Polyethylene terephthalate (PET) is produced from terephthalic acid and ethylene glycol; the ethylene terephthalate repeat unit crystallizes slowly enough that rapid quenching yields a transparent amorphous state. The selection between amorphous (APET) and crystalline (CPET) PET is not a nominal resin substitution but a processing and application boundary. Amorphous (APET) and Crystalline (CPET) PET: The Biggest Selection Fork in machine design, thermal tooling, barrier packaging, and end-use compliance. Intrinsic viscosity measured by ISO 1628-5 for sheet and tray grades typically falls between 0.75 dL/g and 1.00 dL/g. Differential scanning calorimetry per ASTM D3418 or ISO 11357-3 records a glass transition at 76–80 °C, cold crystallization in unmodified material near 125–135 °C, and melting at 245–255 °C. APET sheet is quenched from melt to below glass transition in a controlled roll stack to hold crystallinity below 5%; CPET contains nucleation chemistry or is subjected to annealing to achieve 25–35% crystallinity and an opaque, higher-modulus morphology. Density measured by ISO 1183-1 shifts from approximately 1.33 g/cm³ for APET to 1.45 g/cm³ for highly crystalline CPET.

What Limits APET Quench Clarity on a Three-Roll Stack?

APET cast film and sheet lines use single- or twin-screw extruders with 30:1 to 40:1 L/D and melt temperatures from 270 °C to 285 °C. Hydrolysis is controlled by desiccant drying to ≤50 ppm residual moisture at a dew point of −40 °C; dryer hoppers are commonly held at 160–170 °C for 4–6 h. A three-roll stack at 15–25 °C must remove heat quickly enough to prevent spherulitic haze. If roll surface temperature rises above 30 °C or line-speed fluctuations create variable contact time, haze measured by ASTM D1003 increases above 2% and sheet can develop non-uniform in-plane shrinkage during later thermoforming. Roll-speed mismatch greater than 1% between adjacent rolls produces surface chatter and caliper variation exceeding ±2%. For 0.5 mm sheet, the quench interval from melt to below 70 °C is typically targeted at 4–8 s depending on gauge and roll configuration; published data for specific line configurations is limited. Intrinsic viscosity loss across the melting section should remain below 0.05 dL/g per ISO 1628-5, which limits melt residence time to approximately 15 min at full-rate operation.

CPET sheet uses the same intrinsic viscosity range but incorporates a nucleating package at 0.3–1.5 wt%. Alkali metal carboxylates and inert mineral co-nucleants shift the non-isothermal crystallization onset recorded by cooling at 10 °C/min under ISO 11357-3 from below 140 °C to 165–180 °C, depending on additive loading and copolymer content. The higher crystallization onset reduces the oven heat input needed to develop crystallinity inside a thermoforming mold because the sheet solidifies against tooling at 140–170 °C rather than remaining a supercooled amorphous web. Processing equipment must tolerate plate-out: nucleant-rich films can deposit on chill rolls and guide bars when melt temperature surpasses 290 °C or when additive dispersion is suboptimal. Sheet crystallinity is verified by density per ISO 1183-1 or by DSC enthalpy per ASTM D3418.

When CPET Oven Surface Temperature Variation Exceeds ±2.5°C

CPET thermoforming operates within a narrow thermal window. Sheet surface temperature must reach 160–180 °C before forming; mold temperature is held between 140 °C and 170 °C. Top and bottom ceramic infrared emitters with peak wavelengths of 3–4 µm are used because PET absorbs strongly in that band. When surface-temperature nonuniformity exceeds ±2.5 °C across the sheet, the crystallization rate changes sharply relative to adjacent regions, producing bands of higher spherulite density. The resulting part shows differential shrinkage and tray flange waviness after ejection. Oven dwell for 0.75 mm CPET sheet is typically set at 8–15 s; longer dwell above 190 °C generates acetaldehyde above 10 ppm, measured by ASTM F2013, and can initiate thermo-oxidative yellowing. Capillary rheometry per ISO 11443 at 280 °C and 100 s⁻¹ for 0.80 dL/g PET reports apparent viscosity in the 200–300 Pa·s range. Crystalline domains do not contribute below the melt point, but nucleant particles can increase shear heat generation in the extruder, altering the effective melt viscosity and face-out on mold surfaces. Parts must be cooled below 70 °C before removal to prevent post-mold crystallization warpage.

