Polycarbonate is the material you specify when the part absolutely cannot shatter — and then it cracks anyway, three weeks after it shipped, in a spiderweb of fine lines radiating from a drilled hole. Nothing hit it — the machine guard just sat there and crazed.
That is usually not bad material or bad machining. It is stress the part carried out of the shop meeting a chemical it should never have met — a cutting oil, a thread-locker, a cleaning solvent. Polycarbonate machines beautifully when you respect the stress you put into it and the chemistry you expose it to.
What is polycarbonate?
Polycarbonate (PC) is a transparent, amorphous engineering thermoplastic, sold as sheet and rod under trade names like Lexan (SABIC), Makrolon (Covestro), and TECANAT (Ensinger). Three things define it:
- It is the impact material. The Makrolon GP data puts the sheet at 47 ft·lbs in instrumented impact (ASTM D3763, 0.125" sheet) against 2 for acrylic and 0.5 for glass — why it glazes machine guards, riot shields, and security windows.
- It is genuinely clear. 86–88% light transmission at about 1/8" — close to acrylic, far tougher.
- It is amorphous, and that cuts both ways. It softens gradually, thermoforms and bends well — but machining stress stays locked in the part, and certain solvents find that stress and open it into cracks.
Polycarbonate properties at a glance
Three primary sources, each basis-marked: Ensinger's TECANAT natural extruded stock shapes (the form most machined parts start from), SABIC's Lexan 9034 sheet, and Covestro's Makrolon GP sheet. Where they disagree, both numbers are shown — the spread is real and grade-driven.
| Property | Value | Method | Source / basis |
|---|---|---|---|
| Density / specific gravity | 1.19 g/cm³ (Ensinger) · 1.20 (SABIC, Covestro) | ASTM D792 | SABIC, Covestro; Ensinger publishes 1.19 g/cm³ with no method stated |
| Tensile strength, yield | 9,300 psi (TECANAT stock) · 9,000 psi (Lexan 9034 & Makrolon GP sheet) | ASTM D638 | as marked |
| Tensile modulus | 340,000 psi (TECANAT, Makrolon GP) · 345,000 psi (Lexan 9034) | ASTM D638 | as marked |
| Flexural modulus | 328,000 psi (TECANAT stock) · 345,000 psi (both sheets) | ASTM D790 | as marked |
| Elongation at break | 90% (TECANAT stock) · 110% (both sheets) | ASTM D638 | as marked |
| Notched Izod impact — sources differ | 2.5 ft·lb/in (TECANAT extruded stock) vs 12–16 ft·lb/in @ 1/8" (Lexan 9034 sheet) vs 18 ft·lb/in @ 0.125" (Makrolon GP sheet) | ASTM D256 | grade and form drive a ~6× spread; don't assume glazing-sheet toughness in thick stock |
| Rockwell hardness | M70 (all three) · R118 (Ensinger, Covestro — SABIC publishes M70 only) | ASTM D785 | see basis |
| Coefficient of linear thermal expansion | 3.75 × 10⁻⁵ in/in·°F (both sheets) · 3.8 × 10⁻⁵ (TECANAT) | ASTM D696 | ~37.5 µin/in·°F — about 6× steel |
| HDT @ 264 psi — sources differ | 270 °F (Lexan 9034, Makrolon GP sheet) vs 295 °F (TECANAT stock) | ASTM D648 | as marked |
| Max continuous service temperature | 250 °F long-term, 275 °F intermittent (Ensinger TECANAT); suppliers publish ~240–250 °F depending on grade | — | Ensinger |
| Water absorption, 24 h | 0.07% (TECANAT) · 0.15% (Lexan 9034, Makrolon GP) | ASTM D570 | as marked; saturation 0.35% |
| Light transmission | 86% (Makrolon GP clear, 0.118" sheet) · 88% (Lexan 9034, 1/8" disk) | ASTM D1003 | thickness-marked |
| Dielectric strength | 380 V/mil, in air, 0.125" sheet (Makrolon GP) | ASTM D149 | medium + thickness matter; do not quote bare |
| UL 94 flammability | HB @ 0.060" · V-0 @ 0.394" (Makrolon GP) | UL 94 | rating is thickness-specific |
Treat these as typical published values, not a specification — the Izod row alone spans 2.5 to 18 depending on what you buy. Work from the datasheet for the exact stock on a critical part.
The real enemy: stress plus chemistry
Every cut puts stress into polycarbonate, on top of the residual stress extruded stock carries from the die. A stressed PC part can look perfect and still be primed to fail, because its chemical resistance collapses under stress — the Makrolon environmental-resistance listing is explicitly based on 70 °F and 0% strain.
