Designing a PET Thermoform Tray: Draft, Radius and Wall Distribution begins with sheet-grade selection rather than tool geometry. Polyethylene terephthalate sheet for food and industrial trays is typically converted as amorphous APET or crystallized CPET. Intrinsic viscosity for thermoforming grades is usually held between 0.70 dL/g and 0.80 dL/g when measured in 60/40 phenol/1,1,2,2-tetrachloroethane at 25 °C per ISO 1628-5. Sheet destined for thermoforming must be dried to a moisture content below 50 ppm before heating; higher residual moisture hydrolyzes ester linkages during the forming cycle, reducing molecular weight and causing visible haze or brittle sidewalls. Differential scanning calorimetry per ISO 11357-3 places the glass transition onset near 70 °C and the melting endotherm between 245 °C and 255 °C, but the practical forming surface temperature for amorphous sheet is narrower: 95 °C to 130 °C. Because PET does not melt sharply, the interaction among draft, radius, and wall distribution is governed by strain-rate-dependent elongation and crystallization at the mold surface.

Extrusion and forming equipment set the initial gauge tolerance. On a production sheet line using a 120 mm single-screw extruder with 33:1 L/D, melt pump, and three-roll stack, thickness variation on 0.5 mm sheet is commonly held to ±0.02 mm. Thermoformers for PET trays are generally servo-driven plug-assist pressure formers with clamp force from 150 kN to 600 kN, forming pressure from 0.5 MPa to 0.8 MPa, and vacuum capability below −0.08 MPa. A 2 % gauge band on incoming sheet transfers directly into the sidewall-thickness histogram; wall-distribution failures are often traced to roll-stack temperature gradients before the tool is considered defective.

Why PET Tray Draft Angles Depend on Crystallinity and Surface Texture

Draft angle is the taper on vertical cavity walls that permits ejection after the sheet has cooled below the glass transition. For PET, draft is not only a friction allowance. The linear coefficient of thermal expansion for amorphous PET ranges from 60 × 10⁻⁶ K⁻¹ to 80 × 10⁻⁶ K⁻¹, while semi-crystalline PET may lie between 20 × 10⁻⁶ K⁻¹ and 50 × 10⁻⁶ K⁻¹ depending on filler and orientation, as measured by ISO 11359-2. Cooling from 80 °C mold temperature to 20 °C ambient therefore produces differential contraction on the order of 0.4 % to 0.5 %. A tray with a 60 mm deep sidewall can lose approximately 0.24 mm to 0.30 mm of wall-to-tool clearance after ejection. A draft angle of 1° creates roughly 1.05 mm of cumulative lateral clearance over that same depth, sufficient for polished cavities but marginal for textured surfaces. More generally, radial clearance per side is depth multiplied by the tangent of the draft angle; at 60 mm depth, 0.5° yields 0.52 mm, 2° yields 2.10 mm, and 5° yields 5.25 mm.

Production-scale thermoforming tools for APET trays often specify 0.5° to 1.5° of draft on polished female cavities. Textured sidewalls with Ra 4 µm to 8 µm, measured by ISO 4287, require 2° to 5° because the surface topography mechanically locks the part until the shrinkage-generated gap exceeds the peak-to-valley height. Mold-release load-cell data from production tools show that a sidewall finish increase from Ra 0.2 µm to Ra 6 µm can raise ejection force by approximately 20 % to 30 % at equivalent draft. CPET trays formed in a hot mold at 160 °C to 180 °C usually demand the upper end of the draft range because in-mold crystallization reduces free volume and increases ejection force. Ejection-force spikes recorded below 40 °C below the glass transition indicate that the part has begun to grip the core; frequent release-agent cycling or secondary cooling is then required. This is a process conflict: excessive draft reduces usable tray volume, while insufficient draft increases scrap or delamination in barrier-coated sheet.

Corner Radius Thresholds, Strain Localization, and Die-Cut Notch Sensitivity

In a PET tray, sharp inside corners concentrate biaxial deformation and control the lower bound for wall thickness. Industrial design guidelines generally set the minimum inside radius at 1.0× the initial sheet thickness for APET and 1.5× to 2.0× for CPET trays intended for frozen distribution. For a 0.5 mm APET sheet, an inside radius of 0.5 mm may be acceptable in shallow trays; for a 1.0 mm CPET sheet, the inside radius should not fall below 1.5 mm if the tray is exposed to impact at −20 °C. The outside radius is simply the inside radius plus the local sheet thickness, because the sheet rotates around the corner as it is drawn. When the radius-to-thickness ratio falls below 1.0, localized true strain can exceed 1.0 and the bottom corner may thin below 25 % of the original sheet, creating a hidden weak line that is not always detected by visual inspection.

