CNC Plastics Selection Guide
Written by
David Mayer

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On
November 17, 2017
In
Engineering

So you’ve got a part to design, and the first step is to determine what material it should be made out of. You decide the part should be plastic, and it should be CNC machined, but does the selection stop there? Nope, there are a plethora of plastics to choose from, each with its own combination of properties.

In this guide, I’m going to help you align your project needs with the right plastic material for the job. For the purposes of this article, I’m going to narrow the selection to plastic materials offered by the Fictiv platform for CNC machining.

I’ll provide a material properties overview + benefits, drawbacks, and common uses for:

We’ve also created a handy comparison chart for you to download that clearly lays out the material properties at a glance, as well as common use cases, benefits, and drawbacks for each plastic material. Download the chart instantly using the form below:

P.S. If you know your part should be plastic, but you’re not sure if it should be CNC machined or 3D printed, check out this article first.

ABS Plastic

Overview

ABS is a well rounded general use plastic. It offers high impact strength, toughness, and electrical resistance at a low price. It is also easy to finish, as it can be readily painted, glued, or welded together. If left as-machined, it will have a matte finish, although it can be somewhat shiny, depending on how it is machined.

ABS plastic

Benefits

ABS is an excellent choice for general purpose prototyping, pre-molding prototypes, parts that will receive impact and require toughness, or when low cost is desired.

Drawbacks

ABS does not have good abrasion or chemical resistance and will melt in acetone. It is also not an exceptionally strong plastic. Additionally, due to stricter environmental regulations on the West Coast of the U.S.,  large pieces of ABS are only manufactured in the Midwest and on the East Coast. This means that stock over two inches thick often takes a week to ship, leading to long production time for large ABS parts.

Common Applications

The most common application of ABS is injection molding, to make electronics enclosures, household appliances, and even the iconic Lego bricks.

Learn more in the Capabilities Guide

Nylon

Overview

Nylon 6/6 is a strong and durable plastic, applicable to a wide variety of purposes.

Nylon 6/6

Benefits

Nylon has a high strength and rigidity that it maintains over a wide temperature range, good electrical insulation, and good chemical and wear resistance. Nylon 6/6 is a good choice for applications in which strong, durable parts are needed at a low cost.

Drawbacks

Nylon 6/6 will absorb moisture, causing it to swell and lose some dimensional accuracy. It can also warp if there is a large amount of asymmetric material removal during machining, due to internal stresses inherent in the material.

Common Applications

Nylon is most commonly found in medical devices, circuit board mounting hardware, automobile engine compartment components, and zip ties. In many of these applications it is used as an economical alternative to metals.

Learn more in the Capabilities Guide

Acrylic

Overview

Acrylic, also known as Plexiglass or Luctie, is the trade name for PMMA plastic. It is tough, with decent impact strength and scratch resistance, and can be easily glued using acrylic cement.  

Acrylic

Benefits

It is well suited for any applications that require optical transparency or translucence, or as a less durable but cheaper alternative to polycarbonate.

Drawbacks

However, acrylic is a brittle plastic and will fail by cracking or shattering, rather than by stretching. Any machined surfaces on a piece of acrylic will lose their transparency and take on a frosted, translucent appearance. Because of this, is it usually a good idea to note if an acrylic part should be left at stock thickness to preserve transparency or not. If transparency is needed on a machined surface, it can be polished as an additional post processing step.

Common Applications

As-machined, acrylic is transparent and is most commonly used as a lightweight substitute for glass or for light pipes.

Learn more in the Capabilities Guide

Delrin

Overview

Delrin is a particular brand of acetal homopolymer (aka POM). It has a slippery, low friction surface, excellent dimensional stability, and high stiffness.

Benefits

Delrin is a good plastic for these or any other applications that will experience lots of friction, require tight tolerances, or need a high stiffness material.

Delrin

Drawbacks

The downside to Delrin’s slippery, wear resistant surface is that it’s very hard to glue. The material also has internal stresses that make it prone to warping in areas that are thin or have large amounts of asymmetric material removal.

Common Applications

Delrin is commonly used in gears, bearings, bushings, and fasteners, or to make jigs and fixtures for assembly.

Learn more in the Capabilities Guide

HDPE

Overview

HDPE stands for high density polyethylene.

HDPE

Benefits

Despite its name, HDPE is the least dense plastic we offer, and it also has excellent chemical resistance, electrical insulation, and a slippery surface. It is well suited for making plugs and seals, due to its chemical resistance and slipperiness, but is also a good choice for weight-sensitive or electrically sensitive applications.

Drawbacks

HDPE’s main disadvantage it its poor strength, particularly in tension and flexing.

Common Applications

HDPE is often used in fluid applications, such as fuel tanks, plastic bottles, and fluid flow pipes.

Learn more in the Capabilities Guide

Polycarbonate

Overview

Polycarbonate is our most durable plastic.

Polycarbonite

Benefits

It has a very high impact resistance and stiffness, and remains functional over a wide range of temperatures. It is also optically clear, although it can be dyed black if opaqueness is needed. Polycarbonate is best suited for applications in which a very tough or very strong plastic is needed, or where optical transparency is desired. Because of this, polycarbonate is one of the most used, and most recycled, plastics.

Drawbacks

Pure polycarbonate does not have good wear resistance and is prone to scratches. Anti-scratch coatings and vapor polishing can be added as a post processing step to improve wear resistance or optical clarity, if needed. It is also not readily available in pieces over two inches thick, which limits the size of parts that can be made of polycarbonate.

Common Applications

Polycarbonate’s durability and transparency mean it is used to make optical disks, safety glasses, light pipes, and even bulletproof glass.

Learn more in the Capabilities Guide

Main Takeaways

Hopefully, this guide has provided insight into which plastic is best suited to your application.

The table below can be used as a quick summary of Fictiv’s CNC plastics selection and provides select material properties for each plastic. For more information on CNC machining in general, visit our Capabilities Guide.

Written by
David Mayer
Manufacturing engineer at Fictiv. I'm a mechanical engineer who loves to take things apart to see what makes them tick, and then use the pieces to make something that ticks faster, longer, and louder.
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