What attacks it, per the Makrolon environmental-resistance listing: alkalis, amines, ketones, esters, and aromatic hydrocarbons. In shop terms: acetone, benzene, toluene, xylene, gasoline, brake fluid, lacquer thinner, ammonia cleaners — and cutting oils. Methylene chloride and chloroform outright dissolve it. What it tolerates well: water, dilute mineral acids, aliphatic hydrocarbons, and isopropanol — though not every alcohol: methyl and benzyl alcohol both sit on the not-resistant list, and Covestro rates n-propanol non-resistant too.
Three rules fall out of that:
- Get the coolant chemistry right. Machining references are consistent that coolants for amorphous plastics — polycarbonate, acrylic, polysulfone, Ultem — must be non-aromatic and water-soluble, because petroleum-based fluids risk stress cracking. Air blast or spray mist is the conservative default — a fluid that is fine on acetal can quietly ruin a polycarbonate job that gauged perfectly.
- Anneal when a lot of material comes off. Stock-shape suppliers publish a specific stress-relief cycle for machined polycarbonate: air-circulating oven to 250 °F (heating no faster than 20 °F/hr), hold 30 minutes plus 15 minutes per 1/8" of cross-section, then cool to 150 °F over about 10 hours, and let the oven fall to room temperature before the parts come out. Slow both ways — quenching the stress back in defeats the point. For heavy asymmetric removal or parts headed into chemical service, it is cheap insurance against the three-weeks-later crack.
- Design the downstream chemistry, too. Solvent-based adhesives, thread-lockers, and aggressive cleaners belong on the drawing review checklist for any PC part, and repeated steam cleaning can craze it hydrolytically — the Makrolon listing puts good water resistance at up to roughly 150 °F.
Heat, smearing, and the acrylic comparison
Acrylic and polycarbonate get quoted against each other constantly, but on the machine they are different animals. Acrylic is brittle at the cutter — it chips before it smears. Polycarbonate is tough and heat-sensitive: machining guides flag its tendency to soften and smear at the cut when tools are dull, speeds too high, or feeds too light. A rubbing tool builds heat, and the smeared surface it leaves hides the stress you just added.
The cure, applied strictly: sharp, polished, positive-rake carbide, a real chip load so the tool cuts instead of rubs, and multiple lighter passes on finish work rather than one heavy one.
Drilling is where polycarbonate cracks on the machine rather than in service. The published drilling practice here is general machined-plastics guidance rather than polycarbonate-specific (San Diego Plastics, among others), and it matters more on PC because of what a grabbing drill does to a notch-sensitive material: a 118° point with the lip rake dubbed off and the web thinned, 9–15° lip clearance, a positive, machine-controlled feed of roughly 0.005–0.015" per revolution on pilot holes at 600–1,000 rpm, then dropping the spindle to 400–500 rpm at 0.008–0.020" per revolution to open the hole up — and never hand-feed: a grabbing drill leaves microcracks at the hole wall that seed later crazing. Holes near edges and hardware torqued directly against PC deserve the same suspicion.
What tolerances are realistic
Machined-plastic tolerance guidance in published references runs about ±0.001" per inch of dimension as the practical benchmark. Polycarbonate sits on the cooperative end of that range: it absorbs little water, and its thermal expansion — about 37.5 µin/in·°F, roughly 6× steel — is half of what a polyethylene moves. The usual plastics discipline still applies: let parts normalize before inspection — a warm part off the machine reads differently an hour later. A print carrying metal-shop tolerances on a 24" polycarbonate window is worth a conversation before material is cut.
When polycarbonate is the right call
- Impact with visibility: machine guards, sight windows, safety glazing — nothing else matches its toughness-to-clarity ratio.
- Structural clear parts: manifolds, fixtures, covers where acrylic's brittleness is a liability.
- Moderate heat: service into the 240–250 °F range (grade-dependent) — far above acrylic.
- Electrical: a good insulator (380 V/mil in air at 0.125" for Makrolon GP) — spec the thickness with the rating.
When it is not
- Chemical exposure it can't take. Ketones, aromatics, chlorinated solvents, strong alkalis, ammonia cleaners — under stress, even splash contact matters. HDPE or PVDF for the wash-down chemistry PC hates.
- Scratch-critical optics. PC scratches far more readily than it breaks, and uncoated standard sheet has limited outdoor weathering; hard-coated grades exist — specify them, don't assume them.
- Sliding wear. It is not a bearing material; that's acetal's job or UHMW's.
- Above ~250 °F. Ultem or PEEK territory.
Getting polycarbonate parts quoted
Send the drawing and tell us the service environment — chemicals, cleaners, temperature, and whether clarity matters. Pro Plastics has served OEMs from Linden, NJ since 1968 — machining and fabricating plastics, and only plastics — and polycarbonate is core stock for our fabrication department: cutting, routing, drilling, bending, bonding and assembly. Properties, stock forms and typical applications are on our polycarbonate material page. We stock PC sheet from 1/16" to 2" and rod from 1/4" to 6" diameter in clear, black, white, gray, and bronze. Quotes in 24 hours.