Originally Published MDDI April 2002MEDICAL PLASTICS Knowing the issues unique to machining plastic components enables designers and engineers to make better choices and develop better devices.

April 1, 2002

12 Min Read
Plastics Machining: Understanding the Basics

Originally Published MDDI April 2002

MEDICAL PLASTICS

Knowing the issues unique to machining plastic components enables designers and engineers to make better choices and develop better devices.

Thomas Rohlfs

0204d40a.jpg

Because plastic is soft by nature, part configuration plays a dominant role in machining methodology.

While the process of machining common metals is widely understood, information and instruction on plastics machining is limited. In general, plastics machining is not regarded as a process separate from metal machining. Because molding is used for most high-volume part runs, the total amount of plastics machining is small in comparison to metal machining. And because plastic is soft by nature, part configuration plays a dominant role in machining methodology.

Engineers sometimes must face the challenges of designing plastic parts that require machining because of their low quantities, close tolerances, or unusual shapes. Traditionally, these engineers will turn to a machining data handbook for guidance and will attempt to apply standard principles; however, these resources typically fall short.

Individuals involved in the design or procurement of plastic components can successfully address the inherent challenges by choosing the right materials, following through with the proper dimensional tolerancing and finishing processes, and selecting the right shop for the manufacture of their parts.

MATERIAL SELECTION

The conventional approach to material selection is usually a cost-versus-performance comparison. Decisions concerning a material's chemical resistance, mechanical properties, and thermal capabilities are all found in manufacturers' catalogs. In addition, the Web site at http://www.matweb.com can serve as a helpful on-line data source. Unfortunately, even after research has been performed in catalogs and on the Web, the job of material selection still might not be complete.

Unlike metals, plastics are not created equal. Each manufacturer approaches the plastics process differently. Selection criteria must comprise more than the properties found in various catalog specification sheets.

Obtaining high-quality material is critical to making a high-quality part. The integrity of plastic materials varies widely between manufacturers. Poor-quality plastic is often soft; it will display voids, contamination, or color variation; and it might be poorly annealed. One helpful source of materials expertise is a machining specialist. Through their own experience, specialists have learned which manufacturers to use and which to avoid. Engineers sometimes research material through a distributor, but distributors can be unreliable sources of information because they may be tempted to recommend only materials that produce high profit margins.

Another critical materials selection consideration is stock size. Size becomes increasingly important as the desired plastic components get larger. While small-diameter rods usually come exactly as ordered, larger-diameter rods are oversized so that they can be cut to their exact size. For example, polypropylene rods are typically oversized. Acetal rods are purposely oversized from 0.005 in. for small diameters to 0.03 in. for large diameters. Acrylic sheets are often available in metric sizes with a 10% tolerance, e.g., ¼-in. stock is actually 0.236-in. ± 10%. Those selecting the material should note that stock sizes and tolerances vary among the different plastics.

Once the material and a manufacturer are chosen, these selections should be carefully documented. Trade names and ASTM specifications should be included to guarantee the material is purchased from an approved source, and adding a note requiring a material certificate of conformance for verification is a good idea. Obtaining material certification directly from the manufacturer is the best guarantee; a distributor's material certification is inadequate.

Unlike the metal industry, in which materials are fully traceable, the sophistication of plastics traceability is low. Traceability is frequently lost in the plastic distributor's stock. For high-end medical applications especially, careful research is necessary to ensure lot traceability.

COMMON MATERIALS PROBLEMS

Not Getting What You Need. Many materials can be produced by different manufacturing processes. Extruding, casting, and molding are common methods used to process Teflon, nylon, and acrylic. Consideration of the manufacturing process is important to achieving the desired properties of the component. Acrylic, in particular, varies in hardness between a cast and an extruded sheet. Extruded acrylic is much lower in cost and generally best used only for display-type work. For superior machining and dimensional stability, cast acrylic should be specified. Engineers creating prints without a clear designation of acrylic type run the risk of a component not meeting their expectations.

Similar principles are applicable to nylon and Teflon components. Extruded nylon is a better choice for smaller parts, and cast nylon is a good choice for near-net shapes and larger components.

