Temperature transfer molding

J. W. Connell, J. G. Smith, Jr., and P. M. Hergenrother. High temperature transfer molding resins. US Patent 6124035, assigned to The United States of America as represented by the Administrator of the (Washington, DC), September 26, 2000. 

The precerammed billets, already having appropriate tooth color, are loaded into a dedicated high temperature press and transfer-molded at 1100°C into phosphate-bonded molds. 

Thermosets are processed by compression molding, transfer molding and injection molding or by extrusion, depending on their structure. The pressing temperatures are about 150 to 190°C. Corresponding requirements are placed on the pigments. Dyes are often also used to color thermosets. 

Phenol Formaldehyde (PF). Phenol formaldehyde is known for its high strength, stiffness, hardness and its low tendency to creep. It is also known for its high toughness, and depending on its reinforcement, it will also exhibit high toughness at low temperatures. PF also has a low coefficient of thermal expansion. Phenol formaldehyde can be compression molded, transfer molded and injection-compression molded. Typical applications for phenol formaldehyde include distributor caps, pulleys, pump components, handles for irons, etc. It should not be used in direct contact with food. 

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. 

Ring opening polymerization of cyclic oligomers of BPA can be performed in the melt (200-300°C) or in solution at ambient temperatures. ° ° This process can be used to obtain unique copolymers and ultra-high molecular weight polymers and enables processing techniques such as resin-transfer molding for production of composites. 

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. 

The operations of mold and melt removal from the oven and melt transfer should be conducted as rapidly as possible to prevent premature cooling of the melt. Fluoroplastics have fairly high viscosity and require relatively long transfer time. To avoid premature cooling and freezing prior to the completion of transfer, mold temperature should be maintained at above the melting temperature of the polymer. 

The important variables of transfer molding are the polymer type, melt temperature, pot hold time, transfer pressure, transfer rate, hold-up time in the filled mold, and the mold cooling. These variables are briefly reviewed in this section. 

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. 

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