The gate design should deliver a rapid, uniform and preferably Linidircctional mold-filling pattern,the gate location should allow the air present in the cavity to escape during the injection phase. Failure to vent the air will result in short shots or burn marks on the molding.
If the gate location is likely to cause weld or meld lines, it should be positioned so that these will occur at approptiate positions to preserve part quality and appearance.Gate location and size should avoid the possibility of jetting {the string appearance or spaghetti strands on short shots). Jetting may be prevented by enlarging the gate or by locating the gate in such a way that the melt is directed against a cavity walL
The freeze-off time at the gate should ensure maximum cavity packing time, A well- designed gate freeze-off time will also prevent back flow of the injected material.The gate location should be at the thickest area of the part, preferably at a point where the function and appearance of the part are not impaired.
The gate length should be as short as possible to avoid an excessive pressure drop across the gate* A suitable gate length ranges from I mm to 1.5 imn {0-04—0, 06 in)
Normally the gate thickness is 50—80% of the gated wall section thickness For manually trimmed gates^ the gate section can occasionally be the same as the gated wall section thickness* For automatically trimmed gates, the gate thickness is typically 80% of the gated wall section thickness to avoid part distortion during de¬gating. Typical diameters at the gate end pin and submarine gates are 0.5 mini for the former and 0.25—2.0 mini for the latter.Fibre-filled materials require larger gates to minimise breakage of the fibres when they pass through the gate. Using small gates such as submarine, tunnel or pin gates can damage the fibres in filled materials.
Gates should always be machined smaller when the mold tool is first made, so that they can be enlarged later if necessary. Enlarging gates is relatively easy but reducing them can be both difficult and expensive.