The Troubleshooter, Part 40: Tapering drops for good flow

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.

When I do seminars I'm amazed at the number of designers and moldmakers who do not finish out the drops that feed material into their parts. I see a lot of restrictions to material flow built into the gate areas of many molds. 

It's not that the material won't flow through these poorly designed drops. It's just that I see molders using extra heat in the drop area to keep the material from freezing off prior to proper fill and pack. The extra heat often causes cosmetic defects, sometimes warpage, and almost always a slower cycle because of the extra heat in the mold. I also see flow lines, blush, and jetting at the part's gate area. 

Correcting Surface Defects
I received a runner this morning that demonstrates this exact problem. The runner is ABS, but it could just as easily have been polyethylene, polypropylene, or polycarbonate. The only difference is that polyethylene or polypropylene would have stringing or drool at the gate; the ABS and polycarbonate materials would have blush and/or jetting on the part at the gate. 

This time the material is ABS and I can see blush at the gate. The part, molded in a three-plate tool, is pretty thick at .370 inch, but it is cored out in some areas. The mold core has multiple levels, staggered in thickness: .370 inch, to .250 inch, to .220 inch, all of which is fed by a single gate (see drawing). The gate feeds into a center post that is .220 inch in diameter, .220 inch tall, and fed by a .075-inch-diameter gate with a .080-inch land. This design isn't so bad except for the gate land and diameter. The diameter of the gate, coming in from a drop like this, should be only 50 percent of the part wall for easy-flow materials, and 75 percent of the wall for most higher-viscosity engineering resins like the ABS and polycarbonate. 

In this case we are working with a wall dimension of .370 inch so we go into a default sizing exercise. Instead of the .075-inch-diameter gate, I would start with a gate diameter of .125 inch. The land, which is now .080 inch, should be reduced to .002 inch by tapering the gate at a 45° angle from the surface of the part back up into the flow channel. The .002-inch land ends up being a sharp edge at the surface of the part. This ensures the part is molded without any vestige, strings, or drool attached, and the gate breaks off flush with the surface of the part. 

Subrunners, Hot Runners
Although this was a three-plate runner system, the same corrective action is appropriate for hot runners and heated sprues. The only difference is that the three-plate system also has to be looked at for runner size as well as sprue O diameter size. 

The sprue O diameter needs to be larger than the main runner to make sure the entire runner is pressurized, and the nozzle orifice needs to be just a little smaller-usually 10 percent-than the sprue O diameter to make sure the sprue pulls each shot. 

For ABS I usually make the subrunner that feeds the gate .180 inch or so in diameter, and taper the drop down to the surface of the part at a 45° angle. Then I size the main runner that feeds the subrunners at either .250 or .275 inch, depending on how many subrunners come off the main runner. Then I size the sprue O diameter at .312 or .343 inch, depending on how big the main runner is. You can use full-round, deep-half-round, or trapezoidal runners to feed three-plate drops. 

Now, if this were a hot runner mold, there would be a few additional steps to perform to complete optimization. The first of these steps would be to drill out the molding machine nozzle to match the flow tube diameter of the hot runner manifold. Most hot runner systems have a .5-inch-diameter flow tube from the sprue bushing to the crossover tubes in the center of the mold. The crossover tubes are usually .375-inch, and feed to .250-inch drops at each part. 

If you don't drill out the nozzle you will have shear points inside the nozzle, pressure loss in the hot runner system, and dead areas inside the heated sprue bushing that could collect and trap material. 

Results
Now we are ready to call the molder and give him the good news about the changes he needs to make to get rid of the flow lines at the gate. Of course, he can pack out the sinks using a little higher holding pressure than normal, and he can bring the barrel heats down to the low side of the manufacturer's recommendations. In this case, I suspect he can run his barrel heats at 425F instead of the current 500F. 

He can also speed up the cycle some 10 to 25 percent over current cycle times since less heat will be transferred to the mold steel. Cosmetics will be dramatically improved in the center of the part and the processing window will be opened greatly, which always enables the molding technicians to do their job more easily. 

In the end, whether you're feeding a center gate with a three-plate design, a heated sprue bushing, or a hot runner system, you will optimize the tool in a similar fashion with slight variation from system to system. This is what I told the molder when I finally got him on the phone. He listened intently for a few minutes and then asked me to fax him the information so it didn't get changed from my original suggestions. A few days later I heard from the molder and he said the parts were running just fine and that the cosmetics were better than he hoped for. The most important thing he said was that the customer was no longer climbing up his back looking for parts.