In troubleshooting bubbles, many molders make the mistake of guessing at what the bubbles are, and then immediately start adjusting process parameters to eliminate them. I urge you to resist the temptation to start tweaking, and start by defining what the bubbles actually are.
There are only two possibilities:
1. Trapped gas, which includes air, moisture vapor, volatiles from the resin, or decomposition gases from the polymer or additives. 2. Vacuum void.
One source of bubbles that is rather odd is the venturi effect.
Where and when do the bubbles show up? Examine the flow pattern of each part to see if the plastic flow front is coming around on itself, or if there is a hesitation of the flow front in filling a thin section of the part. Are the bubbles always in the same area? If so, that means there is a fixed place from which the bubbles are coming. Note if there is a race-tracking effect or jetting that can cause air to become trapped in the polymer.
Check ribs or any projections off the nominal wall. If they are short it means there is air trapped in that area and it is being pushed out to form bubbles when the rib is filled. Sometimes you actually see a trail of bubbles coming from this projection. Do the bubbles only appear after the part is 85% full? If so, it could be a venting issue. Check the vents.
One source of bubbles that is rather odd is the venturi effect. There are several possibilities for a venturi effect to suck air into the melt flow: ribs, ejector pins, poor mating of nozzle tip to sprue bushing, nozzle misalignment, and separated plates in a hot runner. These are much harder to detect but as you rule out other sources, the tool must be checked. Apply bluing agent near the hot-runner drops and on plate mating surfaces, being careful not to allow any onto the flow path. If the bluing agent shows up on startup, you have found the source of the problem. Another common source of bubbles is too much decompression, especially in hot-runner molds.
Yet another source is the screw, more specifically the rear zone or feed section. General-purpose screws with L/D of 18:1 or lower can be the culprit. Try using a lower rear-zone temperature and/or higher backpressure. Another solution may be to pull a vacuum on the mold just before injection.
ABOUT THE AUTHOR:
John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties
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