Extrusion Gets a Boost from New Equipment

Originally Published in MDDI May 2001Technological advances promise many improvements to extrusion
processes and products.

William Leventon

The process begins with pellets or powder and ends with medical-grade tubing. In between is the multistep, multicomponent process of extrusion. Despite manufacturers' continued reliance on conventional processes, new technologies are being implemented more frequently up and down the extrusion line. Among other things, this new equipment is intended to boost extrusion accuracy and repeatability, reduce scrap and downtime, improve cooling processes, and enhance product quality.

A KEY ELEMENT

Extrusion system designs vary, but the screw is a key component in each of them. Screw design was a major focus of engineers at Davis-Standard Corp. (Pawcatuck, CT) during the development of the company's Corporate extruder line. The engineers acquired two patents for the design of the extruder's barrier screw. "As far as I know, it's the first new thing to happen to barrier screws in 10 years," says Charles Sparacino, systems sales engineer at Davis-Standard.

The function of the barrier screw is to separate melted from solid material inside the extruder. In typical systems, this process has little effect on the material in the solid pool. To address this, Davis-Standard added a second flight to the barrier screw—locating it in the solid pool. The new flight configuration, according to Sparacino, mixes material in the solid pool more thoroughly and reduces the time needed for the melting process, which can decrease the generation of imperfections in the extruded product.

PUMPING UP THE PROCESS

As extrusion equipment pushes material through the system, gear or melt pumps are often used to perform a metering function. What's the advantage of this arrangement? "Everybody in the medical field talks about accuracy and repeatability rather than speed," says Richard DeCew, sales manager for Harrel Inc. (Norwalk, CT). "Everything has to be accurate." To meet the needs of these manufacturers, gear pumps must be capable of metering material volume with 1% accuracy. DeCew explains that his company's gear pumps have pressure sensors at both ends to stabilize pressure changes along the length of the system, which boosts metering accuracy to 0.1%.

Increasing the accuracy of new systems, however, can also boost both equipment and operating costs. "When you have a small machine like a medical tubing extruder, the gear pump costs almost as much as the whole extruder system," notes Frank Nissel, president of Welex Inc. (Blue Bell, PA). More over, the pumps must be cleaned thoroughly whenever extrusion materials are changed. The cleaning process can take hours and often can fail to remove all of the residual material from the pump.

Chuck Keyes, an extrusion engineer for Chase Medical (Athens, TX), explains that after he took over the extrusion process for a former employer, he immediately removed the gear pump from the line. As a result, "I was extruding better product." He reasons that his employers "weren't getting the pump cleaned very well, so they were getting black specks in a lot of their extrusions."

But what about the need for accuracy? Extrusion precision can be achieved without the use of a melt pump, according to Robert Bessemer, extrusion product manager for Conair Group Inc. (Pittsburgh). "A melt pump is a Band-Aid for an improperly designed screw," says Bessemer, quoting a colleague. "It's a great device if you're trying to use a general-purpose extrusion screw to move various materials. But a general-purpose screw isn't a great screw for any one material."

Bessemer believes that a screw designed for a specific material will perform well in most instances. "Some extruder manufacturers will tell you that when they're putting a line together, they try their best to use a screw designed for a particular material—and only use a melt pump as a last resort."

DRIVE FOR ACCURACY

In addition to the screw and melt pump, the drive is another component that affects extrusion accuracy. Because they offer greater precision, ac drives have replaced dc drives in most extrusion systems. According to Joseph Scuralli, president of Wayne Machine & Die Co. (Totowa, NJ), ac drives "are more accurate because they give you full torque across the whole speed range. A dc drive has to attain a certain speed before you get full torque out of it." He adds that advanced ac drives have become so accurate that some users have been able to eliminate the use of melt pumps in their systems.

Use of ac drives offers other advantages over their dc counterparts. The ventilation systems required for dc motors can blow carbon dust and other pollutants into the atmosphere. In contrast, ac drives are enclosed, which can promote cleaner operation. Welex's Nissel adds that ac drives require less maintenance than dc drives and are less expensive in smaller size ranges.

To manage the extrusion process, Nissel's company now offers a microprocessor-based control system. The system's Windows-based software facilitates the recordkeeping process. "Users can take all the machine's operating parameters, analyze them in many different ways, and make spreadsheets," Nissel says. "And they can store the data forever."

To satisfy FDA requirements, tubing manufacturers must keep detailed records of their extrusion process. Small manufacturers, unable to afford expensive process controllers, often keep records by hand. For these manufacturers, Davis-Standard developed midrange process controllers that are less expensive than the company's Epic controllers. Unlike the Epic, the firm's Mesa controllers do not communicate with every piece of equipment in the extrusion process; however, communication can be maintained with important equipment, such as the extruder, melt pump, and downstream puller.

Mesa controllers also provide limited levels of reporting. "You'll have to depend on something other than the Mesa to do all the reporting you need," Sparacino notes. "But if you have a gauge in the line, you can get reports out of it for dimensional control."

Process control can be especially challenging during extrusion of multilayer products. In multilayer extrusion, each separate material requires its own extruder. With several extruders around a single die, an operator would have difficulty adjusting the individual drive speeds during the extrusion process. Instead, separate drives are often linked to a master in a configuration called a coordinated drive system. Sparacino estimates that Mesa controllers can probably handle up to six drives in such a configuration.

Process controllers are linked to equipment that monitors the dimensions of extruded tubing. To ensure that the cross section is round, the Harrel system uses x-y lasers that take outside diameter (OD) measurements in two directions. In previous versions, individual measurements were generated on displays in such rapid succession that operators had difficulty reading the numbers. Now, almost all laser measuring devices display only the average value of every 100 measurements taken.

