Transfer molding heating

CM is the most common method of molding TSs. In this process, material is compressed into the desired shape using a press containing usually a two-part closed mold and is cured with heat and pressure. This process is not generally used with TPs. TM, also called compression-transfer molding is a... 

Molded articles may be prepared from the prepregs by compression and transfer molding. In general, several sheets of the prepreg are cut and stacked in a mold. For example, a prepreg was cured by heating for 80 s at 150-180°C at 100-200 bar. The cured sample had the physical properties shown in Table 11.4. 

Resin Transfer Molding (RTM). Reinforcing fibers are distributed uniformly in the mold and the mold is closed. Liquid resin is injected into the mold until the excess comes out of the vents. The mold is pressed and heated, similarly to preform molding, until cure is complete. 

Because the pressures of injection are low at approximately 25 to 50 psi (172 to 345 kPa) very fragile inserts can be molded and mold wear is at a minimum. Some formulations for LIM also may be molded at temperatures as low as 200F (93C) which permit the encapsulation of some heat-sensitive electronic components that do not lend themselves to encapsulation at conventional transfer molding temperatures of 300F (149C) or higher. 

Let us still consider the microelement in fig.l, which filled with melt consisting of reinforcement particles and alloy liquid. The temperature of melt and mold are To and Tm, respectively. In order to produce FGM with particles gradiently distributed in the direction of centrifugal force, the transfer of heat must be in the same direction. Hence, the heat conduction equation can be simplified as ... 

All samples were prepared from a commercially available epoxy cresol novolac-phenol formaldehyde novolac-tertia-ry amine based molding compound. Pelletized preforms were heated to 85°C in a RF preheater prior to being transfer molded at 180°C/68 atm. for 90 sec. Molded samples were cooled in air to room temperature and stored in a desiccated environment until testing or subsequent thermal treatment. Post mold curing, PMC, was accomplished in a gravity oven at 175°C for a period of 4 hours. Samples without post mold curing are designated by NPMC. 

Progress has also been made in part cooling. Portable and centralized chillers have been developed to cool the part in a minimum time. These closed loop systems use water or water-ethylene glycol combinations as the heat transfer medium. The application of heat transfer principles allows one to computer model the cooling process (35). For the new engineering resins such as polyamide-imide and polyphenylene sulfide, one must use mold heating. The use... 

Figure 6.53 shows actual data for melt-pot transfer molding of FEP into a liner for a 5 cm diameter T-fitting. Melt temperature at the center of the pot and the mold temperature were monitored using thermocouples. A time period of three hours was required to bring the melt pot and the mold to 340°C which was the selected process temperature. After the desired temperatures have been obtained, the next step is to transfer the melt under pressure into the heated mold cavity that will form the final shape. 

Owing to experimental difficulties, there are but few publications on uniaxial deformation of blends. To prepare specimens for testing, samples usually are transfer molded and relaxed, both operations requiring relatively long heating time, during which only well stabilized blends will not coarsen. 

The two most widely used methods for molding thermoset are compression and transfer molding. Thermosets, you will recall, undergo a permanent set, i.e., become essentially insoluble and infusible under the action of heat. Consequently, techniques for fabricating thermosets must take due cognizance of and make allowance for the fact that sprues, runners, and gates are not reusable and therefore constitute rejects. 

FIGURE 2.4 Molding qrcle of a plunger-type transfer mold, (a) An auxiliary ram exerts pressure on the heat-softened material in the pot and (b) forces it into the mold, (c) When the mold is opened, the cull and sprue remain with the molded piece. (After Frados, J. ed. 1976. Plastics Engineering Handbook, 4th Ed., Van Nostrand Reinhold,... 

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