Polycarbonate Sheet: How One Extruded Board Became Seven Different Products

A single extruded polycarbonate (PC) board manufactured from bisphenol A polycarbonate resin is converted into at least seven service-specific articles through machining, thermoforming, cold forming, lamination, coating, and additive compounding. The base board is produced from resin with a melt mass-flow rate of 6 g/10 min to 10 g/10 min when tested to ISO 1133-1:2022 at 300 °C and 1.2 kg nominal load. Pellets are dried at 120 °C for 4 h in a desiccant dryer to reduce residual moisture below 0.02 %; excessive moisture hydrolyzes the carbonate linkage, reducing melt viscosity and producing surface splay and internal voids. Extrusion is performed on a vented single-screw extruder with a 30:1 L/D ratio and barrel set points between 280 °C and 310 °C. The melt is polished in a vertical three-roll stack with roll temperatures between 120 °C and 140 °C. Sheet thickness ranges from 2 mm to 15 mm, with a thickness tolerance of ±5 % for commercial grades and ±2 % for machinable grades. Density is 1.20 g/cm³ per ISO 1183. Clear grades exhibit visible light transmission of 88 % to 91 % per ASTM D1003 and a haze value below 1 % for 3 mm sheet. Notched Izod impact values exceed 600 J/m at 23 °C for 3.2 mm specimens per ASTM D256. Residual thermal stress from roll-stack cooling is measurable by photoelastic analysis per ASTM D4093; annealed grades show lower birefringence and are specified for machined parts.

The resin selected for extrusion has a weight-average molecular weight in the range of 28,000 g/mol to 32,000 g/mol, which balances melt processability with impact strength retention. Polycarbonate is shear-thinning, with melt viscosity at 300 °C and 100 s⁻¹ shear rate commonly between 800 Pa·s and 1,200 Pa·s for sheet grades. The extruder screw typically includes a barrier section and a vacuum vent at -0.08 MPa to remove residual volatiles. Roll-stack polishing is the primary method for controlling surface texture; gloss levels above 95 GU at 60° are achievable on polished chrome rolls. The board is cooled at a controlled rate to limit thermal stress, and sheet above 6 mm is often annealed at 125 °C for 1 h per 25 mm of thickness. These conditions establish the baseline properties that the seven downstream products modify or preserve.

Clear security glazing fabricated from the flat extruded board is cut with diamond-tipped circular saws and edge-finished using carbide or polycrystalline diamond tooling. Flame polishing or machining removes edge microcracks that would otherwise initiate brittle failure at impact loads. The product is used in machine guards, detention windows, and riot shields where impact penetration resistance is the controlling property. Impact testing of 6 mm monolithic polycarbonate sheet to ISO 6603-2 at 23 °C produces ductile deformation without brittle through-cracking; the same thickness retains notched Izod impact values above 650 J/m when tested to ASTM D256. The material transmits 89 % of visible light at 3 mm per ASTM D1003, but uncoated sheet has low scratch resistance; taber haze increase exceeds 10 % after 100 cycles with a CS-10F wheel under 500 g load per ASTM D1044. A polysiloxane hard coat is applied at 8 µm to 12 µm dry film thickness, with adhesion tested by ISO 2409 cross-hatch rating of 0 or 1. The coating requires solvent flash-off at 23 °C to 30 °C and thermal cure at 80 °C to 120 °C. Operational boundaries include continuous service temperature below 120 °C and avoidance of ketone, ester, chlorinated solvent, and aromatic hydrocarbon contact, all of which produce environmental stress crazing. Ammonia and amine-containing sealants are incompatible because they catalyze hydrolysis of the carbonate linkage. When used as security glazing, the fabricated sheet is often tested to EN 356 categories for manual attack or to UL 972 for burglary-resistant glazing if specified. The framing system therefore must retain the sheet edge at a bite of at least 20 mm for 6 mm sheet to prevent pull-out. Continuous exposure to high humidity does not corrode the polymer, but uncoated surfaces will hydrolyze slowly at temperatures above 60 °C in high-pH aqueous environments.

