Transfer Molding Process Variables

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. 

Selection of the fluoropolymer depends on the end use requirements, part design, and process conditions. Viscosity of the molten pol5nner has a great influence on the transfer rate into the cavity. Temperature changes can alter polymer viscosity as long as it has sufficient thermal stability in the given temperature range. Cleanliness of the melt can affect the quality of the part and thermal stability of the molten resin. 

Surface Finish Finer than 32 rms finish. Surface finish should be monitored, e.g., pitted mandrels are hard to extract. 

Geometry Mandrels and pin cores should be straight. Thickness nonuniformity will result in a part that is not true, subsequently leading to premature liner failure. 

Casting Surfaces A casting such as a fitting serves as a substrate for coating and must be smooth and free of oil and rust. Sandblasting is effective for surface preparation. 

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In addition to material variables, as reviewed in Chapter 6, there are a number of factors in equipment hardware and controls that cause processing variabilities. They include factors such as accuracy of machining component products, method and degree of accuracy during the assembly of component products, temperature and pressure control capability particularly when interrelated with time and heat transfer uniformity in metal components such as those used in molds and dies. 

Figs. 4.63 - 4.66 illustrate the location of lines of constant values of temperature, degree of conversion, velocity and viscosity for five consecutive positions of the front of a stream, which correspond to the following values of the axial coordinate xf 0.2, 0.4, 0.6, 0.8, and 1.0. These lines of constant values of the process variables are calculated for the flow and property values designated by the point D in Fig. 4.61. In this case, the mold temperature Tm = 70°C, the initial temperature of the reactive mix To = 40°C, and the initial temperature of the insert Ti = 20°C. An area above the horizontal line of symmetry of the mold cavity (i.e., the upper part of the cavity) contacts the "hot" surface of the mold and the lower part is in contact with the surface of the cooler metal insert. Thus, we can conclude that the distributions of temperature, degree of conversion, viscosity and velocity of movement of the reactive mix along the mold are related to the ratios between the transfer rate and the chemical reaction, which are characterized by the values of the Da and Gz Numbers. 

Chang, C.-Y., Houmg, L.-W. and Chou, T.-Y., 2006. Effect of process variables on the quahty of compression resin transfer molding . Journal ofReirtforced Plastics and Composites, 25, 1027-1037. 

Process Variable Compression Molding Transfer Molding... 

After the molten polymer has been stored in the pot and reached the required temperature, it is transferred into the mold cavity under positive pressure. The pressure and rate of transfer are the significant variables of the process. Critical shear rates at which the molten fluoropolymers exhibit unsteady flow are fairly low when compared to other pol5miers (Table 6.42). Critical shear rate increases with temperature as seen in Figs. 6.54-6.56 for three commercial grades of FEP, PFA, and ETFE. Careful gate design and transfer rate is necessary to keep the flow below the critical shear rates. 

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