Technology Notebook: Screenchangers play critical role in extrusion

Screenchangers on extrusion lines meet a critical need in preventing contaminants from getting to the die and even into the end product. At least a dozen suppliers are active in marketing screenchanging equipment to North American processors. Despite this, screenchangers frequently receive scant attention from those evaluating the performance of a line.

Screenchangers are occasionally used with injection molding systems, but are far more common with continuous-flow extrusion setups. Injection molding setups commonly have magnets and other devices such as screens and electronic contaminant detectors at the hopper at the beginning of the line to filter out coins, screwdrivers, pens, metal shavings, and every other imaginable object?and some not so imaginable objects?before they cause damage to the screw.

Screenchangers function at the opposite end of the screw, just before material is fed to the die. The material filtered out by the screen, therefore, has already gone through the barrel and past the screw, and is on its way to the die. Contaminants are more on the order of gels, unmelted solids, dirt, dust, and unmixed additives.

We have sought out processors with practical experience in screenchanger use to comment on screenchanger-related issues that affect production in the field. We pass along those comments here, followed by notes on the various kinds of equipment being promoted in the marketplace.

Maintenance Minimized

Bob Donaldson of Econo-Blast Inc. (St. Petersburg, FL), says he has had great success with screenchangers from High-Technology (Hackensack, NJ). Donaldson says the unit is maintenance free, other than requiring screen replacement every couple months. He previously had hydraulic screenchangers, which he says leaked and needed maintenance.How did he settle on this type of screenchanger? Donaldson says he saw an ad in a magazine, called up the company, and bought a unit. He has had at least one of the units for 25 years, which means he dealt with the founder of High-Technology, John Alroy, generally known as "Dr. Alroy." Dr. Alroy has since died, and the company is managed by his capable daughter, Aline.

Econo-Blast has six single-screw lines running profiles and a twin-screw unit doing compounding. The extruders are from Davis-Standard (Pawcatuck, CT) and Entek (Lebanon, OR), with the singles in sizes from 2- to 4.5-inch diameters and the twins in a 36-mm size. The units run 80 to 400 lb/hr, handling engineering nylons and polycarbonates.

Rapid Payback

Marty Danco of Wellmark Inc. (Asheboro, NC) says the company has a staff of 35 and operates three shifts, 5.5 days/week. The company?s chief business is recycling and compounding to grays and blacks. It typically cross-blends output in 10,000-lb batches in mixing silos to make sure that material meets Izod melt-index and impact-test requirements.

Wellmark buys some material in an already granulated condition, but can?t pay a lot and remain profitable in the recycling business. It handles mostly PP, but some HDPE and HIPS.

Much of the material comes from plastics die tubes (cores for yarn used for yarn manufacturing?one use only). Scrap is ground up with a Conair granulator. Danco says that a continuous-flow screenchanger from PSI/ Polymer Systems Inc. (Hickory, NC) paid for itself in three to four months. Wellmark originally got a demo unit to try and was offered a good purchase price. The company had formerly been making 30 to 50 screen changes per hour in preparation for underwater pelletizing.

It mattered to them that they deal with a good supplier; the fact that PSI was located in the same state was considered favorable as well. Location matters, function matters, and price matters. Who can be surprised at that?

In addition to the line with a PSI screenchanger, the company has another line with a Kreyenborg screenchanger. It bought a reburbished extrusion line that already had a Kreyenborg unit on it. Danco says the Kreyenborg unit is fine and has presented minimal problems.

Danco?s advice to screenchanger suppliers is to make parts available, even used parts if necessary; and have a technician available as needed. His summary of the duty of a screenchanger: "Changes screens, keeps the crud out."

Consistency is the Ultimate Requirement

The project manager at a Charlotte, NC film producer that requests that it not be otherwise identified, must have consistency that is as close as possible to absolutely, positively perfect. The company has about 300 employees and operates 24/7. This PE blown-film producer says that one little imperfection can create a hole in the film. The film is 1 mil and under in thickness. Extruders come from Filmaster (Parsippany, NJ), Egan (now with Davis-Standard; Pawcatuck, CT), and Brampton (Brampton, ON). Production is limited to about 200 lb/hr to maintain precision.

Six of the eight extrusion lines use Gneuss (Matthews, NC) screenchangers, in part because Gneuss worked with the producer on their screen-pack needs. Two lines use manual Beringers (Marblehead, MA); these lines don?t need automatic screenchanging because of the low speed at which they operate.

The decision to go with Gneuss was made by a group of individuals at the plant. The group tested a Gneuss screenchanger and got the consistency it wanted. It also saw good backflush, and found the units to be sufficiently "robust."