Thermoforming Draw Ratio and Wall-Thickness Mapping

Thermoforming of the base board into equipment guards and machine enclosures converts the flat extruded sheet into three-dimensional articles. The process window is constrained by polycarbonate’s moisture sensitivity and by non-uniform wall-thickness distribution. Blanks are re-dried at 120 °C until residual moisture falls below 0.02 %; a 1 mm sheet can regain enough moisture to form bubble defects after 4 h at 50 % relative humidity. Heating is performed in a quartz or ceramic infrared oven with sheet surface temperatures between 175 °C and 190 °C, monitored by an optical pyrometer. Tool temperature is held at 80 °C to 100 °C. The critical forming parameter is the areal draw ratio: total part surface area divided by blank area is maintained below 3:1 for thin-gauge sheet and below 2:1 for thicknesses above 6 mm. Larger draw ratios produce corner thinning below 0.5 mm and reduce notched Izod impact values below 300 J/m in the thinned region. Plug-assisted forming with syntactic foam or polyoxymethylene plugs is required for deep pockets; the plug is maintained at 100 °C to 120 °C to prevent chill marks. Post-forming residual stress is assessed using the environmental stress cracking principle of ISO 22088-2; a short immersion in isopropanol or ethyl alcohol reveals crazing in overstressed regions within 30 min. Wall-thickness mapping is performed by ultrasonic thickness measurement at a grid spacing of 10 mm across the formed part; readings below 0.4 mm in high-wear corners are rejected. Finite-element analyses of the thermoforming process, calibrated with biaxial extensometer data at forming temperatures, are used to optimize plug geometry. The production rate is limited by heating time rather than cooling time; forced-air cooling after forming is typically 60 s to 90 s for 3 mm sheet. Reject sorting includes optical inspection for bubbles, chill marks, and burn marks that indicate local sheet temperatures above 200 °C. Production-scale thermoforming lines record higher rejection rates when ambient relative humidity exceeds 60 % because moisture absorbed during blank staging is not fully reversible in the hot forming step. The formed parts are annealed at 125 °C for 1 h per 25 mm of wall thickness to relieve draw-induced residual stress.

For electrical isolation components, the flat extruded board is machined on a CNC router with single-flute polycrystalline diamond tooling at spindle speeds between 12,000 rpm and 20,000 rpm and feed rates that avoid melt-back at the cut edge. The resulting parts are used as busbar supports, switchgear barriers, and terminal blocks. Polycarbonate’s dielectric strength is 17 kV/mm to 20 kV/mm for 1 mm specimens tested to IEC 60243-1, and surface resistivity exceeds 10¹⁴ Ω per ASTM D257 at 23 °C and 50 % relative humidity. Comparative tracking index is typically above 175 V per IEC 60112 for unfilled grades. Flame-retardant versions used in electrical enclosures are rated UL 94 V-0 at 3.0 mm. Melt-back at the cut edge is controlled by chip thickness; a chip thickness below 0.05 mm causes frictional heating and edge whitening. Coolant is generally avoided because some water-miscible coolants contain amines that stress-crack polycarbonate. If cooling is required, clean compressed air or amine-free deionized water is used. The machining process introduces edge stress; annealing at 125 °C for 1 h per 25 mm of thickness relieves residual stress and reduces stress-cracking in service. After annealing, dimensions can shift by up to 0.5 %; therefore, finishing cuts are performed after annealing for tight-tolerance electrical parts. The product is unsuitable for continuous use above 120 °C or in the presence of ammonia and amines, which attack the carbonate linkage and reduce molecular weight. Threaded inserts are pressed into undersized holes rather than bonded with cyanoacrylate, because accelerated testing shows adhesive-induced stress crazing on polycarbonate substrates.

When a Flat Sheet Enters a Cold-Bending Radius

Architectural glazing can be produced from the extruded board without heating when the bend radius is controlled. Cold bending of polycarbonate sheet into skylight or canopy profiles is performed at ambient temperatures below the glass transition temperature of approximately 147 °C. The minimum permanent cold-bending radius is specified as 100 times the sheet thickness for thicknesses up to 6 mm; for a 4 mm sheet, the minimum radius is 400 mm. The maximum surface tensile stress due to cold bending can be approximated by the Euler-Bernoulli relation σ = E t / (2 R), where E is the flexural modulus of 2,300 MPa for polycarbonate, t is sheet thickness, and R is bend radius. At the 100-t radius limit, the calculated bending stress is therefore approximately 11.5 MPa; this value is below the short-term tensile stress that produces crazing, but long-term weathering and chemical exposure lower the craze threshold. Design engineers therefore apply a safety factor of 2 on the minimum radius when the canopy is exposed to cyclic wind load or temperature swings from -20 °C to 60 °C. Bends tighter than the threshold induce tensile surface stress above 10 MPa, which can be measured by photoelastic inspection and is correlated with stress crazing when exposed to isopropanol or ethyl alcohol per ISO 22088-2. The edge frame must apply uniform line load; localized clamping with a radius deviation above ±5 % produces stress concentration and surface microcracking after 500 h of accelerated weathering to ISO 4892-2. For glazing bars, the bearing width is sized to keep bearing stress below 3 MPa at maximum design wind load. Thermal expansion of polycarbonate is approximately 6.5×10⁻⁵ m/m·K; framing slots must accommodate this movement to avoid additional bending stress at the edge. When the required curvature is tighter than the cold-bending limit, the sheet is thermoformed rather than cold bent. In all cold-bent installations, edge seals must be neutral-cure silicone; acid-cure silicone and polyurethane formulations that release amines are excluded because they initiate environmental stress cracking at the bend apex. Aluminum frames are isolated with EPDM gaskets to avoid direct contact with plasticized PVC, which can migrate plasticizer into the polycarbonate and reduce impact strength.

