Polycarbonate (PC) thickness specification is governed by the interaction between short-term mechanical load, long-term weathering, secondary fabrication stress, and the dimensional tolerance of the delivered sheet or molded part. Solid extruded PC sheet is commercially produced from 0.5–20 mm in standard grades, while multiwall, laminated, and bullet-resistant constructions extend the available range to 38 mm and above. The tensile yield stress of unfilled PC is typically 60–70 MPa under ASTM D638-14, and notched Izod impact strength at 3.2 mm is commonly reported at 600–900 J/m under ASTM D256-10(2018). These data do not by themselves define the correct thickness because flexural deflection, puncture resistance, and formed-part wall distribution depend on geometry, clamping, strike velocity, and thermal history. The following four-column table is a preliminary selection matrix, not a replacement for a project-specific risk assessment, structural calculation, or certification test.

Which Polycarbonate Thickness for Which Job: A Four-Column Selection Table

Thickness ranges in the table reflect commercial PC film, extruded solid sheet, multiwall sheet, and laminated assemblies. The governing standard column identifies the test or specification that supplies objective data for that job. Where no standard sets a universal pass/fail thickness, the table records the required engineering assessment method.

Thickness verification on incoming PC sheet is performed with a digital micrometer or ultrasonic gauge according to the producer’s inspection protocol, not with a single-point caliper. For extruded solid sheet, ISO 11963:2019 defines the dimensional classes and tolerances; substituting a 4 mm panel for a 3 mm panel without re-evaluating the frame slot depth and thermal movement can produce buckling or edge fracture. On a production thermoforming line, batch-to-batch thickness variation above ±0.2 mm changes plug-assist settings and sheet sag time. Operators have observed higher corner-thinning reject rates when mill sheet is run without thickness sorting because the sheet is often supplied at the upper tolerance band while the tool is cut to nominal dimension. The resulting wall-thickness deficit in deep-draw areas is corrected by shortening the preheat zone or increasing the starting gauge, not by raising forming temperature alone.

Why Does the 3–6 mm Extruded Band Dominate Flat Glazing?

Flat indoor glazing and observation windows are normally specified at 3–4 mm because panel mass remains near 3.6–4.8 kg/m² at a density of 1.20 g/cm³ while light transmission remains above 88% under ASTM D1003-13. With flexural modulus approximately 2.3 GPa under ASTM D790-17, small-deflection beam theory makes deflection inversely proportional to the cube of thickness. A simply supported 3 mm panel across a 1 m span therefore deflects substantially more than a 6 mm panel under the same uniform load, so edge engagement is held to at least 15–20 mm. Outdoor sign faces and light boxes move to 4.5–6 mm not only for impact resistance but because the sheet must resist wind suction, snow load, and the bending induced by thermal expansion. With a linear coefficient of thermal expansion of approximately 6.7 × 10⁻⁵ K⁻¹, a 2 m panel changes length by about 6.7 mm across a 50 K temperature excursion. Hard fixing a 3 mm sheet at ambient temperature can induce buckling at summer surface temperatures above 40 °C.

Machine guarding specification bypasses generic thickness charts when rotating tool speed, projectile mass, and guard distance are known. The primary framework is risk assessment under ISO 12100:2010, and the selected panel must contain a defined impactor. Instrumented puncture testing per ASTM D3763-18 reports peak force, energy at peak load, and fracture mode. PC sheet at 8–10 mm absorbs high-energy impact, but the failure mode changes with mounting: a rigidly clamped panel may crack at bolt holes because local stress concentration exceeds the material’s environmental stress-cracking threshold, while a panel retained in flexible EPDM gaskets dissipates energy over a longer deflection path. Industrial guards machined on a CNC center require an edge radius of at least 3 mm at interior corners and through-holes; sharp internal cuts have produced radial cracks during installation when thread-cutting screws are used without pilot holes. Secondary splash shields are often specified at 6 mm, while operator-protective guards against ejected workpieces commonly move to 10–12 mm with the frame and anchor bolts engineered to the same impact energy as the panel.

Outdoor sound barrier panels and highway glazing add cyclic wind loading and acoustic fatigue to the thickness calculation. A solid PC barrier at 8–15 mm is selected by acoustic transmission loss and panel deflection under EN 1793-2:2018, but the supporting post spacing and panel joint seal often determine performance more than the sheet thickness alone. Acoustic fatigue cracks have been observed at hard-fixed bolt holes when expansion provisions are less than 3 mm per linear meter; rubber-gasketed channels with slide bearings eliminate this failure mode. In coastal locations, salt spray combined with solar heating accelerates coating loss, so a UV-capped grade with an anti-graffiti layer is standard for rapid transit and roadside installations.

When Thermoforming Draw Ratios Exceed 2:1, Thickness Stacking Becomes the Controlling Variable

Thermoformed PC parts such as equipment housings, transit interior panels, and medical device covers are typically produced from 3–6 mm extruded sheet. The processor must account for thickness distribution across the formed part. As the draw ratio increases, final wall thickness in corners falls below the starting gauge; for a 3 mm sheet drawn at 2:1, local wall thickness of 1.0–1.5 mm is common. This is not a material defect but a geometric consequence of area expansion, and the starting sheet gauge must be selected from the minimum required formed wall. Sheet surface temperature is normally set at 170–190 °C for forming, and the sheet must be dried to below 0.02% moisture to prevent bubbling. On a production-scale thermoformer with an infrared heating tunnel and plug assist, a batch thickness shift of 0.2 mm is sufficient to move sag depth by several millimeters. Published data for the exact sag-to-thickness correlation is machine-specific and therefore limited; empirical adjustment of preheat time and top/bottom heat balance is the standard corrective action.

