Page 329 - Injection Molding Advanced Troubleshooting Guide

324 33 Gloss Variation

Verify that the melt temperature matches the documented process. If the melt tem-
perature is wrong then evaluate the following:
ƒ Barrel temperatures set point versus actual
ƒ Back pressure
ƒ Screw recovery rate

33.3.2„ Gloss Variation Troubleshooting Mold Issues

One of the biggest impacts on gloss levels of a molded part is the mold itself. The
main factors that impact the gloss level are:
ƒ Mold surface finish/texture
ƒ Cooling
ƒ Venting
ƒ Inconsistent wall stock

33.3.2.1„ Mold: Surface Finish/Texture
One of the biggest contributors to the gloss of a molded part is the mold surface
finish. A sand-blasted mold surface will not produce a high-gloss piano black gloss
level no matter what is done to the process.
There is a wide range of finishes that can be used for an injection mold. They in-
clude:
ƒ Polish
ƒ Texture
ƒ Sand/glass blast
All three of the above options also have many different levels that can impact the
part appearance. For example, a polished surface can be polished to a variety of
gloss finishes.

If nonuniform gloss is a problem on a part, the surface of the mold should be in-
spected for areas of variation on the surface. To effectively do this a strong light is
often needed to avoid shadows on the surface. Many times upon inspecting the
mold surface, areas of worn texture or buildup will be visible. Sometimes a thor-
ough cleaning of the mold surface will eliminate the variation and other times the
mold will need to be sand blasted. Inspecting the mold surface can eliminate a lot
of troubleshooting time because if the gloss problem is on the mold surface, pro-
cessing will not resolve it. Buildup can also be an issue on polished surfaces (see
Figure 33.3).