When any of the four variables is compromised the tool should be flagged as incapable not due to statistical measures, but due to the fact that process control has been compromised. In other words, if you have to pack the part with just enough pressure to keep from over-packing it (a tooling or design issue) and those two pressures are within a narrow range from each other, it is certain that you will make a short and once it is made it is almost impossible to keep it from getting out the door.

If you have to fill the mold at an extremely slow rate, it is certain that you will make vastly different parts if not shorts and flash along with it. This depends on the pressure loss, but if your pressure loss is large and you are on the wrong side of the shear rate curve, you will get a short or flash. However, the short is more likely and flash would depend on the process window.

Now it is time to explain to the customer that it is highly likely that a reject part will be made and shipped. It isn’t that you want to or don’t care; it’s just inevitable if the process is weak. Of course, they don’t like to hear that and they hate it even more when it happens, but when you didn’t build the mold and weren’t part of the qualifying process, it is up to you to explain that the product they thought they were getting isn’t going to pass the test.

Forces of Change
Lately, the trend has been to go overseas. Get the molds built faster and cheaper and that is exactly what is happening. Molds are put through rigorous mold tryout and then it’s discovered that you have weeks and sometimes months of work to do to make the tool capable of withstanding normal variation.

In the case of transfer tooling, there has been success with avoiding rejects because incapability is documented upfront. When the reject happens they can’t reject you because you restated the terms based on the mold’s capability. Of course alternatives are offered to get out of the situation, but it doesn’t go over well when the cost to get a tool into shape is $5 to $10,000. So instead of fixing the tool they would rather keep getting shorts.
This type of situation is so pervasive because that’s the way it’s always been done. Why should we change?

What is forcing the changes now are customers’ new demands for “0” rejects. No more ppm acceptance criteria. Just don’t ship one ever. If you do they will rip into you and put you on a ridiculous sorting program. Is it possible? It is if you don’t break the rules and have a robust tool.

So, the industry is changing and it is forcing the molder to be extra careful when bringing in new tooling. Unfortunately, the method of making a few parts and throwing the mold over the wall to the molder is still very common. It isn’t working and the mold builders need to get on board with the concept.