APET Barrier Layer Selection Under Modified Atmosphere Packaging

APET trays for modified atmosphere packaging are selected for clarity and thermoformability, but monolayer oxygen barrier is moderate. Oxygen transmission rate is measured by ASTM D3985 at 23 °C and 50% RH; published values for 0.5 mm APET sheet scatter between 10 cm³/(m²·day·bar) and 20 cm³/(m²·day·bar) depending on comonomer, orientation, and moisture history. CPET of equivalent thickness at 25–35% crystallinity typically reduces oxygen transmission by 10–20% relative to APET because crystallites reduce accessible free volume; however, the opaque mineralized appearance limits CPET to dual-ovenable trays where transparency is not required. High-barrier APET structures therefore use coextruded or laminated ethylene vinyl alcohol layers or deposited barrier coatings.

Mechanical response diverges with crystallinity. APET sheet at 0.5 mm tested under ISO 527-3 commonly exhibits tensile elongation at break of 150–250% and a ductile puncture response under ISO 6603-2 at room temperature. At refrigeration or freezer temperatures, published data for standard homopolymer grades show a ductile-to-brittle transition between 5 °C and 15 °C; total penetration energy at −18 °C may fall by 30–50% relative to 0 °C, creating sidewall fracture in dairy containers. CPET at 25–35% crystallinity raises flexural modulus but reduces elongation at break to 5–20%, so sharp corners in dual-ovenable trays require radii that avoid frozen-in stress concentration. Moisture absorption is low, below 0.3% at 23 °C and 50% RH, but it acts as a plasticizer at the glass transition and shifts impact response.

Does Recycled CPET Flake Retain Crystallinity After Repeated Heat Histories?

Post-consumer APET flake is amorphous in the source article, but CPET trays retain crystallinity through scrap regrind. Reprocessing of CPET requires higher melt temperatures because crystallites must be melted before shear heating; if the extruder barrel is set below 270 °C, unmelted crystalline domains survive and generate optical defects or weak weld lines. Moisture remains the dominant degradation variable. At residual moisture above 50 ppm, hydrolytic chain scission reduces intrinsic viscosity by 0.02–0.05 dL/g per extrusion pass under ISO 1628-5. Solid-state polymerization rebuilds IV to 0.82–0.85 dL/g after washing and decontamination. Compliance is governed by FDA 21 CFR 177.1630 for PET food-contact resin, Regulation (EU) No 10/2011 for overall migration with a limit of 10 mg/dm², and Regulation (EU) 2022/1616 for recycled plastic decontamination technologies.

Crystallinity Gradients That Alter Thermoformed Tray Radius

Dimensional stability in both APET and CPET trays is affected by the spatial distribution of crystallinity. A sheet heated to 160–180 °C and stretched against a 140–170 °C mold develops orientation-induced crystallization, but corners and bottom radii often cool faster than flanges. The resulting crystallinity gradient creates differential shrinkage when the empty tray is reheated; tests per ASTM D1204 at 150 °C for CPET or 70 °C for APET can reveal localized shrinkage variation above 0.5%. In tray-forming trials, a flange-to-base crystallinity difference of 10% absolute is sufficient to produce visible warpage in a 250 mm long rectangular tray, though published data for specific tooling geometries is limited. Tool designers compensate with extended cooling of the flange, controlled mold-release timing, and post-mold fixtures that hold parts below 70 °C.