Nozzle tip design eliminates black magic

On the outside it looks like any other general-purpose nozzle tip, but with this nozzle, it's what's on the inside that counts. In fact, it's the insides that allow the tip to control or eliminate stringing, drool, and cold slugs when used with most any material—amorphous, semicrystalline, and even shear-sensitive. Sound too good to be true? That's exactly what Michael Matanic, process engineer at ACM Southwest, thought until he tried it himself. 

"It's one of those things that makes you envision a snake oil salesman," says Matanic, "but this worked. It eliminates stringing. And it was as simple as changing the nozzle tip out; the problem we were having went away immediately." And what a problem it was. 

String, String, Everywhere 
"You could pull a 10-inch string off the sprue bushing," laments Matanic. "It would just string and string and string. We would load up the mold face with string in a matter of 5 minutes." It was a problem that was costing the company, a custom molder in El Paso, TX, both time and money. 

The molds in question are new tools, six four-cavity molds processing a flame-retardant ABS (GE's Cycolac) in 18-second cycles. ACM typically runs two of these tools a day to produce an average of 19,000 parts/day. With the stringing problem the company cleaned each mold four to six times a shift, says Matanic, accounting for 30 minutes of downtime per mold per 8-hour shift. 

On top of the manpower and cycle time inefficiencies, material waste was becoming a problem as well. "In a single shift, with the amount of string generated in addition to the dirty parts run out after cleaning the mold each time, we were wasting about 10 to 15 lb of resin a day on each machine," he says. With two molding machines processing this resin each day at a cost of $1.80/lb, that adds up to approximately $27 in lost material per machine every day. 

The company also worried about the potential cost of repairing mold damage if the problem persisted. "We would clean the mold and make adjustments, but nothing would work," says Matanic. "There was simply not a way to control the stringing problem without doubling the cooling time that the part actually needed." 

Stringing, as with many ABS resins, is a common problem with the flame-retardant ABS ACM was running. However, though alternative resins were available, they were cost-prohibitive, according to Matanic. 

The answer proved to be a newly designed nozzle tip, appropriately named the Eliminator, which was being beta tested by Spirex. ACM agreed to try the tip in November 2000. "As soon as we put the nozzle tip on, the stringing went away," says Matanic. "We did not have to change any process parameters other than to remove the various black magic techniques—super low or high nozzle temps, excessive decompression, and sprue breaks—that technicians had been using to try to control the stringing problem." 

The Science Behind the Tip 
To find out how exactly the Eliminator works its own brand of magic, IMM went to the source, the tip's inventor Sam Kern, a consultant and the principal of Swistzer Select Enterprises (SSE), based in Woodruff, SC. Unfortunately, since the patent is still pending, the details are limited. Here's what we learned: 

"The mold, when in contact with the nozzle, acts as a heat sink," explains Kern. "That heat sink pulls heat away from the nozzle tip at a fairly consistent rate—that is, unless you're using cardboard, a sprue break, or another artificial means to 'fix' the process, all of which further interrupt the thermodynamic process and the natural heat sink. The design of this tip's opening area provides the best physical contact to allow the heat sink to be beneficial rather than a problem," he says. 

"There is a contradiction in the molding process; the sprue must freeze in order to remove the part, and yet we want the plastic to stay molten so that we can get the next shot. This nozzle incorporates geometry that promotes the natural behavior of plastic to want to cool, yet controls it in an area within the nozzle tip to provide minimal pressure drop and hot material for the next shot." 

It's a design, says Kern, that allows the injection molding process to be more repeatable, making the tip more integral to the whole system. "The tip separates the nozzle into controlled heat sink areas," he explains. "Rather than allowing freeze-off to happen immediately at the physical interface, we've created zones within the tip." Yet, the tip, which is constructed of hardened steel, has no internal components, and requires no machine or nozzle modifications, adds Kern. 

Kern has tested the tip on a variety of materials, and Spirex has conducted several beta site tests to determine that the nozzle does, in fact, control or eliminate string, drool, and cold slugs on all materials. 

Matanic is certainly sold. ACM has ordered two more nozzles to tackle a cold slug problem in a glass-filled nylon 6/6 resin and a stringing problem in a low-density polyethylene material. The Eliminator nozzle tip, currently available through Spirex, sells for $50, but Matanic says that even if it cost twice that it would be worth it. 

"We will buy more of these nozzles, and they will become the way we fix problems like stringing in our shop," he says. "No more black magic."