Fabrication of ballistic laminates from the base board proceeds by bonding multiple polycarbonate sheets with interlayer adhesives under vacuum-lamination pressure. Laminate constructions are tested to UL 752 or NIJ 0108.01 according to threat level; however, published data for this specific configuration is limited because required thickness and areal density vary with projectile mass and velocity. A common level-glazing build uses three or four polycarbonate plies with a total thickness above 19 mm. Vacuum bag lamination is conducted below 80 °C to avoid thermal distortion; adhesive interlayers must be free of plasticizers that migrate into the polycarbonate and reduce impact strength. Interlayer adhesion is promoted by surface preparation with an alcohol wipe and plasma or corona treatment to raise surface energy above 42 mN/m before lamination. The vacuum-bag pressure is maintained between 0.08 MPa and 0.09 MPa during cure. After lamination, the sandwich is edge-trimmed and conditioned at 23 °C and 50 % relative humidity for 48 h before ballistic testing because polycarbonate impact resistance is moisture-sensitive. A single 15 mm sheet is not equivalent to three 5 mm plies because interlayer shear affects energy distribution. This application has the narrowest tolerance for lamination defects, because any trapped air pocket larger than 1 mm in diameter can initiate delamination under impact. Edge delamination is controlled by routing the edge to a 2 mm chamfer and applying a polyurethane edge seal. Testing includes a 9 mm full-metal-jacket projectile at velocities defined by the standard; the laminate resists penetration by absorbing energy through large plastic deformation rather than brittle fracture. In multi-ply assemblies, the interlayer bond strength is checked by lap shear testing to ISO 4587; cohesive failure within the polycarbonate substrate is preferred over adhesive failure at the interlayer boundary. The product is limited by low scratch resistance and requires a glass or hard-coated sacrificial face ply in abrasive environments.

What Fire-Safety Limits Apply to Mass Transit Panels?

Mass transit interior panels fabricated from the extruded board use a flame-retardant polycarbonate grade containing halogen-free phosphorus-based additives. The compound is rated UL 94 V-0 at a thickness of 3.0 mm; the afterflame time is below 10 s per individual specimen when tested to IEC 60695-11-10. The limiting oxygen index of this grade is typically between 35 % and 40 % per ISO 4589-2, compared with 25 % to 27 % for general-purpose polycarbonate. Cone calorimeter testing to ISO 5660-1 at an incident heat flux of 50 kW/m² shows lower peak heat release rate than general-purpose sheet; however, the additive package increases melt viscosity and narrows the extrusion processing window. Barrel temperatures are reduced by 5 °C to 10 °C relative to unfilled resin to prevent thermal degradation of the phosphorus synergist, and a vacuum-vented screw with 25:1 L/D is required to remove volatiles. The melt temperature reduction is critical because the phosphorus-based flame retardant can undergo thermal degradation at temperatures above 300 °C, producing surface silver streaks. Throughput on a 50 mm diameter extruder is typically reduced by 10 % to 15 % compared with general-purpose PC to maintain melt stability. The flammability performance is thickness-dependent; a 1.5 mm sheet may only achieve UL 94 V-1 in some formulations, so end-users must verify the yellow card for the specific thickness. The additive also reduces tensile elongation at break by approximately 15 % to 25 % when tested to ISO 527-2, so panel mounting inserts must be designed to avoid point loading. Compliance with EN 45545-2 hazard level HL2 requires additional smoke density testing to ISO 5659-2 with a maximum Ds of 300 in typical specifications, evaluated at 25 kW/m² incident heat flux with and without pilot flame. The phosphorus-based chemistry does not produce the hydrogen chloride emissions associated with brominated systems. The material is not suitable for outdoor glazing unless a UV-stabilized cap layer is coextruded, because the phosphorus flame retardant can reduce weatherability.

Backlit signage uses the same extruded board as the glazing and electrical grades, but the sheet is screen-printed or digitally printed on the reverse face and die-cut to shape. Weathering resistance is provided by a coextruded UV-absorber cap layer with accelerated weathering to ISO 4892-2; the yellowness index increase for a 3 mm clear sheet after 5,000 h of xenon-arc exposure is typically below 5 when measured per ASTM E313. The product is limited to flat or slightly curved sign faces, because the printed surface cannot withstand post-print thermoforming without ink cracking.