The part that caught my attention this month represents two problems that have plagued molders for as long as I have been in plastics: poorly designed pin gates and an unbalanced runner system.
Often a customer refuses to allow any gate marks on the sides or top of its parts. As a result, the molder has just two gating options. The first is to use a gate on the underside of the part, such as a curved tunnel gate. The second option is to cut a sliver off one side of the end of an ejector pin and use a subgate to feed material into this sliver and then into the part cavity. The part we’re discussing uses this latter pin gate design.
Because the pin gate is often designed incorrectly, the restricted flow creates cosmetic defects on the part surface directly above the pin gate—blush or gas-trap cosmetic issues.
Let’s think about this flow path configuration for a minute. Usually the sliver is .5 inch long (in this case, it’s 1.5 inches long). A pin gate is typically located in roughly the same location as an edge gate. When you analyze the function of this kind of gate, you can see that the uniform width of the sliver essentially creates a long gate land, causing problems such as flow marks in a straight line out from the gate; flow lines at a 90° angle to the material flow; and, in some cases, gas marks just inside the gate area.
Also, the subgate that feeds this pin gate is commonly sized the same as if we were feeding the sidewall of any part. This is wrong. We don’t really care if the subgate is bigger when feeding a pin gate since any peeled or torn material resulting from the larger subgate gets removed in degating.
Going to the wedge
What needs to be done here? All it takes is an adjustment in the sliver from a uniform width to a wedge shape. Sometimes this wedge can be cut or ground into an oversized ejector pin, preferably using the size of the existing ejector pins.
The wedge dimensions for amorphous material wedge gates are always bigger than their crystalline material cousins. For amorphous materials like PS, ABS, ASA, acrylic, or PC and for .100- to .125-inch-thick part walls, the wedge-shaped pin gate should be .100 inch thick or deep where the wedge attaches to the part’s underside. The thick dimension of the wedge should be .150 inch where the subgate feeds it (see drawing).
When modifying the ejector pin to get this wedge shape, it’s a good idea to avoid sharp corners. This keeps the ejector pin from breaking at the subgate feedpoint when the part is ejected and the wedge is flexed. It’s just like putting a radius on an inside corner of a plastic part to keep it from cracking or breaking.
Remember that the gate depth for PC should be 90% of the wall being gated into; the other amorphous materials I mentioned need a gate depth equal to 75% of the wall where the wedge attaches to the part. Then add .050 inch or so to the depth to get your wedge dimension, or to where the subgate feeds into the wedge shape. For crystalline materials such as PE, PP, PA, or PBT, use .050 inch where the wedge attaches to the underside of the part and .100 inch where the subgate feeds into the wedge.
This month’s part is made of an amorphous material (PC/ABS) with a wall thickness of approximately .100 inch. For a wall thickness that is .025 inch thicker, add .025 inch to each of the wall attachment and wedge gate dimensions.
Finally, be sure the wedge-shaped angle goes straight from the .150 inch dimension to the .100 inch dimension of the attachment point. I have found this wedge shape works well with all materials. Just remember that if you use filled materials in the wedge gate design, you will still get abrasive wear problems just like any other gate and occasionally you will have to replace the ejector pins. Also, don’t forget to lock these ejector pins in place so they don’t rotate during use.
Pump up those runners
The note that came with the part mentioned ribs shorting out, knitlines around or near the cored-out holes, and a gas mark or blush just above the pin gate areas (the part’s top surface). With my trusty ballpoint pen, I checked the ribs and bosses for at least a .025-inch radius everywhere they attached to the nominal wall. Most had been radiused properly because I got a single line when I drew a 45° mark where the ribs and bosses intersected with the part wall. A couple of rib sections near the end of the part produced a double line, so I knew they weren’t radiused enough and could break when stressed.
After checking the part with my magnifying glass, it looked like most of the problems were related to the pin gates and maybe packing pressures. The parting line near the lineup of pin gates seemed to have a problem with the material or the additive package because I could see some roughness on the part’s surface. Also, a dull line down the length of the parting line pointed to trapped air.
The sprue, runner, and subgate sizing were all too small, and the runner was unbalanced, meaning the distance from the sprue bushing to each gate or set of drops was a different dimension. The sprue O-diameter was .210 inch. At the other end of the sprue, where it attached to the main runner, the diameter was .325 inch—a big jump for a 3-inch-long sprue. The main runner (full round) was .195 inch and the full-round subrunners feeding the pin gates were .125 inch in diameter. The subgate diameters were .125 inch.
The drawing on p. 32 shows how I balanced the runner system. These sizing calculations are based on PC or a PC alloy. For other amorphous materials or for filled crystalline materials, the dimensions can be about 10% smaller. For an unfilled crystalline material, reduce each of these dimensions by 25%.
Venting tips
The dullness along the parting line pointed to a venting problem. Most of the venting was in place and doing its job, but missing were runner venting and parting line vents along the side of the part where the pin gates were located. Remember that we add individual vents at one vent per parting line inch. When using pin gates, the entire parting line should be vented using perimeter venting. I also like to double-vent the corners of the part when using individual vents.
Runner vents should be much deeper than part vents: .003 inch deep, as wide as the runner channel being vented, with a land of .060 inch; then drop into a .040-inch-deep channel-to-atmosphere, and don’t forget to polish the vent lip at each end of the runner to a mirror or A1 finish to make the vents self-cleaning. Do this at the end of each runner and your venting problems will pretty much go away.
Parting line vents for a PC alloy should be .001 inch deep, .200 inch wide, with a land of .040 inch, and drop into a .040-inch-deep channel-to-atmosphere. Polish the vent lips as mentioned above.
I suspect the reason the toolmakers didn’t vent along the wall length where the pin gates were located is that they thought they were trapped by the pin gates and couldn’t go anywhere with the air. A solution is to use blind pocket vents. These are relief areas—.040 inch deep and .500-1 inch long. They should be ground in or burned somewhere between the parting line and the runner channels. The idea is to place these blind pocket vents closer to the parting line than the runner channels, one vent per parting line inch along the sidewall. The parting line should then be vented as explained above.
When I called the molder and spoke with him and his toolroom supervisor about my proposed changes, I found them entirely receptive to my recommendations. Then, about three weeks later, they called back to thank me for the help and let me know that the mold was producing good parts for the first time ever, and they were able to reduce the barrel heats and speed up the cycle a little bit thanks to the larger runner and gate dimensions. Who says you can’t teach new ideas to a couple of experienced guys?
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.
Let’s think about this flow path configuration for a minute. Usually the sliver is .5 inch long (in this case, it’s 1.5 inches long). A pin gate is typically located in roughly the same location as an 
After checking the part with my magnifying glass, it looked like most of the problems were related to the pin gates and maybe packing pressures. The parting line near the lineup of pin gates seemed to have a problem with the material or the additive package because I could see some roughness on the part’s surface. Also, a dull line down the length of the parting line pointed to trapped air.