Polycarbonate (PC) is an amorphous thermoplastic produced from bisphenol A and phosgene or via melt transesterification, with a density of 1.20 g/cm³ per ISO 1183-1:2019. Unfilled general-purpose grades typically exhibit tensile stress at yield of 60–70 MPa per ASTM D638-14, Charpy notched impact strength of 65–75 kJ/m² per ISO 179-1:2010, and a glass transition temperature near 147 °C per ISO 11357-2:2020. The value-added services of cut-to-size processing, drilled holes, and edge finishing alter surface integrity, residual stress distribution, and dimensional tolerance in ways that are material-specific. Fabrication parameters must account for PC’s notch sensitivity, hygroscopicity, thermal expansion, and susceptibility to stress cracking when exposed to certain process fluids.

Does Moisture Absorption Create a Processing Window for Drilled Polycarbonate?

Moisture uptake in polycarbonate follows Fickian diffusion below 60 % RH; equilibrium moisture content at 23 °C and 50 % RH is approximately 0.15 % by mass according to ISO 62:2008. When moisture content exceeds 0.2 %, drilling operations may generate localized steam at the tool–polymer interface if cutting temperatures exceed 100 °C. The resulting splay, microvoiding, and bore-wall dimensional shift are not reversible by post-machining edge finishing. Production drilling therefore uses pre-dried sheet where fabrication history exposes stock to high humidity or outdoor storage. Desiccant drying at 120 °C for 2–4 h is typical before machining sheet above 8 mm thickness. Thinner sheet is conditioned at 23 °C and 35–40 % RH for 24 h. Moisture verification by Karl Fischer titration or thermogravimetric analysis is required where hole tolerances fall within ±0.05 mm. Published data for specific moisture thresholds in room-temperature drilling of all PC grades is limited, so process validation is advisable.

On vertical machining centres, carbide twist drills with polished flutes, high helix angles of 30°, and point angles of 60–90° are used. Spindle speeds from 18,000 rpm to 24,000 rpm for hole diameters between 3 mm and 10 mm minimize polymer melt-back. Feed rates of 0.05–0.15 mm/rev reduce frictional heat. Peck cycling every 2–3 mm of depth prevents chip packing in holes deeper than 6 mm. A sacrificial polycarbonate or acrylic backing plate suppresses breakthrough chipout on the exit face. Compressed air at 0.4–0.6 MPa is preferred to flood cooling because aqueous coolants can introduce stress cracking in certain PC grades, particularly when amine-based additives are present.

On CNC panel saw lines equipped with automatic scoring units, cut-to-size sheets are processed with a scoring blade depth of 0.1–0.2 mm to prevent top-surface chipping on both general-purpose and UV-stabilized PC grades. Main blades are carbide-tipped with triple-chip tooth geometry, typically 300 mm in diameter with 80–100 teeth. Cutting speeds for polycarbonate sheet fall between 40 m/s and 70 m/s; feed speed for 3 mm sheet is usually 10–15 m/min. Sheet thickness variation from extrusion lines, commonly ±0.1 mm for 4 mm stock, influences final linear accuracy unless the CNC touch probe zeroes on the fixed fence and compensates for thickness drift. Cut-to-size linear dimensions are inspected to ISO 2768-1 class m unless mating features require fine tolerances. For class m, a nominal dimension between 120 mm and 400 mm has a permissible linear deviation of ±0.5 mm; dimensions between 30 mm and 120 mm allow ±0.3 mm.

Edge-Finishing Parameters for Polycarbonate and Residual Stress Control

Diamond-tipped router bits and orbital sanders are used for edge finishing of cut-to-size polycarbonate. As-cut surface roughness is typically Ra 6.3 µm or higher when measured by ISO 1302; sequential sanding at 320–600 grit followed by buffing with 1–3 µm alumina slurry reduces roughness to Ra 0.8 µm or lower. Because the glass transition region of PC is near 147 °C, edge surface temperatures above 120 °C produce heat-affected zones, microcracking, and local yellowing. Continuous routing of edge profiles uses a 6 mm two-flute carbide router at 18,000–24,000 rpm with feed rates reduced to 0.5–1.5 m/min to limit adiabatic heating.

