Compression molding cycle steps

Figure 13.44 represents the various stages of the compression molding cycle from the point of view of the plunger force needed to close the mold at a constant rate. In the first region, t < the force increases rapidly as the preform is squeezed and heated. At tf, the polymer is presumably in the molten state and, as such, is forced to flow into the cavity and fill it. Filling terminates at tc, when compression of the polymer melt takes place, to compensate for the volume contraction that results from the polymerization reaction. The bulk of the chemical reaction occurs after tc. We now comment on each of the steps of the compression molding process. 

Interim storage of the fiber composite material is realized in a heat accumulator, from which it is discharged in cycles by a ram, chopped into molding compound units by a cutter at the end of the accumulator, and transported on a discharge belt. The plasticized material produced in this way is transferred into a compression mold, then formed into a structural component in an extrasion process. The finishing steps are the same as in other CM methods used for long fiber-reinforced thermoplastics. 

For economic efficiency, overall cycle time must be minimized. Note that the molding time is the time under full pressure and does not include other steps such as loading, mold closure, degassing, and part ejection, all of which contribute to the overall cycle time. Compression molding time must be adequate... 

The material system used for this effort was the SiC/SiNC system with CG-Nicalon fibers (a non>stoichiometric SiC fiber). The matrix of Si, N and C was arrived at by set iterations via the polymer pyrolysis process. The initial processing step was compression molding followed by repeated infiltration cycles. For the present worfc, material subjected to 5,7,9 and 11 PIP cycles were considered. The lay-up of all the panels considered here is [0/90]2s resulting in an 8 ply composite. Additional material information has been published by the authors [3,5]. For this test series, each panel was 150 mm x 150 mm x the as-received thickness of the material. This allowed 8 tensile bars to be machined out of the panel as well as 4 interlaminar tensile buttons. 

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