By utilizing state-of-the-art production equipment and processes, machining tolerances are held extremely close on today's multi-lumen and multilayer medical tubing. It is important to note that any misalignment of the tools may be exaggerated in the final product output. 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 in. (0.08 mm), and since there are many such surfaces in a quality tool, cleanliness is critical.
Checking tools for any deformities is also important. Burrs, scratches, and scrapes are usually a result of careless handling and/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. These are highly precise parts but can also be heavy and bulky to remove by hand. Use of a dedicated work cart exclusively reserved and equipped for extruder head maintenance is 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. The following procedures should be part of the process:
Maintain a clean, organized work area with soft and clean renewable work surfaces.
Use a vise with soft jaws, such as copper.
Use special equipment, such as tip removal tools, etc. Standard tools include wrenches, soft-faced hammers, etc.
Maintain an adequate supply of soft, clean rags.
Use cleaning solutions in spray bottle.
Use spare parts as suggested by the tooling supplier, properly organized and stored.
Keep handy the equipment's repair and maintenance manuals.
Have a small surface plate 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, hydraulic lifts, etc.
In most situations, the head and tooling will still be at elevated temperatures; therefore, lined gloves are needed when handling. Today, tubing manufacturers compete with companies all over the world. To be a successful and profitable company, quality and efficiency are essential. This is especially true in extrusion, where material costs are usually much higher than labor costs. Like a racing car stuck in the pit, many extruders sit idle because of poor or damaged tooling, plus excess maintenance time. Overhead costs add up and losing money is the result. 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 50–90 percent of the product cost. The tooling supplier goes to great lengths so that tips and dies are machined to a determined specification, ensuring perfect concentricity and alignment. The material is then distributed in the proper location as part of the finished product.
Understanding Maintenance Procedures
It is important to get organized before you even start. Here are some examples:
Example 1. In this example, with an improperly centered tool, a calculated out-of-tolerance area of 0.059 in 2 (38 mm2) was derived. When the two surface areas were compared, the calculated material waste was 11.8 percent of the finished product.
Example 2. Alternatively, if the percentage wall can be increased from 80 to 95 percent, a savings of about 12 percent of total cost can result. Savings will vary depending on the designs, of course.
Get help for heavy parts and awkward situations. Surfaces and edges are hard and therefore somewhat brittle, so dropping a part or striking parts together can result in damage. Store tools properly in a dry, clean area — a dedicated spot for each tool is best. These areas should have soft surfaces, and each instrument should be covered after cleaning. Also, tools should be segregated so that they do not come into contact with each other. And tools and all instruments should be cleaned thoroughly before storage.
For disassembly of tools, it is imperative to use purpose-built tooling to facilitate disassembly. These should be available from the supplier. If they are not, consult with a reputable tooling house for replacements. The cost of these tools is easily offset by potential damages, frequently caused by improper equipment such as hammers and drifts. Follow the guidelines outlined in the operator's manual. Individual tools may have specific recommendations, so contact the supplier if anything is unclear. The supplier understands that optimum performance relies on proper care and maintenance. Here are some useful tips:
Clean 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; 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, which 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 available in different widths for cleaning flat exposed surfaces.
Brass bristle tube brushes that are available in diameters from 1/16 to 1 in. in 1/16 in. 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 — for plastic only. Follow manufacturer's recommendations. If no temperatures are specified, do not exceed 850 °F (454 °C). Don't 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.
Optimum Machining Efficiency
Remove excess material for optimum machining efficiency. Clean parts are critical to extrusion tooling performance and quality manufacturing. This is especially true for the sealing and locating surfaces that control uniformity of the production process. For general maintenance of the tools, before storage or tooling changeover, a thorough cleaning and removal of the excess material ensures the precision machining alignments required to produce end products to the precise tolerances.
Equipment should be cleaned while it is still hot because residual polymer and rubber will be easier to remove. Be sure to follow all Material Safety Data Sheet (MSDS) recommendations when heating the tooling. Thermal gloves are used to protect the hands from the heated tooling surfaces. A brass scraper, as well as a brass or copper wool cleaning cloth, are recommended because they are soft enough not to scratch the surface.
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 in. (51 mm) flange adapter should be cleaned by carefully using a brass brush.
Examine all surfaces for any irregularities such as burrs and scratches since these must be repaired 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 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 Hastelloy® are typically not heat-treated, requiring special care and handling to avoid any damage.
Don't Overlook Repairs
Tooling maintenance helps ensure a quality extruded product — one that meets dimensional specifications, maintains the specified minimum tolerance, and is economically produced. Dirty, neglected, and improperly adjusted tools contribute to excessive compound applications, which in turn complicate maintenance of minimum thickness tolerance. Excess material results in unnecessary costs, and these directly affect the profitability of the company and the relationships with customers.
The Important Final Step — Reassembly
Working from a 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. 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 and is generally in a star pattern.
Tighten gradually until the proper torque is achieved to prevent distortion of the tooling. One of a die manufacturer's main goals is to form a concentric cone 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 the final product retains memory of this imbalance and unpredictable die swell occurs.
This article was written by Glen Guillemette, President, Guill Tool & Engineering, West Warwick, RI. For more information, Click Here.