Glen Guillemette, president of Guill Tool & Engineering, explains how manufacturers can optimize the extrusion process.
Tooling maintenance
In order to optimize extrusion efficiency, quality and overall productivity for medical tubing applications, tooling maintenance is necessary.
The use of a dedicated work cart exclusively reserved and equipped for extruder head maintenance is also recommended. This cart, along with a supply of spare components and hardware is easily justified, especially when examining the potential cost savings that result from well-maintained tools.
It is really important to realize that any misalignment of the tools may be exaggerated in the final product output. Therefore, machining tolerances are held extremely close on today’s multi-lumen and multi-layer medical tubing by utilizing relevant production equipment and processes.
Cleanliness is critical
Clean parts, especially with sealing and locating surfaces, are key to product performance and successful end products. These surfaces receive the most care and attention during manufacturing and are the control surfaces that ensure uniformity throughout the tubing. Remember, precision-machined alignments are affected by even a speck of dirt measuring only a few thousandths of an inch. A human hair is about 0.003" (0.08 mm), and since there are many such surfaces in a quality tool, cleanliness is critical.
It is also critical to check the tools for any deformities. Burrs, scratches and scrapes are usually a result of careless handling and/or storage of equipment.
Key factors to consider during the process
- Maintain a clean, organized work area with soft and clean renewable work surfaces
- Use a vise with soft jaws e.g. copper
- Use special equipment e.g. tip removal tools
- Standard tools include wrenches and soft-faced hammers
- Maintain a supply of soft and clean rags
- Use cleaning solutions in a spray bottle
- Use spare parts as suggested by your tooling supplier, which are properly organized and stored
- Keep your equipment’s repair/maintenance manual handy
- Have a small surface plate in order to provide a true flat surface
- Use a set of appropriate gauge and tip pins for initial tool location adjustment
- Make sure you have all the proper lifting aids available, including overhead hoists and hydraulic lifts. In most situations, the head and tooling will still be at elevated temperatures, therefore lined gloves are needed during handling.
Case study
In this example, with an improperly centered tool, a calculated out-of-tolerance area of 0.059 in2 (38 mm2) was derived. When the two surface areas were compared, the calculated material waste was 11.8% of the finished product. The formula is % wall = min. wall thickness, max. wall thickness X 100.
Alternatively, if the % wall can be increased from 80% to 95%, savings of about 12% of total cost can result. Savings will vary depending on the designs.
Useful tips for the procedure
- Clean your equipment while it is still hot as the residue is easier to remove. It helps to remove and clean one piece of tooling at a time in order to maintain elevated temperatures.
- When cleaning a dual compound crosshead (plastic and rubber), clean the plastic tooling first and the rubber second.
- Never use steel tools such as scrapers or screwdrivers because these can scratch and mar the tooling.
- Do not use open flames because this generates excessive heat especially in thin sections. This can affect hardness, concentricity and tolerances of components. Recommended cleaning tools and materials include:
- Brass pliers to grip material and aid in pulling.
- Brass scrapers that are available in different widths for cleaning flat exposed surfaces.
- Brass bristle tube brushes that are available in diameters from 1/16" to 1" in 1 /16" increments (ideal for cleaning holes and recesses).
- Brass rods - Different diameter rods are good for pushing material out of 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. Also, polishing compound restores polished surfaces.
- Compressed air which is more effective for releasing plastic but also aids in rubber removal. Be careful not to force debris into recesses with compressed air.
- Cleaning solutions may be useful, so remember to use fresh, clean rags (used rags often have metal chips embedded in them, which may scratch polished surfaces).
- Cleaning oven should be used only for plastic. It is vital to follow manufacturer's recommendations. If no temperatures are specified do not exceed 850 degrees F (454 degrees C). Also, do not quench tooling to cool, as this could affect tooling hardness, concentricity and tolerances.
- Purging compounds - several are offered to purge the extruder screw/barrel of residual polymer and rubber compounds.
How to make tool cleaning easier
The quickest way to remove the die is to employ the pressure of the extruder to push it out.
Clean the body by using an air compressor and brass pliers so that the material cools down which increases the melt strength, making it into one-lump versus an elastic, gummy-like substance that is harder to remove. Clean the body feed port using compressed air and brass pliers to simultaneously cool and remove the excess residue from the feed ports. This procedure is followed by brushing with a round brass brush that polishes the surface. The flow area of the 2" (51 mm) flange adapter should be cleaned by carefully using a brass brush.
Most manufacturers recommend using a hand polishing stone to remove the offending burr. Follow stoning with a light application of 600-grit emery cloth if necessary but avoid rounding edges that are intended to be sharp. Flat sealing surfaces can also be cleaned using a stone, followed by a 600-grit emery cloth. Place the cloth on a clean, flat surface, preferably a surface plate, then apply friction in a circular hand motion until the area is clean and even. The parts in question should all be hardened steel alloys and will not be adversely affected using these methods. Inconel, Monel and Hastalloy are typically not heat-treated, requiring special care and handling to avoid any damage.
Reassembly: The final step of the process
Working from your dedicated tool cart, follow the manufacturer’s instructions for reassembly. Give each component a final wipe down with a clean rag before installing. Even the smallest amount of grit, dirt and residual material must always be removed. To avoid any unnecessary mishaps, use mechanical or manual assistance for heavy and awkward components.