Using High-Performance PEEK, PEI Plastics for Machining, Moulding

By Steve Konick

Plastics have evolved over the last 50-plus years—from housings and panels that reduced weight, a reduction in sharp edges, and added product style options, to today’s engineering grades like PEI and PEEK that increase the performance of parts or replace metals.

In engineering terms, we often think of plastics as reducing cost while increasing mass production capability, or increasing design capabilities through mouldable versus formed parts. We can’t always shake the perception that plastic is “cheap” in the eyes of some customers. 

Looking at the resins available in a modern designer's toolbox, we see new options that have been designed for applications far more hostile than what we originally worked with. Medical device design and aerospace come to mind when you have high expectations for performance and safety.

Spring forward 50-plus years. Today’s plastics are lightweight, strong, and versatile, and plastic parts can be injection moulded, CNC milled, or 3D printed.

Two of the most common specialised materials we consider for these environments are PEEK (polyether ether ketone) and PEI (polyetherimide). You may know PEI by its brand name, ULTEM (R). These two plastics share a lot of characteristics, but there are also distinct reasons to choose one over the other.

Ultem Material Block
Pictured above is a block of material of Ultem ready for machining. In some cases, glass fibre is added to the material to further improve its mechanical properties.

The Basics of PEI and PEEK Plastics

Both PEEK and PEI are considered high-performance plastics, and depending on which grade of each you choose, they are available as blocks, rods, powders, or pellets. Often, you’ll find parts made from PEEK or PEI used in environments where temperature or chemical exposure could be an issue. In other words, you likely would not be using these plastics just because you can. They are specialised plastics for very specific applications. PC, ABS, PA, or POM, are more commonly used plastics, and considerably less expensive.

Let’s say you need an injection-moulded part that matches your branding colours. Because PEI and PEEK only melt at very high temperatures, customers can supply their own materials that already contain the compatible colourants. This ensures that the pigments in the plastic can sustain heat without shifting in colour. 

All in all, you want to look carefully at material performance as described in company datasheets, and expectations defined by their grades/formulations, and then cross-reference those to your expectations and needs. Resin manufacturers do an excellent job of describing why a particular resin grade exists and offering technical support to guide you to the correct selection based on your application.

Comparing Types of PEI Plastics

At Protolabs Europe, we stock two types of PEI: Ultem 1000, and Ultem 2300. Three of these are stocked for injection moulding, but only the last two are available to create CNC machined parts. The materials' primary benefits are high heat resistance, impact resistance, and strength.

Material Injection Moulding CNC Machining
PEI | RTP 2100 LF  
PEI | ULTEM 2200 (20% GF)  
PEI | ULTEM 1000
PEI | ULTEM 2300 (30% GF)  
PEEK Material Block
PEEK is a high-performance plastic that is available for both machining and injection moulding designs.

Glass fibre (GF in the table above) is sometimes added to PEI to make it more heat resistant or to counter compression that could take place in certain installations. While glass fibre is an off-the-shelf addition, other additives can be introduced, yielding other benefits.

For example, carbon fibre creates a lighter weight, but strong, part. It also helps deflect heat and reduce electrical conductivity.

Teflon is added to PEI-based parts to add lubricity, or slipperiness. Ball bearings and pivot joints are often made of PEI that has been impregnated with Teflon, providing a strong part with the ability to resist friction.

Silicon dioxide (SiO2) pre-compounded into PEI adds strength to the part.

Comparing Types of PEEK Plastics

There are two basic types of PEEK products. Standard PEEK is used in CNC milling, while Victrex 450G is available for injection moulding. What makes all PEEK products especially important is that they have excellent wear resistance and can be made even stronger when other products are added to them in an injection-moulded part. Other benefits include chemical resistance, sterilisability, stiffness, and strength. Many of the same additives used in PEI can be pre-compounded into PEEK to alter its base characteristics when used in injection moulding.

Material Injection Moulding CNC Machining
Victrex 450G  

Who Uses PEEK and PEI Products?

PEI is everywhere. Remember your last flight? You were surrounded by plastic cabin panels made of PEI. Medical devices made of PEI benefit from the material’s ability to withstand the high temperatures needed to sterilise them. While some PEI is food-grade, you most often see PEI parts in electronics, appliances, the oil and gas industry, and for aerospace application.

PEEK is inert, meaning it is unlikely to react chemically to surrounding materials. That makes it ideal in parts for the food industry. PEEK will resist almost all chemicals, including being attacked by sulphuric or nitric acid. Other common applications for PEEK-based parts include semiconductors and the electronics industry, oil and gas, aerospace, and in medical equipment.

Conclusion: Consider Part Performance Needs and Cost

By now you can guess that performance doesn’t come cheap. PEI costs about 15-20 times more than other common plastics, and PEEK, given its excellent specifications, can double the cost of a PEI part. PEI tends to be more forgiving with injection moulding processing and tooling, but PEEK can be trickier and more difficult to process. Because of its high-temperature performance, using PEEK may result in potentially more costly tooling.

Your decision to use PEEK or PEI for your parts comes down to two factors: part performance needs and cost. If you expect to install the part in chemically hostile or prolonged high-temperature environments, you need a plastic that’s non-reactive to give your part longevity. While we often tend to look at cost over capability, a glance at industry standards or approved material lists may cause you to adjust your piece part and total cost of manufacturing. The added cost might be worth it, given the part's performance and longevity. Sometimes, PEI and PEEK are the answer.
There is also one other plastic you might consider for CNC machining PPSU (Polyphenylsulfone or Radel (R)) like PEEK and PEI is also flame retardant to UL 94 V-0.

Detailed information on all of the thermoplastics Protolabs stocks is available in our extensive data sheets for injection moulding and CNC machining.