Electrical discharge machining (EDM), which “cuts” metal from a work piece with a series of electrical sparks that precisely shape the material into simple or complex geometries, is common in the manufacture of parts for surgical tools, implants, and other medical devices.

As one of the few North American companies that manufacture what are called bimetallic cylinders or barrels, our company takes a more unusual approach. EDM helps make the cylinders, which become components of large machines such as extruders and injection molders that produce everything from complex medical devices to product packaging. The interior of the thick-wall metal barrels is a centrifugally cast alloy that is highly resistant to abrasion, corrosion, and adhesion wear.

A lot of machining is required to make bimetallic cylinders. Besides a wire EDM, the company uses centrifugal casting, sub-arc welding, multi-axis CNC machining, OD grinding, lathe, hone, and boring technologies. We also work with hard-to-machine metals such as Inconel and super-hard alloys such as tungsten carbide.

The barrels are configured to fit with other associated machinery. For example, when combined with components on plastics-processing machines, the assembly is referred to as the plasticizing unit. In other systems it may be called the masticating unit, accumulator assembly, or process-cylinder assembly. Most often, the cylinders serve as a containment vessel for a feed screw, reciprocating plunger, or other ancillary device used to feed, work, heat, cool, melt, pressurize, or discharge whatever material is being processed.

The cylinders process polymers, rubbers, metals, chemicals, and a wide variety of other materials. Because the cylinder-bore linings are free of porosity and iron content, the barrels especially suit the processing of materials such as fluorosilicone or Teflon, a fluoropolymer, used to make gaskets, seals, and other parts for medical devices.

For these applications, Wexco developed BO22, a nickel-rich trademarked alloy for the bores of barrels that will be processing severely corrosive materials. Other alloys were engineered to resist moderate to extreme abrasion, corrosion, and adhesion wear. Wexco 555, 777 Durocast, and B022 alloys are FDA-approved as suitable bore materials for the processing of plastics in medical and pharmaceutical applications.

Our company keeps a competitive edge by investing in technology and equipment that lets us make cylinders for processes including thixotropic (magnesium) molding and chemical processing. Our company also engineered Dura 686XL, a patent-pending alloy with extended wear life for the high screw-speeds found in blow molding and thin-wall injection molding. The alloy has up to 25% greater wear life when used with most resins as compared to conventional iron-boron bimetallics.

Several years ago, Wexco purchased an Agiecut (us.gfac.com) 350 HSS wire EDM machine, one of the largest operating in North America. The original equipment supported the production of figure-eight barrel segments used on twin-screw extruders. We have since upgraded the controls, software, and wire-feed mechanics to expand the machine's capabilities. It now delivers high accuracy and versatile cutting on runs of multiple-piece jobs for both large and small parts that require tight tolerances. The modifications have also helped us lower costs and improve quality in the manufacture of cylinders with complex features, such as risers and short, grooved sections.

Along with the manufacture of barrel segments, we now use the wire EDM to make other medical products such as implants. In fact, many of the alloys commonly used in implants are chemically similar to alloys the company has been using for years. The company also EDMs items from simple tools and fixtures to complex part geometries. For most work, Mastercam (mastercam.com) software develops the cutting profiles. The well-known software can import geometry from many CAD packages, so parts can be made to complex specifications. Because of the specialized nature of wire EDM, the company has also found it helpful to develop NC code for specific applications to further reduce the cost of the process.

A senior process designer works side-by-side with production personnel to carry out EDM fixturing, programming, and operating tasks. This along with good setup tactics lets parts be machined, unattended, in single run or multiple quantities for maximum cost reduction. The machine is thereby production-orientated, maintaining consistent tolerances and surface finishes across multiple burr-free pieces - process capabilities that are especially important to medical designers, but which are not usually obtainable by conventional machining. Common design features that lend themselves to wire EDM include thin-wall sections, complex cavities, sharp edges, small radii, intricate detail and features, and extremely hard or soft materials requiring special toolfeed speeds.

Our company also performs electrode EDM, also known as ram, sinker, or plunger electrical discharge machining. These machines use a custom-made electrode to remove material in a detailed pattern. Our machinery typically makes precision grooves and other geometries such as port openings in the inner bore diameter of cylindrical pieces.

RESOURCES FOR MEDICAL-MANUFACTURING EQUIPMENT DESIGNERS

Resource	Primary business focus
atwfeedscrews.com	Distributor and designer of custom feed screws and barrels
americankuhne.com	High technology extrusion machinery
pima.asn.au	Repair services and distribution
concortool.com	Tool screw experts
diamondamericacorp.com	Extruders, screws, and systems
donnellyplastic.com	Plasticating components and mold greases
flitetech.com	World-wide distribution screws and barrels
plasticsnews.com	North American plastics industry news source
strictly-extrusion.com	Energy saving platform for the extrusion industry

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