Best Plastic for Machining: Material Selection Guide

Best plastic for machining

The best plastics for machining include ABS, nylon, Vespel®, acrylic, Polyethylene (PE), Polypropylene, polycarbonate, phenolic, and PTFE (Teflon). Others worth considering are Acetal (POM/Delrin), HDPE, Polyethylene Terephthalate (PET), and PVC (polyvinyl chloride).

When considering which type of plastic to use in a CNC plastic machining project, it is crucial to know the suitability and compatibility of each. Specifically, the plastic should be easy to manufacture and meet the functional requirements of the desired part. 

Other important considerations are resistance to factors – heat, temperature, scratches, etc. –  and desired surface smoothness. Since each of these plastics offers unique properties, the producer can choose one that meets specific requirements. 

This guide explores the key plastics for CNC machining applications. It covers standout characteristics, including advantages, limitations, machinability, and common applications. 

 

Acetal (POM/Delrin)

Acetal is easily one of the most machinable engineering plastics. Key characteristics contributing to this are a relatively high thermal conductivity, a semi-crystalline structure, and high dimensional stability. High thermal conductivity means the plastic’s cutting zone rarely exceeds the thermal degradation point during machining. The dimensional stability is largely derived from the material’s moisture absorption of below 0.25% in 24 hours. 

Delrin/POM (Acetal) part

Pros and cons of Acetal (POM/Delrin) machining

Pros of Acetal (POM/Delrin) machining	Cons of Acetal (POM/Delrin) machining
Low moisture absorption High machinability  Electrical insulation Abrasion- and impact-resistant  Creep- and chemical-resistant	Heat sensitivity with a maximum operating temperature of approximately 100 °C [1] Flammability  Poor response to adhesives

Applications 

Acetal machined parts are commonly found in mechanical components such as gears, rollers, springs, and housings. Hinged housings, fuel transmitter units, and door lock systems in automotive applications are also manufactured from acetal. 

 

Nylon/Polyamide (PA) 

Nylon/Polyamide (PA) dates back to the 1930s, following extensive research into polyesters and polyamides. The two common nylon grades are Nylon 6 and Nylon 6/6. Nylon 6 is usually used in high-strength applications where low density is a requirement. Nylon 6/6 is often reserved for applications requiring higher stiffness, strength, and abrasion resistance. It generally absorbs less moisture than nylon 6. 

Nylon (PA)

Pros and cons 

Pros	Cons
High fatigue resistance High wear resistance High strength-to-weight ratio	Moisture sensitive Tool gum-up if not unchilled

Applications: Bearings, gears, bushings, structural brackets 

 

ABS plastic (Acrylonitrile Butadiene Styrene)

ABS plastic machines cleanly even at high speeds. The tool wear is minimal. However, the machinist should take into consideration the material’s proneness to softening under heat. The two main strategies against this issue are expert feed control and cooling. 

CNC milling ABS plastic

Pros and cons

Pros	Cons
Low cost Easy to machine Good impact strength	Limited heat resistance (softens at approximately 80°C) [1] Can warp when overheated  Poor weather resistance

Applications:  Prototypes, enclosures 

 

Polycarbonate (PC) 

Polycarbonate is on the list of best plastics for machining because it offers an unmatched balance of optical clarity and impact resistance. Machining such a plastic requires controlled speeds and sharp tooling. A common machining challenge is melting.

Polycarbonate machining

Pros and cons 

Pros	Cons
Optical clarity  Impact strength  Some grades are flame-retardant	Stress cracking when exposed to solvents Can gum or melt during machining  Pricier than acrylic CNC machining or ABS CNC machining

Applications:  Electrical enclosures, lenses, safety guards 

 

Acrylic/Poly(methyl methacrylate) (PMMA)

Acrylic CNC machining parts

PMMA is one of the most optically clear machinable plastics available. Machining this material requires sharp tooling and slow feeds because it can easily crack and chip. 

Pros and Cons of PMMA

Pros of PMMA	Cons of PMMA
Excellent optical clarity  More cost-effective than polycarbonate  Weather resistant	Chipping/cracking  Brittleness  Poor heat resistance, with a maximum temperature of approximately 80°C [1]

Applications:  Protective covers, signage, displays

 

HDPE (High-Density Polyethylene)

HDPE plastic is easy to machine and offers good chemical resistance and moisture resistance. However, it is not recommended for tight tolerances due to its relatively soft nature. Instead, consider it for workhorse applications. 

