Compression molding limitations

It should be re-emphasized that although our block copolymers do not display spherulitic morphology when they are compression molded, they are nevertheless crystalline. Hence, this indicates that under this mode of film preparation, aggregation into well developed superstructure is apparently kinetically limited. 

Figure 16.7 Compression molding. (Source Wittcoff and Reuben II and Reuben and Burstall 1974, Reprinted by permission of Pearson Education Limited.)...

Amino resins are fabricated principally by transfer and compression molding. Injection molding and extrusion are used on a limited scale. Urea resins are not recommended for outdoor exposure. The resins show rather high mold shrinkage and some shrinkage with age. The melamines are superior to the ureas insofar as resistance to heat and boiling water, acids, and alkalis is concerned. 

Pretreatment of starch granules by esterification with ethylene-acrylic acid iono-mers has been reported to improve the properties of compression molded PE-starch materials compared to simple melt mixing of the three components.54 Improvements over no treatment with ionomer were limited to starch contents less than about 20%. 

The final product may take any one of a vast multitude of final forms. These include, but are not limited to, construction materials, toys, automotive parts, kitchen aids, industrial parts, sports and recreational equipment, containers of all sizes and description, and an unlimited variety of packaging products for foods, drugs, cosmetics, and other consumer products. The processes that are used to produce or mold or fabricate the final products are also varied and depend on the design and purpose of the product. Some of these processes include extrusion, compression molding, roto-molding, blow molding, sheeting, and films. 

Compression molding is the most common forming method for thermosets. Thus compression molding was the preferred processing technique in the early part of the 20th century. Its relative importance diminished as thermoplastics were introduced, and other forming methods better suitable for the new materials were developed. Compression molding of thermoplastics has always been limited to special cases. 

Compression molding involves limited flow and, consequently, less strain than injection molding. Consequently, shear sensitivity typical of polymer blends is less critical. On the other hand, the cycles are usually longer and the long soak times at high temperatures can affect blends morphology through coalescence. 

Examples 11.9 Explain why compression molding of thermoplastics is limited to small quantity production while screw injection molding is thermoset is also currently used on a limited scale. 

Melamine sheet molding compound Developed by DSM and Perstorp, is under the name Remel, as a dry sheet for compression molding, for applications especially in the electrical, public transport, domestic appliance, and sanitary ware industries. It complies with all recognized industrial fire test standards without need for halogen additives. With a limiting oxygen index of 95%, the V-O standard in... 

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