Screw design cures splay problems.

When injection molding splay problems strike, instead of blaming the material, take another look at the screw. That lesson emerged over the past year at two major automotive molding operations, both of which overcame stubborn splay problems with polycarbonate blends after adopting a proprietary screw design from Great Lakes Feedscrews (GLFS) of Tecumseh, Mich.

This "ET II" screw is built under license from Robert Barr, Inc. of Virginia Beach, Va., with GLFS having the sole license to this design for the injection molding market. The screw features an "energy-transfer" (ET) section that takes up the last third of the screw, right after conventional feed and transition sections [ILLUSTRATION OMITTED]. According to GLFS national sales manager Mike Walper, the ET section produces a tumbling effect on the melt. "It works sort of like a taffy puller," he says. The result is a distributive mixing and melting process in which heat energy transfers from melted to unmelted polymer.

Though on the market for nearly a decade, the ET II screw has only recently been applied to solving splay problems in polycarbonate blends. Although the causes of splay remain somewhat of a mystery, the low-shear action of the screw is believed to be the secret of its effectiveness against this cosmetic problem.

Unlike conventional barrier screws, the ET does not separate the melted and unmelted polymer and does not use shear as the mixing/melting mechanism. ET also has deeper flights than most barrier screws. Together, these features mean that at a given rpm the ET II produces about 25% less shear and 15-20% more output than a typical barrier screw, according to Walper. "Depending on whether you're recovery-limited, you can run at a higher rpm without imparting extra shear. Sometimes cooling time is longer anyway, so recovery time is moot. Then you can just reduce the rpm to lessen shear and reduce temperature," he explains.

FIGHTING SPLAY IS JOB ONE

The Ford Motor Co. plant in Milan, Mich., runs about 18 million lb/yr of GE Plastics' Xenoy PC/polyester blend, producing bumpers on 2500-4000 ton injection machines. Last year, plagued with splay problems on the bumpers, Ford performed design of experiments (DOE) analyses to identify the cause and find a solution. Once the most common splay culprit - moisture in the resin - had been eliminated as a potential cause, Ford began to look at shear-induced breakdown of PC, which could release gas bubbles into the melt. "Xenoy is very shear sensitive," explains Carl McBride, a Ford engineer who worked on the project.

Once the ET II screw was installed, splay problems decreased by 5% on average - a significant reduction for an operation running 18 million lb/yr. Ford Milan's operations superintendent, Bob VanderMolen, attributes the ET's splay-reducing ability to the lower shear and melt temperature it achieved relative to barrier screws. Ford ultimately put six ET II screws on its machines.

Saturn Corp. in Springville, Tenn., experienced similar splay difficulties when using Dow Plastics' Pulse ABS/PC blend to make body panels on eight 5000-ton machines. "We were getting volatiles in the melt," recalls Greg Young, a senior manufacturing engineer. The resulting splay was responsible for a 3% reject rate.

Saturn adopted the ET screw on all eight presses because "it allowed volatiles to escape before they could enter the melt," says Young. "Now, we have no splay."

RELATED ARTICLE: Good for TPOs, PPOs Too

Polycarbonate blends are not the only applications for related ET designs. Recently Ford and Saturn found other uses:

* Because of more complex bumper designs, Ford Milan has been switching from PC/polyester to TPOs, which don't typically suffer from splay. But even with the new material, Ford engineers found that another ET screw (the ETI) helped achieve faster cycle times on bumpers. Carl McBride, a floor engineer for Ford, explains that ET's relatively deep flights give it more output per rpm and allow faster recovery.

* Saturn and Great Lakes Feedscrews have fine-tuned the ET's compression ratio to come up with a screw for GE's Noryl GTX PPO/nylon blend, which Saturn is increasingly using. While there are no production results to report as yet, the screw has helped Saturn to standardized its production equipment so that any job can run on any appropriately sized machine.