How to Get The Gate Design Right

By Bob Hatch

Surface defects are often a result of poorly designed gates and sometimes an undersized runner system.

This month I dug into my retained sample boxes and pulled out a great example of what not to do with a spoke runner and gating system. It’s not that I’m pointing fingers, but somehow I have to get the point across that gate design is responsible for producing a cosmetically acceptable part. However, once in a while we see that the runners or the sprue diameters create other issues, too.

Actually, the design of this part appears to be pretty good after you get past the swirls on the surface. They’re probably the result of not running the mold temperature hot enough. The material in this part looks like a glass-filled nylon 6/6—probably a 33% glass load if I were to guess. Glass-filled nylon isn’t a tough material to run; we just have to make sure it’s dried properly before molding, which means 3-4 hours at 180°F in a desiccant-type dryer. The silvery look to these swirls makes me think the material was not dried as well as it should have been.

But the drying issue takes me off track. What I really want to point out is why the gates seem to have more swirls and gloss than they should have, and an overall underpacked condition.

Working with filled nylon
I start by measuring the part’s wall thickness: .240 inch on the interior flat walls. A lip around the circumference rises up .180 inch in the direction opposite the cold sprue. The opposite side of the part from where the lip is

	protruding contains a scallop detail that could be functional, an attempt to reduce material content, or possibly to offset the protruding rib on the other side. Next, I check the flatness of the part and it certainly has a warpage issue. My guess is that the gate issues are connected to this problem. If you want to keep a part flat, you need to edge gate it. If you want to keep it round, you should center gate it. If you want it flat and round, then you need to drop gates into the interior of the part—usually three, five, or seven gates equally and carefully spaced around the center of the part, about halfway between the center of the circle and the outside edge of the part. Each of the runner diameters is .260 inch, the sprue O-diameter is .265 inch, and the nozzle orifice is .230 inch. It’s not quite a direct quote out of my book but very close to what the runners, sprue, and nozzle orifice should be: .250 inch, .312 inch, and .290 inch, respectively, for a runner system like this.
Swirls and gloss marred the surface of this glass-filled nylon part, pointing to inadequate drying and undersized sprue and nozzle diameters. It also exhibited some warp, which could most likely be corrected through adjustments to the edge gate design and location.	The problem here is that the flow ratio of glass-filled nylon is about 40% less than that of unfilled nylon. This means that when we design a sprue, runner, and gate system for glass-filled nylon, we need to increase the diameter of everything by 40% or more to get the same flow lengths and packing pressures as needed to fill and pack an unfilled nylon part. The sprue and nozzle orifice dimensions for this part are undersized for glass-filled nylon, but close enough to give them a try.
	The gates have been tapered down from the top of the full-round runner to the inside diameter of the part. This is, of course, not correct for an edge gate; it must be designed so it comes off the center of the full-round runner. We also want the edge gate depth to be 75-90% of the part wall, the width about twice as wide as it is deep, and the land length half of the gate depth but not more than .030 inch. This means the modified chisel gate design will never mold this part successfully without a lot of work on the tool and in the processing adjustments. The edge gate dimensions for each of the four gates should be .180 inch deep, .250 inch wide, with a .030-inch-long land instead of .050 inch deep, .250 inch wide and a tapered land length. With the increased dimensions of the sprue O-diameter and the nozzle orifice, we should get a good fill and pack condition as well as very little pressure loss through the runner system.

Some molders would stop everything right here, send the mold to the toolroom, and get the sizes adjusted until they are just right; other molders would try everything they could to avoid sending the mold back to the toolmakers.

What do you suppose I would do? Those of you who know me are correct if you said, “Bob will send it back to the toolroom right now.” And that’s because every molding manager or molding technician I know would have already tried raising the barrel heats and the injection pressure, and slowed the injection speed, which is about all they have to work with. Sometimes they try running all virgin material to see if it’s the regrind messing them up, but that’s about it for them. That is why I know this mold is ready for some TLC by the toolmakers.

Avoid homemade nozzle fixes
I can also see a little cardboard stuck in the nylon at the point where the nozzle seats up against the sprue bushing, which tells me these guys either have a slightly bent nozzle or they aren’t used to running glass-filled nylon.

In the old days, we used cardboard between the nozzle and sprue bushing to prevent the nozzle from freezing off between shots. Of course, we were running unfilled nylon through a reverse taper nozzle when this condition would rear its ugly head. These days, we know better and use a GP nozzle of the correct orifice size when we run glass-filled nylon, which means we can get rid of the cardboard.

Why are these guys using cardboard? I would guess they are using a GP nozzle, which is correct, but they don’t have full coverage of the nozzle length by a nozzle heater that will keep the nozzle from freezing off between shots. They probably have a 1-inch heater band stuck on the end of a 2-4-inch-long nozzle in a vain attempt to keep uniform heat throughout the length of the nozzle. This, of course, causes the end of the nozzle to freeze off between shots and out comes the cardboard.

To summarize, the silver streaking at the gates is connected to the lack of proper drying. The gloss at the gate is due to friction heat of the material being forced through the improperly designed high-shear edge gate. The flow marks coming straight out from the gates are caused by gate restriction and pressure losses through the runner system and nozzle orifice due to undersizing the design parameters.

Did these changes straighten out the problems for the molder? Of course they did, and he’s running parts like crazy. Just another case of mistakes being made by the tool designer or possibly the moldmakers because no one has taken the time to pass the proper information on to them about what is appropriate for sprues, runners, gates, and vents for a particular material.

How should they find out? Well, they could get help from the material manufacturer. Or, they could come to one of my seminars, but I just pick on the toolmakers and scold the resin companies. Who wants to listen to that?

August, 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.