Polyethylene terephthalate (PET) sheet and rigid tray stock, specified by thickness, width and gauge naming, comprise the roll-fed input to thermoforming lines producing trays, clamshells, cups and insertable packaging. The PET backbone is semicrystalline, but amorphous PET (APET) sheet is quenched to retain clarity, while crystallizable PET (CPET) sheet is formulated to crystallize during thermoforming and to tolerate reheating. Thickness in converter records is normally stated in mils, microns or millimetres. Width is reported either as the as-cast web width before edge trim or as the final slit roll width. Under the film-gauge convention, 100 gauge equals 1 mil (0.001 in) and is equivalent to 25.4 µm; therefore a 25 mil rigid tray stock at 0.635 mm is equivalent to 2,500 gauge. This equivalence is arithmetic, but its commercial interpretation varies by market and product type.

Rigid tray stock for food packaging is commonly extruded at nominal thicknesses from 0.20 mm to 1.20 mm. Retail produce trays and bakery trays are typically between 0.25 mm and 0.75 mm because that range balances stiffness, thermoforming cycle time and material cost. Extrusion lines for roll-fed PET sheet are generally built for web widths of 600 mm to 1,800 mm, while slit rolls commonly supplied to thermoformers are 350 mm, 450 mm or 560 mm. Width and gauge are not independent: sheet weight per linear metre, which determines roll length and shipping weight, is computed from thickness, slit width and PET density. At the nominal density of APET, 1.33 g/cm³, a 0.35 mm sheet at 560 mm width weighs approximately 0.261 kg per linear metre; a 300 kg roll therefore contains approximately 1,150 m of stock, excluding core weight.

When Does a Film Gauge Unit Apply to Rigid Tray Stock?

Ambiguity arises when a purchase specification uses “gauge” without stating the conversion basis. Many PET packaging films use 100 gauge = 1 mil = 25.4 µm. Rigid tray stock is more often specified directly in mils or microns because the film-gauge expression becomes large above 10 mil. A 20 mil (508 µm) APET tray sheet would be expressed as 2,000 gauge; a 30 mil (762 µm) sheet would be 3,000 gauge.

A purchase order for “30-gauge PET tray stock” can be interpreted as 0.3 mil (7.6 µm) if “gauge” is read in the film-unit convention, or as 30 mil (762 µm) if “gauge” is incorrectly used as a synonym for mil; the first is not rigid tray stock and the second is outside the normal food-tray thickness range. Thickness measurement by ISO 4593:1993 or ASTM D6988 does not resolve that nomenclature problem. Converter documents should state the nominal thickness in millimetres or microns and should treat any gauge-unit value as supplementary.

Within the extrusion and polishing stack, thickness uniformity is established at the die lip and modified by chill roll temperature, gap and line speed. On a single-screw extruder with L/D 30:1 to 36:1, melt temperatures for APET are typically controlled between 270 °C and 285 °C, with die zones held at 270 °C to 280 °C. The polished three-roll stack is operated at 15 °C to 40 °C to quench APET rapidly enough to suppress crystallization, while CPET sheet may use a heated stack or a nucleation package to control crystallinity depending on the downstream forming process. Cross-web thickness variation is managed with automatic die bolts or a flexible lip, with target profiles generally within ±2% to ±5% of nominal; published data for specific die geometries is limited. Scanning beta gauges or infrared sensors measure the web at intervals of 25 mm to 100 mm, and the calculated average profile is used for closed-loop die adjustment. Edge bead is selectively trimmed because the local thickness at the web edges can exceed the average by 10% to 20% depending on melt strength, die lip settings and polymer IV. Neck-in between the die exit and the first chill roll can reduce web width by 20 mm to 60 mm per edge; this loss must be included when specifying a die width for a given trimmed roll width.

The gauge measured at the beta gauge is not identical to the final slit roll gauge at the point of thermoforming. PET sheet continues to shrink or relax after winding, and compressive stress through a tightly wound roll can reduce apparent thickness at the core. Thickness verification is therefore performed after conditioning at 23 °C and 50% RH, and the sample is taken at least 25 mm from the roll edge to avoid edge-bead carryover. For rigid tray stock above 0.75 mm, thickness variation across the web is a more critical forming variable than average thickness because it changes the oven residence time required to reach forming temperature.

Edge Bead, Neck-In, and Slit Width Tolerances in Thermoforming Supply

After edge trim removal and slitting, width control for PET tray stock is managed as a roll-dimension and tension problem rather than as a simple slitter setting. Thermoformer roll widths are commonly specified with tolerances of ±1 mm to ±2 mm for sheet above 0.5 mm, but the tolerance must be confirmed against the knife set and winding tension. A production slit roll 560 mm wide may be wound on 76 mm or 152 mm cores, with roll diameter limited by tension transmission through the PET sheet. Excessively high winding tension can block the roll surface or crush the core, while low tension produces telescoped rolls that jam thermoformer unwinds. Edge trim is removed as a continuous strip, usually 10 mm to 25 mm per edge, and is reground only after drying to prevent hydrolytic degradation in subsequent extrusion. In-line slitting of APET sheet above 0.75 mm requires heavy shear knife loads and more frequent blade replacement because PET is abrasive relative to polyolefins. Because cross-web gauge variation remains after edge bead removal, slit rolls taken from an edge lane may contain the upper end of the thickness profile. Thermoforming ovens compensate poorly for lane-to-lane thickness differences: thin lanes overheat and sag, while thick lanes remain underheated and produce incomplete corners. Slit rolls are therefore recorded by lane number, and a thickness profile printout is shipped with each roll when the downstream forming process is controlled by statistical process limits.

