Corner warpage

The temperature of molded part corners should particularly he taken into account, as the well-known corner warpage can be avoided. In the arrangement of the temperature control channels, heat dissipation is inevitably better in the counter sunk area than in the core area. This can be explained by the smaller contact area in the core. This leads to a displacement of the plastic polymer core in the direction of the core (Figure 2.86), wherein the contraction of the shifted residual melt causes a distortion of the corner geometry. 

As reviewed there is much to consider. Examples include cooling as the product sets up results in different shrinkage rates for thicker versus thinner sections in the different processes. This results in either external waviness or sink marks, or warpage and internal voids, as the product contracts. Flat surfaces are difficult to maintain but not impossible to attain using certain processes. High speed of flow to fill the cavity of the mold is impeded going around square corners, so provision for radii and fillets are important. 

The lower processing limit represents the lowest heat at which the sheet can be formed without undue stresses. This means that the sheet should touch every corner of the mold prior to reaching this lower limit otherwise problems develop such as stresses/strains that can cause warpage, brittleness, or other physical changes in the part. 

For an L-shaped molded part, its flanks come closer to each other because of the local shrinkage at the comer. This is called the comer effect. The inner warpage of box-shaped parts is also due to this corner effect. For a center-gated disk, if the inner shrinkage is larger than the outer one, the disk will twist, otherwise it will dome. These phenomena are called buckling. 

FIGURE 58.15 Depopulated corners to improve warpage-related assembly yield. ... 

PennStateCool Program involves corner cooling to warpage analysis. 

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