This article continues the Troubleshooter’s Rules of Thumb series. 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.

Gate sizing is based on the type of material being molded and the wall thickness of the part. Since all good part designs use uniform wall thickness, I size the gate depth based on the nominal wall. If the part design includes some thicker sections, then I base the gate depth on the thickest wall section.

The depth of the gate determines the gate freeze-off time, the gate width is based on the volume requirements of the mold cavities, and the gate land is related to the gate depth (see “Gate dimensioning,” below).

Gate depth for an ABS part needs to be 75 percent of the part’s wall thickness. For a .100-inch-thick wall, the edge gate depth should be .075 inch. Since the land is based on the depth I follow the rule that the gate land should be half the gate depth, but not exceed .030 inch. In this case, I use the default dimension of .030 inch.

The gate width is at best a guess. Through experience I can usually come pretty close by looking at the size of the part and how thick the walls are. If the part is small, like a thimble, then I make the gate about as wide as it is deep. If the part is bigger, like a coffee cup, then I make the gate twice as wide as it is deep. If the part is bigger than a coffee cup then I make the gate three times wider than it is deep. Admittedly, this approach is not very scientific, but it works.

Using the same 16-cavity layout as in Part 1 of this article, I suspect the parts would be bigger than a thimble and smaller than a coffee cup. So I default to the width being twice the depth. In this case, with a wall thickness of .100 inch, our edge gate is .075 inch deep, .150 inch wide, and has a .030-inch land.

Variations on a Theme
Now, this works well for an edge gate, but what if we want to use a subgate so an operator doesn’t have to trim each part by hand? You have to think in terms of volume.

If the edge gate works well with the dimensions we picked, then we need to calculate the volume of the edge gate by multiplying the gate length by the width. Then, for a round subgate we have to find out what number multiplied by itself equals this number. If your calculator batteries are dead, just look at the numbers you have, which are .075 and .150 inch, and pick the midpoint number: .1125 inch. Otherwise, do what I do and use a round subgate diameter of .110 inch to equal the edge gate volume.

You could also use a rectangular subgate that’s .075 inch by .150 inch instead of using the round subgate of .110 inch if the latter’s bigger cross section bothers you when it comes to shearing the thicker gate off at ejection.

We have used our rules of thumb on gate depth to size our ABS gate, but what if you are using some other material? Easy-flow polyethylene and polypropylene gate depths are based on just 50 percent of the part wall thickness. Edge-gate depths for polystyrenes, ABS, SAN, acrylics, and acetals work better at 75 percent of the part wall, and some materials like polycarbonate need to have an edge-gate depth that’s about 90 percent of the wall thickness. Since the gate width and land length are based on the gate depth, you can see how the numbers will move around.

Telltale Signs of Poor Sizing
For troubleshooting purposes, you can use a visual observation of the gate area on the molded part as a guide to whether the gate is sized correctly. If you see blush at the gate, then the gate isn’t deep enough. If you see a flow line just inside the gate, the land is too long. If you see jetting on the part, the gate is much too small. In any of these cases the corrective action is to make the gate deeper or shorten the land.

Another troubleshooting procedure I like to use is to observe where pressure starts to build up as the material injects into the mold. Since I know I won’t see any pressure buildup until the material meets resistance, I have something to look for.

On older machines you’ll want to read the injection pressure gauge directly, but on newer machines you’ll read the trace on the computer screen. If pressure starts to build almost immediately, then the nozzle orifice is too small. If pressure starts to build about 20 percent into the fill, then the problem is undersized gates. If you get to 90 percent or more of the fill before pressure starts to build, you’re right on.

You don’t want to see pressure build up until you hit the back wall of the mold cavities. It is because of flow restrictions like these that molding technicians run up material heats and injection pressures in an effort to force the material through the restrictions and get the parts filled and packed. The result is poor part cosmetics and long cycle times.

Have I mentioned this point before?