Polycarbonate (PC) CNC Machining

The impact plastic

Polycarbonate’s position in the material list is simple to state: it is the plastic that survives being hit.

Its impact strength is in a different class from every other engineering plastic on this site. Where ABS is “tough enough to be handled” and acrylic shatters, polycarbonate absorbs a genuine blow and deforms rather than breaking. And unlike many materials that become brittle in the cold, it retains that toughness at low temperature — which is exactly why it is the material behind machine guards, visors, face shields and protective housings.

The rest of the property set supports the same applications:

Property	Roughly	What it means
Impact strength	Outstanding	Survives blows that shatter acrylic
Light transmission	~88–90%	Transparent, slightly below acrylic
Service temperature	~115–120 °C	The highest of the common transparent plastics
Tensile strength	~60–70 MPa	Reasonable — stronger than ABS, similar to acrylic
Modulus	~2,300–2,400 MPa	Adequate stiffness, below acrylic
Scratch resistance	Moderate	Softer than acrylic — it marks more easily

And the limits are equally specific:

It stress-cracks. Polycarbonate is attacked by alkalis, ammonia, amines, ketones, esters and chlorinated solvents, and where chemical exposure coincides with stress — from machining, assembly, a press fit or a sustained load — the attack opens fine cracks that spread into fractures.

It yellows in UV. Unstabilised polycarbonate degrades in sunlight, changing colour and losing impact strength. Stabilised and coated grades exist for outdoor service.

It scratches. Less hard than acrylic, so a polycarbonate window that has to stay optically perfect is usually the wrong choice.

Stress cracking: the failure to design around

Most failures of polycarbonate parts come from stress cracking, and it is worth understanding because the fix is not a material change.

Two things have to be present: a chemical that attacks the polymer, and stress in the part. Neither alone is usually enough. A part exposed to a mild attacking agent with no stress in it survives; a part under stress with no chemical contact usually survives. Put them together and cracks open and spread.

The stress does not have to be large. Sources that are easy to overlook:

• Machining stress left in the part by cutting and clamping
• Assembly stress from a press fit, an over-tightened fastener, or a snap fit that has to be forced
• Sustained service load, including a part bolted down flat against a surface that is not actually flat
• Thermal stress from a part constrained between materials with different expansion

And the chemicals are not only industrial. The list that catches people out includes:

• Ammonia-based cleaning agents, which are ubiquitous in kitchens, laboratories and washrooms
• Alkaline cleaners and degreasers
• Amine-containing products — some epoxy hardeners, some corrosion inhibitors
• Silicone sealants and adhesives that release amines or acetic acid as they cure

So the practical guidance is: keep stress out of the part, and name the cleaning regime and the chemicals at the enquiry. Relieving machining stress by annealing after the part is cut is the direct control, and it is worth specifying on any polycarbonate part that is stressed, visible, or exposed to chemistry.

Machining polycarbonate

Polycarbonate machines well — it is tougher in the cut than acrylic, so it does not chip and craze as readily, and it is far less gummy than nylon. The standard plastic discipline applies:

• Sharp, high-rake tooling. In plastics, edge sharpness is the mechanism, not a preference.
• High spindle speeds with moderate feeds, so material is removed before heat accumulates in the workpiece.
• Good chip clearance. Polycarbonate produces stringy chips that must be able to leave the cut.
• Air blast or light coolant. Polycarbonate absorbs little moisture, so either works; air is usually enough and avoids cleaning the part afterwards.
• Restrained clamping. This matters more on polycarbonate than on most plastics, because clamp stress becomes stress cracking later rather than showing up as a mark now.
• Radius sharp internal corners where the design allows, since a sharp corner is both a stress concentration and a crack initiation site.

Annealing after machining is the standard preventive step, in the same way it is for acrylic. A controlled heat cycle around 120–125 °C, held and then cooled slowly, relaxes the internal stress left by cutting and clamping. On a stressed, visible or chemically exposed part, it is part of making the part correctly.

Precision features are held to ±0.01 mm with ISO 2768-m as the general tolerance block — see tolerances we hold.

