Eliminating Pin-Hole Burn Marks in a 32-Cavity Connector Housing Mold

Introduction

A Tier-1 automotive connector supplier was running a 32-cavity housing mold with a 16-pin terminal layout, molding in glass-filled PBT. The part is a workhorse of modern vehicle electronics — a terminal housing that has to stay clean inside all 16 of its pin holes, cycle after cycle, at high volume. But on this program, the inside of those holes was the problem. At any given time, 12–18% of the 32 cavities were producing parts with burn marks on the pin-hole walls.

The root cause was geometry. The pin holes are blind cavities at a 5:1 depth-to-diameter ratio, so the air trapped at the bottom of each hole could not escape during fill. As the melt front advanced, that air compressed and carbonized against the core steel, leaving dark burn marks on the pin-hole walls. With an annual volume of 1.8M shots, an 18% defect rate and a 6.2% scrap rate meant real money leaving the line every shift — and under IATF 16949 traceability, every batch had to be fully inspected before it could ship.

The supplier came to DieStrike for a venting solution that would clear the trapped gas at the moment of fill — without flashing the part, without adding cycle time, and without turning the mold into a different tool. They had tried opening the mold, re-polishing the cores, and dialing back fill speed; every workaround traded one problem for another. What they needed was a change to the core itself.

The Snapshot

Here is the project at a glance — the part, the material, the tool, and the numbers that mattered.

• 16-pin connector housing molded in glass-filled PBT across 32 cavities.
• Blind pin holes at a 5:1 depth-to-diameter ratio trapped air that burned the core steel.
• 12–18% of the 32 cavities produced burn-marked parts; the program ran at an 18% defect rate and 6.2% scrap.
• Surface-vented core pins cleared the gas — defect rate down to 0.3% at 1.8M shots/year.

The Challenge

Injection molds fail in two ways: slowly, through wear, and suddenly, through a defect that lives in a specific geometry. This program had the second kind, and it was hiding inside the deepest feature on the part. A blind hole at a 5:1 depth-to-diameter ratio is effectively a dead end for air. The melt front flows around the core pin and seals the hole entrance first, trapping the air column at the bottom. With no vent path through a solid core, the air has nowhere to go except to compress.

Compressed air heats up, and hot, compressed air against steel is exactly the recipe for carbonizing. The result was burn marks on the pin-hole walls — a defect in a location that is anything but cosmetic, because a terminal housing with burned pin holes is a reliability risk at the connector assembly. At any given time, 12–18% of the 32 cavities produced burn-marked parts. That variability was the second problem: the mold was not failing all at once, it was failing unpredictably, cavity by cavity, cycle by cycle. Operators could not predict which cavities to watch, quality could not sample around the defect, and the maintenance team could not point at a single root cause to fix.

The commercial impact compounded the technical one. The program ran at an 18% defect rate with 6.2% scrap at an annual volume of 1.8M shots. Every percentage point of scrap is a real cost, and at that volume a stable double-digit defect rate is a line item, not an inconvenience. On top of the yield loss, IATF 16949 traceability forced full inspection of every batch — burn marks on the dark walls of a 5:1 hole are exactly the kind of defect automated vision struggles to catch reliably, so the inspection burden alone was a bottleneck.

How We Solved It

The fix had to let air out at the exact moment of fill without letting resin follow it. That is what a surface-vented core (SVC) pin does. Instead of a solid pin that seals the hole entrance, the vented core carries a controlled vent channel along its length — a precisely sized escape path for the trapped gas that opens at the moment of fill and closes before the melt can flash into it. We manufactured the pins in hardened SKH51, so the solution carried its own wear resistance rather than trading one defect for another. The channel is sized and positioned against the part geometry and the fill behavior of glass-filled PBT — close enough to the deepest point of the hole to catch the gas before it compresses, tight enough that the melt never follows it out.

The manufacturing detail is where vented pins win or fail. These were Ø1.2 mm vented cores, ground to ±0.002 mm, with the vent geometry EDM'd in-house and the pins heat-treated to HRC 62. At that diameter, the vent channel is a feature you can barely see — it has to be cut precisely, positioned precisely, and held precisely, because the difference between venting and flashing at Ø1.2 mm is measured in microns. Grinding the pin body to ±0.002 mm and cutting the vent geometry by EDM in our own shop is what made the channel repeatable across all 32 cavities. Venting layout of this kind is exactly the decision we work through in our mold design and DFM review before steel is cut.

We also planned for the material chemistry. Glass-filled PBT is aggressive at the core surface, so where the resin chemistry also demands corrosion resistance, we offered an S136 ESR variant of the vented core — same vent geometry, same precision, added corrosion margin. And because a vent channel is a wearing feature, we added the vent condition to the 50,000-shot PM schedule with a written report on every service. Maintenance becomes part of the solution, not an afterthought — the same discipline we apply across our mold-making operation.

The Results

The results show up on a P&L, not just on a mold shop report. The defect rate fell from 18% to 0.3% — a 98% reduction — and scrap dropped from 6.2% to 0.8%. At 1.8M shots per year, that scrap recovery alone pays for the pins many times over. And because venting is a feature of the pin rather than an added process step, cycle time stayed exactly where it was: zero loss.

The deeper win is throughput. All 32 cavities are back in service, running burn-free at the full 1.8M shots/year the program was designed for. The unpredictable cavity-by-cavity failure is gone, and with it the full-batch inspection bottleneck — the quality team trusts the pin now, not just the inspection. Yield planning became honest again: the mold finally runs at the rate the capacity plan assumed, and the 16-pin holes that used to be the first place a defect appeared are now the first place a clean part comes out.

Why It Held

Venting solutions are judged the same way every mold fix is: does it hold after the novelty wears off? This one held for three reasons. First, the vent channel is a controlled, machined geometry — cut by EDM in a pin heat-treated to HRC 62, with the Ø1.2 mm body ground to ±0.002 mm. It vents at the moment of fill and does not flash, run after run — and because the geometry is machined rather than improvised, every pin in the mold behaves the same way. Second, the pin steel carries the wear: hardened SKH51 in the standard build, with an S136 ESR variant where resin chemistry demands corrosion resistance. The fix did not move the wear problem somewhere else; it put the wear where the steel was chosen for it.

Third, the program treats venting as a serviceable feature. The vent condition is on the 50,000-shot PM schedule with a written report, so the channel is measured, cleaned, and documented before it can degrade into a defect. That combination — precision geometry, the right steel, and a maintenance schedule that is actually followed — is why 1.8M shots/year has been sustained across all 32 cavities, burn-free, and why the defect rate has stayed at 0.3% rather than creeping back up.