Corner thinning is also strain-rate dependent. At forming temperatures near 100 °C, PET displays strain hardening, but the corner radius controls local deformation rate. Reducing plug speed over the final 10 mm of travel can improve bottom-corner thickness retention by 5 % to 10 % in shallow tray geometries, although deep-draw CPET configurations may not respond equally because crystallization begins at the mold surface before plug retraction. The die-cutting operation adds a second notch sensitivity. Trimmed PET edges with sharp corners or microcracks may fail through brittle fracture in CPET at frozen temperatures. Multiaxial puncture testing per ASTM D3763 on tray corners shows that a radius below 1.0 mm combined with a trimmed edge can reduce energy absorption significantly, although published data for every tray configuration are limited. For that reason, some converters specify a minimum corner radius of 2.0 mm at the cut line and use matched-metal die punches with 0.01 mm to 0.03 mm clearance to avoid crushing the sheet. The notch-sensitivity concern also influences material selection: under ASTM D3763 puncture conditions, APET absorbs more impact at ambient temperature, while CPET retains dimensional stability but has lower frozen-temperature puncture resistance.

When Sidewall Thickness Falls Below 20 % of Initial Sheet, PET Trays Fail in Cold-Chain Transit

Wall distribution in a PET thermoform tray is the measured thickness map from flange to base, and it is rarely uniform. A 0.5 mm APET sheet may produce a flange thickness of 0.48 mm to 0.50 mm, a sidewall of 0.26 mm to 0.32 mm, a bottom corner of 0.12 mm to 0.18 mm, and a bottom center of 0.20 mm to 0.28 mm after plug-assisted pressure forming. The local draw ratio, defined as initial thickness divided by final thickness, is therefore highest at the bottom corner. Contact micrometers on cut sections per ISO 4593 or ultrasonic thickness scanning calibrated against that method are used to verify the distribution, but the process itself controls the map.

Plug-assist tooling dominates wall distribution because the plug redistributes sheet before final vacuum or pressure. Servo-driven plug speed is normally set between 100 mm/s and 300 mm/s for APET; slower speeds allow more sheet cooling and create thick flanges with thin bases, while higher speeds leave more material in the base but may tear at the plug edge. Plug materials for PET include syntactic foam, POM, high-density polyethylene, and heated metal. Plug temperature is held between 100 °C and 140 °C for APET to prevent chilling the sheet, and the plug surface may be roughened to Ra 1 µm to 3 µm to prevent early slip and improve material carry. If the plug is too hot, PET sticks and the tray will show a circular stress mark; if too cold, the sheet loses formability before reaching the base.

CPET tray production imposes an additional wall-distribution constraint: the mold must be hot enough to crystallize the polymer but not so hot that the sheet sticks and tears. The practical CPET mold temperature window is 160 °C to 180 °C. Below 150 °C, crystallization is slow and the part warps after demolding; above 180 °C, polyester degradation accelerates and acetaldehyde generation increases, which is controlled in food-contact applications under FDA 21 CFR 177.1630 and EU 10/2011. Sheet surface temperature for CPET forming is commonly 130 °C to 150 °C. The residual wall-distribution tolerance is wider than for APET: ±0.08 mm on a 1.0 mm sheet is typical because crystallization causes localized shrinkage and thickness nonuniformity. Deep-draw CPET trays with depth-to-width ratios above 0.4 may require pre-stretch plugs with heated oil channels and variable-speed profiles to keep the bottom corner above 15 % of the original sheet.

Sheet heating is a further determinant of wall distribution. On a four-zone contact halogen IR oven, APET sheet at 0.5 mm gauge is usually heated to a center-line surface temperature of 105 °C to 120 °C, with edge zones set 10 °C to 15 °C lower to prevent excessive sag and flange thickening. Sag compensation in the forming station, typically by pre-stretch vacuum or plug pre-blow, is applied when sheet width exceeds 500 mm. Rolls with moisture above 50 ppm must be rejected; otherwise hydrolytic degradation reduces intrinsic viscosity below 0.60 dL/g and the resulting tray shows erratic wall thickness and reduced drop performance under ASTM D5276 free-fall tests.