Pros and cons of HDPE 

Pros of HDPE	Cons of HDPE
Good impact resistance  Low weight  Chemical and water resistant	Surface softness Limited dimensional stability

Applications:  Marine parts, tanks, chemical reservoirs 

Precision-machined HDPE

 

PET (Polyethylene Terephthalate)

PET is another plastic that is easy to machine. On top of that, the material has low moisture absorption, good chemical resistance, and good dimensional stability. Some PET grades can be used for food-contact applications if they comply with the relevant FDA requirements. 

Pros and cons of machined PET parts 

Pros	Cons
Good strength  Low moisture absorption Some grades are FDA-compliant	Moderate heat resistance  Can undergo stress whitening

Applications: medical devices, food processing equipment  

 

Polypropylene (PP)

While lightweight at a density of 0.89–0.92 g/cm³, PP exhibits good wear, impact, and chemical resistance. With a relatively good heat resistance and electrical insulation capabilities, PP can be used for a wide range of general industrial applications. 

Pros and cons of PP machined parts

Pros	Cons
Lightweight  Impact resistance  Fatigue resistance  Excellent chemical resistance	Poor tolerance holding Gumming and warping during machining Low stiffness

Applications:  Living hinges, chemical tanks, lab devices 

 

Phenolic Plastic

Often associated with Bakelite, phenolic plastic is a hard and brittle material. This thermoset plastic tends to produce short fragmented chips accompanied by fine dust during machining. Note that the specific chipping behavior can vary based on the specific phenolic grade, machining conditions, and laminate structure. Phenolic’s thermal stability and electrical insulation are excellent. As expected, phenolic machining wears cutting tools quickly. 

Phenolic parts

Pros and Cons of Phenolic Machined Parts

Pros	Cons
Dimensional stability even under heat Superior electrical insulation  Good wear resistance	Abrasive – faster tool wear  Brittle

Applications:  Phenolic machining is ideal for Insulating spacers, cams, gears, bushings, and washers

 

PVC Plastic

Machined PVC plastic parts offer durability and chemical resistance. When done expertly, these parts are valuable in a range of industries, including plumbing and chemical handling systems. 

Pros and cons of PVC machined parts 

Pros	Cons
Corrosion and chemical-resistant  Self extinguishing  Easy to machine	Brittle  Poor heat resistance Limited load-carrying capacity

Applications:  Plumbing fittings, valves 

 

Vespel® (DuPont’s polyimide)

Although difficult to machine, Vespel® produces parts with tight tolerances, temperature tolerance, and durability. This unfilled polyimide grade has a glass-transition temperature (Tg) of approximately 249°C. [2] This makes this plastic one of the best for machining for critical applications. 

Vespel machined parts

Pros and cons of Vespel machining parts

Pros	Cons
High friction and wear resistance Superior thermal stability  Good chemical resistance  Isotropic machined characteristics Strong dimensional stability	Complex material production – high-pressure sintering  Machinability depends on grade

Applications:  Defense and spacecraft equipment 

 

Polystyrene (PS)

PS material has excellent optical clarity and is stiff. Its stiffness can be enhanced using rubber – High Impact Polystyrene (HIPS). This cost-effective thermoplastic combines these properties with good dimensional stability and availability in different shapes for stock.

Pros and cons of polystyrene machined parts 

Pros	Cons
Good dimensional stability  Good machinability  Natural rigidity  Low cost	Brittle  Poor heat resistance  Limited chemical and wear resistance

Applications: Polystyrene material is rarely used in CNC machining; it is ideal for injection molding. 

As you consider which one on the list is the best plastic for machining, remember that PS is included for completeness. Procurement teams new to plastics may specify it for its wide availability, unaware that its poor thermal performance and high brittleness pose serious challenges in machining.

