PET Sheet Buying Guide: Gauge, Sheet Size and Weight per Square Metre

Polyethylene terephthalate (PET) sheet is specified by three linked commercial parameters: thickness gauge, panel size, and mass per unit area. Mass per unit area is not an independent property but a calculated value derived from thickness and bulk density. Extruded amorphous PET sheet, commonly designated APET, normally exhibits a bulk density of 1.38 g/cm³ when tested by ISO 1183-1 or ASTM D792; therefore, a 1 mm thick sheet has a nominal mass of 1.38 kg/m². Crystallised PET sheet used for dual-ovenable trays shifts density into the range 1.43 g/cm³ to 1.46 g/cm³, so the same 1.0 mm gauge can weigh up to 1.46 kg/m². A procurement specification that omits density is incomplete because the mass difference between amorphous and crystallised sheet can exceed 5 % at equal gauge.

What gauge tolerances are specified for extruded PET sheet?

Commercial PET sheet gauges range from 0.2 mm to 6.0 mm as cut panels and from approximately 0.1 mm upward as roll stock. Thickness measurement is performed by mechanical scanning according to ISO 4593:1993 or ASTM D374-16; rigid sheet above 1 mm is often checked with a flat-anvil micrometer. On a three-roll polishing stack line driven by a 30:1 L/D single-screw extruder, transverse gauge uniformity is set jointly by die lip gap, roll nip geometry, and melt temperature spread. Published converter data show that a common commercial tolerance is ±5 % for sheet above 1.0 mm and ±10 % for film gauges below 0.5 mm, but these are supplier-grade tolerances, not ISO-defined classes. A buyer should specify a target average gauge plus a minimum point thickness because edge thinning on a 2 mm sheet can exceed 0.05 mm without violating the average tolerance. Profile error is read at the die centre and at points 25 mm from each edge; a well-tuned polishing stack with roll gap parallelism better than 0.03 mm across 1000 mm can hold transverse variation within ±3 % on APET sheet above 1.5 mm. At gauges below 0.5 mm, the same line may show ±7 % variation because of die swell and polymer melt viscoelasticity. A ±5 % average tolerance can still hide a localised thin band of 0.09 mm on a 1.0 mm sheet if only the arithmetic mean is reported.

Panel dimensions are selected from downstream clamp frame geometry, trim waste, and transport mass. In European distribution, common cut sizes include 1000 mm × 2000 mm, 1220 mm × 2440 mm, and 1250 mm × 2050 mm; North American stock includes 48 in × 96 in and 60 in × 96 in panels. Dimensional tolerance on guillotine-cut sheet is typically ±1 mm for panels up to 2 m, while CNC-router cutting can hold ±0.25 mm. Laser cutting is generally avoided where transparent edge finish and low melt burr are acceptance criteria because PET develops a heat-affected edge with localised crystallinity. Panel squareness is checked as the difference between diagonal measurements, with a tolerance of ±2 mm on a 2 m panel. The panel weight is obtained by multiplying the mass per square metre by the panel area. For a 1220 mm × 2440 mm × 2 mm sheet at 1.38 g/cm³, the calculated panel mass is 8.22 kg; a pallet of 200 such sheets contains 1644 kg of product excluding pallet and packaging. The same panel count in CPET at 1.45 g/cm³ contains approximately 1727 kg.

For roll-fed pressure formers and in-line printing presses, gauge is specified in roll stock rather than cut panels. Common slit widths include 300 mm, 500 mm, 700 mm, and 1000 mm; roll lengths at 0.5 mm gauge can reach 2000 m on 152 mm cores. Roll mass is not assessed per square metre alone but by total roll weight, which determines unwind brake pressure and machine acceleration limits. A 500 mm wide roll of 0.5 mm APET containing 1000 m weighs approximately 345 kg; exceeding the unwind station’s load cell capacity by specifying a longer roll produces registration errors and web chatter. Thickness variation in roll stock is measured across the web by a traversing beta gauge; a profile error above ±3 % on a 0.5 mm film can generate a tension variation proportional to the thickness deviation because the differential roll diameter changes at the high-tension side.

Weight per square metre as a direct function of sheet density and thickness

The calculation is W = t × ρ × 1000, where W is in kg/m², t is thickness in mm, and ρ is density in g/cm³. At 1.38 g/cm³, a 0.8 mm sheet produces 1.10 kg/m², a 1.5 mm sheet produces 2.07 kg/m², and a 3.0 mm sheet produces 4.14 kg/m². A filled or heat-set sheet with density 1.45 g/cm³ at 3.0 mm weighs 4.35 kg/m², a difference of 0.21 kg/m². For imperial-unit orders, 1 kg/m² converts to 0.204816 lb/ft², and a 1 mm APET sheet weighs 0.283 lb/ft². The same calculation supports pallet weight, vacuum forming clamp force estimation, and sheet-fed printing press feed loads.

