Hot Runner Underfill: When to Stop Adjusting the Process

When cavities come up short, the first move is almost always to the process. More melt temperature, more injection pressure, a faster fill, a longer hold. Often that is the right call. But when the same cavities are short week after week, and every change works for a shift and then fades, the process is telling you something: it is not the problem.

The case: two years on one mold

Bob Gray was Tooling Manager at a family-owned injection molder in rural Iowa. An 8-cavity hot-runner mold making water connectors for residential PEX lines kept producing underfilled cavities. Process engineers had worked on it for two years. The mold was repeatedly torn down and checked, with no lasting fix.

Bob moved the investigation from the process to the hot-runner manifold. He had the manifold sent to a specialist in Northern Illinois for cleaning. When several cleaning cycles did not fix it, he had the channels inspected with a video scope.

The scope found it. A ceramic insulator, left in the channels during a cleaning two years earlier, was blocking the flow. With the obstruction removed, all 8 cavities ran at full capacity, and two years of recurring tear-downs ended. Read the full case study.

Why process changes can hide a tool problem

A restriction in the flow path can be partly overcome from the press. Raise the temperature or the pressure and more plastic gets past it. The parts look better, the job ships, and the change becomes the new setup.

The cost shows up somewhere else. The cavities that were filling well are now overpacked or flashing. The cycle gets longer. The process window gets narrower, so normal variation in material or ambient conditions is enough to bring the short shots back. Then the process is adjusted again.

In a multi-cavity tool there is a simple test of logic. Every cavity sees the same machine settings. If some fill and others do not, the difference between them is not in the settings. It is in the tool: the path the plastic takes to each cavity.

Signs the problem is in the tool

• The same cavities are short every time, whatever the settings
• A process change helps for a while, then the defect returns
• The process window keeps getting narrower
• The problem started after a repair, a cleaning or another maintenance event
• A short-shot study shows the cavities filling out of balance

A short-shot study is worth the time it takes. Reduce the shot size until no cavity fills completely, then compare the parts. Cavities that lag behind the others every time point to a restriction on their side of the system.

Where to look in a hot-runner system

• Manifold channels: degraded material, restrictions, and anything left behind after service
• Nozzles, tips and gates: wear, damage and partly blocked orifices
• Heaters and thermocouples: a zone that reads correctly on the controller but is not heating the way it should
• Valve pins, where the tool has them: timing, travel and sticking
• Vents at the cavities that fill last

None of these can be judged from the setup sheet. They have to be looked at.

Inspect, do not assume

The most useful detail in this case is what happened after the manifold was cleaned. Cleaning was a reasonable step, but it rested on an assumption: that whatever was in the channels would come out. It was repeated several times before the question changed from "what should fix this?" to "what is actually in there?"

A video scope answered that question. The same thinking applies to any tool that has been torn down more than once for the same defect. A tear-down without a specific question to answer is just downtime. Before the mold comes out of the press, write down what you expect to find, and decide how you will check for it. If the answer is "we will have a look", the tool will probably go back in the way it came out.

It also helps to keep a record of every service event on the tool: what was done, by whom, and what went in and came out. In this case the obstruction dated from a cleaning two years earlier. A service record makes that kind of link much easier to find.

What a recurring defect costs

The case does not come with a dollar figure, but any molder can do the sum for their own tool. Add up the scrap, the capacity lost on cavities that are short or blocked off, the press time lost to each tear-down, and the hours of the engineers and toolmakers involved. Then multiply by how long it has been going on. A problem that is "managed" for two years is usually far more expensive than the investigation that would have ended it.

The lesson

When process changes keep failing, stop adjusting the process and inspect the tool. Thorough investigation beats repeated tear-downs.

If you have a mold that keeps coming back with the same defect, that is the kind of problem production efficiency consulting is built around: finding the cause in the tool, the process or the maintenance practice, and fixing it once. You can start with a free 15-minute call.