The Troubleshooter, Part 23: Delamination at a fan gate

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 manager of technical service and custome support for Prime Alliance, the Des Moines-based resin distributior. Before his present assignment, Bob managed a molding operation for 25 years.

It started out as a pretty quiet day before the phone rang. A long-time customer was on the phonasking if I could drop everything and help him with a major production problem. It was close by, so I agreed to try and help him. I grabbed my briefcase and took off to see how I could be of assistance. Besides, it would be about lunchtime by the time I got there, and I could probably get the molder to buy lunch. Maybe we could even squeeze in a round of golf after we solved his problem.

Upon my arrival, I was ushered into the company's training room where good and bad parts were laid out side by side for me to look at. Lunch was in sacks brought in from the local deli, and I could tell we wouldn't be playing golf this time around.

The problem was bad. The molder was getting severe delamination problems on a large ABS part being molded in its 500-ton, 48-oz Cincinnati. The part was a window frame with long thin sections. Not really thin, but like .150-inch walls for the frame and .125-inch walls in the thinner sections. Material was being fed to the part through a hot runner mold and the ABS was a fairly common grade that has been around for a long time.

Hot Runner Gating

The hot runner was feeding directly into the part in two areas and into a tab in another area. The tab portion was a fan gate arrangement that took the material from the hot drop and fed it under and up into the edge of the part. No doubt the hot runner designer had been given orders not to gate into a cosmetic area and this arrangement was all he could come up with. The delamination problem was isolated to the area of the part being fed by the tab. The areas of the part being fed directly from the two hot drops looked fine.

The first thing I could see was that the orifice of the hot drop feeding the tab was the same diameter (.090 inch) as the two drops feeding directly into the part. This is a classic mistake shared by many of us when designing fluid delivery paths to carry material from the barrel of the molding machine to the cavity of the mold.

I probably see this undersized gate condition more often when heated sprue bushings are used to feed runner systems. My usual comment to seminar audiences is to use a .125-inch or less diameter orifice to feed a part, but always feed a runner with at least a .250-inch - if not a .375-inch - orifice. Having the orifice too small also restricts the volume capability of the drop to feed enough material into the tab that in turn feeds the part. As a result, you will end up with a short shot or at least a hard to fill molded part.

In this case, we had to get enough material through the drop to fill and pack approximately one third of the mold in the same amount of time and with the same speed and pressure as the other two gates were seeing to fill and pack their areas of the part. The delamination was coming from the ABS being injected too fast through the restriction and the result was delamination due to shear from the small hot drop orifice.

The solution was to open the .090-inch orifice up to .150 inch and see what happened. Opening up the diameter did two things for us. First, it eliminated most of the shear at that drop. Secondly, it gave us more volume of material through that drop to more easily fill and pack out the part.

Faulty Fan Gate

That corrected the immediate problem, but we had a secondary problem with this arrangement. The drop we opened up was feeding into a tab that was shaped like a fan gate, but it wasn't dimensioned correctly to provide proper control of the pressure through the tab. It also restricted the volume of material that we could get through the tab into the part itself.

This fan-shaped drop had a .300-inch runner being fed by the .090-inch hot drop (which we opened up to .150 inch), then the runner fed a fan gate that was .320 inch wide where it attached to the runner and 1 inch wide where it attached to the part (see drawing). The fan gate was .270 inch thick from where it attached to the runner to where it attached to the part.

For volume considerations alone, we want the width and depth of the tab to be the same or slightly larger where it attaches to the runner as it is where it feeds into the part. When we started out, this tab was .270 inch deep by .320 inch wide where it attaches to the runner and .270 inch by 1 inch where it attaches to the part. This is the problem I felt we had to solve.

We deepened the area under the hot drop from .300 inch to .400 inch and tapered the fan gate from that point to the edge of the part. We changed the width of the fan gate from the .320-inch by .270-inch dimensions where it attaches to the runner to .400 inch deep by .700 inch wide. This change would give us about the same material volume at that point as the 1 inch by .270-inch dimension where the fan gate attached to the part.

Even More Improvements

In this case, we were running the fan gate under and up into the wall instead of making it an edge gate into the side wall as we usually do. If we had wanted to make this fan gate feed a traditional edge gate we would have changed the dimensions to fit the situation. The molder did not want to optimize the fan gate dimensions. He was satisfied with the results he got from just opening up the orifice in the hot runner drop that feeds the fan gate. At least he has the information he needs to finish the job if he ever wants to.