CNC machining is a well-established method for fabricating metal and plastic parts by removing material from raw stock in a highly controlled manner. Machining dates back to the Industrial Revolution, with metal and steel being the primary materials used in the early years of machining. The evolution of machining and advancements in material science paved the way for the machining of plastics. However, metal and plastics behave differently under pressure, stress, and heat, bringing unique challenges for those machining plastics. Therefore, a manufacturer that can machine metals doesn’t necessarily have the experience and knowledge to machine plastics. Below, we look at some of these unique challenges to help you better understand any quality issues you may have and to underscore the importance of working with a partner experienced with plastic CNC machining.

Choosing the Right Plastic Material for Machining

The first challenge is selecting the right plastic material. Each plastic and even variations of plastics will have characteristics that will impact machinability. Understanding why some plastics are better than others can help in making an informed decision. Some plastics exhibit superior properties, such as high strength, excellent dimensional stability, good heat resistance, low friction coefficient, and chemical resistance, that make them ideal for CNC machining.

Some of the plastics that are less challenging to machine include:

  • Acetal – Acetal is known for its high strength, stiffness, and dimensional stability, making it less likely to deform during machining, and its coefficient of friction makes it ideal for precision parts.
  • Polypropylene (PP) – Polypropylene is softer than other plastics, which makes it easier to machine, but care must be taken to avoid burrs. It also has excellent weldability.
  • Nylon (PA) – Nylon is known for its ease of machining, but moisture absorption can affect its dimensions.
  • High-density polyethylene (HDPE) – HDPE has a high strength-to-density ratio and is easy to machine.
  • Polyvinylidene Fluoride (PVDF) – PVDF has good thermal stability and chemical resistance and is relatively easy to machine.

While these plastics listed above may be favored for predictable behavior under machining processes, it is important to use the right tools and techniques to achieve the desired results.

Other plastics can be challenging to machine due to specific properties. Precise control of parameters, as well as the use of specialized tooling, may be necessary to machine these plastics.

  • Polymethyl methacrylate (acrylic) – Acrylic is prone to cracking and melting, notably thicker sheets, and requires careful process control to achieve a good surface finish.
  • Polycarbonate (PC) – this tough material tends to chip rather than deform, which can make it difficult to achieve precision. It also requires a coolant to keep it from melting or burning.
  • Polytetrafluoroethylene (PTFE) – with a high degree of softness and flexibility, PFTE can easily deform and smear when machined. It also doesn’t dissipate heat well, which can create difficulties in maintaining precision.
  • Polyvinyl chloride (PVC) – This material is easy to cut, but it generates a large amount of chips and swarf and, when heated during machining, can release chlorine gases.
  • Ultra-high molecular weight polyethylene (UHMWPE) – while this material is tough and abrasion resistant, its low melting point can lead to issues with burring or melting during machining.
  • Fiberglass-reinforced plastics – these abrasive materials can wear down cutting tools, and the fine dust created during cutting can pose a health hazard.

By considering the specific requirements of your project along with the desired properties of the plastic material, you can make an informed decision on which plastic will best suit your CNC machined part.

Issues with Chip Control and Swarf Management

Chips and swarf are by-products of plastic machining. Sometimes the terms are used interchangeably, but they differ in their shape and size. Swarf is spiral pieces of plastic, and chips are pieces that break off from the stock in irregular shapes. While these are typical by-products from CNC machining, they pose unique challenges with machining. Plastic chips and swarf are unlike metal, which falls away freely. Plastic becomes soft, so it can clump together and clog tool paths, coolant lines, and machine components. Static electricity can build up with some plastics, further exacerbating the problem. And because plastics have poor thermal conductivity, accumulation of chips and swarf around the cutting zone can insulate the heat buildup, which can soften the material, leading to inaccurate cuts, tool wear, and possible thermal decomposition of the material. Additionally, the chips and swarf may scratch or mar the surface, creating issues for parts with tight tolerances or requiring optical clarity.

Manufacturers with plastic machining experience will employ strategies to reduce issues caused by chips and swarf. Chip formation and clogging can be controlled by optimizing cutting parameters (tool geometry, feed rates, spindle speeds). Using the proper coolant and delivery method can help to evacuate chips and swarf and control heat buildup. Equipment can be grounded, and anti-static lubricants can reduce static electricity.

Maintaining Dimensional Stability and Preventing Warping

Some materials and part geometries are more susceptible to warping during machining. When more material is removed from the centerline of a part than from the other, the part can tend to cup toward the side with less material. This is caused by the redistribution of internal stresses. The lower the stiffness of the plastic, the more prone it will be to warping. Improper clamping or fixturing can also introduce additional stress during the machining.

Thermal expansion and contraction can also create warping when the expanded material (from heat generated by machining) cools too quickly or unevenly, causing contraction at different rates. Excessive heat and stress can also be introduced to areas of the part by aggressive machining or improper tool paths. Thin-walled or intricate parts are more prone to warping because of their reduced ability to dissipate heat and withstand machining forces.

Working with a partner who understands how various plastics react to machining, uses proper cutting tools and processes to reduce heat, and employs strategies critical for the proper machining of specific plastics will ensure you receive the highest quality parts.

Achieving High Precision and Surface Finish Requirements

Plastic generally has a higher coefficient of thermal expansion than metals. The higher expansion and contraction rate with temperature changes that can occur during machining can make it more difficult for those unfamiliar with the material to achieve precision and meet tolerances. If the material absorbs moisture, this can lead to swelling and dimensional challenges that add complexity to maintaining high precision. Warping during machining will also lead to deviations from the desired dimensions. Additionally, tooling deflection and wear can impact the consistency and accuracy of machined parts.

Some characteristics that create challenges in maintaining tolerances do the same for surface finishes. The heat from machining can melt or deform the surface. One cause of this is a speed rate that is too high. PVC, acrylic, and other brittle materials can experience cracking or chipping. Additionally, dull tools can tear at plastic, negatively impacting the surface finish.

A partner with experience in machining plastics will understand the importance of temperature control, pre-conditioning materials, using correct tooling, and optimizing machine parameters. In some cases, post-processing steps may be necessary. Each plastic material may require a unique approach to achieve the desired precision and surface finish.

Get the Highest Precision Plastic Machined Parts

With decades of experience in the machining of engineering and high-performance plastics, we understand the intricacies and challenges associated with machining these plastics. We are equipped with state-of-the-art manufacturing and finishing technology dedicated to CNC machining plastics, allowing us to adhere to the tightest tolerances and provide essential part finishing capabilities. Contact us to get started with your next project.