Transfer Molding TM

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... 

TM transfer molding TMA thermomechanical analysis TMA Tooling Manufacturing Assoc, (formerly TDI)... 

The transfer molding process is a development of the previously discussed CM process. For the TM method, the same machines (presses) are used, which are also necessary for the CM method. The main difference between the two methods is that plasticization and cross-linking takes place in two separate systems. The TM technique is mainly used for small parts (maximum diameter is 100 mm) and for large quantities. The tolerance of the process is a material hardness of 80 to 85 Shore A. 

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. 

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 ... 

A further development of TM is to plasticize the mbber mixture in a screw-type extmder solidly attached to the press, heat it up and inject it into the transfer cylinder. Closing the clamping unit then fills the mold cavities as in normal TM, Fig. 44 [7]. 

The majority of TS compounds are heated to about 300°F for optimum cure. Higher heats could degrade their performance or could cause them to solidify rapidly, particularly in TM, where material could solidfy before the cavity completely filled. Lower heats extend the cycle time. The molds are heated by electricity, steam, or hot circulating heat-transfer fluids. 

As higher temperatures are required to process polycarbonate, heat transfer through the mold to the material can be a problem. The best results are achieved using oil-jacketed molds and finer powder than the 35 mesh (500 tm) commonly used for polyethylene. However, bubbles can still occur in parts and it is common to see textured mold surfaces used to disguise their presence. The use of a nitrogen atmosphere can reduce the yellow color caused by degradation. 

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