Page 66 - Injection Molding Advanced Troubleshooting Guide

6.1 Mold Texture 49

Figure 6.2 Magnified micro texture

When acid etching of a cavity is complete, to create the texture a blast medium is
used to clean up the surface and achieve the desired gloss for the specified gloss of
the plastic part. Aluminum oxide is an abrasive, basically a rock with sharp edges
creating peaks and valleys on the surface finish of the texture geometry, which will
produce a duller finish on the plastic part. There are numerous sizes of aluminum
oxide, referred to as grit size, that impact the depth and surface finish also. Glass
bead is round and smooth like a marble, and will produce a smoother finish on the
texture and a glossier part. So the aluminum oxide surface finish on the texture
will provide the dullest gloss on the plastic part and the glass bead surface finish
on the texture will provide the highest gloss with media blasting.
The two media can be combined at different ratios to achieve any gloss between the
low and high gloss they produce alone. To achieve the mid-range between the two
media you will need to have a 1 : 2 ratio: one part aluminum oxide and two parts
glass bead. The aluminum oxide has a bigger impact on the surface finish than the
glass bead, which is why the ratio is not 1 : 1. Various blends with ratios that go
from 1 : 1 to 12 : 1 are not uncommon. It all depends on what type of gloss you are
trying to achieve. Typically try to use three media variants: lowest (aluminum
oxide), mid (1 : 2 ratio), and high (glass bead) gloss. When the gloss is critical and
cannot be achieved with these three go with what we refer to as custom blends/
ratios in between the three.