The Troubleshooter: Blush with a filled PC

This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Bob Hatch is manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years. 

This month I get to tell you about a problem that I run into quite often. I received a package from a customer I have worked with many times before. He sent me an unbalanced runner system and one of the parts he was currently molding. The material was a glass-filled polycarbonate but it could have been just about any filled material and the problems would have been roughly the same. 

The runner system was sized fairly well. It wasn't perfect, but they seldom are. The main runner was .300 inch and the sprue was .250 inch, which was a little backwards since a main runner of .300 inch should be fed by a sprue O-diameter of .343 inch with a nozzle orifice of .312 inch. 

The short subrunners were sized at .312 inch each, which was also slightly off target. If the main runner is .300 inch, then the subrunners should be slightly smaller—maybe .275 inch. These were little mistakes, but in some cases could cause big problems. 

To this point, all I'd done was nitpick because I didn't even know what the specific problem was. So I called the molder and he said all he wanted help with was the blush at the gate. I asked him if flatness or cycle time or anything else was a problem and he said "just the blush." 

Well, blush is always caused by the gate being too small—not deep enough—or the land being too long. When subgates are involved it can be that the tunnel of the subgate is too narrow as it goes from the subrunner to the part. I looked at the subgates and could see that they would be better suited to feed polystyrene than glass-filled polycarbonate. The next item to check was the gate itself. 

I looked at the tunnel portion of the subgate and could see blush on the tapered portion of the tunnel. This told me that the gate was too restricted to shoot glass-filled polycarbonate through it. I measured the subgate diameter, which was .080 inch. The blush stopped on the tunnel at the .120-inch diameter. This told me that the correct size for this subgate, where it contacts the part, should be .120 inch instead of .080 inch. 

Tricky Slivers 
Next I looked at the gate and, lo and behold, I found a pin gate. Toolmakers love to cut a small sliver off the side of an ejector pin to use with a subgate to feed material into the pin; the material then tries to force its way to the end of the pin and into the part cavity. The mistake that is commonly made is that the sliver removed from the ejector pin isn't thick enough to feed the material into the cavity without requiring extra material heat and a lot of injection pressure. 

This was exactly what was happening here. The part walls were .120 inch with .090-inch ribs—perfect for this material. The boss walls were more like .180 inch and in this application that was also OK. So what was the problem? The trouble was that the pin gate sliver was only .060 inch thick where it contacted the part and .065 inch where the subgate fed into it. 

So we had two problems. First, the sliver was not thick enough to fill and pack the part without extra heat and pressure. Second, the subgate that fed the sliver was too small in diameter. 

First let me discuss the undersized subgate. Toolmakers tend to keep the subgate on the small side when feeding a pin gate. I guess they think that all subgates should be small. This thinking goes back to where we use a subgate to gate directly into a part. It's probably an issue of staying steel safe. 

To feed a part, the subgate should be small enough to prevent tearing or stringing of the material when the gate is sheared away from the part during ejection. But feeding a pin gate is different. We don't care if we get tearing or stringing at the gate during ejection because we cut the entire sliver away from the part after it is ejected. 

Now that we knew why the toolmakers did what they did, it was time to figure out how to design a pin gate that did a better job of getting the material into the cavity. 

I started with a sliver that was wedge-shaped instead of flat all the way from where the subgate fed into it down to the surface of the part. I thought about an edge gate and what its depth should be for this size part and this kind of material. For glass-filled PC the gate should be about 90 percent of the wall thickness for depth. For this part that would mean an edge gate would be .090 inch deep; the pin gate should be .090 inch thick where it feeds into the part. 

Now for the wedge part of the pin gate. I knew the wedge would be .090 inch thick where it fed into the part and I wanted to taper it back up to where the subgate fed into it. The part of the wedge that was fed by the subgate should be about twice as thick as it was at the part surface. 

Sometimes we can only make the location of the subgate feed 50 percent thicker and sometimes we can double it. Mostly it depends on the diameter of the ejector pin being used. If the wedge dimensions are too big for a small ejector pin, then it is necessary to put an oversized ejector pin in place of the small ejector pin. 

In this case I recommended the oversizing, calling for a pin size of roughly .218 inch instead of the existing 3/16-inch-diameter ejector, to handle the requirements of a wedge that was .090 inch at the part surface and .150 inch where the subgate fed in. At this point we had a .120-inch subgate feeding into a .150-inch-thick wedge that tapered down to .090 inch at the surface of the part. 

I called the molder and gave him all of this information and he said he would get to work and see what happened. It was only a couple of days later that he called and told me that he was molding great-looking parts with no signs of blush or flow marks. 

I asked him if he wanted to work on the sprue and runners to optimize the rest of the mold to maybe get the cycle time down a little bit but he declined, saying he didn't want to rock the boat and take a chance of losing what he had gained. I told him that he sounded more like a toolmaker than a molder and he reminded me that he started out as a toolmaker.