While the Harrel system uses lasers to check the OD, ultrasound equipment measures wall thickness. These dimensions are then communicated to the process controller. If the product is out of tolerance, the control equipment makes the necessary correction. Although this control process is not new, the system is faster than previous versions, Sparacino reports. With signals traveling more quickly to and from control equipment, extrusion systems produce a smaller amount of flawed tubing than in the past.

SMALL EXTRUDERS

To make small plastic tubing, medical manufacturers are turning to a new generation of small extruders. "All of a sudden, 1/2-in. extruders are becoming very common," Bessemer notes. "And there are companies that make them even smaller." Small extruders are also available for products fabricated from silicone rubber. "For a long time, everybody had to use large extruders for rubber because nobody was making small ones," says Scuralli, whose company now offers silicone rubber extruders as small as 5/8 in.

According to Scuralli, manufacturers often find it more difficult to extrude silicone rubber than plastic because the rubber must be roll-fed into the extruder. To simplify this process, Wayne developed a special roll-feeding system that is electronically linked to the screw drive. The system is intended to simplify operation by allowing speed adjustments to be made electronically. The use of different feed sections enables the system to be adapted to different applications.

Instead of the worm-drive gearboxes that are commonly used in small extruders, Wayne's equipment incorporates helical gearboxes that provide the additional torque that is required to process silicone rubber. Helical gearboxes also give manufacturers a level of power transmission that minimizes product variance, Scuralli states.

Another small extruder incoporating a heavy-duty gearbox is the portable MD extruder manufactured by Merritt Davis Corp. (Hamden, CT). Says Sandy Guthrie, president of Merritt Davis, "It's the same [gearbox] normally found in a 6-in. machine." In fact, the entire extruder includes components typically found in larger machines. Using these components, the portable extruder gives manufacturers the degree of ruggedness generally found in floor-mounted models, Guthrie claims. The MD extruder also incoporates a logarithmically curved hopper. Conventional hoppers have angles and flats that can impede flow. Using the curved hopper design, on the other hand, has been shown to produce an even flow that can improve material mixing and product quality.

DOWN THE LINE

Farther down the extrusion line, the processed tubing is pulled from the die and moved through a cooling tank. Variations in pulling speed can cause inconsistencies in tubing OD and wall thickness. Not surprisingly, manufacturers want drives that can minimize these changes in pulling speed.

A few years ago, the majority of medical tubing manufacturers were using digital puller drives. The current choice among manufacturers, however, is the servo drive. "The servo drive is the Lamborghini of drives," says Bessemer. "It's both an advanced high-speed controller and a high-speed motor. Its accuracy and repeatability are much better over a wide speed range than anything else on the market."

Current extrusion lines race along at higher speeds than were possible previously. Of course, higher rates require additional cooling. Guthrie estimates, however, that 90% of medical tubing manufacturers have difficulty providing adequate cooling space on the factory floor. "Say you need 60 ft of cooling for a certain line speed," Guthrie explains. "Then your line speed increases and you need 100 ft of cooling. If there's a wall at the end of your line, you can't just add another 40 ft of cooling."

To address the space problem, Davis-Standard has developed multiple-pass cooling tanks. Extruded tubing can require as many as four trips through these shorter tanks rather than a single pass through a much longer tank. In another variation of multiple-pass cooling, tubing is wound around a capstan and spray cooled while the capstan rotates. "Spray cooling is starting to catch on in the industry," says Guthrie. "It's evaporative cooling, which is the most efficient kind."

AUTOMATE FOR ACCURACY

Most extrusion systems can be run manually. But proponents of automation say it delivers more accuracy with less scrap and less downtime than manual operation. "Our rule is that if you do it manually on a repetitive basis, you should automate it," says DeCew. Harrel's extrusion equipment, he notes, is fully automated. "By that, we mean that you drop in pellets or powder at one end and get out a finished product that meets specifications at the other end."

Following setup, an automated extrusion process takes care of itself, Scuralli explains. "If you fill up the hopper and start the machine, the thing will just make tubing all day. In fact, the operator doesn't even have to be there. Lasers watch everything. If the product starts to go out of spec, corrections are made automatically."

DeCew describes extrusion automation as "a trend, but a very slow one from where we [at Harrel] sit. We moan and groan about it all the time." He explains that resistance to automation is partly a matter of economics for some manufacturers. "They're very reluctant to get rid of the equipment in their factories because they put a lot of money into it," DeCew says.

There can be other, noneconomic, reasons as well. "If you're running the same product all the time, automation would be fine," says Keyes. "But I'm running three or four different products every few days, and I have to make major changes to my setup." Far from making the process easier, automated equipment can make changeovers more difficult and expensive, he asserts.

CONCLUSION

From one end of the line to the other, new equipment has made its mark on the extrusion process. Among these are enhanced screw designs, melt pumps for better metering functions, new drive components, and microprocessor controls and automation systems.

Some of the equipment has raised new operational issues, with experts questioning whether implementation of such systems cause more problems than they solve. For tubing manufacturers looking to upgrade their systems, however, the equipment offers the promise of both better extruding and better end products.

The importance of maintaining quality in the extrusion process for manufacturing medical devices is often emphasized. Says Bessemer, "In extrusion, there are so many variables that there's always going to be tweaking. So you still need an elite operator." He adds that, "It's not like making a garden hose. If medical tubing bursts, it can kill somebody."

William Leventon is a freelance writer living in Somers Point, NJ.

Copyright ©2001 Medical Device & Diagnostic Industry