming blind holes is subjected to the bending forces that exist in the cavity due to the high melt pressures. Calculations can be made for each case by establishing the core pin diameter, its length, and the anticipated pressure conditions in the cavity (3). 

TSs has experienced an extremely low total growth rate, whereas TPs have expanded at an unbelievably high rate. Regardless of the present situation, CM and TM are still important, particularly in the production of certain low-cost products as well as heat-resistant and dimensionally precise products. CM and TM are classified as high-pressure processes, requiring 13.8-69 MPa (2,000 to 10,000 psi) molding pressures. Some TSs, however, require only lower pressures of down to 345 kPa (50 psi) or even just contact (zero pressure). 

In practice, most molds are flexible and will deform when the fiber volume fraction becomes too high (which corresponds to a high compaction pressure). The forces associated with the compaction can be so high that the mold surface becomes dented or even that the entire mold breaks (if the clamping device is powerful enough). 

Use a higher temperature on the mold side, where high surface finish is required combined with internal mold pressure during cure... 

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