Gauge bands visible as transverse ridges are therefore classified as a surface quality defect when optical clarity or print registration is part of the acceptance criteria. Clear APET sheet for printing or display glazing is evaluated by total luminous transmittance and haze according to ASTM D1003-21; a 1 mm sheet typically shows total transmittance above 88 % and haze below 2 %. Thickness deviation across a print sheet changes the effective caliper at the print nip. A variation of 0.05 mm on a 1 mm sheet is sufficient to alter the flatbed press gap setting and produce local ink transfer differences. For that reason, sheet buyers in print applications specify a maximum point-to-point thickness variation rather than only a mean thickness tolerance.

Mechanical performance is not solely controlled by gauge; sheet extrusion tension, molecular weight, and crystallinity shift mechanical data. Tensile strength measured by ASTM D638-14 on 1.0 mm APET sheet is typically reported between 50 MPa and 75 MPa in the machine direction, with transverse direction values lower by 10 % to 20 %. Flexural modulus measured by ISO 178 or ASTM D790-17 is commonly reported from 2000 MPa to 3000 MPa. These values provide a pre-selection baseline and are compared against the supplier certificate of analysis before a gauge is released for production.

For food-contact applications, the purchase specification is not complete unless the polymer identity and migration properties are referenced. FDA 21 CFR 177.1630 applies to polyethylene phthalate polymers in direct food-contact service. European compliance is evaluated under Regulation (EC) No 1935/2004 and plastics-specific migration testing under Regulation (EU) No 10/2011. The overall migration limit under the European regulation is 10 mg/dm². Specific migration limits for antimony are published at 40 µg/kg food simulant. In high-temperature filling, bottling, or retort conditions, residual acetaldehyde is a sensory rejection parameter; extruded PET sheet for food trays is commonly specified at ≤ 10 ppm acetaldehyde by headspace gas chromatography, while bottle-grade PET is often controlled below 5 ppm. That distinction is not a regulatory limit in most jurisdictions but a commercial organoleptic limit with published data limited to specific supplier resin grades.

If the sheet is thermoformed, what is the minimum retained wall thickness?

Thermoformed part geometry is specified by purchased gauge and minimum retained wall after forming. On a 2.0 mm amorphous PET sheet, plug-assisted negative forming with an areal draw ratio above 2.5:1 commonly thins corner walls to 0.6 mm or less when the plug speed, sheet surface temperature, and plug material are not matched. Production rotary thermoformers heat amorphous PET sheet to a surface temperature of 100 °C to 120 °C; below 95 °C, the sheet exhibits elastic recovery and optical haze, while sustained surface temperatures above 135 °C promote crystallisation and sag. Top and bottom quartz heater banks are typically operated at 350 °C to 450 °C surface temperature. A 4 mm sheet may require 60 s to 90 s to reach core forming temperature, whereas a 1 mm sheet can exceed 120 °C at the surface in 10 s to 15 s. Infrared pyrometers with an 8 µm to 14 µm spectral response are used on production lines; clear PET emissivity is typically set between 0.90 and 0.95. An incorrect emissivity setting is a frequent source of surface-temperature error. The minimum retained wall is measured in the finished part with an ultrasonic thickness gauge calibrated to ISO 16809:2017 or by sectioning with a micrometer calibrated to 0.001 mm. A buyer that specifies only the incoming 2.0 mm gauge without a minimum point thickness can accept parts that are outside the tensile and barrier requirements of the final application.

Moisture uptake shifts thermoforming response and final panel weight

Moisture interacts with gauge because desorption is diffusion-controlled. At 23 °C and 50 % RH, amorphous PET sheet absorbs approximately 0.2 % moisture by weight at equilibrium; thin sheet reaches equilibrium within hours, while 6 mm sheet may require days. If the sheet is fed to a thermoformer without drying, the rapid heating above 100 °C causes hydrolysis. Sheet re-drying is performed in desiccant dryers at 60 °C to 80 °C with a dew point of -30 °C and a residence time of 3 h to 5 h for gauges above 3 mm. A supplied intrinsic viscosity near 0.80 dL/g may fall below 0.65 dL/g under severe hydrolysis, leading to corner pinholes and reduced gas barrier. The effect is amplified in sheet with post-consumer recycled content above 30 %; published data for the batch-to-batch intrinsic viscosity range in that specific configuration is limited. A gauge specification that omits these linked variables is underdetermined.