Sink-mark resistance

Because of low injection pressure, some cost savings are possible in mold and press constmction. Mol ding cycles are somewhat longer than for injection molding. The part must be cooled in the mold long enough to be able to resist swelling from internal gas pressure. In stmctural foam parts there is almost a total absence of sink marks, even in the case of unequal section thickness. Stmctural foam has replaced wood, concrete, sohd plastics, and metals in a variety of appHcations. 

OTHER COMMENTS used in the manufacture of acid-proof coverings in floors, tables, sinks, etc. used in switchboard panels of high electrical resistance sue in crayons for marking cloth, metal, and glass has also been used in ceramics, cosmetics, pharmaceuticals, putty, plaster, oilcloth, and as a filler in paints. 

Cissing is the appearance of small, saucer-like depressions in the film s surface. These are caused by particles or droplets of incompatible material, which either land on the film during drying, or are present in the paint itself. The weight of the particle causes it to sink but, since there is no attraction between paint molecules and molecules in the particle s surface, the surface tension of the paint resists the particle s entry into the liquid film. Immediately under the particle, the paint molecules have no alternative but to be in close proximity to the particle. In the zone surrounding the particle, the liquid skin is depressed by the particle s weight, but attractions from within the paint film pull the surface molecules away from the particle. There is said to be a high interfacial tension between paint and particle. The net result, as Fig. 10.4 shows, is a ciss mark... 

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