Pick up any engineering plastics catalogue and you’ll find dozens of grades listed side by side, often with very similar-sounding descriptions. In practice, the differences between them matter a lot. Choose the wrong material and you’re looking at premature wear, dimensional creep, or a part that simply doesn’t hold up under load. Choose the right one and it’ll outlast cheaper alternatives by years.
Engineering plastics broadly sit above commodity plastics like PVC and polypropylene in terms of mechanical performance. They’re designed for applications where strength, dimensional stability, temperature resistance, or low friction are genuine requirements.
Acetal: Precision, Stiffness and Low Moisture Absorption
Acetal (also known as POM or Delrin) is the go-to choice when you need tight tolerances and good surface hardness. It machines cleanly, holds its dimensions well in humid environments, and has a naturally low coefficient of friction. That combination makes it popular for gears, valve seats, pump housings and small precision components in food processing and medical equipment.
It comes in two main forms: acetal homopolymer and acetal copolymer. Homopolymer is slightly harder and stiffer, while copolymer offers better chemical resistance and less risk of centre-line porosity in thicker sections. For most machined parts, either will work, but it’s worth specifying which you need rather than leaving it to chance.
Nylon: Load-Bearing Performance at Mid-Range Cost
Nylon, or polyamide, covers a wide family of grades. The two most common in sheet and rod form are Nylon 6 (PA6) and Nylon 6/6 (PA66). Both offer good tensile strength, solid impact resistance and a useful ability to self-lubricate under sliding contact. They handle moderate-to-high mechanical loads well, which makes them a practical choice for bearings, wear pads, guide rails and structural brackets.
For medium-to-high load structural and wear applications specifically, cut-to-size Nylon 6 sheets are widely used because they can be ordered to exact dimensions, reducing waste and machining time. Nylon 6 is produced through extrusion, which gives it good toughness and machinability. It’s available in natural and black, with black carrying enhanced UV resistance for outdoor use.
One thing to keep in mind: nylon absorbs moisture, which can affect dimensional stability in precision applications. If your part needs to maintain very tight tolerances in a wet environment, you may want to account for moisture uptake in your design tolerances or consider a drier-running alternative.
PTFE: Chemical Resistance and Ultra-Low Friction
PTFE (polytetrafluoroethylene) sits in a different category from most engineering plastics. Its mechanical strength is lower than nylon or acetal, but its chemical resistance is practically unmatched. It’s non-reactive with almost every industrial chemical and fluid, and its coefficient of friction is one of the lowest of any solid material.
This makes PTFE the natural choice for seals, gaskets, liners and slide plates where chemical attack or contamination is a concern, particularly in pharmaceutical, chemical processing and food-contact environments. It’s also FDA-compliant in its unfilled form. The trade-off is that it creeps under sustained compressive load, so it’s not well suited to high-stress structural components.
PEEK: High-Temperature Performance Under Load
PEEK (polyether ether ketone) is at the top end of the engineering plastics spectrum in terms of both performance and cost. It retains its mechanical properties at temperatures up to around 250°C, resists a wide range of chemicals, and offers excellent fatigue resistance. Those characteristics make it the preferred choice for demanding applications in aerospace, automotive and medical device manufacturing.
Where PEEK earns its price premium is in applications where other materials simply can’t cope: bushings in high-heat environments, structural components close to heat sources, or parts that need to resist both chemical exposure and mechanical stress simultaneously. For less demanding applications, there’s little justification for the cost over nylon or acetal.
Polyethylene and Polypropylene: Lightweight and Chemical-Resistant
Polyethylene (PE) and polypropylene (PP) are at the lighter end of the engineering plastics range. Both resist a broad range of chemicals and are easy to fabricate. They’re commonly used for tanks, liners, cutting boards and conveyor components where hardness and load-bearing strength aren’t the primary concern.
UHMWPE (ultra-high molecular weight polyethylene) is worth a separate mention. It has outstanding wear resistance and is used extensively in wear liners, chute liners and marine fendering. It won’t machine with the same precision as acetal, but in high-abrasion applications it can significantly outperform harder materials.
Final Notes
There’s no single engineering plastic that suits every application. Acetal wins on precision and dimensional stability, nylon handles structural and wear duties at a sensible price point, PTFE covers chemical resistance and low friction, and PEEK steps in where high temperatures and demanding loads coincide.
Getting the specification right at the outset will save money and rework later. If you’re unsure where a particular grade sits on that spectrum, talking to a specialist supplier before committing to a material is always time well spent.
