Runners: A question of balance

Last month yours truly penned an article for Canadian Plastics taking a brief look at hot runner systems, and it reminded me how little processors think about runner systems, hot or cold.

That’s understandable; molding is about making parts, not conveying hot resin to mold cavities, but take a moment to consider runners from the resin’s point of view.

At its simplest, cold runners are a bunch of channels that pipe hot resin to the gates. An optimal cold runner system would use the minimum linear run to deliver resin to multiple gates at identical pressures and temperatures, then break cleanly at mold opening and weigh almost nothing.

For hot runners systems, all the above still apply, but you have to add in the minimal need for heaters in the fewest possible number of control zones.

Hot runners are a form of voodoo all by themselves, but even the cold runner community has something to think about here, because runners have a part to play in the mold’s cavity design, the required shot, heat, packing and other factors, too.

Balancing act

Mold designers think a lot about ‘balance’, the ability of a runner to deliver resin at the same temperature and pressure as possible to each cavity.

The need to squeeze the largest number of cavities into the smallest base at the lowest possible cost means that perfect balance is often sacrificed, but often it’s plenty good enough to get close. But sometimes, perfect balance is detrimental.

For example, I once worked with a cold runner moldmaking end-gated high-aspect ratio (that’s engineering babble for ‘skinny’) parts in polypropylene (PP). It used very short runners relative to the part mass by fanning the cavities out from the mold centreline.

Theory versus practice

It looked good on paper, because the short runners meant a compact mold, a good resin mass fraction going into the part instead of the grinder, a short cycle time with fast injection speed and a mold footprint small enough to run the job on a small press.

The problem was the outer cavities dropped the parts onto the lower tie-bars, where they picked up the inevitable grease.

As usual, there was no time to engineer telescoping deflectors, so my solution was to hammer two sections of 1/4-inch copper tubing flat, solder the ends and slot one side. Supplied with utility air through a small flow controller, the resulting air film eased the parts past the tie bars. Oddly, very little pressure worked better than a big blast, which I can’t explain to this day.

What’s the point of this tale?

The mold was beautifully engineered and built, we ran a small press to its maximum capability (which I love to see), the set up was easy and it dry-cycled perfectly.

But getting the minimum runner mass reduced one problem while creating another (the grease issue) that I didn’t spot at the initial setup. But that’s just the life of an injection molder: One damned thing after another.