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Telltale surface defects point to the need for a new gate.

This month I received a part from an old customer that gives me a chance to explain how to be successful with ejector pin gates. The part is flat, molded of ABS. It appears to be more of a corner piece that would go on a computer table or possibly a plaque of some kind.

I don't even need to call the molder on this one. This could be any amorphous material with a flatness callout that requires a certain amount of decent surface cosmetics. I would imagine the only reason for ABS here is for some chemical resistance qualities. I am guessing that high-impact polystyrene just doesn't have the chemical resistance required for this application.

The surface of this part shows jetting across at least half of the visual top surface and a little blush and some gloss at the area just inside the pin gate, on the part's outside wall. We usually see a little sink in this area opposite the gate but not this time.

Surface defects on this ABS part were caused by an improperly shaped gate.	Go for the wedgie First of all, it is important to use a wedge shape for the ejector pin gate design. This way we can gate into a thicker part of the ejector pin gate, and then we can taper the wedge shape up to the underside of the part where we want the gate to freeze off. Typical sizing for a wedge-shaped gate like this is .100 inch thick where it attaches to the part and .150 inch thick where the subgate feeds material into the gate. This works well for ejector pin gates feeding material into a part with a wall thickness of .125 or .150 inch. This part is much thicker than the .125 or.150 inch we usually see using pin gates, so I will have to adapt my sizing dimensions to accommodate the
By switching from a half-round ejector pin gate (right) to a wedge-shaped gate (left, not attached), the blush, gloss, and jetting disappeared.	wall's .330-inch thickness. I won't have to go to extremes on my sizing because there is a point of diminishing return when working with parts with thicker wall sections. The runner that feeds the gate should be at least as thick as the thickest section of the part. I do this to be sure I can get enough pack and hold pressure to pack out any sinks or voids common to thick-wall parts. In this case, I will not try to size my runners to the .330-inch wall thickness, but from past experience I still expect to get this part filled and packed out with an injection pressure of 600-800 psi and hold pressures of 1000 psi or so.
The Troubleshooter uses his own standard runner sizing when working with ABS as well as a full-taper nozzle.	I typically like to work with the following dimensions when working with ABS: .250-inch main runners; .225-inch subrunners; a cold sprue with an O-diameter of .312 inch; and for amorphous materials such as this ABS I like to use a full taper nozzle, opened up in this case to .290 inch. Will this sizing work with this part or not? We won't know until we make the changes and sample the mold, but I am encouraged by past results with similar problems. A faulty sliver So far we have specified the runner sizing, selected a style of nozzle, and now we are ready for the ejector pin gate recommendations. I start with the subgate dimensions because I want to be sure the ejector pin is large enough in diameter to accept the subgate diameter.

With a part this thick I usually try to size the subgate diameter at .125 or .150 inch. I could also use a rectangular subgate that is .150 inch wide and only .100 inch deep so the effective size would be approximately .125 inch. I like to use this design when I can, mostly because it makes the gate easier to shear off when the part ejects. These rectangular gates are best for larger ejector pins such as this one, which is .250 inch. I haven't used them yet on thinner-wall parts but I bet they would work just as well there.

With gate design, we size for volume and try to reduce as much shear as we can during injection. In this case I suggest a rectangular subgate feeding into the .250-inch-diameter ejector pin. The subgate dimensions should be .125 inch deep and .175 inch wide, resulting in an effective subgate size of .150 inch and the ability to shear off relatively easily when the parts eject.

The jetting I see on this part is from the long land length of the “sliver” of .100 inch that has been removed from the ejector pin. This .100-inch-thick-by-.600-inch-long sliver just doesn't give us enough room to allow the ABS to flow; it cools the material quickly from the outside-in, gradually squeezing off the flow of material into the cavity. This is probably causing the molding technicians to increase the heats on their nozzle in an effort to keep the sliver from freezing off. It's also likely why I am seeing jetting, gloss, and blush on the visual side of the part opposite the ejector pin location.

Going to this new wedge-shaped design will allow the ABS to flow from a thick to thin-wall section of the wedge-shaped ejector pin gate without having the material set up and squeeze off flow to the cavity. This is all we have to do to get this part back to the visual quality levels they were hoping for in the first place.

I called the customer and gave him my recommendations and waited for his response. I heard what sounded like a moldmaker in the background say something like, “We haven't ever done anything like this before, but I guess we can give it a try. Just remember that if this doesn't work I'm not going to tell you I told you so.” I didn't get a call back for a couple of weeks, but when I did it was a simple “thank you” and then a little more excitement for getting them to the point where they could sell the parts they were molding.

The molder also thanked me for teaching their toolroom a couple of variations on an old idea that their moldmakers can use from now on.

February, 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

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