Page 180 - Injection Molding Advanced Troubleshooting Guide
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170 18 Bubbles
more in-depth evaluation to resolve as a root cause. To determine mismatch areas
the mold may need to be disassembled and blued up to determine potential areas
of concern.
18.3.3 Bubbles Troubleshooting Machine Issues
The main issues that can create bubbles that are related to the machine are as fol-
lows:
Machine performance
Crack in feed throat leaking water
Screw design
18.3.3.1 Machine: Machine Performance
See Chapter 8 on machine performance.
18.3.3.2 Machine: Crack in Feed Throat
If there is a crack in the feed throat of the machine it is possible for the feed throat
cooling water to enter the barrel along with the plastic pellets. As this water is in-
troduced to the plastic it will turn to steam as it is heated, and the steam gas will
then be trapped in the melted plastic which can cause gas bubbles in the part.
To check for feed throat cracks the barrel must be emptied and the hopper removed
from the feed throat. Use an inspection mirror to look in the feed throat for any
water drips. If a crack is detected it will need to be fixed to resolve the scrap issues.
Extreme caution should always be used when examining a feed throat because
pressure in the barrel can blow back through the feed throat. Figure 18.2 shows an
inspection mirror for examining a feed throat. A face shield is another safety pre-
caution to be used when dealing with feed throats. Never look directly in or stand
directly above the feed throat.
Figure 18.2 Inspection mirror for safely examining a feed throat
