Page 261 - Injection Molding Advanced Troubleshooting Guide

252 26 Dimensions

26.3.4.1„ Material: Filled Material
A filled material will be more dimensionally stable and experience lower overall
shrink than will an unfilled material. Experimentation with a filled version of the
material may yield better results and produce a dimensionally acceptable part.
This potential will require a great deal of effort between the molder and the cus-
tomer to verify overall impact and suitability to the application but it may be the
only choice other than building a new mold.

26.3.4.2„ Material: Nucleating Agents
Addition of a nucleating agent to a semi-crystalline material will cause the mate-
rial to crystallize faster, which can reduce some of the shrinkage of a semi-crystal-
line material. Use of a nucleating agent may impact the overall physical properties
of the material so care should be taken to evaluate the overall impact of running
the process with a nucleated material.
Various additives to the material such as glass and mineral will also create a nucle-
ating effect on the material. If the filler content is varying the nucleating impact
may also be varying and impacting the dimensions of the part.

26.3.4.3„ Material: Type

It is important to understand that at a basic level the difference in shrinkage be-
tween an amorphous material and an unfilled semi-crystalline material can be
very large. In some cases, an unfilled semi-crystalline material may shrink 4–5
times the amount of an unfilled amorphous material. See Table 26.2 for a basic list
of material types.

Table 26.2 Basic List of Material Types

Amorphous Semi-crystalline
Polycarbonate PPO (Acetal)
PMMA (Acrylic) PET
ABS Polypropylene
Polystyrene Nylon
ASA PBT

The material type must be planned for ahead of time and the appropriate shrink
factors for the chosen material must be used (STOP, Think). If a mold is cut to the
wrong shrink factor it can be extremely difficult to get the part to the required
dimensions. Sometimes a material change will occur during process development,
which will result in trying to achieve dimensions even though the wrong shrink
factor was used. This makes planning for shrink very difficult.