Part Details Review
| Uneven Filling |
We have been taught to maintain uniform wall thickness in all our injection molding applications. By and large the teaching is true, but there are exceptions. Sometimes, it makes sense to purposely introduce non-uniformity to assist the melt to fill and pack uniformly.
A center-gated cylindrical part of uniform wall thickness fills evenly, but a center-gated rectangular or square shaped part does not. Consider a square box-shaped part of constant wall thickness with a center gate as shown in figure. Due to different flow lengths created by the part geometry, a radial flow pattern originating from the center of the base, will reach the corners later than it reaches the sides.
Ideally, melt should flow from the gate to all extremities at the same time achieving a balanced fill. A balanced fill avoids the overpack / underpack situation that could cause shrinkage differential in the part, leading to warpage and increase undesirable residual stresses.
Locally increased wall thicknesses, known as flow leaders or internal runners, help equalize the pressure drops through different flow lengths in the part and avoid the filling and packing imbalance. To avoid shrinkage and warpage problems when using amorphous resins, limit the flow leader thickness to 1.25 times the nominal wall. For unfilled semi crystalline resins, limit it to 1.15 times the nominal wall.
When flow leaders can't be incorporated in parts to achieve balanced flow for some reason, flow restrictors can be used to retard the flow of the faster moving melt. See the topic on picture framing for an example using flow restrictors.
