Fabrication of stock bisphenol-A polycarbonate sheet, rod, and plate in a machine shop differs from metal working in one decisive respect: the material is amorphous, notch-sensitive under certain chemical conditions, and viscoelastic at temperatures only modestly above ambient. The bulk mechanical properties establish the boundary conditions for cutting and clamping. Density is 1.20 g/cm³ per ISO 1183-1. Tensile stress at yield for general-purpose machining grades is commonly reported between 60 MPa and 65 MPa per ISO 527-2, with tensile elongation at break often in the range of 50% to 120%. Flexural modulus is approximately 2,300–2,400 MPa per ISO 178. Notched Izod impact values for unfilled polycarbonate typically fall between 600 J/m and 850 J/m per ASTM D256. The heat deflection temperature under a load of 1.8 MPa is reported at 125 °C to 130 °C by ISO 75-1/A, while the Vicat B50 softening temperature is commonly 145 °C to 150 °C per ISO 306. Glass transition, as determined by differential scanning calorimetry under ISO 11357-2, falls near 147 °C. Poisson’s ratio is approximately 0.37.

Typical short-term property envelope for unfilled machining-grade polycarbonate

Property	Standard method	Reported range
Density	ISO 1183-1	1.19–1.21 g/cm³
Tensile stress at yield	ISO 527-2	60–65 MPa
Tensile elongation at break	ISO 527-2	50–120%
Flexural modulus	ISO 178	2,300–2,400 MPa
Notched Izod impact	ASTM D256	600–850 J/m
Heat deflection temperature	ISO 75-1/A, 1.8 MPa	125–130 °C
Vicat softening temperature B50	ISO 306	145–150 °C
Glass transition temperature	ISO 11357-2	145–148 °C
Poisson’s ratio	ISO 527-2 axial measurement	0.36–0.38

These values are not merely reference data; they define shop risk. The glass transition near 147 °C means that cutter-tip surface temperatures generated in dry or poorly cooled machining can reach the softening range, producing smeared chip roots, re-welded debris, and a heat-affected surface layer with residual tensile stress. The tensile yield range of 60–65 MPa is far above the stress needed to initiate environmental crazing, which is a separate failure mode driven by localized strain concentrations and chemical diffusion into the polymer surface. The high notched Izod value does not protect a machined part once a craze array has formed at a sharp corner, a drilled hole wall, or a clamp-contact perimeter.

Which Coolant Chemistries Leave Polycarbonate Susceptible to Solvent Crazing?

The selection of a cutting fluid for polycarbonate cannot be separated from the solvent-cracking response of the polymer. Polycarbonate exhibits a solubility parameter near 9.5–10.0 (cal/cm³)^0.5. Low-molecular-weight organic liquids with solubility parameters in this range diffuse into the surface and reduce the energy required for craze nucleation and propagation. Aromatic hydrocarbons, halogenated solvents, ketones, esters, and certain amines are capable of initiating environmental stress cracking in stressed polycarbonate. Even methanol and ethanol, which are classified as mild ESC agents for this polymer, can reduce the critical strain for craze initiation from a published air reference of approximately 0.8% to 1.0% down to 0.3% to 0.5% at 23 °C. Aromatic hydrocarbons can depress the critical strain below 0.2%. The practical consequence is that a coolant contact is not a neutral event when the part carries machining residual stress, a bolt-torque preload, or a clamp-induced bending moment. Water-miscible metalworking fluids formulated for ferrous alloys are frequently unsuitable because they contain amine-neutralized rust inhibitors, aromatic co-solvents, or high-pH buffers. Aliphatic ethanolamines and diethanolamine-bearing additives have been documented as aggressive toward stressed polycarbonate and should be excluded. The concentration of the diluted emulsion or semisynthetic fluid also matters. A heavy-duty water-miscible coolant diluted to 5–10% by volume may provide adequate heat removal, but the same fluid at higher concentration can leave an oily film whose residual solvent fraction remains in contact with the surface after the part is unclamped. The pH of water-miscible coolant should be maintained between 7.5 and 8.5 for polycarbonate work. Alkaline hydrolysis of the carbonate linkage accelerates above pH 9.0, particularly when interface temperatures rise above 80 °C during interrupted cuts. Straight mineral cutting oils with low aromatic content and no chlorine perform acceptably in sawing and threading operations, but the film must be removed by a compatible degreaser; a degreaser containing toluene or methyl ethyl ketone will introduce the same chemical stress-cracking risk the coolant was selected to avoid.

Coolant class compatibility for stressed polycarbonate machining

Coolant class	Representative composition	Observed polycarbonate interaction	Applicable test method
Straight mineral cutting oil	Naphthenic or paraffinic base, aromatic content below 1%	Low interaction; residual oil staining possible	ISO 175 immersion; ASTM D543-21
Water-miscible semisynthetic	Amine-free, pH 7.5–8.5, concentration 5–10%	Low interaction when pH and concentration controlled	ASTM D543-21; ISO 22088-3
Water-miscible synthetic with amine rust inhibitor	Diethanolamine or triethanolamine neutralized	Moderate-to-high crazing risk on stressed surfaces	ISO 22088-3 bent strip method
Solvent-based aerosol cutting fluid	Toluene, xylene, methyl ethyl ketone	Severe crazing and surface softening; prohibited	ISO 22088-3; ASTM D543-21
Compressed air	No liquid chemical contact	No chemical interaction; risk of thermal buildup only	Not applicable to chemical resistance testing

