The Troubleshooter, Part 109: Another classic case of a ­polycarbonate part being ambushed

An undersized runner system causes an abundance of cosmetic and filling problems.

A few weeks ago the FedEx driver showed up at my door with a big thank you. The package he was delivering was smaller and lighter than most of my deliveries and he was just expressing his appreciation.

I opened the box and found a black polycarbonate part with an intact runner and sprue system attached. I could see sink on the top surface and a filling problem on one end. I couldn’t see any parting line flash so the tonnage of the molding machine seemed to be OK.

Now, I have found that the more I work on problem parts, the quicker I can figure out the problems. So I could quickly see that the sprue was undersized, and the vestige of the gate orifice told me it was much too small to be effective as well. Also, the gates were not deep enough for a part with a 0.100-inch wall thickness, and the land lengths were too long—possibly twice as long as they should have been for a part of this size and wall thickness. The gate width on the long subrunners was pretty close to being the only correct dimension on the part, but it was too wide for the short end subrunners.

The venting appeared to be functional—there were no burns on the surface around the parting line or in those end-of-fill areas that like to trap air. It’s pretty hard to see burns on a black part, but if you look closely, you can see a grainy condition or black carbon appearance.

The runner system

After my preliminary examination, I turned my attention to the details. I like to start at the subrunner or runner that feeds the gate. In this case, the part had six gates. This meant the runners should all be sized pretty much the same.
For a polycarbonate part with 0.100-inch walls like this one, I use a formula I came up with many years ago: Multiply 0.100 by 2.25 to get a subrunner diameter of 0.225 inch. Then add 0.025 inch to get 0.250 inch for the main runner diameter.
Here’s where I found the first problem. Both the main runner and subrunner diameters were 0.250 inch. What happens when the subrunner and main runner diameters are the same? You get pressure loss in the runner system. The correct sizing would be 0.225 inch for the subrunner and 0.250 inch for the main runner.

Given this change, the moldmakers would probably need to make some adjustments to the runner plate. They could either recut it to make the subrunners 0.225 inch and the main runner 0.250 inch, or they could leave the subrunners at 0.250 inch and open the main runner to 0.275 inch. This would remove the pressure loss and save a little money for the molder.

Now it was time to move to the sprue. The sprue O-diameter was 0.180 inch and the nozzle orifice diameter was 0.125 inch. In my formula, I add 0.050 inch to the main runner diameter, which would give us a sprue O-diameter of 0.300 inch. I usually tell the toolmakers just to use a sprue bushing of 5⁄16 or 0.312 inch to make it easier for them. They probably have a sprue bushing with a 0.312-inch orifice but not a sprue bushing with a 0.300-inch orifice. This slight sizing difference doesn’t usually hurt because bigger is often better when it comes to sprue bushing O-diameters.

The nozzle needed to be a full-taper design for amorphous materials and the nozzle orifice needed to be 10% smaller that the sprue O-diameter. Following this rule would make the nozzle orifice diameter approximately 0.280 inch to maintain pressure throughout the nozzle, sprue, and runner system.Next, I moved to the gates. I started with a depth of 0.075 inch since we could always go deeper if needed. For polycarbonate, the depth of an edge gate should be 75-90% of the part wall thickness. The edge gate depth was 0.040 inch, but based on a wall thickness of 0.100 inch, the gate needed to be at least 0.075-0.090 inch deep.

Once I determined edge gate depth, I could determine the gate land length. It was 0.070 inch on the black part. The rule is that the gate land length must be equal to half the edge gate depth but should not exceed 0.030 inch. So the land length needed to be 0.030 inch for all six gates.

Now for the gate width, which I measured at 0.275 inch: A typical edge gate width is twice as wide as the gate is deep, so it needed to be 0.150 inch wide (2 x 0.075). Since the width determines material flow capabilities and since the length of the two subrunners at the end of the main runner was shorter, the width of the end gates would be about 25% less than the gate width on the other four runners, or 0.110 inch. All of the gates could be welded up and recut to the proper depth, width, and land lengths.
That was all I could see to do so I called the molder. He was out of town on business so I gave the good news to his daughter. Just two or three hours in the toolroom and this mold would be ready to sample, most likely creating parts without the cosmetic defects and with a faster ­cycle.

The molder called back a few days later and said the mold changes had been made, the samples run, and his customer was very pleased. I couldn’t ask for a better result than what he achieved.

The Troubleshooter’s notebook

Part/material: Black PC.
Tool: Single-cavity cold runner.
Symptoms/problem: Sink, short shots.
Solution: Decrease diameter of subrunners; increase sprue and nozzle orifice ­diameters; decrease gate width and land length but increase gate depth.