Page 116 - Injection Molding Advanced Troubleshooting Guide

102 12 Cavity Balance

Figure 12.2 Moldflow fill plots for the tensile bar mold in Figure 12.1

A single-cavity mold will always provide the most consistent part; however, it may
not be cost effective. Multicavity molds will normally produce parts at lower cost
provided that the scrap does not become prohibitive.
During mold design it is critical to ensure that a multicavity will be balanced to
provide equal process conditions to each cavity. If a cavity is filling behind the
others the process that the plastic experiences in that cavity will be different,
which in turn will produce a different part. One of the critical challenges in multi-
cavity molding is to ensure that every cavity is as equal as possible throughout
the process. Keep in mind that this must extend to all plastic conditions that each
cavity is experiencing (fill, pack, hold, gate seal, shear, cooling, etc.).
Cavities must have the following conditions applied equally:
1. Flow length
2. Flow diameter

3. Shear
4. Cooling
5. Venting
6. Clamp pressure