Summarized Selection Table for the Best Plastic for Machining

Below is a material selection table featuring key engineering requirements

Plastic	Dimensional stability	Wear resistance	Chemical resistance	Machinability	Thermal resistance	Relative cost
Acetal (POM/Delrin)	High	High	High	Excellent	Moderate	Low
Nylon (PA)	Moderate	High	Good against fuels and oils; poor against acids	Good	Moderate – chilling required to prevent gumming	Low to moderate
ABS	Low	Moderate	Poor weather resistance	Excellent	Low	Low
Polycarbonate(PC)	Moderate	Good	Poor	Moderate	Moderate – gumming, melting	Moderate to high
Acrylic (PMMA)	Moderate	Low	Weather resistant	Moderate	Low – slow feeds and sharp tooling	Moderate
HDPE	Low	Good	Good	Good, tolerances not tight	Low to moderate	Low
PET	Good	Good	Good	Good		Moderate
Polypropylene (PP)	Low	High	Excellent	Moderate – warping and gumming		Low
Phenolic	High	Good	Good against oils and solvents	Poor		Moderate
PVC	Moderate	Moderate	Good	Good		Low
Vespel®	High	High	Good against most fuels and solvents	Poor-to-moderate-to-difficult, grade-dependent		High
Polystyrene (PS)	Good	Low	Poor	Good, but not machined regularly	Low	Low

Machining Processes for Plastics 

As you consider which of the highlighted plastics suit your machining project, take note of the common machining processes: Turning, boring, drilling, milling, tapping, and reaming. 

Plastic Turning and Boring

These lathe-based operations work on internal diameters and produce cylindrical components. Turning uses a stationary single-point tool on a rotating workpiece to produce bushings and shafts. Boring precisely enlarges existing holes. For a plastic to withstand these operations, it must have high dimensional stability – the cutting forces can be too much. Acetal and PTFE are some of the plastics with the best dimensional stability. The chip evacuation method should be fast and effective enough to prevent melting. The plastic’s heat tolerance should also be considered when setting the spindle speed. 

Plastic Drilling 

Plastic drilling produces straight circular holes in the material. The frictional heat buildup in plastics can melt and deform the hole edges in ABS, PS, and other plastics if the speeds are not well managed. The best practice is to use peck drilling, a technique that enhances chip clearance and heat dissipation. Continuous drilling is not a challenge for PET and other plastics with higher melting points. 

Plastic Milling 

Milling is a process whereby a rotating multi-point cutter is used to produce pockets, 3D geometries, and flat surfaces on the plastic. It is a highly versatile process applicable to hard and soft plastics. The milling strategy used for specific plastic matters. For instance, gummy plastics such as HDPE require sharp tooling and faster cutting speeds to prevent material smearing and enhance surface finish. 

Plastic Tapping 

Tapping is the production of internal threads in a pre-drilled hole. It is popular for dimensionally stable plastics, including acetal and phenolic. Since softer plastics tend to undergo thread deformation and stripping, metal inserts are used instead. 

Threaded metal insert in plastic

Plastic Reaming 

Reaming helps attain the precise final diameter for a drilled hole – enlarging and smoothening. It is a requirement in the manufacture of bushings and bearing seats, especially in acetal parts where predictable hole diameters are essential in assemblies. 

Each of these processes comes with distinct mechanical and thermal requirements. Each plastic should be matched with a suitable machining process to ensure optimal performance of the machined parts. 

As you prepare your machining plan, ensure the plastic’s properties match the process parameters. For instance, the material’s heat sensitivity should determine the feed rate, cutting speed, and cooling rate.

Conclusion 

The best plastics for machining offer the right machinability, thermal stability, mechanical properties, and other properties, depending on real-world requirements. From acetal’s flawless precision for tight tolerances to PC’s unmatched impact resistance, each of the plastics has a unique suitability for industrial applications. For procurement managers seeking reliable manufacturing partners and engineers expediting plastic material specs, there are nuances that only an experienced plastic processing company can understand. 

Your plastic part should be machined right the first time. For a flawless transition from design to production of CNC-machined plastic parts, share your drawings with ProleanTech’s engineering team. You can also review our plastic machining service for more details regarding precision machining for different types of plastics.  

FAQs

What is the best plastic for machining?

The best plastic for machining depends on the application requirements in terms of tensile strength, heat resistance, chemical resistance, and impact strength, among others. Each plastic offers distinct properties; no plastic is a one-size-fits-all. 

Resources 

[1]https://ideambox.com/assets/downloads/engineering-materials-datasheet.pdf

[2]https://www.materialdatacenter.com/ms/en/tradenames/Vespel/DuPont/VESPEL%C2%AE+TP-8311/7e482917/835