Gate Related Defects

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Even if four out of five measurements in the tool are correct, you’ve still got to fix the fifth for a defect-free part.

I have a great story to tell this month. I received parts and runners the other day that show how easy it is to get a part defect from a poorly designed edge gate. It sounds similar to other stories I’ve related, but this one shows how important it is to make all the necessary runner and gate changes, not just a few of them.

	The part looked like a taillight lens for a trailer or a reflective lens that could be mounted on a post to mark a driveway or mailbox. The material was polycarbonate, but it could just as well have been acrylic or another amorphous material. The problem indicated by a note in the box was that the parts had a surface defect opposite the gate, right at the end of fill. Sometimes it appeared on just one part and sometimes it appeared on all parts in the same location.

It wasn’t an air trap issue like the molder thought, but it took a comment from the customer for management to give the toolmakers the time to make changes needed to resolve the defect in this reflective lens. In this case, the runners were mostly sized correctly but the gate was too shallow, narrow, and long.	Gate depth, width, length As usual, I looked over the part and applied my nozzle, sprue, runner, gate, and venting review (NSRGV, for short). The sprue size appeared to be adequate for polycarbonate, since the O-diameter was .375 inch. The shorter subrunners were all .312 inch—also right on target. The only mistake I saw was that the connector runner, between the main runner and the Y-shaped subrunners, was sized the same as the subrunners rather than the main runner. This could cause a pressure loss in the subrunners, but probably not enough to cause many problems, especially the kind indicated on the note. The gates were near the center of the full-round subrunners. They weren’t exactly centered and this could cause a problem that I hadn’t yet identified. The gate depth seemed a little shallow for PC; it should equal 90% of the wall being gated into, but was only 50%. The lip being gated into was .100 inch by .150 inch and the .050-inch-deep edge gate was not deep enough to fill and pack properly these 4-inch-diameter lens parts. The gate width was only .070 inch; it should have been closer to .225 inch or the width of the subrunner feeding the gate. The width depends on how much material needs to flow through the gate. If we get the depth right, we shouldn’t see any blush or jetting at the gate.

The land was also too long for an edge gate. For PC, it should be half of the gate depth, but not more than .030 inch. The gate land feeding these gates was .080 inch, which was much longer than it should have been and would definitely cause cosmetic issues in an area straight out from each of the edge gates.

Let’s review our findings: The sprue and runner sizing was pretty close to what it should have been. The main-to-subrunner connector was slightly undersized, the edge gates needed a lot of help as they weren’t deep or wide enough, and the land was too long. The off-center gate would probably be a problem, but I usually make the easy changes first, and then go for the tougher changes if we need them. By making the gate depth modification all on one side of the gate, that would help center it on the subrunner.

A handy checklist
I called the customer and recommended the gate changes needed to bring his mold into compliance with the rules of thumb that I like to use for PC parts. The molder said he had just started getting reject notices from his customer about an area of concern on these parts that was straight out from the gate. The customer thought the defect looked like a small area of surface splay just opposite the gate and right at the end of fill. This problem had looked like an air trap issue to the molder, but he couldn’t get rid of it by working on the runner and part vents. He had not thought about it being a gate-related cosmetic problem. When he listened to my review, he realized quickly that he might have missed the proper corrective action. Not one to irritate his moldmakers, he wanted to run the changes past them and see if they agreed with my gate changes.

He called back a couple of hours later and said the moldmakers were in complete agreement. They were even a little peeved because they had been asking for the opportunity to make similar changes on these gates for some time but had been told that the production schedule would not allow the mold to be pulled for changes.

Apparently it took a call from the customer to “create” the time needed. They also liked my NSRGV checklist to avoid missing any necessary gate changes. Another great story for my scrapbook!

The Troubleshooter’s Notebook
Part/material:	4-inch-diameter PC taillight len
Tool:	Eight-cavity, cold runner
Symptoms/problem:	Splay-like surface defect opposite the gate, at end of fill.
Solution:	Increase size of connector runner (between main runner and subrunners) to the size of the main runner; increase gate depth to 90% of wall thickness all on one side to make the gate more centered; increase gate width to the size of the subrunner; shorten land length.