Polycarbonate (PC) is an amorphous engineering thermoplastic produced by interfacial polymerization of bisphenol A and phosgene or by transesterification with diphenyl carbonate. Commercial unfilled PC sheet typically exhibits a density of 1.20 g/cm³ when tested in accordance with ISO 1183-1, a tensile modulus near 2300 MPa per ISO 527-2, and a notched Izod impact strength in the range of 60 kJ/m² to 70 kJ/m² per ISO 180/1A. When sheet is extruded to a nominal thickness of 6.000 mm, the specification “Polycarbonate Thickness Tolerances: What Plus or Minus Five Percent Means on 6 mm” defines a bilateral tolerance of ±5%. That tolerance converts to +0.300 mm and −0.300 mm. The accepted sheet thickness therefore ranges from 5.700 mm to 6.300 mm, a total tolerance band of 0.600 mm. This band is not a purchasing abstraction; it interacts with cutting, thermoforming, clamping, optical quality, and structural mass.

How does ISO 11963 classify thickness tolerance for solid PC sheet?

ISO 11963:2019, clause 5, addresses dimensional tolerance classes for polycarbonate sheet. The standard provides thickness-tolerance tables for solid flat sheet, but many purchaser specifications impose a symmetrical ±5% band across selected nominal thicknesses. On 6.000 mm sheet, a ±5% band is often a commercial or application-specific requirement rather than an automatic default class. The tolerance applies to thickness measurements taken away from trimmed edges, with the exact edge exclusion zone set by contract or reference to the purchaser’s acceptance test plan. Thickness verification on production-scale sheet is commonly performed using a traversing laser gauge or beta gauge calibrated against contact measurements per ISO 4593.

Thickness measurement records on extruded PC sheet typically include a transverse profile at intervals of 10 mm to 25 mm across the usable width. A 6.000 mm sheet that exhibits a center thickness of 6.120 mm and an edge thickness of 5.880 mm remains within ±5%, yet it carries a 0.240 mm cross-web wedge. That wedge creates non-uniform clamp compression in CNC routing fixtures and variable draw behavior during thermoforming. The total permitted thickness asymmetry across a sheet can approach 0.600 mm only if the two extreme limits occur within the same inspection plane; most processors reject sheets with edge-to-center wedge above an internal threshold because downstream fixtures are designed for a single nominal geometry.

On single-screw extrusion lines used for PC sheet, thickness control begins with melt-delivery stability. A typical configuration uses a 90 mm single-screw extruder with a 30:1 L/D ratio, a vacuum vent, and a gear pump between screw tip and die. Barrel temperatures are profiled from 260 °C behind the feed section to 300 °C at the metering zone, with melt temperature held near 290 °C. PC must be dried to a maximum moisture content of 0.02% before extrusion; desiccant dryers operating at 120 °C for 3 h to 4 h with a dew point of −40 °C are standard. Residual moisture above this limit hydrolyzes PC at processing temperatures, reducing molecular weight and generating gaseous defects that disturb sheet thickness profile and surface quality. Published data for the exact transverse capability of a specific production line is limited; the following values represent typical industrial ranges rather than a universal guarantee.

Die lip, roll gap, and haul-off tension as interacting thickness variables

Thickness variation on a 6.000 mm sheet is dominated by three mechanical settings. Die bolt adjustments alter local mass flow from the flex lip; a change of 0.10 mm in die lip gap can shift local sheet thickness by the same order before draw-down. The calendering stack, typically a three-roll vertical or 45° configuration with roll temperatures of 120 °C to 140 °C, sets final gauge by squeezing the melt web. Roll gap variation of 0.05 mm across the working width appears directly as sheet thickness deviation unless roll bending is compensated by crown or roll crossing. Haul-off speed ratio between the polishing stack and downstream pull rolls controls orientation and final thickness; an increase in haul-off tension decreases sheet thickness in the machine direction and increases widthwise neck-in. Production lines with melt pumps typically hold a transverse thickness range of 0.15 mm on 6 mm sheet, while lines without melt-pump stabilization may drift by 0.30 mm to 0.40 mm over an eight-hour run as polymer viscosity shifts with recycled content. Polycarbonate extrusion grades have a melt volume-flow rate of 6 cm³/10 min to 12 cm³/10 min measured at 300 °C under 1.2 kg load per ISO 1133-1:2022. A shift of 2 cm³/10 min from recycled material alters die pressure and can increase transverse gauge variation on 6 mm sheet unless gear-pump speed is adjusted.

