Polyethylene terephthalate (PET) is synthesized by melt-phase esterification of purified terephthalic acid with ethylene glycol or by transesterification of dimethyl terephthalate with ethylene glycol. Molecular weight is controlled through intrinsic viscosity; injection-grade homopolymer PET commonly falls between 0.65 dL/g and 0.80 dL/g when measured in 60/40 phenol/1,1,2,2-tetrachloroethane at 25 °C according to ISO 1628-5. Homopolymer PET displays a glass transition near 78 °C and a crystalline melting point near 265 °C, but its slow crystallization from the melt creates spherulitic gradients, opacity, differential shrinkage, and brittleness in thick sections. For that reason, durable injection-molded and extruded parts are frequently produced from glycol-modified or TMCD-modified copolyesters. The selection of PETG, PCTG and Tritan for durable parts involves balancing drying discipline, melt-temperature headroom, chemical resistance, post-molding dimensional stability, and regulatory documentation.

How Does Comonomer Incorporation Suppress Crystallization and Shift Mechanical Response?

Glycol-modified poly(ethylene terephthalate-co-1,4-cyclohexanedimethylene terephthalate), commonly called PETG, replaces approximately 30–35 mol% of ethylene glycol with 1,4-cyclohexanedimethanol, or CHDM. The cycloaliphatic comonomer disrupts chain symmetry sufficiently to eliminate practical crystallinity; PETG solidifies as an amorphous glass with a glass transition near 80 °C. PCTG uses higher CHDM content or acid modification to further suppress crystallization and improve toughness while retaining clarity. Tritan introduces 2,2,4,4-tetramethyl-1,3-cyclobutanediol, or TMCD, a fused-ring cycloaliphatic diol that raises the rotational barrier along the backbone and lifts the glass transition to approximately 110 °C. Differential scanning calorimetry of these amorphous copolyesters typically shows no significant melt endotherm above 200 °C for PETG and PCTG; selected Tritan grades also show no crystalline melting point across the standard heating range. The practical consequence is isotropic shrinkage, typically 0.4–0.8% when tested under ASTM D955-21, and transparent amorphous parts with wall thickness up to 6 mm can be molded without crystallite-induced haze.

Desiccant drying is the first process boundary for all three copolyesters. PETG is dried at 65 °C for 4–6 h to reach a residual moisture level below 0.02% by weight. PCTG is dried at 70–75 °C for 4–6 h. Tritan requires a higher hopper setpoint of 88 °C for 4–6 h. A closed-loop desiccant dryer with a dew point no higher than -20 °C is mandatory because melt-phase hydrolysis at 230–280 °C scissions ester linkages, reduces melt viscosity, and generates acetaldehyde. On production-scale injection molding lines, general-purpose screws with L/D ratio 20:1–24:1 and compression ratio 2.0:1–2.5:1 are used, with medium-shear check rings and free-flow nozzles. Barrel temperatures for PETG are held at 230–260 °C, PCTG at 240–280 °C, and Tritan at 260–280 °C. Mold temperatures are typically 10–40 °C for PETG, 15–40 °C for PCTG, and 30–65 °C for Tritan. Pack pressure is held between 40 MPa and 60 MPa with hold time sufficient to seal the gate; short hold times produce sink marks, while excessive hold pressure increases gate stress and oriented birefringence. Melt residence time is limited to 5 min for PETG and PCTG and 4 min for Tritan. Amber streaks, silver streaking, or acetaldehyde odor indicate hydrolytic or thermal degradation. In ambient relative humidity above 60%, open-container pellet transfer should be limited to 30 min or the material must be re-dried before processing.

Sheet extrusion of PETG for durable medical or point-of-purchase housings uses a single-screw extruder with L/D 30:1, a barrier screw, and a polished three-roll stack maintained at 50–70 °C. Thermoforming requires sheet surface temperature between 110 °C and 130 °C for PETG and PCTG; Tritan sheet requires higher surface temperatures, typically 140–170 °C depending on grade and draw ratio. Draw ratios above 2:1 can produce localized thinning and stress whitening, particularly in deep-draw housings. Published data for rapid-cycle thermoforming of Tritan under production-scale conditions is limited; material suppliers recommend pilot trials because heating uniformity and mold temperature have a stronger effect than sheet thickness alone.

