Whenever possible locate gates on non-visual surfaces thus eliminating problems with residual gate vestiges after the gate has been removed.
Avoid areas exposed to high external stress (mechanical or impact). The gate area has high residual stresses and also rough surfaces left by the gate act as stress concentrators.
Locate the gate in the thickest section to ensure adequate pressure for packing out the part. This will also help prevent sink marks and voids forming.
Molecular orientation becomes more pronounced in thin sections, the molecules usually align themselves in the flow direction. High degrees of orientation result in parts having unaixal strength, resistance to loading only in one direction. To minimize molecular orientation the gate should be located so that as the melt enters the cavity it is diverted by an obstruction such as the cavity wall or an ejection pin.
Place gates to minimize the number and length of weld lines or to direct weld lines to positions that are not objectionable to the function or appearance of the part. When weld lines are unavoidable try to locate the gates close to the weld line location this should help maintain a high melt temperature that is beneficial to a strong weld line.
Glass fiber reinforced materials
Fiber-filled materials require larger gates to minimize breakage of the fibers when they pass through the gate. Using small gates such as submarine, tunnel, or pin gates can damage the fillers in filled materials. Gates that deliver a uniform filling pattern (such as an edge gate) and thus, a uniform fiber orientation distribution are preferable to point-type gates. Fiber orientation will normally be the determining factor for warpage problems with this type of material and the gate location and choice of gate type are 2 of the primary factors in controlling the orientation.
In general, there will be a higher glass fiber orientation in thinner wall sections, e.g. less than 2 mm and as injection speed increases. A high injection speed is required to obtain a smooth surface. The direction of orientation is influenced by gate type and location and, of course, by the shape of the product (see figure below).
Influence of gating on glass fiber orientation and shrinkage of the product.