Mold Concerns

Gate position can have a direct impact on part moldability, performance, appearance, and cost. The location of the gate determines the filling pattern and maximum material flow length. Ideally the gate would be positioned to balance filling and minimize flow length, typically near the center of the part or at strategic intervals for multi-gated parts. Cavity layout restrictions and mechanisms in the mold such as slides or lifters may restrict gating to less-than-ideal locations. The best gate position is often a compromise between molding ease and efficiency, part performance and appearance, and mold design feasibility.

Gate position determines the filling pattern and resulting flow orientation. Plastics typically exhibit greater strength in the flow direction. Glass-fiber-filled plastics can often withstand more than twice the level of applied stress in the flow direction as in the cross-flow direction. Keep this in mind when choosing gate locations for parts subjected to mechanical loads. When feasible:

Position gates to direct filling in the direction of applied stress and strain.

As a general rule:

To minimize warpage and dimensional problems in glass-filled plastics, position the gates to provide uniform flow orientation along the part length.
In parts with varying thickness, always try to gate into the thickest section to avoid packing problems and sink.

Avoid thin-to-thick filling scenarios. When gating must feed a thinner wall, consider adding a thickened channel or flow leader from the gate to the thicker wall sections to facilitate packing and minimize shrinkage variations.

Gates typically generate elevated levels of molded-in stress in the part area near the gate. Also, gate removal often leaves scratches or notches that can act as stress concentrators that weaken the area. For these reasons, avoid gating into or near areas that will be subject to high levels of applied stress such as screw bosses, snap arms or attachment points.

The flow length resulting from the chosen gate locations must not exceed the flow capabilities of the material. Check the calculated flow length, usually the shortest distance from the gate to the last area to fill, against the published spiral flow data for the material.

The pressure imbalance from uneven flow around long, unsupported cores can bend or shift the cores within the mold. This core shift increases the wall thickness on the side nearest the gate and reduces the wall thickness opposite the gate. Such parts require symmetric gating around the core or wall-thickness adjustments to balance flow around the core.

[ Close This Window ]