Shrinkage during molding

Composites cured by the polyaddition mechanism are especially interesting for reactive molding, because by-products are not formed and there is no appreciable shrinkage during the reaction. This is the principal difference between polyaddition and polycondensation. 

After mixing of several streams, the reactive mixture goes into a mold where polymerization or solidification takes place. The behavior of materials in a mold depends substantially on the chemical nature of the initial raw materials, on mixture viscosity, on shrinkage during polymerization and solidification, on adhesion to the mold walls, and on temperature. The main feature of free casting is that the mold is either unpressurized or the pressure is no higher than 0.5 MPa if a mold is... 

Two extreme cases can be predicted for the shrinkage of an LCP molded part based upon 3-dimensional CTE data. In the first case the part is assumed to shrink from the melt temperature (Ts) to room temperature (Tf). Here it is postulated that no real part packing occurs during molding. For the second case, the part is assumed to cool from the mold temperature (Tm) to room temperature. For this case, maximum packing would occur. For cooldown from melt temperature, the equations are ... 

All corners and edges must be generously rounded (minimum radius 2 mm) to avoid stress concentration in the molded liner. Fitting liners that contain sharp comers may fail at these locations in chemical service after multiple temperature cycling. Unrounded liners may fail because of stress concentration that occurs by the hindrance of resin flow during the transfer operation and subsequent shrinkage during the cool-down. 

Shrinkage during solidification always tends to reduce the dimensions to values lower than those of the mold. The magnitude of this shrinkage is shown in Figure 23.12. It appears that for amorphous polymers a reduction in volume of circa 10% occurs when the polymer is cooled from the processing temperature to the ambient for crystalline polymers, this reduction may amount to 20-25% (Fig. 23.12). 

Shrinkage Any filler will decrease shrinkage the most commonly-used fillers are silica, clay, calcium carbonate, alumina talc, powdered metals, and lithium aluminum silicate. Low-shrink or low-profile additives are used in TS systems such as SMCs and BMCs to minimize shrinkage during and after molding, and improve the surface appearance of a molding. 

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