Right Part, Wrong Color. A plastic's color can pose problems as well. Typically, when the product's end-user can see the component, color uniformity and repeatability are important. As a carbon additive, black is an easy choice. Within a material type, black is very consistent with little variation between lots or manufacturers.

White, on the other hand, is problematic. Some materials are labeled "natural," which may or may not be a true white. Other materials are labeled "white," but they may not actually appear white. Still other material types are offered in both natural and white.

Polypropylene is an excellent example of a material whose color varies. Some manufacturers' material is an opaque white. Other polypropylene is more of a translucent beige or yellow. Next to each other, these materials do not appear the same. The opaque white polypropylene may be confused with a polyethylene.

Color variations can be quite pronounced, especially among various manufacturers, so it is useful for engineers to communicate directly with their plastics machining specialists to discuss the issue of color. The color characteristic is sometimes not repeatable by a single manufacturer from lot to lot or through material size changes. Natural ABS, for example, is notorious for unstable coloring. The material can run from nearly white to yellow in all different sizes—all from the same manufacturer's stock.

Importing Poor Quality. Like several consumer items on the market today, some plastics are manufactured outside the United States. Imported acrylics, for example, are popular because they are inexpensive. Unfortunately, sheet thickness at the bottom of the tolerance band tends to be thinner than that of the domestic sheet counterparts. Additionally, these acrylics may not be as stable and usually have greater internal stress. Imported material is suitable for the display industry, in which cost control is paramount, but for high-end and medical applications, specifying a domestic source is recommended. The material will provide greater stability and better overall machinability.

0204d40b.jpg

Plastic components are micromachined to close tolerances.

Achieving Close Tolerances. A common assumption in the designing of plastic components is that they cannot be toleranced as close as metal parts. The real difference between metals and plastics is that plastic tolerances are more affected by material type and part configuration. Under the right conditions, it is possible to machine plastic components with tolerances as tight as ±0.0002 in. With some plastics, however, maintaining a tolerance of ±0.005 in. is admittedly difficult.

Contamination. Contamination can come from coolants and metal filings, and it can pose serious threats to the quality, durability, and appearance of machined plastic parts. Plastic machine shops are typically aware of contamination issues, so they maintain the appropriate plastic-compatible coolants. Metal shops, on the other hand, employ coolants that work well with metals. Some high-quality metal coolants on the market today will actually cause stress cracking in plastic components. For more-sensitive materials, metal coolants will not only potentially reduce the machining quality, they will attack the plastic as well.

Another potential contamination problem is that material chips from previous jobs are often present in the machine tools. While metal shops that also machine some plastics frequently struggle with this problem, plastic machine shops are rarely, if ever, contaminated with metal filings.

Improper Handling. Any type of custom-made components should be handled carefully. As one would expect, plastic components are very sensitive. Parts banging against one another can result in damaged surfaces and dented corners. Extra care should be taken to avoid rough handling, from the manufacturing process to packaging the parts for shipment.

Instability. A plastic's stability is strongly influenced by the part's configuration and its material dependence. Stability includes three characteristics: thermal expansion, outgassing, and material stress.

Thermal expansion is material dependent. Plastics such as PEEK and Torlon (polyamide-imide) are relatively resistant to thermal expansion, while polypropylenes are highly susceptible. In large components with close tolerances, body heat alone is sometimes sufficient to cause the part to move out of tolerance.

Outgassing is a process during which a material continues to emit a gas after it is processed. DuPont's Delrin acetal exhibits a classic case of this condition. Because a key ingredient of acetal is formaldehyde, acetal components left in a sealed container for a few weeks will emit a strong formaldehyde odor. This outgassing actually reduces the mass of some components, causing them to shrink.

Material stress is the least controllable and most troublesome characteristic. For instance, a skim cut placed on a flat sheet will curl. In situations like this, special handling is required. Fortunately, plastic machine shops have developed special techniques to ensure components will not arrive warped and distorted, or become that way later.