Where polycarbonate is the right choice

• Machine guards, safety windows and shields, where impact resistance is a safety requirement
• Visors and face shields
• Protective housings and covers that have to survive handling and knocks
• Light covers and diffusers — transparent or opal, and tolerant of lamp heat
• Inspection windows and transparent enclosures
• Electrical enclosures and insulating components needing toughness
• Medical device housings, where gamma or ethylene oxide sterilisation is used
• Manifolds and transparent fluid components where impact and temperature both matter
• Any part that must be transparent or tough and will see service above 80 °C — where acrylic would soften

Where polycarbonate is the wrong choice

The part meets alkalis, ammonia, amines or aggressive solvents. Polycarbonate stress-cracks, and the combination of chemical plus stress is common in the field. Name the chemicals and the cleaning agents.

The part is machined, assembled under stress, and cleaned with an ammonia-based product. This is the classic combination and it is worth checking specifically, because the three elements arrive from three different people — machining, assembly and facilities.

The part is outdoors long-term and not UV-stabilised. Polycarbonate yellows and embrittles. Specify a stabilised or coated grade, and say the part is for outdoor use.

Optical perfection is the requirement. Polycarbonate scratches more readily than acrylic. A display window, a lens or a light guide that has to look flawless is usually acrylic.

The service temperature is above roughly 120 °C. Polycarbonate softens and creeps. Higher-temperature polymers are a different family.

Repeated steam autoclaving is the sterilisation route. Polycarbonate hydrolyses under repeated autoclave cycles. The method of sterilisation has to be stated, because it decides the material.

A cheap part with no impact requirement. Polycarbonate costs several times what ABS does. Where toughness is not needed, that money buys nothing.

Finishing polycarbonate

• Annealing — the stress-relief cycle, and the most important finishing step on a stressed part
• Hard-coat application — a coating that improves scratch resistance and blocks UV, common on glazing and guards where both properties are wanted
• Vapour polishing — a solvent vapour that clears a machined surface, done through our vetted partner network with proper solvent handling
• Mechanical polishing — for flat faces and simple geometry, working through grades
• Screen printing and painting — possible with appropriate inks and surface preparation
• UV-stabilised grades — a material choice rather than a finish, but the one that decides whether a part survives outdoors

One point that follows from the chemistry: cleaning matters more on polycarbonate than on most parts. A mild soap solution and a soft cloth is the safe method; ammonia-based glass cleaners and alcohol wipes are exactly the agents that cause crazing, and they are the default in many workplaces. Where a part will be cleaned regularly, that regime belongs on the enquiry.

All finishing runs through our vetted partner network, managed under one quality process. The grades we run across every process are listed on the materials page.

What to put on the drawing

Instead of	Write
Plastic	Polycarbonate (PC), where impact resistance is the requirement
Polycarbonate, chemicals or cleaning regime	Name them — alkalis, ammonia and amines cause stress cracking
Polycarbonate, stressed or press-fitted	Specify annealing — stress plus chemistry is what cracks it
Polycarbonate, outdoor service	Specify a UV-stabilised or coated grade
Polycarbonate, sterilisation	State the method — gamma and EtO are fine, repeated steam is not
Polycarbonate, must be optically perfect	Wrong material — acrylic is harder and clearer
Polycarbonate above ~120 °C	Wrong material — it softens and creeps
Polycarbonate, cheap part, no impact duty	ABS costs several times less

The three lines that decide most polycarbonate specifications are whether impact resistance is genuinely required, what the part touches and how it is cleaned, and whether it is outdoors. The first decides whether polycarbonate is worth its cost. The second and third are where it fails, and both are cheap to settle at the enquiry.

Material is bought against your drawing for your job, and every batch ships with a material certificate (MTR) tracing the grade and stock form to its mill lot plus pre-shipment photos of the actual parts. Orders start at 50 pieces per style and material, samples from a single piece in 5–10 days, and production runs in 25–35 days after sample approval. Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the chemical exposure, the annealing step and the UV grade where any of them need settling.