The Troubleshooter: Part 87: Basics of tab gates

This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Consultant Bob Hatch of Bob Hatch & Assoc. has more than 45 years of experience finding solutions to processing challenges. You can reach him at [email protected].

When demonstrating gloss and texture patterns, it’s critical that the samples accurately represent the surface desired. This shear splay showed up as the result of an improperly positioned and sized tab gate. Rule of thumb: Always make the tab the same thickness as the wall.Correcting cosmetic defects always makes sense. Here’s how to do it with a tab gate.This month’s discussion revolves around a flat, edge-gated part. In this case, a tab gate is used as the edge gate.An edge gate is the best way to keep a part flat. The tab part of this edge gate helps prevent either a flow mark in the direction of flow or a smaller line perpendicular to the flow direction.The older design of this tab gate is not employed as often as it used to be. Today we would use a curved tunnel gate, sometimes called a peanut, cashew, or (in Texas) a horn gate. With enough volume, you can also try a hot tip gate or a valve gate as part of a hot runner system.The fact that this mold is designed with a tab gate tells me the volume is low. The molderhttps://www.plasticstoday.com/moldmaker used a cold runner system and is trying to avoid cosmetic defects at or near the gate.Let’s review why I like the tab gate. You use it on any part that needs a section between the runner and the part to swirl material around and avoid cosmetic shear splay or flow marks. The swirls and flow marks end up in the tab area, which is clipped off and reground. About the only mistake you can make with a tab gate is in not making the tab as thick as the part wall it is feeding into.That’s the rule for tab gates: The thickness should be the same as the part wall. I have even made the tab thicker than the part wall, but didn’t notice any improvement in the cosmetics. However, if you make the tab thinner than the part wall, you’ll run into shear splay and flow mark problems.Center the gateI looked for a note but all I could find was a phone number. I called the number and asked if anyone there had sent me a package. The receptionist told me the company I was calling had more than a thousand people working there and could I give her any more information? Well I couldn’t, so I put the box on the corner of my desk and waited for someone to call me.In the meantime, I decided to check out the part and runner.The big issue here was the hemisphere-shaped cosmetic defect about 41?2 inches in from the gate on this 81?2-inch-long part. It looked like shear splay perpendicular to the flow direction. Since this was a plaque designed to show what different gloss or texture patterns should look like, the defect in section 24 was totally inappropriate. The sprue O-diameter was just about the right size for ABS—.320 inch. The runner was .250 inch in diameter, again just right for ABS. The main runner went straight out from the sprue, took a right angle turn toward the part, and a right hook into the tab, all at the .250-inch diameter for the full-round runner. Couldn’t be better.I moved on to the edge gate feeding into the tab and could see without measuring that the land length was too long. It should be one-half the gate depth, or at the most, .030 inch.Being the cautious type, I pulled out my calipers and found the land length was only .045 inch long. What confused me? I twisted and turned the part and runner around until I could see it from all sides. Then it hit me. The edge gate was not in the center of the full-round runner.You always use a full-round runner to feed an edge gate, and you always want an edge gate to come off the center of the full-round runner. Then you make sure the edge gate depth, land, and width are designed according to the wall thickness and volume requirements of the mold.Sizing the gateFor ABS or any amorphous material, the gate depth should be at least 75% of the wall thickness. As mentioned before, the land length should be one-half the gate depth, but never exceed .030 inch. The width depends on the volume required to pass through the gate(s). A little part (think thimble-size) should have a gate that is only as wide as the depth. A medium-sized part, like a pack of cigarettes (but thinner) should have a gate twice as wide as it is deep. For bigger parts, I make the gate width three times the depth, but I try not to exceed the runner diameter that feeds the gate.What else was wrong with the gate design? Well, the tab (.130 inch) wasn’t as thick as the part wall (.230 inch). It was a big difference, but that was all that needed correcting.The last thing I did was look up stiffer-flow ABS materials in the IDES Prospector database (www.ides.com), in the free section, of course. Reducing the flow promoters might also improve the cosmetics of this part. That was about all that was needed to put this one to bed. Was all this review work worth the effort on a part like this? Absolutely, since it’s a cosmetic part and your customers should always see your very best work when they open a box of parts that you ship to them. Now this molder can hold his head high every time he walks into a customer’s office to take him to lunch and thank him for the business.