Vented barrels for small machines

Vented barrels don’t work on small tonnage molding machines, right? Screw and barrel size restrictions on small machines cause vent flooding, and few custom molders today have time to waste in cleaning out plugged vents.

Take Nypro Carolina, for instance. Nypro Carolina II runs lights-out, 24/7 in Burlington, NC. It runs 16 30-ton all-electric molding machines custom-built for Nypro by Ferromatik Milacron. Each is equipped with a three-axis servo robot and produces small, tight-tolerance parts in cavity-pressure-sensitive tooling. Nypro Carolina II also runs fully automated QMC systems that allow it to do changeovers and be up and running again in less than 5 minutes.

So, Nypro Carolina II cannot afford four to five hours of predrying lead time. It wanted to use vented barrels like the other two Nypro facilities in Burlington running bigger machines. But vented barrels don’t work on small machines . . . or do they?

Jay Needham, the molding group leader at Nypro Carolina II, wanted to find out. He found his answer online.

Cyberspace Solutions
"We had been looking at vented barrels for small machines for about two years," Needham recalls. "I’d tried using cavity-pressure control to compensate for screw recovery inconsistencies, but it didn’t work. The vents still flooded."

"The operating costs of our electric machines are comparable to the cost of running our dryers," he continues. "We felt that the savings we could achieve eliminating dryers could be used for better things. Of course, I still wanted to dry my material, but we do 5-minute mold changes. How could I get around predrying downtime?" He went to the Net and asked.

Needham posted his question in a popular Web forum. He wanted to mold gears in his quick-change 30-tonners, 24/7, with no predrying. A PTFE and carbon-filled PC compound was the specified material. The gears, with an OD tolerance of only half a mil, were to be run in a two-cavity mold with a 28g shot. The screw had to have a 20:1 L/D and a second set of pumping flights to work. Its diameter could go down to 18 mm.

Carriage-retract limitations in the design of its custom-built all-electrics added further restrictions. Nevertheless, either there had to be zero drool, or vents should have to be cleaned only once every 24 hours. Can it work? Needham asked. Harmut Jahnke, vp of technology at Xaloy, responded by e-mail that he had an idea. Offline, they started talking.

Balanced Screw Staging
"Usually it’s not easy to get such small diameter screws to work with a vented barrel. Small screws do not follow all of the conventional rheological models," Jahnke explains. "Because of the size limitations, you get restrictions. It is extremely difficult to optimize root diameters and make them small enough."

Jahnke continues, "Also, the channel height is very small. These restrictions affect the behavior of the melt, often resulting in inconsistent recovery and part quality problems, not to mention uncontrollable vent flooding. It is difficult to get the screw geometry tweaked to achieve good flow and also decent venting characteristics. The trick is to balance the output from the first stage of the screw with the receiving ability of the second stage."

In time, an optimized 22-mm, 20:1 L/D screw was built by Xaloy to run in Nypro’s existing vented barrels. Xaloy had never installed such a small vented barrel set before.

"In three months of run time, we’ve never once had to clean vents," Needham says. "My process techs are the best judge and jury in the world. They were skeptical, and I didn’t say anything to influence them. After watching it run, they came back to me and said, ‘Man, this thing is great.’ I’ve ordered two more sets. The implementation of the vented systems will significantly decrease downtime caused by material drying, which will increase revenue. That’s the key thing. But, the elimination of drying will also save Nypro $1000 a year on energy costs per machine."

Needham says he intends to order more, each optimized to run different types of filled and unfilled hygroscopic materials. He’s done his math: "If we had them on only 10 machines, we’d save $10,000 in a year. That’s a good slush fund for me to buy more cavity-pressure transducers.

"I intend to have every machine daisy-chained into an RJG Dartpak monitoring network, so my customers can dial in and watch their parts running, right along with me," he adds.