PET vs PETG on the Machine: Drying, Shrinkage and Recyclability

On high-volume injection molding and sheet extrusion lines, the substitution of polyethylene terephthalate with a glycol-modified copolyester is not a simple materials change. PET and PETG diverge in crystallization behavior, drying tolerance, mold shrinkage, and the reclaim stream to which the plant can return the material. The differences appear first at the dryer, then at the mold, and finally in the plant’s post-consumer regrind strategy.

What Changes When Cyclohexanedimethanol Replaces Ethylene Glycol in the Backbone?

PET is a semi-crystalline polyester based on terephthalic acid and ethylene glycol. PETG is produced by partially replacing ethylene glycol with cyclohexanedimethanol (CHDM). The CHDM comonomer disrupts chain regularity and suppresses the rapid crystallization that allows PET to be sold as crystallized pellet feed for high-temperature drying. Differential scanning calorimetry under ISO 11357-2:2020 places the glass transition temperature of PETG near 80 °C, while amorphous PET grades show a similar Tg near 76–80 °C. The operational difference is that PETG remains amorphous after drying, whereas PET crystallizes and can therefore be dried at 120–160 °C without pellet fusion. On a production floor, this means PETG cannot be exposed to PET dryer settings because the pellets soften and agglomerate when the bed temperature exceeds the glass transition by more than a few degrees.

Desiccant-wheel dryers with a closed-loop air dew point below -40 °C are specified for both resins, but the thermal profiles diverge. Crystallized PET grades are commonly dried at 120–160 °C for 4–6 h to a target moisture content below 0.005 % (50 ppm) when measured by ISO 15512:2019 or an equivalent Karl Fischer method. PETG datasheets specify a drying temperature of 65–71 °C for 4–6 h to a moisture target of 0.04 % (400 ppm). The lower temperature is not a minor suggestion; pellet clumping in the hopper, bridging at the throat, and screw feed starvation occur when PETG is dried at PET temperatures because the pellets exceed Tg and fuse. Conversely, PET that is dried only at PETG conditions may retain hydrolyzable moisture; when processed at 270–285 °C, residual water attacks the ester linkages and lowers intrinsic viscosity, causing loss of melt strength and increased injection-molding reject rates.

On production lines where both resins are dried from a central desiccant system, the conflict is not theoretical. A central desiccant bed supplying 160 °C air cannot be branched to a PETG hopper without a post-cooler; PETG hopper inlet air must be reduced to 65–71 °C before contacting the pellets. If the post-cooler fails or the control thermocouple is located too far from the hopper cone, PETG pellets in the lower cone can reach the glass transition and form a fused plug at the dump throat. In PET service, the opposite failure is more common: an undersized dryer or a leak in the return-air loop allows the dew point to rise above -30 °C, and the pellets leave the hopper with surface moisture that is not detected by a simple moisture balance but is sufficient to hydrolyze the melt at 285 °C. The resulting splay, silver streaking, and loss of intrinsic viscosity are frequently misattributed to mold temperature or injection speed.

The practical drying window for PETG is therefore narrow: 65–71 °C with a residence time of 4–6 h in a hopper designed for conical mass flow. PET crystallized feed has a wider temperature tolerance but a stricter moisture limit. Drying below 50 ppm is necessary because at 270–285 °C, each 0.01 % moisture can reduce melt viscosity through hydrolysis. Published data for bottle-grade PET often report a drop in intrinsic viscosity of more than 0.05 dL/g when improperly dried material is processed above 280 °C. That threshold is critical because blow molded and sheet-extruded PET articles rely on intrinsic viscosity above 0.70–0.82 dL/g for impact resistance and environmental stress crack resistance.

Shrinkage Measurements Separate Into Mold, Annealing, and Service-Temperature Components

Linear mold shrinkage values alone do not capture the PET/PETG processing difference. Unfilled PETG injection-molding grades typically show flow-direction shrinkage in the range of 0.002–0.006 mm/mm per ISO 294-4:2018 or ASTM D955-21. Amorphous PET can fall within a similar datasheet band, but the difference emerges after ejection. PET continues to crystallize when the part is exposed to heat above its Tg, causing additional shrinkage and warpage. PETG remains amorphous and therefore shows lower post-mold crystallization shrinkage; however, its heat deflection temperature is lower, and it can creep more under sustained load at the same service temperature. Moldmakers compensate by adjusting cavity dimensions, but a tool cut for PETG may not produce acceptable dimensions in PET when annealing or hot-fill conditions are part of the specification.

Shrinkage on the machine is influenced by mold temperature and part thickness. PETG with a Tg near 80 °C can be ejected from a mold at 10–30 °C with minimal post-mold crystallization because there is no spherulitic growth to drive late shrinkage. PET homopolymer, by contrast, solidifies as a supercooled amorphous glass if the mold is cold but will crystallize when the part is exposed to 100–120 °C during annealing or hot-fill. That secondary crystallization can add measurable linear shrinkage from the molded dimension depending on thickness and orientation. This is the key reason PET and PETG are not interchangeable in hot-fill packaging or under-hood components. The mold can be cut to compensate for one condition, but not for both unless the part is annealed in the mold, which requires mold temperatures above 130 °C and extended cycle times.

Post-mold crystallization cannot be read from the mold shrinkage band alone. Published datasheet shrinkage values represent a specific plaque mold and conditioning protocol, not the final dimension after annealing. Comparative studies on injection-molded amorphous PET plaques show additional radial shrinkage after annealing at 100–120 °C, while PETG plaques under the same conditions generally show smaller additional linear change because the CHDM comonomer prevents crystallization. The exact value depends on gate design, packing pressure, and wall thickness; published data for a specific tool configuration is limited.

If Mixed Flake Enters the Reclaim System, PETG Is Not a Drop-In Diluent

Resin identification code 1 under ASTM D7611-20 applies to PET; PETG is classified as 7 other. The separation problem is not solely labeling. Near-infrared sorting arrays used in material recovery facilities are trained on the C–H and O–H overtone spectra of PET bottle flake. The CHDM modification in PETG shifts the spectral response enough that clear PETG sheet and packaging can be misrouted into the PET bottle stream when the sorter is configured only for broad PET classes. Once PETG enters the PET wash line, it does not hydrolyze or crystallize like PET under solid-state polycondensation conditions. It can soften in the dryer at temperatures used for rPET crystallization, form agglomerates on the dryer walls, reduce intrinsic viscosity of the blend, and increase haze in the final pellet.

Published recyclability protocols such as the Association of Plastic Recyclers critical guidance for PET bottle-to-bottle reclaim classify PETG as a contaminant that must be controlled at low levels. Commission Regulation (EU) 2022/1616 also distinguishes PET from copolyesters in food-contact recycling authorizations. Under ASTM D7611-20, PETG remains a 7 other resin, and products molded from PETG should not be placed in the PET bottle collection stream unless the local material recovery facility has explicitly validated a separate copolyester stream.

On the machine, regrind strategy follows the same boundary. PETG runners and sprues should be kept in a separate closed loop; mixing PETG regrind into a PET melt at more than trace levels changes melt rheology and can reduce the crystallization rate of the resulting part. Conversely, PET regrind introduced into a PETG melt raises the processing temperature requirement and may create crystalline domains that show up as surface haze or internal stress concentrations. Color and transparency are also affected: PETG is naturally clear, but PET homopolymer parts may be opaque or white when cooled in a cold mold unless the mold is run above the crystallization temperature. This optical difference is an early indicator that the resins have been cross-contaminated before off-spec parts reach quality inspection.