Compression molding processing

BMC can be molded by compression-, transfer-, or injection molding processes. Compression molding is the oldest and most commonly used process. It consists of forming the material between the heated metal punch and die. This process is the most economic one however there are a few components that cannot be compression molded owing to their intricate shapes. Another drawback of compression molding process involves weighing of the charge before it is laid out... 

Softening Behavior. The softening behavior of the PS control, a sodium C-PS, and a sodium S-PS are illustrated in Figure 4. These softening curves were obtained on materials that were compression molded. Both ionomers contained about 5 mol % ionic functionality and at that level are extremely difficult to melt process. Compression molding can be effected at temperatures of 250°C for the S-PS and 200°C for C-PS, provided that the molding is effected over a sufficiently long time to permit the viscous flow processes to occur. A Du Pont Thermo Mechanical Analyzer was used (10°C/min) under the same conditions for all three materials. 

As in the hot-compaction processes, compression molding is also carried out in a tool that possesses a tempering higher than the melting temperature of the self-reinforced thermoplastic textiles. Equivalent to hot-compaction, a selective melting of the films, tapes or fibers can be induced using the same functional principles. As is already known, it is necessary to differentiate between mono- and co-extruded semifinished t5rpes. 

Although an old process, compression molding is not well described technically or analytically in the literature. Basically, what is needed is an analysis that combines rheology, heat transfer, chemical reaction (the curing process), and even mass transfer (outgassing) in a dynamic (time-dependent) system. The situation is also complicated by changing temperatures and pressures. 

TABLE 2 Water vapor permeability (WVP), oxygen permeability (OP), and mechanical properties (tensile strength, TS elastic modulus, EM and % elongation, %E) of various edible films fi-om dry process (compression molding and extrusion)... 

Above the shut-off edge, the remaining space between the force and female mold is extended to demold the parts more easily. Due to the type of process, compression molds have to be operated with an excess of material (small material overdose). 

Suggested process Compression molding Molding extrusion Molding extrusion... 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

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). 

Eoamable compositions in which the pressure within the cells is increased relative to that of the surroundings have generally been called expandable formulations. Both chemical and physical processes are used to stabilize plastic foams from expandable formulations. There is no single name for the group of cellular plastics produced by the decompression processes. The various operations used to make cellular plastics by this principle are extmsion, injection mol ding, and compression molding. Either physical or chemical methods may be used to stabilize products of the decompression process. 

Manufacture of friction elements includes the impregnation of fabrics and subsequent lamination, the wet-dough process, and the dry-mix process. Elements from the last two are prepared by compression-molding the formulation for up to an hour at 150—175°C. Thick brake elements require a carefully controUed heating-and-cooHng cycle to minimize stresses created by expansion and contraction (see Brake linings and clutch facings Fillers). 

Hybrid Processes. There are also hybrid processes that have evolved to meet specific product needs. As an example, automotive leaf springs utilize a filament winding system to prepare impregnated fiber bundles that are then compression molded to final configuration. 

Acrylics processed by compression-molding techniques give dentures as satisfactory and dimensionally stable as the estimated 5% of the dentures made with special resins and elaborate processing equipment (208). Promising fiber-reinforced dentures have been reported (209—212). 

A compatible blend should have good processing properties along with a smooth surface and cross-section. If needed, further tests on mechanical properties should be carried out on testing samples made from 2-mm films produced by compression molding. 

The manufacture of elastomeric radial lip seals centers around the molding operation. Figure 2 [1] illustrates the manufacturing flow diagram for the two primary processes used compression molding and injection molding. 

Figure 2 Typical process flow diagrams (a) compression molding, (b) injection molding.

Excess flash—Indicates the compound flow was excessive during the mold closing process (shelf loaded compression molding) [3]. 

In the two typical shaft seal manufacturing processes shown in Fig. 2, the extrusion process is only required if the compression molding process is being used. There are basically two types of extruders in general use, the cold feed and the hot feed. In both cases their role is to produce the preforms for use with compression molding. The critical issues with the preform are the shape, the dimensions, and the weight. 

During the shaft seal molding process, the elastomeric compound has relatively large distances to travel/ flow in the mold, particularly when using shelf loaded compression molding (Fig. 17a) and injection molding (Fig. 17b). History has shown that without the use of a vacuum, either applied directly through the mold or... 

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