Residual stresses in as-extruded sheet can relax during edge finishing and produce dimensional movement after machining. Annealing of cut blanks at 120 °C for 1 h per 10 mm of thickness lowers internal stress before final edge finishing. This cycle is applied after rough cutting and before final dimensioning because stress redistribution can alter flatness by more than the dimensional tolerance of the finished part. Edge finishing should not be substituted for annealing; mechanical polishing removes microcracks but does not eliminate bulk residual stress.

Drilled-hole quality in PC is evaluated by entry circumferential chipout, exit burr height, and bore wall roughness. Drill geometries with point angles of 60–90°, clearance angles of 8–12°, and polished flutes reduce chip accumulation and thermal scoring. Pneumatic clamping pressure is kept below 0.5 N/mm² to avoid surface indentation. A rigid backing plate reduces exit burr height below 0.1 mm for hole diameters up to 12 mm. Where drilled holes require edge finishing, diamond-coated reamers or floating reamer holders maintain hole diameter tolerance H7 per ISO 286-2 for hole diameters of 6–12 mm. Because the linear thermal expansion coefficient of PC is approximately 65–70 × 10⁻⁶ K⁻¹ per ISO 11359-2:1999, a hole measured at 20 °C will change measurably if the component later operates at 60 °C or higher.

When Cut-to-Size Tolerances Collide with Stress Relaxation in Polycarbonate

Polycarbonate exhibits viscoelastic stress relaxation, and cut-to-size plates with high aspect ratios may bow or twist after machining when extrusion-induced internal stress redistributes. Post-machining aging at 23 °C for 24–72 h often stabilizes dimensions before final inspection. Production-scale panels with aspect ratios above 3:1 sometimes display edge lift after cutting; published quantitative data for this specific configuration is limited because stress state depends on sheet extrusion history, cooling rate, and grade. When dimensional stability is critical, flattening anneals at 120 °C are applied after rough cutting, followed by finish machining and controlled cooling at ≤20 °C/h to reduce thermal gradient stress.

For cut-to-size edges, the presence of saw marks or microchips acts as stress concentrators. PC is notch sensitive, and an as-cut edge with Ra 6.3 µm roughness may show reduced impact strength compared with an edge finished to Ra 0.8 µm. Mechanical edge finishing is therefore not purely cosmetic; it removes surface flaws that lower fracture resistance under tensile or impact loading. Flame polishing and solvent polishing are avoided because localized heating above 147 °C or solvent contact with methylene chloride causes stress cracking and reduces molecular weight at the surface.

For compliance documentation, cut-to-size and drilled polycarbonate components are inspected to ISO 2768-1 class m or fine depending on mating features. Surface roughness after edge finishing is specified to ISO 1302. Impact retention can be verified by ISO 179-1/1eA or ASTM D256. Material certification for food-contact or medical applications requires EU 10/2011 or USP Class VI when the specific PC grade is listed. Unmodified polycarbonate grades generally comply with REACH 1907/2006 and RoHS 2011/65/EU, but additive and colourant packages must be reviewed because flame-retardant or specialty grades may contain restricted substances.

Polycarbonate is incompatible with aromatic solvents, ketones, chlorinated hydrocarbons, and amine-based cutting fluids because these agents induce environmental stress cracking on drilled and edge-finished surfaces. Alkaline cleaners above pH 10 attack the polymer surface and should not be used for degreasing fabricated edges. Aliphatic hydrocarbon cleaners are less aggressive but still require validation under service load. End users must avoid contact between finished PC components and plasticizers, greases, or sealants containing phthalates or amines. When adhesive bonding is specified for cut-to-size and drilled assemblies, compatibility testing follows ASTM D6962-17 or equivalent, and stress-crack-resistant grades may be required.