Page 65 - Injection Molding Advanced Troubleshooting Guide

48 6 Mold Texture and Polish

Figure 6.1 Steel with various blast media

Just think of a lake on a very quiet calm day and how the sun reflects itself on the
calm water. It can produce a glare that burns the eye. Now on another sunny day
but with light breeze causing soft ripples you can still get a reflection but the rip-
ples distort it. Now let’s move to a sunny day with a brisk wind causing waves; the
sunlight is even more distorted. Two things are happening. First think of the water
as the texture; the flatter and smoother it is the glossier the part, and conversely
the deeper the texture (waves) the duller the part. The other thing to note is when
the texture is deeper you are creating more surface area with the peaks and valleys
in the same amount of the 2D profile area of the part.
Textures have come a very long way the past decade or so and have some very
complex geometries. The depth, patterns, crispness, or peaks of the valleys of the
texture geometry have a big impact on part gloss. There is also a process called
microetching (see Figure 6.2), which breaks up the geometry of the texture to
make a duller part. This is just like putting waves on the smooth water surface to
reflect/distort the light.

Now that we understand how the texture geometry can affect gloss let’s go a step
deeper. Let’s consider the geometry of the surface finish on the texture itself. We
are now getting into the micro level but the surface finish of the texture geometry
is just another level of texture with flat and smooth versus peaks and valleys. This
is where blast media or polishing come into the picture, in particular in how they
affect gloss. The main two blast media used in the industry to adjust gloss are alu-
minum oxide and glass bead. Silica sand and other media are used but no matter
what blast medium you use it is the effect they have on the surface finish that
influences the gloss. Some also refer to this process as vapor honing but it is all the
same.