The Troubleshooter, Part 12: Hot runners
Published: February 28th, 1997
- Gate diameter for hot runner molds
- Gate diameter of 50 percent of wall thickness for easy-flow PE and PP.
- Gate diameter of 75 percent of wall thickness for ABS, SAN, and acrylics.
- Gate diameter of 90 percent of wall thickness for polycarbonate/acetal/ PVC/glass-filled materials and PPOs.
- Land length for hot runner molds
- Land should be no more than .005 inch.
- Check with your hot runner manufacturer for correct land length.
Problems with hot runners and heated sprue bushings always seem to be in one of three places. The first problem area is usually the gate diameter, second is the gate land, and third is the size of the nozzle orifice.
When troubleshooting hot runner problems, I start with the gate diameter and land length. The gate diameter should be at least half the wall thickness for polyethylene and polypropylene and bigger for the more shear sensitive amorphous materials such as ABS, acrylics, and polycarbonates. The land length of these hot tip gates should be .005 inch, usually indicated as a sharp edge on prints.
When the flow path is restricted because of a small gate diameter or a long land length, you will usually see higher heat settings being used by the molding technicians to get the parts to fill and pack out. The problem with this approach is that it usually causes warpage of the parts, cosmetic defects around the gate area, and long molding cycles.
If the flow path is sized correctly, you will be able to set the same heat settings on the front zone of the barrel, the nozzle, and the heated manifold, such as 450F across the board for ABS or 525F for easy-flow polycarbonate. If you find you have to run the manifold heats higher than the front zone of the barrel you probably have gate sizing problems. If you have to run the heats up on the nozzle, then the nozzle orifice probably hasn't been drilled out to match the flow tube diameter of the heated bushing (Figure 1). Notice that it isn't just sizing the nozzle orifice to match the hole in the heated sprue bushing. First you might have to increase the hole size of the sprue bushing to match the flow tube diameter, then drill the nozzle out to match that.
Figure 1. To optimize the performance of the hot runner, the sprue bushing orifice should be the same size as the main sprue bushing flow channel. Increasing the nozzle diameter reduces the pressure loss during both injection and decompression.
A case in point is a two-cavity hot runner mold that a good friend of mine is running in Arkansas. As often happens, he inherited this mold from another molder. The parts look like upside down banana split bowls (Figure 2) and the material is easy-flow polycarbonate. When he ran the mold for the first time, he found he had to run the barrel heats at a melt temperature of 610F to 620F plus all the injection speed and pressure he could muster and he still couldn't quite pack the flow lines out of one of the two parts.
The molder called the nationally known company that originally made the hot half of the mold and was advised to send the mold out to have it baked in a hot sand oven to clean out the degraded material and anything else that might be restricting material flow through the hot runner system.
He did send it out and when it came back it looked clean; but when he tried to run it, the part still wouldn't fill out. He called me for help again and I suggested he send me sample parts to see if I could find anything we had missed.
I received the parts and took a good look. The parts looked pretty good overall but, as he said, one of the parts had the flow lines he described over the phone. The flow lines looked like waves coming in off the ocean just before they crash onto the beach. It had to be a pressure loss problem. The second thing I saw was a brown streak on the part with the flow lines. The streak started at the gate and was about an inch long. It kind of looked like a candle flame because it started out with a narrow streak, spread out in the middle, then narrowed down again.
Now, I know that any blush or jetting at the gate means the gate is too small, and brown discolorations mean the material is being degraded by too much heat, but how to apply this to the problem wasn't totally clear. I knew it had something to do with the gate, so I started there. First of all, the gate didn't have the traditional recessed area and transition dimple or melt puddle like most hot runner gates. The gate was standing straight up from the part. The diameter was .105 inch, which is a little bit oversize for polycarbonate parts. We usually suggest the gate diameter should be 90 percent of the nominal wall thickness and these part walls were .105 inch thick. This would calculate out that the gate diameter should be .095 inch, so I knew the gate diameter was certainly big enough.
Then it hit me. At .105 inch in diameter, the gate size was okay, but the gate was standing straight up approximately .070 inch, not the sharp edge or .005 inch it should be.
No doubt the molder was running excessive heat in the manifold area to keep the long land gates from freezing off between shots, which was causing the polycarbonate to degrade. It was probably a combination of shear through the long land of the gate during injection and the extra heat needed to keep the gates open that were causing the brown streak and the flow lines.
I called the molder and told him what I had found, but he didn't want to get inside the manifold and start removing steel. He said something to the effect that the hot runner people should know what they're doing and that they wouldn't miss something as critical as the gate land length. I reminded him that the hot runner people probably didn't build the part of the mold that the gates are in, just the hot half. After a few quiet moments, he agreed to have the toolmakers take a look at it and see if he could get in there to remove enough steel to cut the land down to the proper dimension. I soon got a call from the toolmaker and had to convince him that the sharp edge was correct for a hot tip gate. He said he was afraid that reducing the land to a sharp edge would weaken the gate area and it would break out.
I reviewed with him how plastic flows and assured him that the maximum velocity of plastic flow is in the center of the flow channel with zero friction on the side walls, which is ideal for hot runner gates. I told him he could break the gates out if they get plugged with cold pellets or metal, but that is why melt filters are sold. Every hot runner should have a melt filter installed between the barrel and the nozzle. I guess that satisfied him or he just didn't think it would do any good to argue with me anymore; he said he would take a look and see what he could do.
A few days later, I got a call from the molder. He said the toolmaker was able to reduce the land as we wanted and the mold had been sampled with the changes. Not that I had any doubts, but I had to ask him how it ran. "Well," he said, "I'm not having any trouble filling and packing any more, but at first I was still getting the brown streak. After turning the heats down quite a bit (50 to 75 deg F) on the manifold, everything started running great."
Then he said, "You know we're 11 for 11 now," meaning this was the 11th mold I have helped him troubleshoot. He's getting pretty good at fixing up the two- and three-plate molds, but this was the first hot runner we had worked on together. I would say he's close to getting his "Official Bob Hatch Troubleshooter" diploma and badge! Not all hot runners can be fixed this easily; each has its own problems and its own way of being optimized. Sometimes it is stress concentration problems at the sharp edges of a gate recess area that causes breakage, and sometimes it is just the size of the gate and material melt flow that is causing filling problems or longer cycle times. But the majority of the time, it is the three areas covered here that cause most of the hot runner productivity problems.
|Part:||Clear polycarbonate bowl.|
|Tool:||Two-cavity hot runner mold.|
|Symptoms:||Burn marks, sinks, flow lines.|
|Problem:||Mold had to run at very high heats (610F) to even fill incompletely; material was degrading, gate stood out from the part.|
|Solution:||Reduced long land of the gate to minimize shear stresses on the material. This allowed the part to fill properly. Heat could then be turned down to eliminate burning and material degradation.|