Machining Plastics: How Metals Techniques Could Lead You Astray

Understanding the differences between machining plastics and metals is vital to the successful design, manufacture, and performance of medical devices.

MACHINING

A flow cell with a 200-μm square flow channel machined from acrylic plastic.

Although industry practices and guidelines for machining metals are well published and have been substantiated over time, engineers seeking similar technical parameters for machining plastics can find themselves at a loss. Metal machining has existed for decades, but the craft of machining plastics is an evolving specialty that parallels the material science creating new polymers. Providers of plastic stock shapes often offer material-property guidelines for their different products, and some may even provide machining guidelines that advise on materials, speeds and feeds, and cutting tools. However, much of the proprietary experience enabling precision plastics machining has been developed in small, privately held plastics machining operations. Such experience is not found in handbooks.

For a medical designer or manufacturing engineer, considering the dynamics of machining plastics versus metals is critical to the success of a product, from design through its function. Improper machining with the wrong holding tools, cutting tools, coolant, or manufacturing process can prevent the part from meeting all the design requirements. It can also lead to premature product failure.

Fundamental to optimal processing is a solid working knowledge of plastic materials including how they behave during manufacturing and in an application. For an engineer who cannot rely on this capability in-house, outsourcing is a better option. It is important to find a supplier that specializes in machining plastics and has the experience, equipment, and engineering expertise in both design and manufacturing processes.

While machining plastics and metals involves many of the same challenges, each requires very different methods. Machinists attempting to transfer the techniques, tools, and practices of metal processing soon discover that machining plastics is as much of an art as it is a technology, and that the very practice creates new and different problems for the machinist. One of the greatest challenges in successful plastics machining—stress management—is also one of the least discussed or understood.

Material Properties

Polariscopes with cross-polarized lenses show the stress pattern in transparent or translucent parts. In some parts, the color pattern can be visually interpreted to convert to an approximate mechanical value. Other equipment is available with quantitative readout that has less subjectivity but also provides mechanical value of the stresses or strains.

Compared with commonly machined metals, plastics might appear soft and easily machined. Even experienced engineers may assume that material can be more rapidly removed from plastic and that tools won't dull as quickly, but that's not always the case. Some plastics are abrasive and wear away cutting tools very quickly, while others are flexible and require very precise machining to achieve proper feature size.

Imagine cutting a tomato. Even though a tomato is soft, it takes an extremely sharp knife to make a good cut. Many types of softer plastics do exist, but some plastics behave more like metal. Plastics are often reinforced with carbon fillers or glass fibers to make them more structurally or dimensionally stable, or to increase their compressive strength characteristics to support higher loads. Polyether­etherketone (PEEK), for instance, is a biocompatible bearing-grade plastic, designed for a high level of load.

Sidebar:

The different properties of metals and plastics determine how they are successfully handled (see the sidebar, “Material Variables That Can Affect Successful Machining”). Design engineers might choose a plastic for their application for many reasons including light weight, chemical compatibility, corrosion resistance, and optical clarity. Using plastic can often reduce material and machining costs, and may offer enhanced mechanical properties such as for implantable parts that simulate how bone absorbs shock in the human body.

Yet many of those same characteristics alter what tools are most effectively employed in the machining environment. For example, the thermal expansion of plastic can be five times greater than that of metal, while the melting temperature of metal can be 10 times higher than that of plastic. The effect of these differences rapidly becomes apparent to a machinist executing tight-tolerance designs in a plastic part.

A change in the machining environment such as a temperature variation from 60° to 90°F throughout a factory can be enough to affect the precision of features on a machined part. Similarly, a change in the temperature within the machine tool during drilling or milling operations can adversely affect the part. Even the change in coolant temperature during a production cycle can significantly affect part size and positioning accuracy in the machine tool.

Any of these environmental thermal variants would be enough to cause sufficient expansion or contraction to distort a precise dimension or feature on a plastic part. Without the proper environmental controls in place or without the knowledge of how different materials behave under varying temperature conditions, the machinist attempting to transfer the practices and specifications of metal machining to plastic would have limited success.

Each different plastic formulation has particular mechanical, thermal, chemical, and electrical properties that affect how the material machines. Resin manufacturers formulate their products to meet certain application and industry needs. They might include various fillers and additives such as glass and graphite fibers, or additives to improve strength and fire resistance. Materials containing fibers and additives tend to be very abrasive to the cutting tools. The pigment used to color some plastic materials white is very abrasive. So even the color of the material can reduce tool life.

Cutting-Tool Technology

Metals, which are typically much stronger than most plastics, can be machined in a much more straightforward manner. That same process, if applied to plastic, may actually induce excessive stress into the part, which may not be apparent to an untrained eye. A machinist using cutting tools designed for metals or using the wrong cutting tool parameters might be unable to properly turn, drill, or mill a particular plastic. The end result can be an undesirable surface finish or unacceptable feature variation.

