Page 299 - Injection Molding Advanced Troubleshooting Guide

292 30 Flash

30.3.2.8„ Mold: Cavity Balance
If a multicavity mold is not well balanced a common side effect is that some cavi-
ties flash whereas others are under-packed. If all cavities are not balanced for
filling and packing it will be very difficult to find a process window that produces
acceptable parts in all cavities.
The normal industry standard is that all cavities should be balanced within 3% for
a critical part and a maximum of 5% for noncritical parts. The part requirements
for dimensional characteristics as well as performance requirements will dictate if
the balance should be held to 3% or if 5% is allowable.
Family molds will complicate matters even more because the amount of variation
between cavities is driven by the overall size and shape difference between the
cavities. Family tools can create a great deal of flash problems due to the inherent
cavity imbalance.

For more information on cavity balance see Chapter 12.

30.3.3„ Flash Troubleshooting Machine Issues

Some flash causes that are machine related include:
ƒ Tonnage
ƒ Clamp parallelism
ƒ Core pressure
ƒ Mold size
ƒ Toggle wear

30.3.3.1„ Machine: Tonnage
Is the molding machine set to the correct tonnage? Is the machine actually achiev-
ing the desired tonnage? These are a couple of key questions to ask when experi-
encing flash. Some machines have tonnage settings that can be adjusted and as
such can be set at a too low value for a given mold.
Clamp tonnage requirements are based on many factors including:

ƒ Material type
ƒ Flow length
ƒ Wall thickness
ƒ Cavity pressure
ƒ Gate quantity and location
All of the above factors interact to determine the required clamp tonnage per
square inch of projected area. The projected area is the square area of the part that