Injection molding skin formation

Other modified AZs have been developed for the manufacture of integral-skin foams by extrusion and injection molding. These contain additives that modify the usual decomposition process of AZ and suppress the formation of cyanuric acid, which causes plateout on the surfaces of molds, dies, and screws. The additives used include zinc oxide and/or silicic acid (a colloidal silica) with a very low water content. The additives also act as nucleating agents, producing a cell structure that is both uniform and fine-celled. 

The addition of P-nucleating agent raised the onset crystallization temperatnre and the relative crystallinity of the P-crystals of the micro-injection molding specimens becanse of its strong heterogeneons nncleating effect in the iPP matrix. Introdnction of UHMWPE increased thickness of the shish-kebab stractnre in the skin layer and indnced the formation of perfect P-cylindralites. Similar effect was observed with the addition of polyolefin elastomer. ... 

Keywords annealing, application. Pa-recrystallization, hexagonit, P-nucleants, compression molding, extrusion, immature spherulites, impact resistance, injection molding, lower and upper limit temperature of formation of p-iPP, mechanical properties, morphology, skin-core structure, strain hardening, toughness. 

We have subsequently shown (see section 4.3.) that injection molded specimens, which were chilled in air, have mixed polymorphic composition where the skin coasists of a-ciystals while the core comprises /3-spherulites. This fact supports the existence of a lower critical temperature limit for the formation of /3 PP. 

Injection molded parts are known to degrade under the influence of UV or chemical environment (environmental stress cracking) at certain areas of stress concentration. Release agents may play certain role in these processes because they may help to release part before its core has solidified. Also, uneven distribution of release agent on surface may induce tension into a part skin. Any internal stress potentially increases risk of material degradation, under the influence of radiation or contact with solvents. The essential parameters, which contribute to stress formation, are evaluated in a broad research study. ... 

Cold curing permits the formation of dual density foam products. We either sequentially inject different formulations or we mold the first foam then mold the second around or on top of it. When making dual density foams, it is important that the first component develop a continuous skin prior to the injection of the second reactive mixture. 

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