Poliamidas

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Published: September 28th, 1998
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A common part, a cylinder with a shaft running through it, exhibits the typical problems. Here's the cure.
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 DesMoines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years. |
This latest problem is more common than most of us think. The parts are round with a solid molded shaft running through the middle of a disk. The shaft extends 2 inches perpendicularly above and below the disk. The material is a 33 percent glass-filled nylon 6/6 that is precolored with carbonblack at a high enough loading to make the material fairly UV resistant.
How the Parts Were Molded
The problems mentioned in the customer's note were warped parts and voids in the shaft, mostly in the thicker cross section of the part where the shaft and the disk intersect. I reviewed the process conditions from the setup sheets and saw the molder was using high barrel heats (580F) and highinjection pressures (1400 psi), both good indications of flow problems in or near the runner system. From looking at the runner system, I could see the machine nozzle orifice was definitely undersized and the gate, on the end of the runner, also appeared to be undersized based on the wall thickness of the part.
The injection speed was set a little bit slower than I like to see for glass-fillednylon, indicating the technicians may be giving any trapped air time to get out instead of taking care of venting problems in the mold. Maybe the runner isn't vented, or perhaps it is a lack of vents on the part itself, or maybe it's both.
Mold temperatures were okay at 180F, which will bury the glass and give the best looking part surface one can expect from a glass-filled nylon 6/6. Besides, if abetter looking surface were important, the customer would have selected nylon 6 with glass. He must have wanted the higher heat deflection temperatures of the nylon 6/6.
The backpressure was set at 50 psi, which will keep the glass fibers from getting chopped up during screw recovery. The screw rpm was set at a low speed, just enough to get the screw back before the mold opened, which again isperfect for glass-filled nylons. So everything looked good except for the high barrel heats and the high injection pressure.
Usually, I like to watch the pressure gauges on the molding machine to see where the injection pressure starts to build up, so I can figure out where the restriction to the material flow is. This works great on older machines, but on new machines I just look at the computerscreen. Things sure have changed! If the pressure starts to build quickly, then the restriction is in the machine nozzle or in the runner system. If pressure starts to build later in the fill sequence, it probably means the material made it through the nozzle, sprue, runner, and gate and has filled and started to pack the part.
But this time I am not standing beside the molding machine, so Iwas only able take a guess at the reason for the high injection pressure. This mold has a heated sprue bushing feeding a cold runner that in turn feeds directly into the part right on the end of the shaft. I looked at the cold runner again and could see that the orifice in the heated sprue bushing was far too small to feed and properly pressurize the trapezoidal runner. The runner was .250 inch deepand .250 by .300 inch wide and about 4 inches long.
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The orifice in the heated sprue bushing is too small to feed and properly pressurize the runner for this part. The gate diameter feeding into the part is too small as well. |
Checking Both Ends of the Runner
I saw the molder was trying to feed the runner with a .060-inch orifice coming out of the heated sprue bushing, and it should...
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