Runner cross-sections - Diameter

The diameter of the runners influences the temperature of the melt in the runners and thereby the quality of the product and the amount of material waste.

Effects of diameter

A small runner diameter causes shear heating in the runners so the plastic temperature is higher in the runners than in the barrel. Higher melt temperatures reduce residual stress levels and the tendency of parts to warp, but high barrel temperatures can cause degradation of the material.

To minimize material waste and decrease the barrel temperature required, design the runners with a small cross-sectional area.

Note: Change made in the diameters of runners should be gradual. Avoid creating a large difference in size between the runner diameter and the gate diameter, or the gate diameter and the part surface thickness. Sizeable changes in thickness at these intersections can cause the following molding problems.

• Rapid changes in flow resistance
• Flow instabilities
• Increased injection pressure