The Troubleshooter, Part 54: Sizing runners for PBT

This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. 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.

It started out as kind of a quiet day. I only received 12 boxes of parts to look at instead of the 20 or more that often arrive from the next-day-air carriers. Most of the problems were fairly routine and didn't take long to handle.

One of the 12 packages was from a very capable molder, Brian Qualizza with Plainfield Molding (Plainfield, IL). I saved it for last. When I did open the box, I found a small plastic bag with a little runner and many small parts in it. The note said he was having to use higher-than-normal heats and somewhat higher injection pressures to get the parts to fill consistently. The note went on to say that he could make good parts most of the time, but once in a while he would get some short shots, and because of the higher barrel heats, he was having to run a longer cycle than originally quoted.

Impediments to Flow
I looked at the parts and runners. The material was a PBT polyester, colored off-white. The runner system appeared to be on the small side, but with PBT, I like to stay with small runners and small gates. How small? Well, the subrunner that feeds the gate still has to be as thick as the thickest section of the part and the gates still need to be at least as thick as one-half the wall section into which they are gating. I pulled my verniers out and sure enough, both of these requirements appeared to have been met. I didn't want to make a mistake so I took a closer look.

The sprue was a little longer than what I like to see on small runner systems like this so I took a closer look at the O-diameter of the sprue, often called the small end of the sprue by molders and toolmakers alike. I base the O-diameter of the sprue on the diameter or depth of the runner, depending on whether I am working with a full-round or trapezoidal runner configuration. But even more than that, I base the main runner on the size of the subrunner and the subrunner is based on the thickest section of the part.

To be sure I didn't miss anything I measured the thickest section of the part and found it to be .100 inch. This would mean that the subrunner that feeds the gate should also be no less than .100 inch. But the subrunner depth was only .060 inch. The subrunners that fed the gate were connected to the next subrunner level up by a short connector runner that was also .060 inch. The subrunner that fed both the connector section of the runner and the subrunners feeding the gate was .090 inch deep. It wasn't hard to see that the connector runner and the subrunners that fed the gates were not deep enough to allow the PBT to flow through them without having to use extra heat and injection pressure.

Proper sizing of this area meant making the subrunners that fed the gates .100 inch deep. The connector section that fed these subrunners would be .125 inch deep and the subrunner depth that fed the connector would be .150 inch.

This brought me to the main runner and, based on the subrunner depth requirements, it should have been .175 inch deep to feed the subrunners. Instead, the main runner was .100 inch—considerably undersized. If the main runner was .175 inch deep, we would want to see a sprue with an O-diameter of .225 inch and a molding machine nozzle orifice of .200 inch. The sprue O-diameter was .125 inch and the nozzle orifice looked like it was about .100 inch.

Everything was undersized, from the nozzle orifice diameter all the way through the runner system. Even the gates were too small since they only measured .020 inch. With a .100-inch-thick section on the part, the gate should have been at least .050 inch in diameter for PBT.

What Did You Measure?
Is it hard to figure out why this molder was using extra heat in the material and extra injection pressure to get the parts to fill out? Certainly not, but why did these guys miss it? This was one of the best molding shops that I've visited and it has a great bunch of toolmakers led by Joe Messina, one of my best students.

Looking at the runner gave me a little bit of a clue, but to be sure I called the molding manager and asked him what he was looking at and why he thought he was having troubles. He said the runners looked big enough when he applied the sizing rules, and if he knew what the problems were, he would have fixed them already. Extra heat and pressure were the only things he found that helped.

I reviewed my findings with him, and when I finished,

he took issue with my runner sizing measurements. He said his measurements were somewhat larger than mine. I asked him if I received the right runner system to look at and after confirming that I had we both started measuring the runners again. He was still measuring larger dimensions than I was.

Then I discovered what we were doing differently. He was measuring the width of the runners and I was measuring the depth. Who was right? Of course I was, but this gave me a good opportunity to do a mini seminar and explain the effective flow channel size of a half-round, trapezoidal, and full-round runner. The conclusion of this discussion was that you should always base the size of the runner on the depth, not the width, of the runner.

With this hurdle out of the way he was able to understand how to size the nozzle orifice, sprue, runner, and gates to eliminate the need for extra barrel heat and injection pressure. The molder went off to have a conversation with the toolmakers, and he promised to get back to me on the results.

After a little more than two weeks, the molding manager called. He wanted me to know that the changes had been made and after a few hours of working with the processing conditions, he and his crew were able to work the barrel heats back down, and the injection pressure was again less than 1000 psi. He said it took a little longer to make the changes than he had originally thought because they decided to do some extra venting on the runners and a couple of other things since they had the opportunity. The parts were running steady now, with no short shots, and the cycle time was actually a little faster than they had quoted. Not bad for a couple of weeks' work.