Continuous-molding compound

Composites. The history of phenoHc resin composites goes back to the early development of phenoHc materials, when wood flour, minerals, and colorants were combined with phenoHc resins to produce mol ding compounds. In later appHcations, resin varnishes were developed for kraft paper and textile fabrics to make decorative and industrial laminates. Although phenoHcs have been well characterized in glass-reinforced composites, new developments continue in this area, such as new systems for Hquid-injection molding (LIM) and sheet-molding compounds (SMC). More compHcated composite systems are based on aramid and graphite fibers. 

Sheet molding compound (SMC) consists of randomly oriented chopped fibers in a matrix of resin and filler. SMC is produced in the continuous manner shown in Figure 1-16. Note that the polyethylene film protects the roller system from getting gummed up with the resin-filler paste. The rug-like rolls of SMC are then used in compression molding machines to create large parts such as the sides of cars and trucks. 

Contact Molding. Spray-up Pressure Bag Filament Winding Continuous Pultrusion Premix/ Molding Compound Matched Die Molding with Preform or Mat... 

Figure 5.101 Various forms of sheet molding compounds (a) random, SMC-R (b) continuous/ random, SMC-CR and (c) criss-crossed continuous/random XMC L and T refer to longitudinal and transverse directions, respectively. Reprinted, by permission, from Composite Materials Technology, P. K. MaUick and S. Newman, eds., p. 33. Copyright 1990 by Carl Hanser Verlag.

The technique for monitoring the dielectric loss factor is relatively simple. Two metal electrodes are placed opposite each other at critical locations on opposite sides of the mold. When the sheet molding compound (SMC), is placed between the electrodes, a capacitor is formed. The dielectric power loss is monitored continually throughout the molding cycle, as outlined in Section 6.1.2.2. 

Thick Molding Compounds and Continuous Impregnated Compounds... 

The molds should have the ability to maintain constant temperature despite continual heat removal by the relatively cooler molding compounds this can be monitored by placing temperature sensors and heat cartridges to provide uniform temperature. Insulating blankets may prove beneficial in minimizing mold heat losses and variations caused by local air currents around the molds and presses. A cut out should be provided, which will cut off the power to the heat cartridges as and when it senses a few degree rise than the desired temperature. 

Fibers can be produced from a specific grade of PPS polymer. This polymer has the same outstanding heat resistance possessed by the grade of material used in injection molding compounds. As shown in Table VII, continuous filament PPS yarn has been exposed at 200°C for 60 days with only a moderate decrease in strength. 

A number of companies have tried to promote PET as a molding compound. In 1966, the first injection molding grades of PET were introduced however, these early materials were not very successful. The primary problem was that PET does not crystallize very rapidly a molded object composed of a crystallizable polymer caught in an amorphous or partially crystallized state would be rather useless. In service such a part could crystallize, shrink, distort, crack, or fall. The obvious solution was to use hot molds and hold the parts in the mold until the crystallization process was completed. Postannealing also permits continued crystallization. These approaches, especially with glass fiber incorporation, led to... 

Presently, epoxy resins sell for 81< /lb. Compared to unsaturated polyesters at 38< /lb., phenollcs at 40d/lb. and urea molding compounds at 56< /lb., epoxy resins are still more expensive and perhaps will continue to be so. A brief breakdown of the reported uses for these resin in 1978 is given. 

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