Page 21 - Plastics News July 2021

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John Bozzelli
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n troubleshooting bubbles, many molders make the change form. If it is a gas bubble, the gas will warm up
Imistake of guessing at what the bubbles are, and and expand, raising the surface, and often will pop as the
then immediately start adjusting process parameters to part,surface softens. If there is no air in the bubble and
eliminate them. I urge you to resist the temptation to start it’s a vacuum void instead, the bubble will collapse due to
WZHDNLQJ DQG VWDUW E\ GHÀQLQJ ZKDW WKH EXEEOHV DFWXDOO\ the atmospheric pressure pushing on the softened surface
are. There are only two possibilities: of the part. Now you know more of what the problem is.
There are some conditions required for this test to work.
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larger in diameter and make sure the part is no more
than 4 hr old. It is possible for the bubble to start out as
a void, but over time air migrates through the plastic and
the void becomes a gas bubble.

That is perhaps hard to believe, but during my last public
seminar the class and I were treated to some striking
evidence.
Check out Figs. 1, 2, and 3. Figure 1 is the part before
heating. Figure 2 is a part tested a few minutes after
1. Trapped gas, which includes air, moisture vapor, molding and Fig. 3 is a part tested 16 hr after molding. In
volatiles from the resin, or decomposition gases from the )LJXUH \RX FDQ VHH WKH EXEEOH LW ORRNV OLNH D JDV EXEEOH
polymer or additives. 2. Vacuum void. right? Most would assume so. Figure 2 shows the part after
testing, and you can now see a depression or sink. Figure
It is important to determine which type of bubble your 3 is a part tested after 16 hr, and you now see the bubble
part has, and what the root cause might be. Determining expanded. Despite appearances, it was not originally a gas
bubble type will allow you to pinpoint the source and or air bubble but a vacuum void, as the freshly molded
determine your next course of action to eliminate the part provided a sink upon warming.
problem. How do you test to determine if it is gas or a
vacuum void? Many claim (as I once did) that you can tell Trapped Gas
by the shape, location, or some other characteristic of Let’s start our troubleshooting discussion assuming your
the bubble or bubbles. But you can be fooled easily by this test proved that it was indeed a gas bubblethat is, the
approach. There is a simple test you should use instead. It
takes less than 15 minutes but requires a little patience to
conduct.It’s important to determine which type of bubble
your part has, and what the root cause might be.
Test your part by gently warming the part area containing
the bubble or bubbles until it softens. I emphasize gently
because of the tendency of some operators to pick up the
nearest torch and aim it at the part. Plastics do not quickly
transfer heat through the nominal wall, so our friend with
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gun or something similar. Then as you gently warm the
area of the part where the bubble is, the bubble should

21 July 2021 Plastics News