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Emphasis on Tooling: Guidelines for Ensuring Quality When Extruding Medical Devices

Tue, 11/01/2005 - 11:32am
Productivity and profitability are two important goals when working with extrusions. This exclusive report offers advice for producing extrusions that meet dimensional specifications and maintain specified minimum tolerances so that end products can be economically produced.


The thicker a medical tubing wall becomes, the greater the savings. However, wall thickness depends upon application requirements, which may not enable a heavier wall. If the wall can be increased from 80 to 95 percent, the result is savings of about 12 percent of total cost. (Click to open larger image in new window)
AT A GLANCE
• Material and labor costs
• Obstacles to precision
• Proper tool handling
• Basic requirements

By A. Roger Guillemette and Glen Guillemette
In the extrusion of medical products, the cost of materials can be higher than the cost of labor. Any extruder that produces excess scrap is inefficient and expensive to run. Waste material of as little as 10 to 20 percent accounts for 50 to 95 percent of the final product cost. In addition, when an extruder sits idle because of poor or damaged tooling, overhead costs add up and money is lost. Running a product oversize to hold minimum tolerances also can be wasteful.

With these factors in mind, it becomes clear that maintenance is an important part of the on-going quest for extrusion productivity and profitability in medical manufacturing. Proper maintenance and cleaning are critical and controllable. They ensure that extrusions meet dimensional specifications, maintain specified minimum tolerances, and deliver an economically produced end product.
Misalignment and Imperfections
Any misalignment of tools can be exaggerated in the final medical tube or product. It’s important for the tooling supplier to make every effort to ensure that tips and dies are machined to a determined specification so that perfect concentricity and alignment are achieved.

Making sure parts are clean, especially with sealing and locating surfaces, is key to product performance. Such surfaces receive the most attention during manufacturing and are the control surfaces that ensure uniformity. Because a speck of dirt measuring a couple thousandths of an inch affects precision-machined alignments, cleanliness is critical. Also important is checking tools for any deformity. Burrs, scratches, and scrapes are usually the result of careless handling or storage of equipment.

Double- and triple-layer extrusion heads pose an even greater challenge for maintenance. The number of sealing and centering surfaces multiplies and can magnify the results of dirty tools. During changeovers, the head may be disassembled in order to change compounds and/or tips and dies. Foreign matter is usually introduced at this point and residual materials must be thoroughly removed. Physical tool damage often occurs during this phase due to mishandling and poor storage techniques.
Important Guidelines
A dedicated work cart, one that is reserved and equipped for extruder head maintenance, should be used. This cart, along with a supply of spare components and hardware, is justified when examining the potential cost savings resulting from well-maintained tools. Also recommended are an organized work area with soft, clean renewable work surfaces; a vise with soft jaws, perhaps made of copper; special equipment such as tip removal tools; standard tools such as wrenches and soft-faced hammers; soft, clean cloths; a spray bottle of cleaning solution; properly organized spare parts that are suggested by the tooling supplier and listed in the operator’s manual; equipment repair/maintenance manuals; a small surface plate that provides a true flat surface; and a set of appropriate gauge and tip pins for initial tool location adjustment. In addition, proper lifting aids, including overhead hoists and hydraulic lifts, are advised. In most situations, lined gloves are required because the head and tooling are at elevated temperatures.

Tools should be stored in a dry, clean area. Having a dedicated spot for each tool is best. Each area should have soft surfaces and each instrument should be covered after cleaning. The tools should be separated so that they do not touch each other. All instruments should be cleaned thoroughly before storage.

When disassembling tools, it’s imperative to use purpose-built tooling available from the supplier. If it’s not available, a reputable tooling house should be consulted for replacements. Also, the guidelines outlined in the operator’s manual should be followed. Because individual tools may have specific recommendations, the supplier should be contacted if anything is unclear.Additional guidelines include the following:

• Clean equipment while it’s hot when the compound is easier to remove.
• Remove and clean one piece of tooling at a time in order to maintain elevated temperatures.
• When cleaning a dual-compound crossheads, clean the plastic tooling first and the silicone tooling second.
• Never use steel tools such as scrapers or screwdrivers because they can scratch the tooling.
• Do not use open flames, which generate excessive heat especially in thin sections and can affect hardness, concentricity, and the tolerances of the components.

Recommended are brass pliers to grip and pull material; brass scrapers in different widths for cleaning flat exposed surfaces; brass bristle tube brushes in diameters from 1/16 to 1 inch, in 1/16-inch increments, for cleaning holes and recesses; brass rods in different diameters for pushing material out flow holes; copper gauze for cleaning and polishing exposed round or conical surfaces; copper knives for removing residue from recesses and other hard-to-reach areas; polishing compound to restore polished surfaces; compressed air, which is more effective for releasing plastic but also aids in silicone removal; purging compound to purge the extruder screw and barrel of residual polymer; cleaning solutions; and cleaning oven for plastic, not silicone.

If the manufacturer specifies no oven temperatures, do not exceed 850°F. Also, do not quench the tooling to cool it because quenching can affect the hardness, concentricity, and tolerances of the tool parts.
Repair and Cleaning
Examine all surfaces for irregularities, such as burrs and scratches, and repair them before the head is reassembled. Most manufacturers recommend using a hand polishing stone to remove the offending burr. Follow stoning with a light application of a 600-grit emory cloth if necessary, but avoid rounding edges that are intended to be sharp.

Also, flat sealing surfaces can be cleaned using a stone, followed by a 600-grit emory cloth. Place the cloth on a clean, flat surface, preferably a surface plate, and then apply friction in a circular motion until the area is clean and even. The parts in question should be hardened steel alloys, which won’t be adversely affected by this method.
Reassembly Notes
Follow the manufacturer’s instructions for reassembly. Wipe each component with a clean cloth before installing. Even the smallest amount of grit, dirt, and residual material must be removed. Use either mechanical or manual assistance for heavy and awkward components. Reapply anti-seize compound to all fasteners if required. Tighten fasteners to the manufacturer’s recommended specifications as well as in the recommended sequence. This fastening sequence should be specified in the manual and is generally in a star pattern. Tighten gradually until the proper torque is achieved, preventing distortion of the tooling.

One of a die manufacturer’s main goals is to form a concentric cone of compound as quickly and accurately as possible in the primary section of the die when the extrudate first emerges from the die’s distribution capillaries. A properly designed and manufactured die has even distribution close to the extrudate entrance point, but this effort is negated once the die is adjusted, shifting the extrudate off to one side. An eccentric cone is formed in the primary area, and a concentric cone exists at only one point in the process, rather than a smooth, continuous flow path with decreasing volume. A properly manufactured and aligned extruded head, along with well-maintained tooling, should require little or no adjustment.

Another adverse effect of unnecessary die adjustment is the stress introduced to the extrudate caused by unbalanced flow. The net effect is that the final product retains memory of this imbalance and unpredictable die swell occurs.

It is clear that dirty, neglected, and improperly adjusted tools contribute to excessive compound use. In turn, this complicates the die’s ability to maintain a minimum thickness tolerance. The excess material results in unnecessary costs that directly affect profitability.

A. Roger Guillemette is the CEO and Glen Guillemette is the president of Guill Tool & Engineering Co. Inc., 10 Pike St., West Warwick, RI 02893. Guill Tool & Engineering is a leading designer and manufacturer of extrusion tooling. More information is available by calling 401-828-7600 or e-mailing gguillemette@guill.com.
ONLINE
For additional information on the technologies discussed in this article, see Medical Design Technology online at www.mdtmag.com or Guill Tool & Engineering Co. Inc. at www.Guill.com.
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