The Troubleshooter, Part 110: Are you getting the picture?

No matter how many times I present my corrections, parts still arrive needing the same solutions.

I have a part to discuss this month that should be of interest to everyone. It came to me with a request for help because of blush around the gates and sink opposite some ribs inside the part.

The molder didn’t say, but I could see the material was polycarbonate or PC/ABS. These are both amorphous and no matter which one it was, the suggestions would be the same.

The wall thickness of this part, made in a single-cavity cold runner mold, was a fairly uniform 0.085 inch. Just by looking, I could see the sprue diameter was too small to feed the two short main runners, which in turn fed edge gates located on the part’s top surface.

My process is simple: Break down the big problems into individual components, fix each of the components, reassemble the pieces, and see if the problems have been corrected.

Follow the rules for runners and gates

It really doesn’t matter where I start troubleshooting, so, by habit, I began with the main runner (I always start with the runner that feeds the gate). A PC or PC alloy requires a runner that is approximately 2.25 times the wall thickness.

Although the wall thickness was 0.085 inch, I rounded it up to 0.100 inch for the review. I multiplied 0.100 by 2.25 for a main runner diameter of 0.225 inch instead of 0.185 inch deep. The runner sent to me was too shallow and of a trapezoidal design instead of full round.

Second, I determined the size of the sprue O-diameter by adding 0.050 inch to the main runner diameter –0.275 inch for this part instead of 0.160 inch.

Third, I reduced the sprue O-diameter by 0.025 inch to determine the correct nozzle orifice size (0.250 inch instead of 0.090 inch). Since it was an amorphous material, a full-taper nozzle was required instead of the GP variety being used. Noz­zles are not that expensive in the first place, so most molders should be able to afford the correct style for their application.

Next, I looked at the gate design. It was an edge gate feeding into an 0.085-inch wall. The rule for edge gates is depth should be 90% of the wall thickness, width should be twice the depth, and land length should be one-half the depth but not exceed 0.030 inch.

First, I measured the gate depth at 0.040 inch; however, 90% of 0.085 is 0.076, so I rounded it off to 0.075 or 0.080 to make it easy for the toolmakers. The 0.100-inch width was slightly short of the 0.150 needed, and the land length was a tapered 0.080-inch— – so I reduced that to 0.030 inch.

One key point to make about a full-round runner and an edge gate is that the gate should exit the runner from the middle. This creates an easy-flow gate instead of a high-shear gate that will cause blush on your part just inside the gate area.

Final touches

With the nozzle, sprue, runner, and gates determined, all that was left was the venting – and a couple of other suggestions. Venting problems are a little hard to check on a black part, but the rules are pretty straightforward:

• Include one vent per parting line inch.
• Each vent must be 0.0005-0.002 inch deep in most cases. Call the material manufacturer for the correct vent depth.
• Each vent must be 0.200 inch wide with a land length of 0.040 inch out from the parting line.
• Drop into a 0.040-inch-deep channel to atmosphere and draw polish the vent lip to a mirror finish to make it self-cleaning.

All these changes would eliminate blush or flow lines at the gates and reduce knitline visibility. In many cases, the part would not need to be textured in an effort to mask these visual problems.

I called the molder, who, after conferring with the toolroom supervisor, agreed to make the changes I suggested. A week later, they reported that some of the problems were fixed, but not all. I told them to send me the optimized parts and I would take a look to see if we missed anything. The next morning the samples arrived and I could see immediately what had happened. They made all the changes we discussed except for two – increasing the nozzle orifice diameter and changing the nozzle from a general-purpose style to a full taper.

They said they would do so right away and let me know how the parts were looking in 2 or 3 hours. They called back and said the problems all went away with the nozzle change.

I used to get into discussions with toolmakers because they couldn’t believe my optimization ideas were so simple yet worked so well. These days the only questions I get are how to make some of the changes—such as how to design an insert to make any gate into a curved tunnel or how deep to make vent depths.

If you have been reading this column with any regularity, you know my secret: It doesn’t matter if the parts are big or small or what the material is – the changes needed to make the parts look better are pretty much always the same. 


The Troubleshooter’s notebook

Part/material: Black PC or PC/ABS.

Tool: Single-cavity cold runner.

Symptoms/problem: Blush near the gates and sink opposite the ribs.

Solution: Modify gate dimensions; change the runner to a full-round design and increase its diameter to 0.225 inch; enlarge the sprue O-diameter; change the nozzle to a full-taper with an orifice of 0.250 inch.