Flow lines near the gate area on this thick HDPE part indicated that the land length was too long at .150 inch—the Troubleshooter’s rule says it should never exceed .030 inch. An undersized sprue also contributed to cosmetic defects on the part.br>

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 technical programs manager for resin distributor Channel Prime Alliance. Before his present assignment, Bob managed a molding operation for 25 years. You can reach him at bob.hatch@channelpa.com.

Small changes to the gate and sprue fix bigger cosmetic problems.

I recently received a couple of parts and a runner system to examine. The parts had separated from the runner so I gave them a quick once-over, and then took a look at the runner. It seemed to be sized OK with a subrunner diameter of .250 inch and the main runner .312 inch. Maybe they were a little larger than they needed to be, but that was better than being too small.

One of the problems I noticed right away was the O-diameter of the sprue, which appeared to be undersized to properly feed the runner system. What should the sprue O-diameter have been? To find out, I looked at the runner dimensions. You wouldn’t believe how many times I have run into this situation where the runners were sized OK but the sprue bushing was undersized. The sprue O-diameter was .240 inch and fed a main runner of .312 inch. I hope everybody knows by now that the sprue O-diameter should be the largest diameter in the runner system. With a main runner diameter of .312 inch, we needed a sprue bushing with an O-diameter of .344 inch (roughly 10-25% larger). Then the molding machine nozzle orifice should be opened up to .312 inch (about 10% smaller than the sprue O-diameter).

Allow me to digress for a moment: Where does this erroneous runner and sprue bushing sizing information come from? I wish I could find the school that puts it out and straighten them out. The problem is that the technical schools that try to address the design issues of injection moldbuilding are few and far between, and those schools that do a good job of teaching the design of injection molds are only slowly starting to realize that they’ve omitted design issues as they relate to sizing the runners and gates. They spend all their time on part design and not enough time on the rest of the tooling issues. I have started to get calls and e-mails from technical schools here in the United States and even some from overseas on this very subject. I always try to pass on as much as I can about sizing the runners based on the wall thickness of the parts and the type of material being molded.

In this case, I wasn’t sure what material was being used but I suspected it was an olefin of some kind based on the feel of the part and the appearance of the defects around the gate area and near the end of fill. Rather than guess, I called the molder; it was high-density polyethylene and a fairly stiff grade at that. The flexural modulus of this HDPE was 220,000 psi, which made for one stiff part.

The defects were flow lines near the gate area and surface flow marks about halfway between the gate and the end-of-fill area. This told me two things: 1) The gate was too small for this HDPE’s melt flow; and 2) poorly melted pellets were being pressed against the cavity walls and the rest of the material in the shot was channeling around these partially melted pellets. What was causing these problems? The barrel heats could have been a little on the cold side, the gate was probably too small for this material, and the wall thicknesses—.070 inch on the thin walls and .145 inch on the thick gated wall—may have been too different.

Shorten the Land Length

Next, I looked at the gate size—.115 inch deep. That isn’t bad for HDPE with a .145-inch wall thickness. In fact, it was just about right since polyethylene gate depth is supposed to be 50% of the wall, which this one was, plus some. The additional amount in this case could have been used because of the possible industrial-grade quality of the HDPE (a 2-melt). With the gate depth looking pretty good, how about the width and land lengths? The width was .175 inch—not quite the full width of the subrunner (.240 inch), but close enough to do a pretty good job of getting the cavities filled and still have a little width to play with later on if needed.

The gate width is how we get more material through the gate in a reasonable time to fill out larger and thicker parts. The wider the gate, the more material we have to work with. I don’t really like fan gates all that much so I usually don’t let the gate width get any larger than the subrunner diameter that is feeding the gate. If I find I need a gate that is wider than the runner diameter feeding the gate, I first make the runner diameter larger, and then widen the gate and work with the new sizing in this manner until I get what I want on fill and pack.

The land length was .140 inch on one of the parts and .160 inch on the other one. We’ll split the difference and call it .150 inch. It doesn’t really matter because no land length should be longer than .030 inch, so you can see it was much too long. This mistake caused the flow marks near the gate. If the gate depth had been too short, we would have seen blush at the gate. By using these troubleshooting techniques I discovered that the sprue diameter was undersized and the gate land length was too long. That didn’t leave much to figure out. We still had a type of splay pattern on the outside walls about halfway between the gate and the end-of-fill area, but I was sure the restricted gate (with the long land length) caused it.

I called the molder and found out the owner of the company was a moldmaker. After explaining everything in detail, he agreed with my assessment and said he would make the changes right away because his customer was anxious to put these visual defects behind him. I didn’t mention anything to him about running the barrel heats a little higher because I first wanted to see what the changes did to the mold.

He called me a week later and said the problems disappeared after he made the changes to the sprue bushing and gate land. He expressed surprise that these minor corrections made such a big difference in the way the parts looked. I’m not surprised at all.

TROUBLESHOOTER’S NOTEBOOK Part: HDPE thick-walled part. Tool: Two plate, cold runner. Symptoms: Flow lines near the gate, surface flow marks and splay between gate and end-of-fill area. Problem: Sprue was undersized, gate land length too long. Solution: Increase sprue diameter from .240 to .344 inch; shorten land length to .030 inch.