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Pencils out, class. Here are adjustments for the defect-free running of a part made of a crystalline material.

A runner system I received this morning was slightly different from any others I’ve received. It was from a three-plate mold with a heated sprue bushing. The molder (I’ll call him Chuck) mentioned he was having problems with warpage, sink, and poor cosmetics on his parts. I also saw another problem that just won’t go away: sucker pins directly above the drop locations that restrict flow through this part of the runner system. The restriction continues down each of the drops to the parts.

The moldmaker tried to help reduce the flow restriction by increasing the top part of the drop to allow more room for the material to flow around the sucker pin, but while allowing extra room, which is helpful, it does not do anything to stop the flow front from tearing or ripping when injection speeds are unusually high.

So, what needed to be done about the sucker pin locations for this mold or any three-plate mold? Well, how about moving them to the outside of the drops? Usually one on each side of the main runner or subrunners is effective, where you calculate the need for the runner to be held in place until the ejection system kicks the runner off of the sucker pins. Remember, don’t put any sucker pins along the flow length of the runner; they’ll hang down into the material flow as it moves from the sprue bushing to the drop points.

Small gates, big runners
This was a 16-cavity mold with a balanced cold runner. The material was crystalline so I knew the runners and gates would be on the small side. The main runner was a modified trapezoid, .200 inch deep and .235 inch wide. The connector runner between the main runner and subrunner 1 was .170 inch, still a modified trapezoid design. Subrunner 1 was .150 inch deep and .195 inch wide, again a modified trapezoid. The runner connector was .125 inch deep and .150 inch wide, and fed the next subrunner level (sub 2). Sub 2 was .115 inch deep by .135 inch wide. Next, the drops fed by sub 2 were .240 inch where they attached to it, tapered down to .145 inch where the material flows around the sucker pins, and then down to .085 inch at the bottom of the drop. The drops fed into each part though a .050-inch gate diameter.

At first blush, this looked pretty good, but I checked it against my rules of thumb. The part walls were .125 inch, which tells us the gate diameter should be approximately .065 inch (50% of the wall thickness for PE, PP, PA, and PBT polyester). The .050-inch gates on this runner were a little small, but if the molder was using an easy-flowing crystalline material, the smaller size would be an advantage.

I always say smaller is better when it come to gates; otherwise, we could get a dimple around the area just inside the gate if the gate’s too big. This dimple results from a backflow condition that occurs when the gate is too big andthe material flows backward through the gate because the gate did not have sufficient time to freeze off before screw recovery. Filled crystalline and both filled and unfilled amorphous materials typically require larger gates, but it depends on the part’s wall thickness.

The runner depth that feeds the gate should be 11?2 times the wall thickness, or .125 inch, for crystalline materials. That would make sub 2 .188 inch (for simplicity’s sake we’ll round up to .200), but it was only .115 inch. So here is where Chuck’s problems were starting.

With sub 2 undersized, we’re in trouble on every runner level up to and including the main runner, as well as the sprue bushing dimensions. The connector between sub 1 and sub 2 should be the same diameter as sub 1, or .225 inch (.025 inch larger than the previous runner level).

The diameter of the connector between the main runner and sub 1 should be the same as the main runner, or .250 inch (.025 inch larger than sub 1), instead of the .170-inch connector and the .200-inch-deep main runner. It’s safe to say here that the sink resulted from lost pressure through an undersized runner system.

Next is the feedpoint to the runner, which would be the same size as a cold sprue O-diameter if one was used here. This means the orifice diameter in the heated sprue bushing needed to be opened up to .312 inch, if possible, instead of its existing size of .140 inch. This would provide a big volume boost as well as require less injection pressure to push the material.

Sometimes you can’t open the heated sprue bushings up very much because you might cut into the internal heaters that surround the flow tube. If this is the case, then another heated sprue bushing of the proper size would need to be ordered.

The final touches
We must remember that even though the cosmetics will be improved with our runner adjustments, I don’t expect to lower the barrel heats that much because of the crystalline material requirements of these parts. We still have to run a barrel melt temperature high enough to get the pellets to flow.

However, we need to watch the total heat we put into the heated sprue bushing because we don’t want the orifice to string or drool. We have several options:

• Control the barrel heats with setpoints and shear put into the melt by way of the screw rpm and backpressure settings.
  
• Install a nozzle shutoff to take the pressure off of the melt.
  
• Use a little bit of decompression to take the pressure off of the melt, but be careful; with too much, you could suck air into the melt, which causes cosmetic defects on the part surface.

One other important point: Be sure the general-purpose nozzle feeding the heated sprue bushing is drilled out to match the flow tube or bore diameter of the bushing. In this case I would suggest to Chuck that he open the heated sprue bushing up to a flow tube diameter of 3?8 inch, and the nozzle should match exactly. This is not like a cold sprue mold where the nozzle orifice needs to be 10% smaller that the sprue O-diameter. Heated sprue bushings and hot runner sprue bushings are matched exactly with the nozzle orifice diameter.

I looked at the runner again to see if I missed anything and sure enough, I could see some knockout pin half circles on Chuck’s parts, so when I called him with the new runner and heated sprue bushing sizes, I told him the barrel heats looked pretty good so he might want to leave them where they were, but he did have some work to do on the part cores. To get rid of the knockout pin marks, Chuck will need to check the draft angle on each core, and then be sure the cores are draw polished. If necessary, he will have to send the cores out to have them coated with something to reduce the ejection force necessary to push the parts off the cores.

I do hope it works out well for them. I would hate to see another mold or molding job end up overseas.

October, 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.

About Bob Hatch
Bob Hatch is one of the leading on-the-spot problem solvers in the molding industry. Mr. Hatch spent time as the technical programs manager at Channel Prime Alliance and managed a molding operation for more than 25 years. Currently, he writes articles for Injection Molding Magazine under the pseudonym The Troubleshooter.

The Plastics Troubleshooter