Marks on Clear Parts: Mold Maintenance Is Part of Quality

Some defects are solved in the toolroom. Some are solved on the press. This one was solved by asking a different question: not "how do we remove the marks?" but "when do they appear?"

The case: marks every morning

Bob Gray was Tooling Manager at a family-owned injection molder in rural Iowa. The plant molded clear covers for instant tickets, the kind sold at gas stations and grocery stores. The covers showed marks on the side walls every morning. The steel had to be re-polished again and again.

Tracking when the marks appeared showed where they came from: how the mold was handled during maintenance on one shift, including tools used on the polished steel.

Bob changed the maintenance practice at the source rather than continuing to polish out the damage. Everyone who handled the mold learned what careless tool use did to polished steel. The recurring marks and the daily re-polishing stopped. Read the full case study.

Why clear parts show everything

A molded part is a copy of the cavity surface. On a textured or colored part, a small scratch in the steel may never be noticed. On a clear part there is nothing to hide it. Every scratch in the polished steel is reproduced in every part, in the same place, until the steel is repaired.

That makes clear and high-polish tools unforgiving in a way most molds are not. Damage that would not matter on another job becomes a reject on this one.

Re-polishing is not free

Polishing out a mark looks like a routine repair. Done again and again, it adds up.

• The press is down while the tool is worked on
• It takes skilled time, and it has to be done well or the repair shows too
• Each polish removes a little steel, which over time can affect dimensions, edges and shut-offs
• It treats the mark and leaves the cause in place, so the mark comes back

The last point is the expensive one. A plant can re-polish a tool every day and never be closer to the answer.

The pattern was the clue

The detail that mattered in this case was "every morning". A defect that keeps a schedule has a cause that keeps a schedule.

So the useful work was not on the steel. It was tracking when the marks appeared, and comparing that with what happened to the mold beforehand. That pointed to one shift, and to the way the mold was handled during maintenance.

It is an easy step to skip. When a mark appears, the natural reaction is to fix the mark. Writing down the time, the shift and what was done to the tool in the hours before takes a little discipline, and it is what turns a daily repair into a solved problem.

Questions to ask when a defect has a schedule

• When exactly does it first appear, and when is it absent?
• What happens to the mold in the hours before it appears?
• Who handles the tool then, and with what tools and materials?
• Is the mark in the same place every time?
• Does it follow cleaning, greasing, a stuck part being removed, or a mold change?

The answers usually narrow the search quickly, because they turn "the tool keeps getting marked" into a specific event at a specific time.

Handling polished steel

The rules for a high-polish cavity are simple. The hard part is having everyone follow them on every shift.

• Nothing hard touches the cavity. A stuck part comes out with a soft tool made for the job, never a screwdriver or a knife.
• Clean and wipe only with clean, soft materials. A dirty rag carries grit, and grit scratches.
• Protect the polished surfaces whenever work is done near them, in the press or on the bench.
• Apply and remove rust preventive with the same care as cleaning.
• Write the procedure down, and make it the same on every shift.

Train everyone who touches the mold

In this case, everyone who handled the mold learned what careless tool use did to polished steel.

That is worth copying. Show people the mark in the steel and the reject it produces. Someone who has seen the damage and the result handles the tool differently from someone who has only been told to be careful. And include everyone: every shift, part-time staff, and anyone new who will work on the tool.

Maintenance is part of quality

It is easy to think of quality as something the process and the inspection department look after, and maintenance as something separate. On a clear part they are the same thing. The condition of the steel is the quality of the part, and the way the tool is handled decides the condition of the steel.

The lesson

On clear and high-polish parts, maintenance practice is part of quality. Find the cause before you polish the steel again.

If you have a defect that keeps coming back, on a schedule or not, finding the cause is the work behind production efficiency consulting. Start with a free 15-minute call.