When manufacturing engineers design a process for machining a plastic part, they must make numerous decisions regarding equipment and tools for work holding and cutting. Cutting-tool considerations include the selection of tool material, coatings, and cutter-geometry design. Machining parameter decisions include speed, feed, depth of cut, direction of cut, dwell time, peck cycle, and tool path, all optimized to reduce cutting stresses and maximize material removal rates.

A cutting tool used to machine metal or plastic works like a knife blade to shear away material from the workpiece. It commonly rotates like a drill or cuts in a linear fashion like a saw. The proper angle of the sharp cutting edge relative to the surface of a part during the machining process is critical to proper material removal. Cutting tools for plastics require different angles of approach than for metals. With plastics, the primary cutting angle must be more positive and the secondary relief angle more generous to prevent chip buildup, which can cause excessive heat and stress.

Companies specializing in plastics machining conduct extensive tool life studies to help determine the optimal tool design to perform with maximum efficiency and minimal residual stress. Even a consideration as seemingly minor as the types of diamonds used for diamond-tipped tools will affect the finished part. Natural diamond has the lowest coefficient of friction and imparts the least amount of heat and stress, but it is very expensive. Specialty diamond cutting tools for turning or milling operations cost from several hundred to thousands of dollars.

(Left to right) A comparison of cutter inserts for machining metal and precision machining of plastic.

High-precision plastic machining of optical components requires specialized thermally controlled and ­vibration-isolated, multiaxis machines that employ sophisticated controllers, software, and tooling. These machines are capable of producing complex optical lens geometries with just a single cutting point. This type of highly specialized equipment is not commonly found in most metal and plastic machine shops.

Stress and Testing

Improper tool selection or overly aggressive machining creates tool wear. As tools start to wear, cutting edges become dull. Applying dull cutting edges to plastic stresses the material by transmitting too much heat into the plastic. The excess heat can cause thermal damage to the part's surface without leaving visible signs if the stress levels are fairly low. If thermal damage becomes extensive, the material can burn, and cracking or crazing will appear.

Minimizing material stress is probably the most significant consideration during plastics machining. Bending the part puts stress into it. Machining too aggressively imparts stress by straining or pulling apart the individual elements within a plastic structure.

Some of that strain does not relax. It remains in the material permanently and can result in parts that crack under temperature variation or chemical exposure. In many cases, parts will crack after having been installed for a time under normal fastening torques, in environments where they may be subjected to combinations of thermal, chemical, and mechanical stresses.

To some extent, it is possible to relieve stress that has been induced into a workpiece by annealing, a process in which the material is heated slowly, held for a time at a temperature that allows the material to recrystallize, and then cooled very slowly. Stress can be reduced but not eliminated, so the key during the manufacturing cycle is to avoid introducing stress into the part during machining operations.

(click to enlarge)
A typical shell mill used for cutting metal.

It may be difficult to observe the presence of all stresses, unless gross problems have been created. Some gross problems may initially appear as crazing or microcracks on the surface of the part but often may be difficult to see. As surface stress becomes more significant, the part starts to fail and small surface cracks become apparent. Early detection of low levels of cracking or crazing on the surface requires a microscope, proper lighting, and above all, the trained eye of an expert plastics machinist.

(click to enlarge)
A special diamond tool for cutting plastic.

Another way to detect stress in clear parts uses cross-polarization lenses to examine how light refracts when passing through the part. Parts that are stressed will bend or refract light differently. Higher stress produces higher refraction. Ranges of stress show up as different colors when viewed through the lenses. The color can be compared with a special calibration scale that correlates color to a mechanical tensile-stress value.

A different testing protocol, a destructive test, exposes machined parts to chemicals for a defined time period and then observes the surface conditions for signs of accelerated failure. Chemical exposure weakens a material's surface, causing the weakened surface to fail more quickly. Signs of chemical cracking or crazing indicate that stress was introduced into the surface of the part during the machining operation. A properly machined part that is free of excess stress would withstand chemical exposure for a much longer time before exhibiting surface crazing or cracking.

Mechanical tests can mathematically compute stress levels based on deflection and material characteristics. Mechanical tests might be appropriate for opaque materials such as PEEK. In a bend test, an opaque part might be deflected 0.25 in. and then examined for stresses, cracks, and strain in the material. A deficient part would exhibit a visible failure point.

With all testing, the ultimate goal is to determine whether the cutting tools are still sharp and still performing properly to minimize stress at a given stage of the machining operation. Stress can't always be seen, but its effects will be.

Conclusion

Manufacturers come across enough challenges during the product development process. Understanding the differences between machining plastics and metals can keep engineers and designers ahead of the game in choosing the right materials, tools, and techniques.

Acknowledgments

Nathaniel Neinhuis, a manufacturing engineer at Upchurch Scientific (Oak Harbor, WA), which is part of Idex Health & Science, contributed to developing this article.

Frank Molgano is director of new product development for the Eastern Plastics brand of Idex Health & Science (Oak Harbor, WA). Reach him at healthandscience@idexcorp.com.

Copyright ©2009 Medical Device & Diagnostic Industry
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