The Troubleshooter, Part 51: Coping with gate jetting

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 customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years.

I received a record number of parts recently that needed help. The biggest challenge was deciding which part to write about. I received a gas-assist part that wasn't filling uniformly. I could have written about it, but the only problem was that the molder forgot that gas assist still requires you to size the runners and sprue correctly. So I passed that information on and the parts started coming out OK. 

I even got some more acetal parts to review, but I'm not going to work on another acetal part for a while. It's been overdone. Maybe I'll use this example sometime in the coming months. Besides, it's just a runner and sprue sizing issue with a gate problem thrown in for good measure. 

I even have an acrylic flow line problem to share with all of you sometime before the end of the year. 

This month, however, I think I need to focus on gate jetting problems, since I seem to be seeing a lot of them lately. A customer sent in two black ABS parts along with a runner. The problem was jetting (Figure 1) or "snake tracks" starting at the gate of the part and ending some 5 inches inside the gate, making part cosmetics look pretty poor. The black color didn't help matters much, either. 

Anytime I see gate blush or jetting on a part I know the gate is the culprit. Sure, we can reduce injection speed or heat up the material or the mold to reduce the effect, but the side effects can be just as bad, or even worse, when it comes to a high-quality part. Reducing injection speed might jeopardize the packing out of sinks and voids; heating up the material or the mold could make the sink worse and may even cause the cycle to slow. 

The main runner (Figure 2) was a two-level modified trapezoidal shape feeding straight into an edge gate on each part of a two-cavity mold. To understand why this is a problem, refer to the old rule book that says you must feed an edge gate from a full round runner. Also, the edge gate must be centered on the full round runner and not come off the side of the runner. 

This edge gate was coming off the side of the trapezoidal runner, so we know we have a high-shear gate, which causes blush at the gate or, in the worst case, jetting. 

The next problem I could see was that the edge gate was feeding into the cavity in an area where the incoming material was not hitting or impinging upon a core face. This interruption of the material flow front allows the material to move smoothly into the mold cavity. This type of flow problem usually requires a conversion to a curved tunnel gate or a valve gate if it happens to be a mold with a hot runner system. 

A tab gate can also be used quite nicely in cases like this. Put a tab on the part where you want to gate, and then gate into the tab with an edge gate to break up the flow of material. Blush or jetting will then disappear from the part. The key to a tab gate (Figure 3) is that the thickness of the tab should be the same as the wall thickness of the part. 

O My Diameter 
The next thing I looked at was the sprue diameter. The main runner diameter was .250 inch—fine for ABS. Typically, a main runner of this size requires that the sprue O-diameter (narrow end of sprue) be the same size as or larger than the main runner if just one main runner is coming off the sprue. The sprue O-diameter should be .312 inch if the main runner splits into two directions away from the sprue, which is what we had here. The sprue O-diameter was .220 inch. The nozzle orifice was .130 inch; so after we opened the sprue O-diameter to .312 inch we could open up the nozzle orifice to .290 inch. 

Since we were more than doubling the nozzle orifice size, this meant the flow of material through the nozzle would more than quadruple. This would make a big difference in running these parts. Once the flow through the nozzle and runner system was increased, we could reduce barrel heats and speed up the cycle if so desired. In cases like this the processing window typically opens wide and the cosmetics improve dramatically. 

All these changes would greatly benefit these ABS parts. These same changes will help any part, running any material, but it really works great for amorphous materials. All that was left was to call the molder with the changes. 

Well, I did call the molder and faxed my suggestions. After reading my recommendations he wanted to look over the changes to determine which options he should pursue. A couple of days later I received a call from the toolmaker on the project. He wanted to know more about the curved tunnel gate design (Figure 4, above). He was wondering if he could leave the sprue and runner alone and just change the edge gate to a curved tunnel gate (sometimes called a banana, cashew, peanut, or horn gate). The answer was a cautious yes, but I still recommended that he increase the sprue bushing diameter and the nozzle orifice, per the recommendations. 

Toying with Tunnels
The important fact to remember here is that a trapezoidal runner can be used to feed a tunnel gate, but not an edge gate, so the trapezoidal runner is not going to be an issue for the curved tunnel gate. Not everyone can use a curved tunnel gate because the bottom side of a part often needs to be kept absolutely flat. In this case it turned out that the bottom surface did not have to be flat, so I suggested that the curved tunnel gate would probably work fine. This is why I offer two or three suggestions to correct any problems I am presented with. I don't always know everything about the part design and I sure don't want to assume anything. 

The moldmaker asked for as much information on the curved tunnel gates as I could find, so I faxed him what I had and waited for feedback. It wasn't more than a week before I had a conference call with the molder and the toolmaker. They had made the changes and were so amazed at the improvements that they just had to call and talk about it. 

I guess this is what keeps me going—the "light bulbs" that I can turn on in the heads of molders and toolmakers. Maybe it's like the story of Johnny Appleseed, who traveled this great country of ours, planting apple seeds and watching them grow. 

It still gives me a good feeling when I walk into a molder's shop or into a toolroom and find copies of my articles taped to the wall in the office or on the front of a tool box. Then I talk to the molders or toolmakers and find out how many problems they have solved just by referring to these IMM articles. So, here's another problem solved and only thousands more to go.