HTV Silicone Rubber: Why Roller Defects Come From Handling

Are the Colored Specks on Your Silicone Roller Really Dirty Compound?

12,000 pcs shipped. 315 pcs sampled, a 2.25% reject rate. Three defects, all from the bench.

The compound was clean. Leftover material from the previous run stuck to the new parts. The bulge and the shiny marks came from handling too.

The customer sent a quality improvement notice. They wanted the cause by September 22 and visible improvement by September 25. The clock was tight. The investigation was not long.

Why Does Leftover Compound End Up on the New Part?

315 pcs sampled, a 2.25% reject rate. The specks the customer circled had the wrong color, and they sat in random places.

They came from the previous run. The press and the loading plate were not cleaned, so leftover compound mixed into the new batch and cured onto the roller surface.

Changeover cleaning is the step everyone skips

We took the notice and went to the warehouse first. The returned parts had the specks, right where the red circles were. Then I walked out to the press, and the cause was obvious. The operator had finished the previous part number without cleaning the press or the loading plate. The leftover compound was still sitting there.

HTV silicone rubber cures at a high temperature. Whatever is left from the last shot is already fully cured. Its color, its hardness, and its flow are nothing like the fresh compound. The operator loads new compound on top of it. Both get pressed. Both get cured. The roller comes out with those spots on the surface. You cannot pick them off, because they are not dirt sitting on top. They cured into the part. I covered the basics of this material in another post on HTV silicone.

This was not our first dirt case. We had a batch with a very high dirt rate before, and that fix also started at changeover cleaning. The logic is the same.

Why Does the Roller Bulge on One Side?

The bulge was the clearest one. You could feel it with your hand. The side carried an extra ring of material.

The load was not centered. The press pushed harder on one side, the compound squeezed that way, and the side came out bulged.

Load position decides where the compound goes

In compression molding, the compound flows inside the cavity. Load it in the center, and it spreads evenly. Load it off center, and one side gets too much while the other side gets too little. The excess pushes out through the parting line. It cools there, and you get a bulge. The short side may come out with missing material.

A bulge is not just ugly. It pushes the local dimension outside the tolerance band. ISO 3302-1 sets out how dimensional tolerances for rubber products are classed. The customer puts a caliper on the side thickness, and out of tolerance is out of tolerance. Looks do not enter into it.

Loading and unloading is where molding goes wrong most often. I listed a few more of these in this post on molding mistakes.

Is That Shiny Mark a Polished Finish?

The shiny spots followed a pattern. They all sat near the edge. They looked polished. Nobody polished them.

The air gun tip touched the edge when the operator picked the part up. Rubbing left the mark. One puff, one touch.

What an air gun can do to a part

Air guns do a lot of work on a molding floor. They blow off compound, blow off crumbs, blow parts dry. Operators keep one in hand, and the tip ends up close to the part. The tip is usually metal or hard plastic. When it touches silicone that has not fully cooled, it rubs. Silicone that gets rubbed goes glossy, and that patch no longer matches the surface around it.

These marks cannot be reworked. Polishing makes them brighter and wider. So they can only be judged as appearance defects. And they need to be told apart from three other things that also look shiny: mold polish lines, oil film, and bloom.

Why Were the Three Fixes So Small?

The customer gave us days, not months. A big rebuild was impossible. It was also unnecessary.

A changeover cleaning routine, load position lines, and a guard on the air gun. Each one targets a specific motion.

Make the right action the only action

First, we retrained the operators on changeover cleaning. The press does not run the next part number until the shift leader signs off. This one does not rely on memory. It relies on a signature.

Second, we painted position lines on the bench. The preform goes in the middle of the lines, and so does the hand. Before, the operator judged by eye. Now there is a reference.

Third, we fitted a guard on the air gun tip. This is the most direct fix. It puts a physical barrier between the tip and the part. The operator can keep the same motion. The tip still cannot reach the surface.

Three small changes, but they cover people, method, and machine. Training sets the habit. The lines set the method. The guard sets the physical limit. The third one is the most reliable, because it does not depend on anyone remembering anything.

From September 20 to 22, the customer's IPQC came in and sampled the parts. None of the three defects came back, so we passed. What the customer asked for is the loop that ISO 9001 describes: find the cause, take action, then verify the result. The standard does not tell you which tool to use. It does require you to close the loop.

Schlussfolgerung

Three defects looked like three separate problems. They came down to one cause: handling details that were never standardized. A 2.25% reject rate means only a handful of parts out of 315. The volume was small. The cost was not. The customer held the whole batch, and the rework, the re-inspection, and the new paperwork all still had to happen. I have worked in this industry for fifteen years, and I have seen a lot of notices like this one. The presses get better. The molds get more precise. Then one batch fails because someone skipped one wipe of the loading plate, or touched the part once with an air gun. We delivered the improvement before September 25, and IPQC confirmed it. But the lesson for me is not that we passed. The lesson is that patrol inspection and operator training have to nail these points down, before a customer opens a notice. If you mold small parts like silicone rollers, check three things on the floor today. Is the plate wiped at changeover. Are there position lines on the bench. Is the air gun tip guarded. If you need help, you can talk to our team.

FAQ

Q1: How do you tell compound contamination from leftover material on a silicone part?
A: Look at the distribution and the shape. A compound problem shows specks across the whole batch, with a consistent color. Leftover material sits in random places, varies in shade, and has a clear edge. It also will not pick off, because it cured into the part. The fastest check is the changeover record. Compare the color of the specks with the previous part number.

Q2: Does a bulging silicone roller always mean the mold needs work?
A: Check the load first. An off-center preform pushes compound to one side, and the part bulges. That needs no mold work. If the load is centered, the weight is right, and the bulge still lands in the same place every shot, then inspect the cavity and the parting line.

Q3: How clean is clean enough at changeover?
A: Every surface you can see, and the ones you cannot. That means the loading plate, the cavity, the parting line, the press table, the transfer trays, and the tools. The pass criterion can be simple. Wipe with a white cloth. If the cloth shows no color transfer, the surface is clean.

About the Author & Dibo Tech

My name is Kunfa Ye, but friends call me Jeff. I was born in 1986, the year of the Tiger. In 2011, my brother Autumn Ye and I co-founded Dibo Tech, rooting ourselves in the silicone rubber industry for fifteen years. Building from scratch, we navigated industry cycles and learned the true rewards of hard work. We have always built our business on technical expertise and integrity.

The silicone industry touches every corner of modern manufacturing. I deeply believe that behind every product lies trust, and the essence of business is mutual gain. Over these years, my proudest achievement is not personal financial independence, but helping our clients optimize supply chains and boost production efficiency. Their success is far more rewarding than personal milestones. Today, I aim to give back by sharing insights, answering engineering questions, and connecting resources.

Brand Name: Dibo Tech Slogan: Dibo Technology Silicone Manufacturing Specialists Website: https://samxine.com/ Our Mission: Based on strong technical capabilities and quality first, we are committed to innovative and eco-friendly silicone rubber production, providing reliable silicone solutions while fulfilling social responsibilities for a sustainable future.