The edge bead formed at the sheet edge can be 0.2 mm to 0.5 mm thicker than the center because of neck-in and die-edge effects. This zone is removed or excluded from thickness acceptance by agreement. A ±5% band on 6 mm permits a total transverse wedge of 0.600 mm, but many processors impose a narrower internal limit, often ±0.20 mm, for thermoforming-critical and optical applications. Published cross-industry acceptance data for that internal limit is limited.

For transparent 6 mm PC sheet used in glazing or instrument windows, thickness variation changes the optical path length. At a refractive index of 1.586 at 589 nm per ISO 489, a 0.300 mm thickness deviation introduces an optical path difference of approximately 0.176 mm. This affects wavefront quality and visual distortion in flat-panel inspection windows, where optical defects are evaluated under haze and transmittance standards. Thickness variation also modifies local transmission slightly because absorption and reflection losses scale with geometric path length.

When 5.700 mm is within specification but below tooling design nominal

CNC routers, bending jigs, and vacuum fixtures are commonly designed around a nominal 6.000 mm pocket. A sheet at the lower tolerance limit of 5.700 mm leaves 0.300 mm of clearance per side in a fixture cut for 6.000 mm, which may allow vibration and part shift during machining. A sheet at 6.300 mm can bind or require excessive clamp force. The lower-limit condition is particularly relevant for nested cutting where vacuum hold-down force depends on surface contact area and gasket compression. For multiwall polycarbonate panels, thickness tolerance interacts with glazing channel dimensions and gasket compression; a 0.600 mm total band can shift installation force from design nominal to a tight or loose fit.

Thermoforming of 6 mm PC sheet is normally conducted with sheet surface temperatures of 180 °C to 220 °C. A blank at 6.300 mm contains 10.5% more polymer than a blank at 5.700 mm for the same projected area. The thicker blank heats more slowly in the core, sags differently under radiant heating, and requires longer soak time to achieve uniform forming temperature. The thinner blank reaches forming temperature earlier and may overheat or sag excessively if the line cycle is set for nominal 6.000 mm. Local thinning in deep-draw areas is a function of initial gauge, so a 5.700 mm sidewall may fall below the minimum part thickness required by structural load calculations.

Thermal expansion and moisture absorption shift the accepted band less than the extrusion wedge

Polycarbonate has a coefficient of linear thermal expansion of 65 × 10⁻⁶ K⁻¹ to 70 × 10⁻⁶ K⁻¹ per ISO 11359-2:2021. For a 6.000 mm sheet, a temperature change of 30 °C changes thickness by approximately 0.012 mm, which is one order of magnitude below the ±5% tolerance band. Moisture absorption is 0.15% at equilibrium in 23 °C and 50% relative humidity per ISO 62. Moisture-induced thickness swell in solid PC sheet is low, but humidity gradients can produce bow and residual stress relaxation that is more geometrically significant than the absolute thickness change. The dominant thickness error therefore remains the extrusion-related transverse wedge, not post-delivery thermal expansion or moisture uptake.

Machine-guard and glazing applications using 6 mm solid polycarbonate are sensitive to the lower tolerance boundary. If a 6.000 mm sheet measures 5.700 mm at all inspection points, the mass per square metre is 6.84 kg/m² at a density of 1.20 g/cm³, compared with 7.56 kg/m² at 6.300 mm. That is a 9.5% mass difference for the same sheet format, which changes bracket design loads, fastening requirements, and impact energy distribution. For impact-rated glazing, the lower thickness limit should be carried into structural retention and impact calculations because the part may still be conforming but below the nominal design mass per unit area.