PROCESSING CONSIDERATIONS

Polishing. The most common surface-finishing methods are flame, vapor, and mechanical polishing. Flame polishing uses a hot flame to flow a surface. Operator skill is critical with this method. When done properly, however, flame polishing produces the clearest finish, especially on acrylics.

Vapor polishing is performed with a chemical vapor, which attacks the surface of the plastic and smoothes it. The plastics best suited to this process are polycarbonate, Ultem (polyetherimide), acrylic, and polysulfone. Because of the risks involved, this process should not be performed by an inexperienced person. The chemical vapors are harmful if inhaled; special equipment must be employed to prevent the chemicals' contact with the operator. In addition, various government agencies have strong regulations concerning the exposure limits to this group of chemicals. When done properly, however, vapor polishing can provide visually appealing quality finishes.

Mechanical polishing is the most common and easiest to do, and it can be performed on any plastic. One drawback of this method is that it tends to leave very fine scratches on the product's surface.

One caveat regarding all three polishing processes: polishing is all in the preparation. If the machining is not done correctly, all the polishing in the world will not fix the components. One suggestion for achieving optimum polishing work is to avoid machine shops that subcontract their polishing. These shops do not always have technicians with the knowledge or skill to properly prepare the plastic component for polishing. Polished components machined and processed at a single location tend to be of higher quality.

Annealing. Annealing, a process used to strengthen a plastic and reduce its internal stress, is a step not to be overlooked in plastics machining. Without an annealing step, some plastic components will deteriorate from stress cracking. Such cracking results from stresses that occur during the machining and polishing processes.

Annealing components is sometimes useful as a stand-alone treatment. For critical applications requiring maximum plastic stability and protection from cracking, relieving stress should be a mandatory part of the manufacturing process.

Finishing. Some applications require a superior finish to function properly; achieving it is a matter of material choice and part configuration. Plastic compositions like Teflon often produce a porous surface, thereby limiting the degree of smoothness that is possible. Other plastics burnish well.

Where required, a finish of 32 µin. is achievable with only slightly more effort than is required for the standard finish measuring 63 µin. Finishes less than 32 µin. are progressively more difficult—but not impossible—and require special tooling considerations. It is important to remember that polishing can be expected to reduce any finish by 5 µin. on a finish less than 32 µin.

CONCLUSION

Knowledge of the plastics manufacturing process can be helpful when selecting the right shop for a plastics machining job. Extra time and research should be invested if the part has special specifications or is highly technical.

Shops that specialize in plastics machining will likely provide the best and most consistent job. While metal shops can do plastics machining, particularly the most basic jobs, they often lack expertise in materials selection, contamination, burr control, and overall plastics quality. Specialists in plastics machining can also be more accommodating to clients requiring a particular tolerance or surface finish.

After identifying a few potential shops, the customer should write down a few details about the particular job desired. The candidate shops should be questioned about their machining approach and briefed on the customer's needs. Each shop's answers to the following types of inquiries will reveal its ability—or inability—to handle a plastics machining project:

  • Asking if the shop has ever worked in the specific material needed for the job. If a component must be made of PET, for example, and the shop has never worked in PET, then that shop should not be considered.

  • Asking about the particular properties of the plastic that are required for the job, such as its cost and its behavior under different conditions. If a shop cannot engage in a conversation about the required material, then that shop is not a contender.

  • Asking questions on-site to better understand the capabilities of prospective shops to handle the job. During a site visit, a customer should ask to see examples of components similar to the one he or she wants and inquire about how they were produced: Were the projected deadlines for delivery met? Did the machinist encounter any specific problems during the manufacturing process? Does the shop anticipate any similar challenges in handling this particular job?

Finally, a customer should make sure the facility is clean, well-run, and well-organized, with modern machines and knowledgeable leadership.

Armed with the basic but important facts about plastics machining and determined to seek out the shop that will best meet their needs, OEMs stand a good chance of achieving a machined plastic component of high quality and accurate specifications.

Thomas Rohlfs is chief manufacturing engineer at Connecticut Plastics in Wallingford, CT.

Copyright ©2002 Medical Device & Diagnostic Industry

Sign up for the QMED & MD+DI Daily newsletter.

You May Also Like