Page 117 - Injection Molding Advanced Troubleshooting Guide

12.3 Shear 103

„ „12.1„ Flow Length

A balanced runner is a critical starting point for any multicavity mold. A tradi-
tional runner system will have the same linear flow length from the material inlet
(sprue or tip) to each cavity. Flow length should intentionally be designed to be
identical for all cavities. Figure 12.3 shows a traditional eight-cavity runner system
that resulted in unbalanced fill, even though the flow length to each cavity was
equal (see Section 12.3).

„ „12.2„ Flow Diameter

Just as with flow length it is critical that the plastic flows through the same dia-
meters of runners as it moves from inlet to cavity. Fishbone style runner systems
have been built where runner diameters are adjusted based on distance from the
inlet. This method of runner layout guarantees that the cavities will experience
different conditions.
Flow diameters that have been adjusted to try to fix something can be unbalanced,
which leads to additional problems. If the diameter of the runner varies the flow
channel also varies, which leads to variation in pressure drop, shear rate, and cool-
ing effects. These differences will result in differences in cavity fill rates, cavity
pressure, and cavity cooling. Always ensure that runners have matching diameters
for all of the flow paths. A four-cavity mold has four flow paths and all four of them
should have equal diameters at comparable locations for each cavity’s runner.

„ „12.3„ Shear

Even if a runner system is naturally balanced for flow length and diameter, molds
with eight or more cavities will still be unbalanced. Everyone who has been around
molding long enough has heard a variety of possible explanations as to why an
eight-cavity mold is running with imbalance even though the flow lengths and dia-
meters are balanced. The four inner cavities on eight cavity molds will normally fill
before the four outside cavities. This effect has been blamed on mold deflection in
the center of the mold, increased melt or mold temperature near the center of the
mold, or platen deflection, among other possibilities.