On a converting line without a certified length counter, roll length is derived from net weight, slit width, measured thickness and PET density. The same thickness value used in this calculation must be converted correctly. A purchase specification that reads “25-gauge PET” under the film convention denotes 6.35 µm, not 635 µm. A roll of 560 mm wide APET sheet at 0.35 mm thickness and 300 kg net weight contains approximately 1,150 m; the same roll misread as 0.0035 mm would produce an erroneous length above 100,000 m and a non-existent commercial article. Roll-length estimation is therefore a practical check on gauge naming. Width also enters the yield calculation because edge trim and neck-in reduce the saleable slit width from the die width. If a die exits a web at 1,600 mm and each edge trims 20 mm, the available saleable width is 1,560 mm, or three 520 mm slit rolls with no centre trim. The same line can produce four 380 mm rolls, but slitter spacing and gauge profile quality determine whether the outer lanes meet the forming specification.

Before melt processing, drying of PET tray stock resin is a kinetic constraint rather than a simple moisture-removal step. Undried PET hydrolyzes in the extruder when moisture exceeds 50 ppm, causing intrinsic viscosity loss and acetaldehyde formation. Standard practice uses desiccant dryers with a dew point of -40 °C or lower and a residence time of 4 h to 6 h at 140 °C to 160 °C. Intrinsic viscosity for bottle-grade PET is measured by ASTM D4603 and typically falls between 0.70 dL/g and 0.84 dL/g; tray stock may use the same range or a lower IV grade when thick sheet flow and fast cycles are prioritized. A drop of 0.02 dL/g to 0.05 dL/g after extrusion indicates hydrolytic or thermal degradation. Residual acetaldehyde in APET tray stock is often controlled below 10 ppm for sensitive beverage contact, although published values for tray-specific grades vary. Melt temperatures above 290 °C and residence times above 5 min accelerate thermal degradation, producing yellowness and surface roughness. Screw designs with low-shear mixing elements are used to prevent local hot spots, and barrel temperatures are maintained below 285 °C.

Regrind addition changes the drying and IV profile. Post-industrial APET edge trim and start-up sheet are often added at 10% to 30% by weight after grinding, but the dried regrind may have a lower IV than virgin resin and a higher acetaldehyde content. When post-consumer recycled PET is used, the input IV distribution is broader, and the extruder throughput must be reduced to maintain a stable die pressure. Batch-to-batch variance in regrind moisture can shift the final sheet gauge because hydrolytic degradation changes melt viscosity and die flow. Production-scale extruders with vacuum venting can remove some volatiles, but vent port flooding occurs when the regrind fines carry too much moisture.

When Oven Surface Temperatures Reach 120 °C, APET and CPET Diverge

When oven surface temperatures reach 120 °C, APET and CPET tray stock diverge because amorphous sheet softens without crystallizing, while crystallizing grades densify and build heat resistance. APET sheet is heated to 100 °C to 130 °C for vacuum or pressure forming; below 0.30 mm, the sag window narrows and plug assistance becomes necessary to prevent webbing. CPET formulations using nucleating additives or a high crystallization rate are run at oven temperatures of 130 °C to 180 °C, and the formed tray can tolerate reheating to 200 °C or higher. Crystallinity in CPET tray walls can reach 30% to 40%, while APET tray walls remain below 10% unless heat-set. Forming pressures in pressure forming are normally 3 bar to 6 bar, with vacuum alone limited to about 1 bar differential. Plug-assisted forming reduces wall thickness variation, but plug temperature and plug depth must be adjusted to avoid stress whitening at tray corners. A tray formed from 0.50 mm sheet can thin to 0.15 mm in the corner if the pre-stretch ratio is too aggressive; published data for this specific configuration is limited, but sectioning formed trays with an optical comparator confirms the thinning pattern.

For regulatory submissions, the following test designations appear on PET tray stock certificates of analysis. The matrix below separates thickness and conversion tests from food-contact requirements.

Thickness measurements for PET tray stock are typically taken with a flat anvil micrometer or an automated scanning gauge at 23 °C and 50% RH after conditioning. Sampling must include cross-web and machine-direction positions because thickness varies in both axes. Food contact compliance for PET sheet is governed by EU Regulation 10/2011 and FDA 21 CFR 177.1630; for recycled PET, EU Regulation 2022/1616 and individual FDA food-contact notifications control authorization. A compliance statement must identify the specific resin formulation, regrind fraction and extrusion conditions because additives and degradation products migrate differently from the formed tray than from the neat resin. Post-consumer recycled content is not automatically inert in direct food contact; it must be approved through the applicable recycled-plastics process route, and the tray manufacturer must verify that the final article meets overall migration and specific migration limits.