This is something of a tough customer. There is loyalty to performance, but not particularly loyalty to a given source of equipment. It demands 24-hr consistent pressure drop, although the company acknowledge that any screenchanger causes some discontinuity. Saying that all filtration systems have pluses and minuses, it will evaluate any new ideas that emerge.

Unit Pays for Itself

Pat Cain of Alcoa (Hazleton, PA) says it uses continuous screenchanging because slide-plate screenchangers just aren?t right for the company. Its source, Gneuss, provides equipment that?s pricey but pays for itself, according to Cain.

Alcoa has one Gneuss KSF unit in England and five Gneuss RSF units in the U.S. It operates 24/7 at eight locations with about 600 employees. The HIPS, PP, and BOPS film it produces is mostly for food packaging.

Alcoa produces cast 10- to 60-mil PP and HIPS film with 130-mm extruders running at about 2000 lb/hr. Biaxially oriented 7- to 75-mil PS is done on an 8-inch extruder at 4500 lb/hr for food packaging. Welex and Davis-Standard (Egan and NRM) equipment are used, Welex for the cast lines and Davis for the BOPS line.

Cain says the best benefits from screenchanging are seen on 1- to 3-mil lines. Good screenchanging avoids blowing holes in the web. The plants formerly produced 15 to 30 minutes of scrap once every 2 to 6 hours. Now, the screenchanger runs all day.

Alcoa checked out other brands, but liked the straight-through concept of Gneuss. To get hands-on experience with a unit, Cain went to the Gneuss factory and helped assemble one of the units.

Alcoa can put regrind through at 0% to 100% (postindustrial, not postconsumer)?some in house, some from customers. The company gets at least two weeks production out of each screen. Most importantly, it gets no carbon in its film, and smooth output pressure yields a final product without holes.

Suppliers Provide Choices Aplenty

AEC (Wood Dale, IL) markets the EM Manual screenchanger with seals designed to be accessible through the slide-plate opening, allowing service without machine disassembly. Six manual screenchangers are available in sizes from 1 to 4.5 inches. All allow pulling the extruder screw through the screenchanger while it?s mounted on the barrel.

AEC?s EH hydraulic screenchangers come in models from 2.5 to 6 inches. Close coupling to the extruder is available with the standard bolt-through extruder connection for ease of installation and a good seal.

Berlyn Extruders (Worcester, MA) cites the Model SC-12M as an example of its line of slide-plate screenchangers. It has a 12-inch diameter bore, a solid one-piece forged body of 4150 alloy steel, and is reported to create a perfect seal at polymer pressures in excess of 10,000 psi. It will change screens in 1 second or less.

CRG Logics Inc./PHT (Appleton, WI and Ferrara, Italy) announced at the time of NPE 2003 that CRG Logics had entered into an agreement to become the U.S.-based sales and service company for PHT?s line of screenchangers and pelletizing equipment. Under this agreement, CRG Logics? intention was to eventually build and stock PHT?s entire line of screenchangers in the U.S.

Dynisco (Franklin, MA) presents the EC continuous-flow single-bolt screenchanger system, which allows changes to be made at full production conditions without melt-flow interruptions. Changes are initiated by operator input. When used in conjunction with a Dynisco gear pump, screen changes can be made without die-pressure fluctuation.

Dynisco Beringer?s CFW (continuous flow wheel) rotary screenchanger provides continuous filtration of the polymer stream. This hydraulically activated screenchanger also provides leak-free operation at pressures up to 10,000 psi and temperatures up to 600F.

ECON GmbH (Pasching-Wagram, Austria) describes its discontinuous screenchanger as capable of processing even low-viscosity materials such as PET and PA. It is free from leakages, accomplished by air-cooling at piston ends. It has an approximately 65%-effective filter surface for more throughput, and low-optimized design of the melt channels.

Maag/Textron (Charlotte, NC) makes a variety of manual, slide-plate, belt-type, and piston screenchangers. The SSC unit is suitable for various materials, including PET, PC, PMMA, and fluoropolymers. It operates discontinuously with a single piston and one screen.

PATT Filtration (Montreal, PQ) says it will custom-tailor continuous screenchanger equipment to filter any engineering polymer with pressures up to 12,000 psi and temperatures up to 650F.

Trendelkamp (Norcross, GA), provides the TSK screenchanger with double-piston design for discontinuous operation. Screens reportedly change without interrupting production. The TASK diverter valve and screenchanger operate as a combination of startup valve and screenchanger for continuous operation.