The verification path for coolant compatibility is not visual inspection alone. A standard procedure is to expose pre-stressed polycarbonate coupons to the candidate fluid using the bent strip method of ISO 22088-3 or the immersion practice of ASTM D543-21. The coupon is held at a known strain, typically between 0.3% and 0.8%, and inspected after 24 h and 48 h of exposure at 23 °C. Craze formation at 10× magnification under oblique light indicates that the coolant is unacceptable for production where identical strain exists. The same coupon methodology can be used to audit a recovered shop coolant after it has accumulated tramp oil and dissolved solids, because aged fluid chemistry may differ from the as-mixed condition. Without a header to interrupt the clamping discussion, the mechanics of workholding introduce a second stress field that interacts with the machined surface state. In cross-slide milling, drilling, and tapping setups, clamping is often treated as a static boundary condition. For polycarbonate, it is a chemical-mechanical boundary condition. A steel step clamp, a serrated vise jaw, or a pointed set screw creates a local compressive contact zone; immediately adjacent to that zone the material experiences a tensile hoop stress that follows the compressed region and extends into the part body. When the part surface is simultaneously wetted by a marginal coolant or by residual cutting oil, this tensile zone becomes the site where crazing initiates. The phenomenon is not delayed by the part’s bulk toughness, because craze growth is a surface-localized yield process preceded by solvent plasticization and molecular orientation. Clamping force should be calibrated to the minimum required to hold position against cutting forces, not to a fixed torque habit. For unfilled polycarbonate at 23 °C, the compressive yield strength reported under ISO 604 is approximately 75–80 MPa. That value does not define a safe clamping pressure; under sustained contact, creep reduces the allowable working stress to a published continuous bearing range of roughly 10–15 MPa. In shop practice, point-contact pressures above 5 MPa at steel edges are sufficient to produce visible clamp marks and craze arrays on coolant-wetted polycarbonate within 24 h, especially when the part has been machined dry and no stress-relief anneal has been performed. Polished aluminum pads, cast phenolic shoes, or 90A to 95A polyurethane pads should be used to distribute load over the largest available flat area. The contact perimeter should be kept away from drilled holes and notches, because the elastic stress concentration factor for a circular hole in tension is approximately 3.0, and the same factor applies to the tensile fringe around a clamped hole in bending. Vibration during machining worsens the picture. A tooling fixture that permits workpiece oscillation at frequencies above 100 Hz can convert a nominally static clamp force into a low-cycle fatigue input. Fretting at the pad-polymer interface produces fine surface debris, haze, and microvoided regions that act as pre-existing flaws when a coolant film is present. The combination of vibrating clamp contact and a marginal coolant has been observed on production milling lines to produce craze networks within 4 h to 8 h after unloading, even though the part showed no cosmetic defect immediately after machining. This is not a cosmetic failure; it is a precursor to brittle breakage under service load.

When Hole-Making Generates Sufficient Local Heat to Trigger Surface Microcrazing

Drilling, tapping, and reaming are disproportionate contributors to polycarbonate shop craze failures because the tool tip is buried, chip evacuation is restricted, and the polymer’s thermal conductivity is only about 0.20 W/(m·K). Heat generated at the tool edge cannot dissipate through the bulk at the rates achieved in aluminum or steel. The resulting interface temperature can approach the glass transition of 147 °C if feed and speed are not moderated and coolant flow is inadequate. The affected layer undergoes local softening, molecular orientation, and partial recovery during cooling, leaving a residual tensile stress state in the hole wall. When the same hole is then exposed to a cutting fluid, a tapping compound, or even ambient humidity over time, the residual stress reduces the external strain needed to form crazes. Critical strain in a machined hole that has experienced thermal softening can be lower than the 0.8–1.0% air reference for annealed material. Tool geometry controls the magnitude of the thermal input. For drilling polycarbonate, fabricators typically run high-speed steel or polished carbide drills with included point angles between 90° and 118°, helix angles between 15° and 30°, and polished flutes to reduce chip friction. Feed rates are kept low to limit axial thrust, commonly in the range of 0.05 mm/rev to 0.25 mm/rev for hole diameters from 3 mm to 12 mm. The softer the feed, the lower the axial residual stress left in the hole wall. Tapping produces a more severe condition because the thread root is a geometric stress concentrator and the tap flutes load the part with a helical cutting torque while chips are trapped. Roll form taps are not recommended for polycarbonate unless the hole is carefully annealed afterward, because material displacement at the thread crest leaves a high-strain skin that is preferentially attacked by ESC fluids. After aggressive hole-making, post-machining annealing is often required to bring the part back into a low-residual-stress condition. The stress-relief cycle used in fabricating shops for unfilled polycarbonate is commonly 120 °C for 1 h per 6 mm of wall thickness, followed by cooling at 30 °C/h or less to avoid reintroducing thermal gradients. Annealing cannot repair visible crazes or microcracks that have already formed; it relaxes orientation and reduces the stored elastic energy that would otherwise make the part vulnerable to delayed failure. If annealing is skipped, a part that leaves the fixture in acceptable visual condition can develop an intersecting craze pattern around a hole or clamp mark after 48 h to 72 h of ambient exposure, particularly when the machine coolant residue is not removed. The operational boundary is therefore explicit: polycarbonate fabricated with water-miscible coolants or clamped with metal contact must be considered chemically vulnerable until it is cleaned, inspected under oblique light at 10× magnification, and, where hole-making or heavy clamping has occurred, stress-relieved before use.