In piston transfer molding, the material is injected by a separate piston that moves independently from the mold closing movement. The transfer molding process has its advantages in the processing of shear –sensitive materials and in the processing of large volume parts for which extended heating times is generally required. As heat is generated when the material flows from the transfer pot in the cavity, the heating time can be reduced compared to compression molding.
During the molding cycle, the molten polymer is injected from the barrel into the mold through one or multiple piston channels in the processing head, depending on the desired manufacturing process. Once the mold is filled the cycle starts actuating the pistons in a selected sequence. The piston action first develops fluctuating melt pressure that moves and shears the melt on the cavity and gate areas. Then, the pistons are moved to apply compression/decompression forces to the melt. Additional material is introduced to compensate for shrinkage and voids. Primary benefits of this process are claimed to be uniform alignment of fibers in the direction of flow, elimination of “knit lines” or weld lines, minimization or elimination of sink and void, and improved dimensional stability.
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