CLEVELAND — Very high outputs and short residence times are two advantages of the new high-torque, high-speed compounding extruders, equipment makers said at a June 25 presentation during Plastics Fair Cleveland.
Billed as compounding's ``muscle machines,'' high-output machines can run faster than ever before — topping 1,200 revolutions per minute. In the early days of twin-screw extrusion, rates were around 350 rpm. Steady advances in gearbox design and screws have produced machines with enough torque to make the screws fly.
What happens when the screws turn that fast? Some resins don't run as well.
``Everything is not going to mix well at high speed,'' said Brad Gannon, compounding division manager at Davis-Standard Corp. in Pawcatuck, Conn. Although physical blending works well at high speeds, alloying — the chemical bonding of different materials — gets harder, he said.
Extrusion machinery officials said high-speed extruders have become much more application-dependent than traditional, slower extrusion machines, which could run pretty much anything.
``As speed increases, screw design and application flexibility decreases,'' Gannon said.
Peter Wickenheisser of Farrel Corp. agreed: ``The screw design becomes much more critical to the process. The word `general-purpose' screw design is going to disappear at 1,500 rpm. It's going to be very process-specific.'' Screws also will wear out faster.
Although high speed is not for every resin, some materials run much better, a fact resin companies and compounders are discovering in tests.
While melt temperature rises with high speeds, the resin flows through much more quickly, speakers said.
``In certain cases — I'm not saying in every case — because of the shorter residence time you can actually make a better product at higher speeds than you can at lower speeds. That's the excitement of high-speed, high-torque compounding,'' said James Schak, director of marketing and sales at Welding Engineers Inc., which was purchased in April by National Feedscrew & Machinery Inc. of Massillon, Ohio.
Welding Engineers sells SS twin-screw machines from Toshiba Machine Co. Ltd. in Japan. SS machines can run as fast as 1,450 rpm.
In Cleveland, the June 25 session amounted to high-output advocates preaching to the converted. The morning presentation by representatives from six extruder makers drew just 15 people, and just a few of them were from actual compounders. Each company spoke for 30 minutes.
One point of debate came when Davis-Standard's Gannon disputed Welding Engineers' claim that its SS machine has a higher torque per free volume than competing machines. Gannon said Davis-Standard's Alpha Class is higher.
Everyone agreed on the benefits of short residence time, which cuts down on material degradation. At 300 rpm, resin stays in the screw and barrel for 45 seconds, said Daniel Mielcarek, manager of process applications at Krupp Werner & Pfleiderer Corp.
But at 1,000 rpm, he said: ``We're measuring residence times below 10 seconds. That's an advantage for compounding.''
The quick residence time makes feeders more important than ever.
``We need to rely on the feeder manufacturers to provide more accurate feeders than in the past,'' Mielcarek said.
Krupp W&P of Ramsey, N.J., makes the ZSK Mega compounder.
Another challenge is shear, especially its impact on additives, such as color. Mielcarek said concerns about shear of organic pigments at high speeds has prompted machinery makers to introduce color downstream, via side-feed extruders.
Schak said new gearboxes have made the superhigh outputs possible.
``You really needed torque development along with the speed to keep your melt temperature low and get a lower overall average shear rate,'' he said.
Martin Mack, vice president of research and development at Berstorff Corp. of Florence, Ky., said the company's Ultra Torque boosted torque 20-30 percent over the same size of traditional machine, thanks to a spline shaft and improved gearbox.
Mack said the output rate is about more than how fast the screws turn.
``You need to look at more sides than speed and rate. Temperature is definitely a factor in high-speed machines,'' he said.
Berstorff's Ultra Torque machines have two-section screws. Each section has a different diameter. The two sections disconnect to pull out the screw for maintenance.
Wickenheisser, Farrel's director of process applications for compounding, said the company in Ansonia, Conn., focuses on mixing.
``On the old machines, with slower screw speeds, the uniformity of material was very consistent. It may not have been high, but it was consistent,'' he said.
Wickenheisser advised compounding firms not to buy a too-large machine, because torque on each machine is set according to the extruders' maximum output.
``Buy a machine that's suited to what you're going to run,'' he said.