Monthly Archives: April 2017

Machining Acetal Shapes

Highly precise acetal parts in a variety of sizes and complexities can be manufactured economically through machining. In the world of Engineering Plastics, Acetal (POM) stock shapes are considered to be some of the easiest to machine. On a scale of 1-10 with 1 being the easiest, many manufacturers place acetal at a 1, compared to a PBI which is often seen as a 10. In fact, machine shops that traditionally make metal parts find they can machine acetal using the same primary tools used for most of the metals they work with. As with any material, there are some good guidelines that can help ensure your success.

Best General Practices for Machining Parts from Acetal (POM) Engineering plastic and Potential Pitfalls of Machining Thermoplastic Shapes

We love engineering plastics! So we are always touting the many benefits of replacing plastics with metals. But this does not mean they are perfect in every way in every situation. There are some differences between plastics and metals that can trip you up when machining. But once you know the potential problems of machining acetal stock shapes machining them can become as easy as the metals machine shops are used to working with now.

Watch out for the heat! As a general rule keep in mind that, due to heat, thermoplastics expansion can be up to 10X greater than metals. Thermoplastics also hold heat longer than metals. Acetal is a thermoplastic material and has a lower thermal conductive aspect than most of the metals it is used to replace. Heat may not be an issue for metals but in the case of an engineering plastic shape from acetal heat build up from machining needs to be monitored and taken into account. Thermoplastics are more elastic than metals. So in general, heat buildup during the machining process can potentially lead to thermal expansion which can distort acetal parts.

If this leaves you concerned about machining plastics, not to worry! Plastics like acetal have numerous benefits that often outweigh the challenges of heat buildup and once you understand how to work with acetal you can easily machine consistently accurate, detailed high quality parts that your customers will be happy with. Be mindful of heat buildup, but also know that acetal does not typically require a coolant (except when drilling or threading). Sawing and machining can usually be set up to minimize heat buildup. (See the table below for Quadrant Engineering Plastics general recommendations for tools and speeds.)

  1. If cooling is used on acetal, compressed air is the standard method. The great thing is this has two benefits. The air cools the part and it keeps chips blown out of the way and keeps the heat built up in the chips off of the part or tools where it can add to any heat buildup. Other options include spray mists and non-aromatic, water soluble coolants.
  2. Sharper tools = Less friction = Less heat. To help reduce heat buildup use extremely sharp cutting tools.
  3. Chipping. Acetal creates chips when being machined so plan for removal of chips as you machine. Removing chips is very important in deep hole drilling. As the chips add to the heat, hole walls can heat to the melting point and clog the drill.
  4. Pieces may be flexible. Make sure the acetal is supported in a way that the material is not distorted, bent, twisted or allowed to deflect away from the tool.
  5. Make sure machining equipment is running as smoothly as possible, reduce any vibration to help aid in accuracy and part quality.
  6. Acetal shapes can be clamped but be aware of how tight.
  7. Choose the right blade for the job. Start by asking yourself what the end product is going to be.
    1. Band saws are good choice for a support groove and for cutting acetal rod and tube. Heat gets dissipated over the long blade.
    2. Circular saws are a good choice for cutting acetal sheet or blocks that have straight edges. Watch the feed speed (most acetal manufacturers have a recommendation).
  8. Choose the right tool for the job too
    1. Opt for positive tool geometries with ground peripheries
    2. For best tool life use carbide tools with ground top surfaces
  9. Is post machining annealing needed? See our previous blog post on this topic.
  10. Choose a machining cycle that will allow for evacuation of the chips from holes and cutting surfaces. For example, when drilling holes choose a cycle that allows drill to ‘peck’ or withdraw at certain points to draw chips back out of the hole.

The following tables are a good starting point for how to set up machining of acetal materials. The information comes two US manufacturers of acetal materials – Quadrant Engineering Plastics and Engineer Plastics provide guidelines for machining the acetal materials they produce. Depending on the manufacturer acetal materials may go through a stress relieving (annealing process) as part of their manufacturing. This helps to ensure the highest possible consistent quality of materials. Testing and consulting with your local tkEP representative on manufacturer recommendations is always a good way to help prevent machining problems. tkEP representatives not only have a broad range of industry experience, many have worked hands on in the industry, and all tkEP representatives attend manufacturer training so we stay on top of current products and how to work them.


As you can see each manufacturer has their own insights into how acetal should be sawn, milled, drilled, or turned. Their are also some pretty broad ranges when it comes to the numbers they provide. This is because these are truly general guidelines that cover the broad range of acetal shapes. Acetal shapes can be acetal homopolymers, acetal copolymer. In addition there are filled acetals and unfilled acetals. Add that to other variations including thickness and size plus environment and it is easy to see that testing for individual applications is necessary.

