Today's machining tolerances are held extremely close on multilumen and multi-layer medical tubing through the use of state-of-the-art production equipment and processes. Any misalignment of the tools may be exaggerated in the final product output.
With this in mind, clean parts, especially with sealing and locating surfaces, are key to product performance. These surfaces receive the most care and attention during manufacturing and are the control surfaces that ensure uniformity throughout the tubing. It is important to note that precision-machined alignments are affected by even a speck of dirt measuring only a few thousandths of an inch.
Deformities can also affect performance. Burrs, scratches, and scrapes are usually a 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.
If the head is disassembled in order to change compounds and tips and dies, foreign matter can be introduced. Any residual materials must be thoroughly removed. Physical tool damage often occurs during this phase due to mishandling and poor storage techniques. These are highly precise parts, but can also be heavy and bulky to remove by hand.
For proper maintenance a dedicated work cart should be reserved exclusively for extruder head maintenance. 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. The work area must be clean and organized. When maintaining tools, it is important to use a vise with soft jaws, such as copper. In addition, use special equipment, such as tip removal tools, etc. Standard tools include wrenches, soft-faced hammers, etc. Maintain a supply of soft, clean rags, and use cleaning solutions in spray bottle. Use spare parts as suggested by the tooling supplier, properly organized and stored. The equipment’s repair/maintenance manual should be readily available. A small surface plate is essential to provide a true flat surface, and a set of appropriate gauge and tip pins will be needed for initial tool location adjustment. Finally, all proper lifting aids, including overhead hoists and hydraulic lifts, should be easily accessible. In most situations, the head and tooling will still be at elevated temperatures; therefore, lined gloves are needed when handling.
Many extruders experience downtime because of poor or damaged tooling and excess maintenance time, which is costly in terms of time and money. Some start up quickly and make scrap, whereas others start up and run a product oversized to hold minimum tolerance. They waste 10–20 percent of the material, which can run from 50 to 90 percent of the product cost.
To prevent mishaps, it is essential to ensure that the operators have help for heavy parts and awkward situations. Because many surfaces and edges are hard and even brittle, dropping a part or striking parts together can damage the part. Tools should be stored in a dry, clean area, preferably in a dedicated area with soft surfaces. Tools should be segregated so that they do not come into contact with each other, and tools and instruments should be cleaned thoroughly before storage.
Purpose-built tooling, which should be available from the supplier, should be used to facilitate disassembly. The cost of these tools is easily offset by potential damages, frequently caused by use of improper equipment such as hammers and drifts.
The guidelines outlined in the operator’s manual provide specific recommendations for individual tools. But there are some general cleaning guidelines that apply to all tools. Clean equipment while it is still hot because the residue is easier to remove. Remove and clean one piece of tooling at a time. When cleaning a dual-compound crosshead (plastic and rubber), clean the plastic tooling first and the rubber second. Also, never use steel tools such as scrapers or screwdrivers because these can scratch the tooling, and avoid open flames because that will generate excessive heat.
Recommended cleaning tools and materials include brass pliers to grip material and aid in pulling, brass scrapers, brass bristle tube brushes, and brass rods for pushing material out of flow holes. Copper gauze is great for cleaning and polishing exposed round or conical surfaces, and copper knives are best for removing residue from recesses and other hard-to-reach areas. Compressed air aids in releasing plastic and removing rubber. Be careful not to force debris into recesses with compressed air. Cleaning solutions may be useful, so remember to use fresh, clean rags. A cleaning oven should be used only for plastic. It is important to avoid quenching tooling to cool because it can affect tooling hardness, concentricity, and tolerances. Purging compounds are useful for removing residual polymer and rubber compounds from the extruder barrel.
Removing Excess Material
Clean parts are critical to performance and quality manufacturing, especially for the sealing and locating surfaces that control uniformity of the production process. Thorough cleaning and removal of the excess material ensures the precision machining alignments required to produce end products to the precise tolerances. Residual polymer and rubber is easier to remove from equipment while it is still hot. When heating the tooling, refer to all material safety data sheets for guidance. To protect the hands from the heated tooling surfaces, wear thermal gloves.
Using the pressure of the extruder is the quickest way to push the die out. The body can be cleaned by using an air compressor and brass pliers so that the material cools down. This method increases the melt strength, making it into one lump instead of an elastic, gummy-like substance that is harder to remove. The body feed port can be cleaned using compressed air and brass pliers to simultaneously cool and remove the excess residue from the feed ports. Brushing with a round brass brush that polishes the surface will work for this step. Carefully clean the flow area of the flange adapter with a brass brush.
Any irregularities such as burrs and scratches must be repaired before the head is reassembled. Most manufacturers recommend using a hand polishing stone to remove the burrs. If this does not completely remove them, a light application of 600-grit emery cloth can be used. If flat sealing surfaces are cleaned using an 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.
Each component should be wiped down with a clean rag before installing. All grit, dirt, and residual material must always be removed. Use mechanical or manual assistance for heavy and awkward components to avoid unnecessary mishaps. Reapply anti-seize compound to all fasteners if required. Tighten fasteners to manufacturer’s recommended specifications as well as in the recommended sequence. This fastening sequence should be specified in the manual. Gradually tightening the proper torque is achieved prevents distortion of the tooling.
A properly manufactured and aligned extruded head, along with well-maintained tooling, should require little or no adjustment. 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.
This article was written by Glen Guillemette, president of Guill Tool & Engineering Co., West Warwick, RI. For more information, Click Here .