Thermal Deflection and Notched Impact Across Unfilled Copolyester Grades

The property envelope of unfilled injection-molding grades under standardized test conditions is summarized in Table 1. The values are representative ranges assembled from publicly available manufacturer datasheets; lot-to-lot variation and specimen conditioning according to ASTM D618-21 shift the results.

The impact data demonstrate a key selection boundary. PETG and PCTG are sufficiently tough for many housings and covers, but high-impact reusable parts that must survive repeated drop loading are often moved to Tritan when notched Izod values below 300 J/m are unacceptable. The heat deflection difference also matters: PETG and PCTG lose stiffness under hot-fill or hot-air conditions above 70–80 °C, while Tritan retains load-bearing capacity into the 95–110 °C range at 0.455 MPa. These comparisons should be repeated on the actual production tool because melt history, gate frozen-in stress, and wall thickness influence final crystallinity, orientation, and impact retention.

When Alkaline Dishwasher Detergent Exposure Eliminates PETG from Durable Part Shortlists

PETG is not specified for reusable food-service parts that will encounter high-pH dishwasher detergents. Alkaline solutions responsible for stress cracking typically operate in pH 10–12 at wash temperatures up to 85 °C. PCTG and Tritan are selected because their lower molded-in stress and aliphatic/cycloaliphatic backbone reduce attack by aqueous sodium carbonate and sodium hydroxide. Chemical resistance screening is performed under ASTM D543-21 with defined strain fixtures; an imposed flexural strain of 1% in alkaline media at 65 °C differentiates the grades, but published independent comparisons under identical clamp strain and detergent concentration are limited. End-use validation under NSF/ANSI 51 or internal dishwasher cycling is therefore required before selecting a material for reusable tumblers, trays, or handles. Molders reduce residual stress by using generous gate radii, balanced filling, and low clamp-force packing profiles. Stress-relief annealing is generally not used because amorphous copolyesters can distort near their heat deflection temperature.

Food-contact and medical-grade selection requires documentation beyond mechanical data. The relevant compliance matrix is shown in Table 2. PETG and PCTG are commonly covered by food-contact provisions when CHDM content and extractable fractions meet the applicable migration limits. Tritan has separate food-contact clearances in many jurisdictions. Medical durables requiring repeated cleaning may also need ISO 10993-5 and USP Class VI data from the resin supplier.

Weld-Line Strength, Gate Frosting, and Process-Window Constraints

Durable parts with multiple gates or holes exhibit weld-line strength loss. Weld-line tensile strength is assessed with double-gate plaques under ASTM D638-22; retention is strongly influenced by melt temperature, fill speed, and mold temperature. Published independent comparative data for PETG, PCTG, and Tritan under identical double-gate molding conditions is limited, so internal plaque studies are required before cutting production tooling. Gate frosting or haze occurs when melt shearing exceeds the critical shear rate for the grade. For PETG and PCTG, cold-runner gate diameters between 0.8 mm and 2.0 mm with land length to wall thickness ratios of 0.5:1–1.0:1 reduce the defect. Tritan can require slightly larger gates to avoid jetting. In hot-runner systems, pressure transducers are used to monitor pressure loss; total pressure drop above 35 MPa can induce shear heating and degrade the regrind fraction. Color concentrates containing aromatic amines should be avoided because they can promote transesterification and molecular-weight loss in polyester-type matrices.

Medical durables made from PCTG or Tritan are specified for reusable handles, pump housings, and device enclosures where lipid exposure, alcohol wipes, and high-level disinfection are routine. PETG is not suitable for autoclave steam sterilization at 121–134 °C because its heat deflection temperature is below the autoclave cycle. PCTG also loses dimensional stability above 80–90 °C under load, while selected Tritan grades may withstand certain steam cycles depending on wall thickness and mechanical load. Gamma sterilization at 25–50 kGy is commonly used for limited-reuse devices; PETG and PCTG may yellow after gamma exposure, and Tritan may exhibit less color shift. Validation under ISO 11137-2 is required to establish a maximum acceptable dose. Compatibility with quaternary ammonium disinfectants and isopropyl alcohol at 70% concentration is generally acceptable for PCTG and Tritan; PETG may develop stress marks after repeated alcohol contact under molded-in stress.