To read more about acetal plastic shapes check out our online catalog. We have product information as well as a full range of shapes, sizes, and grades of acetal. Read More… For more detailed information on machining from Quadrant EPP and Ensinger Engineering Plastics we’ve included links to pdf files of their machining guides. In these guides you’ll find data for acetals as well as a broad range of other machinable engineering plastics. Last but not least, don’t forget about your friendly local tkEP representative. We are always happy to assist you with finding the right engineered plastics solution for your application. Contact us today 877.246.7700. this one number will put you in contact with your local tkEP branch, or send us a note.

Quadrant Engineering Plastics Machinist Handbook

Ensinger Engineering Plastics Machinist Guide

Stress Relieving of Engineering Plastics

Stress relieving of engineering plastics, also known as annealing, can play an important role in the quality of machinable plastics and machined plastic parts. Many of the high quality materials including engineering plastics and high performance materials are run through a specific annealing process by the manufacturer to reduce internal stress in their plastic shapes that occurs naturally through the extrusion or molding process. Stress relieving of engineering plastics helps to provide machinists and fabricators with the best possible dimensional stability and ease of machining and fabricating.

What Causes Stress in Engineering Plastics?

The above photo shows the affect of stress from the manufacturing process.

Plastic rod that has not been stress relieved cut through the centerline

Plastics that are formed through extrusion into stock plastic shapes are pressed or pushed through a profile die to create the shape. The stress that is introduced into the material during extrusion is not relieved in this part of the process because the plastic resin sets up as soon as it comes out of the extruder. Instead the stress remains in the plastic. The tell tail signs of stress in plastics are:

  • Warping and distortion
  • Reduced physical properties
  • Cracking
  • Changes in finished part dimensions

However, plastics machinists can also do annealing as part of their machining process. Depending on the particular plastic material, a specific cycle of heating and cooling the plastic shape or the machined plastic part is done by using an annealing oven that allows for precise temperature and timing control.  When this process is done properly it can enhance the lifespan of machined plastic parts. The image to the right is an example of a stock shape that has not had any stress relieving (annealing) performed on it and what happens when it is cut down the centerline.

Machining Practices that can help Reduce Stress Build-Up

Internal Stress caused by machining a plastic shape into a part can also be an issue. It can harm the integrity of a part and lead to premature part failure and reduced performance of plastic parts. The best guide on this is the manufacturer instructions and guides for the type of material you are machining. For example acetal performs very differently from nylon etc. But there are a few general causes of stress created during the machining process that you can be on the lookout for:

  1. Be sure your tools are sharp, razor sharp
  2. Make sure the tool is designed for the job. Make sure it is the right angle, size type of material
  3. Watch feed and speed rates to ensure you aren’t allowing excessive heat to build up
  4. Coolants can be used, but be aware of how a coolant may affect a specific type of plastic. If you cannot use a liquid coolant, air cooling might be an option
  5. If you have highly critical dimensions your part must meet. Consider machining these final dimensions with a light cutting after annealing a rough shape
  6. Are you balancing? If machining multiple sides try to plan on balancing the machining of each side to be as equal as possible. Balancing the machining can help to prevent centerline warping

Annealing plastics can directly affect the quality of your finished plastic parts. You can optimize the integrity and dimensional stability of your machined plastics and reduce the potential expansion and/or shrinkage of the plastic. If and when you do an annealing process there are a couple of things to keep in mind for that as well:

  1. Do you have very thin or thick sections? This can alter the heating and holding time needed
  2. Fixture parts in the annealing oven to help prevent distortion
  3. Rod reacts in a radial fashion while sheet most often has curling or flatness issues
  4. Polycarbonate and polysulfone are very sensitive to stress cracking
  5. Do you have screw threads? The stress from the screw partnered with machining stress can add up to shortened part life. Reducing the stress from machining can help boost part life by eliminating the machining stress

Which Engineering Plastics can Benefit the Most From an Annealing Process?

While some plastics may not need stress relieving there are a few engineering plastics that nearly always benefit from a post-machining annealing process. These include:

  • Ultem®
  • PEI
  • Torlon®
  • PAI
  • Transparent materials post-machining annealing can help reduce or minimize stress crazing

Whether or not you opt to stress-relieve an engineering plastic involves any number of factors and a talk with your plastics expert at your distributor or manufacturer can go a long way in helping you to make the best from your engineering plastics.

To learn more about engineering plastics and engineered plastics solutions visit our online catalog at

For questions about engineering plastics or how to work with them please contact us.