Mold Concerns
| Size and Shape |
Runner design influences part quality and molding efficiency. Overly thick runners can lengthen cycle time needlessly and increase costs associated with regrind. Conversely, thin runners can cause excessive filling pressures and related processing problems. The optimum runner design requires a balance between ease of filling, mold design feasibility, and runner volume. Round cross-section runners minimize contact with the mold surface and generate the smallest percentage of frozen layer cross-sectional area. As runner designs deviate from round, they become less efficient (see figure). Round runners require machining in both halves of the mold, increasing the potential for mismatch and flow restriction. A good alternative, the "round-bottomed" trapezoid, requires machining in just one mold half. The runner system often accounts for more than 40% of the pressure required to fill the mold. Because much of this pressure drop can be attributed to runner length, optimize runner length.
Runner thickness has a direct effect on filling pressure, cycle time, packing, and runner volume. The optimum runner diameter depends on a variety of factors including part volume, part thickness, filling speed and pressure, runner length, and material viscosity.
- For sufficient packing, make runners at least as thick as the part nominal wall thickness.
- Increase runner thickness for long runners and runners subjected to high volumetric flow rates.
