Not every void wants more pressure

A dimple in the surface of a thick part is one Mold Doctor kept getting almost right. You see the low spot, or you cut the part and find a cavity where solid plastic should be, and the reflex is the same either way. Raise the pack. Hold it longer. Feed the shrink. Ask Mold Doctor and the read comes back close to that, because most of the time that is where the answer lives. So we turned up the pressure. On a lot of parts the hollow filled in and the shift went on. On a few of them the pressure did nothing at all, and on one job the hollow got worse the harder we pushed. That was the one that kept bothering me.

That read isn't wrong. When the hollow is a true vacuum void, pressure is exactly the lever. The part shrank away from itself as it cooled and left a cavity, and more pack pressure with a longer hold feeds material into that space while the gate is still open. Do that and the void closes. So the instinct to reach for the pressure is a good instinct, and most days it earns its keep. The only place it trips is the assumption underneath it, that every hollow is a pressure problem. Two of these three are hollow, and of those two, only one answers to pressure.

Split it three ways first

The first cut is the simplest one, and you can almost always make it at the bench. Is the trouble on the surface, or is it inside the wall? A sink mark is a shallow dip in the skin over something thick behind it, a rib, a boss, a heavy section. The surface went down but the plastic is solid underneath. A void and a bubble are the opposite. The skin is fine and the emptiness is buried in the middle of the wall. To tell those apart you look inside. Backlight the part if the resin passes any light and the internal cavity shows as a shadow. If it does not, cut a part in half through the low spot. Solid behind the dip means a sink. A cavity means you are dealing with one of the two internal failures, and the work moves to the next step.

Once you know the cavity is internal, one test tells the two apart, and it costs you a fresh part and a little heat. Warm the part gently, soon after it comes off the press. Watch the low spot. A vacuum void sinks further and pulls the surface in with it, because you are letting the skin move toward a space that was already under vacuum. A gas bubble does the reverse. It puffs, it swells, because the gas trapped inside expands when you warm it. Collapse in means vacuum. Rise out means gas. Location and resin back up the read. Vacuum voids favor the thick places, the ribs and bosses and heavy cores, and they show up more on the crystalline resins that shrink hard as they set. A bubble can sit anywhere the gas had to go, and it points you at what the gas was: moisture, air, something that boiled off.

Why one of them fights back

Here is what is happening when it is a void. In a thick wall the outside freezes first. The skin sets against the cold steel and turns solid while the core is still molten and still shrinking. The core wants to pull in from every side, but the frozen skin is holding the outer shape, so the shrink has nowhere to go except inward on itself, and it tears open a vacuum in the center as it cools. This is the one pressure was built for. Pack harder and hold longer so material keeps feeding the core before the gate freezes. Look at the gate too. If it is feeding thin and freezing early, it is starving the thick section it is supposed to fill, and no amount of hold will reach past a frozen gate. Get the pressure into the thick area in time and the core has no reason to pull a cavity.

A gas bubble is a different animal, and this is where the reflex goes wrong. Nothing shrank away here. Something gassed off and got caught. Usually it is moisture, resin that went into the barrel wetter than it should have, and the water flashed to steam in the melt. Sometimes it is air, sometimes a volatile cooking out of the material. The gas expands, the melt traps it, and you get a pocket that looks just like a void. Pack pressure does not fix this, and it can make it worse, because you are squeezing a part that has gas in it instead of getting the gas out. The lever is upstream. Dry the resin properly and most of these never form. Carry a little more back pressure so the screw works the gas out of the melt as it recovers. And check your decompression, the suck back you pull before the screw comes forward, because too much of it draws air in through the nozzle and makes the very bubbles you are trying to chase off. On a gas bubble, reaching for the pack is chasing the wrong thing, and worse, it hides the miss that caused it.

Where this goes wrong

A few ways I have watched this go sideways. The first is pumping pack pressure at a moisture bubble, shift after shift, watching it never quite close. It never will, and every notch of extra pressure hides the real problem, which is that the resin came in wet. The second is too much decompression. Set the suck back too deep to stop drool and you pull air into the nozzle and start making bubbles without meaning to. The third is the easy one to trip on, reading a surface sink as an internal void, and hunting for a buried cavity when the plastic behind the dip was solid the whole time. And one more, for the resins filled with glass. Back pressure is a real tool against gas, but on a filled resin you go gently, because working the screw too hard breaks the fibers and trades one problem for a weaker part. More is not the move. Enough is.

So when the description that comes in is a dimple or a hollow, Mold Doctor runs the split before it reaches for the pressure. Surface or internal. If internal, collapse or puff. It would rather know which of the three it is looking at than hand you a pressure setting that only helps one of them. Mold Doctor takes a photo of the defect along with your process settings, estimates the likely causes, and tells you what to adjust, and your shop's own history builds up as you log each run.