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18.3 Bubbles Troubleshooting 171
18.3.3.3 Machine: Screw Design
If the screw in the machine is of wrong design for the material being molded, it is
possible to have gas entrapped in the melt stream. A key to verify the screw design
is the compression ratio which is the ratio between the feed flight depth versus the
metering flight depth. If the machine has a low compression ratio screw (under
2.5) there may be issues trying to run materials such as PMMA, acetal, nylon, etc.
Note that most machines will have a medium compression ratio screw of between
2.5–3, which will be adequate in most cases. If the machine has a specific low com-
pression screw it could be an issue. Low compression ratio screws are commonly
used for molding materials like PVC.
18.3.4 Bubbles Troubleshooting Material Issues
Material-related issues with bubbles may come from the following:
Moisture content of material
Transparent materials
Contamination
Unmelts
18.3.4.1 Material: Moisture Content
If a plastic material is molded wet the moisture will turn into gas as the material is
heated in the barrel. As this gas is formed in the melted plastic it will tend to form
bubbles. These bubbles can be trapped within the molded part resulting in defects.
All hygroscopic materials must be adequately dried prior to processing.
Moisture content being too high is probably the leading overall cause of bubbles in
molded parts. When troubleshooting bubble problems one of the first items to
check is the moisture content of the material (see Chapter 9 on drying).
18.3.4.2 Material: Transparent Materials
Transparent materials are not necessarily more prone to experiencing bubbles but
rather it is much easier to see bubbles in a transparent part. Clear, hygroscopic
materials like polycarbonate, clear polyesters, and PMMA will display bubble de-
fects if molded wet.
When molded, opaque materials can have bubbles that will not be noticed. Due to
this fact, parts molded with opaque materials will tend to have less scrap for bub-
ble defects. There may very well be bubbles present that are not detectable. Gener-
ally speaking, because bubbles will not be visible in an opaque material, parts will
not be scrapped due to bubble problems; however, they may be suffering the same
root cause and end up being scrapped for splay.