Bullet-Resistant and Forced-Entry Assemblies: 21–38 mm Laminates

For security glazing, nominal PC thickness is only one term in an assembly-level equation. The threat standard—UL 752, EN 1063:2000, or ASTM F1233-19—defines projectile energy, velocity, and shot pattern, while the frame, interlayer, and air gap determine the assembly response. Commercial forced-entry and low-velocity ballistic glazing frequently uses 21–38 mm of PC, often as multiple 6–12 mm PC sheets laminated with polyurethane or ionomer interlayers. A monolithic 25 mm PC panel is not automatically equivalent to a laminated assembly of the same total thickness because delamination and impact energy distribution differ. The edge rebate depth is usually 38–50 mm, and the glazing must be tested in the final frame. Because PC yields under high strain rate rather than shattering, it can capture projectiles, but the reverse-side penetration limit depends on threat level and interlayer design. Substituting a thicker PC sheet without re-testing is not recognized by certification bodies.

Multiwall PC roof and skylight panels are specified at 10–16 mm for U-value and snow load, not for impact alone. The load-bearing response depends on rib orientation, support spacing, and end closure. Panel producers supply wind-load tables, and failure occurs by buckling of the upper skin if support spacing is exceeded or by thermal movement when gutters and curbs do not permit expansion. On production projects, field surveys have recorded panel bowing above 40 mm at midspan when solar heat gain raises the top panel surface to 80 °C while the interior surface remains near 25 °C; this bowing is reversible if the panel is correctly supported, but it can pull poorly fixed gaskets out of the glazing channel.

Extrusion and injection molding introduce a second thickness-dependent variable: moisture removal and melt residence time. PC granules must be pre-dried at 120 °C for 4 h to a residual moisture below 0.02% using a desiccant dryer with a −40 °C dew point. Moisture above this threshold hydrolyzes the resin at melt temperatures of 270–310 °C, producing splay and a measurable reduction in molecular weight and impact strength. Thin-wall injection molded PC covers with nominal wall thickness of 1.5–2.5 mm require higher melt temperatures and injection speeds to fill the cavity, but processing above 330 °C can release volatiles and embrittle the part. For thick-wall injection molded lenses and protective covers above 6 mm, sink marks and internal voids appear if the packing pressure profile is not adjusted; the mold should be run with a surface temperature of 90–110 °C and reduced screw recovery speed to limit thermal degradation in the barrel. Grades are selected by melt volume-flow rate under ISO 1133-1:2022; thick sheet extrusion often uses low-flow PC with MVR below 10 cm³/10 min, while thin-wall molding selects higher-flow PC with MVR above 20 cm³/10 min. Thickness selection cannot be separated from the melt-processing route.

Thermal Degradation Does Not Scale Linearly with Sheet Gauge

Heat history during extrusion and drying has a greater effect on thick sheet because low-flow PC grades require longer residence time and higher barrel temperatures to maintain melt stability. A 10 mm solid PC sheet run on a 90 mm single-screw extruder with an L/D of 30:1 typically operates with a melt temperature of 280–300 °C and back pressure below 100 bar. If melt temperature exceeds 330 °C, polycarbonate undergoes hydrolysis and thermal rearrangement that lower the notched Izod impact strength below the supplier’s specification. The same thermal excursion in a 2 mm sheet line may be less severe because melt residence time is shorter, but the large surface-to-volume ratio of thin film increases sensitivity to oxidative discoloration if the die exit is not shielded. Adequate pre-drying and melt-temperature monitoring are therefore mandatory for all gauges, but the upper temperature limit is especially critical in thick sheet extrusion and thick-wall injection molding.

Fabrication imposes additional thickness boundaries. PC film below 2 mm is typically kiss-cut, laser cut, or die cut, while sheets above 4 mm are CNC-routed or panel-sawed. Producer fabrication guides specify diamond-tipped tooling with negative rake for thick sheet to avoid chipping. Drilling holes near an edge requires an edge distance of at least 1.5 times the hole diameter; for 10 mm sheet, an edge distance below 10 mm creates microcracks that can propagate under service load. On a production CNC line, feed rates for 10 mm PC are typically 0.8–1.5 m/min with air coolant; high-speed steel tooling is avoided because frictional heat causes localized melting, stress whitening, and poor edge transparency. For thermoformed and machined parts, stress relief after machining is sometimes performed at 110–120 °C for 1 h per 2 mm of thickness, followed by slow cooling, to reduce residual stress before solvent-containing labels or coatings are applied.

Chemical contact imposes a final thickness-dependent boundary. Environmental stress-cracking resistance may be screened under ISO 22088-2:2006 using constant tensile strain. Polycarbonate is susceptible to environmental stress cracking in the presence of aromatic hydrocarbons, ketones, esters, strong alkalis, and certain amine-based compounds. Thinner PC sheet fails at lower applied strain because residual forming stress is proportionally higher relative to section thickness. For thermoformed parts, solvent exposure on stressed corners leads to microcracks that are not seen on unstressed flat cut sheet. Cleaning protocols for formed PC should exclude ketone and amine-based cleaners; mild soap and water or a compatible aliphatic hydrocarbon is the standard recommendation. If the job exposes PC to incompatible chemicals, the selected thickness becomes secondary to environmental stress-cracking resistance, and either a chemical-resistant grade, protective coating, or material substitution is required.