Several years ago a customer transferred an old single cavity mold to our facility. The part was a very simple rectangular ring. It had two sub-gates, one at each end of the part on the short side of the rectangle. The material was glass filled and made a very strong part.

We made our first sample submission and were approved for production immediately. The next day we delivered the first parts for their order. A couple of hours after the parts were delivered I received a call from our customers General Manager thanking us for the very fast service, then he went on to say none of the parts were usable because they did not pass “the hammer test”. The first words out of my mouth were “what is the hammer test? And why wasn’t this test requirement talked about before this”. I drove over to the customer’s plant and observed the hammer test. It was quite simple; the part was set on a metal table so it stood up resting on its long axis. A technician then proceeded to beat the daylights out of the part with the pointy end of a steel masonry hammer.

The part broke in the very middle of the long side. I examined the part and asked if they had this problem before. The general manager said this happens all the time and it is a major problem. As it turned out the hammer was used to seat a metal piece between two of the plastic parts to make the assembly. The metal pieces were to stiff to assemble by hand so the hammer was used.

My further examination showed that the failure had occurred at one of the weld lines in the part. Weld lines occur when plastic flows around a core and then joins back together. The solution was very simple, the problem was that no one looked at the way the part was gated and how the part would be assembled. I told the GM that I would be back that afternoon with parts that would not break.

I had one of our mold makers take the mold apart and we simply blocked off one of the two gates. This meant that all the material would flow through one gate and the single weld line that resulted from the new flow pattern would be on the short axis. The old flow pattern produced a weld line in the middle of both of the long sides. We tested the parts with our own hammer and could not produce a failure. I immediately took the newly re-gated parts to the customer and asked them to test them with their hammer. They could not break the parts either. I took the first parts back and scrapped them and we delivered the new parts thenext morning.

The customer never had a weld line failure again. We had solved an assembly problem that had existed for years, saving time and money on every assembly.