Switching a Bottle Line from PET to PETG: What Has to Change

Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester polymerised from ethylene glycol and purified terephthalic acid or dimethyl terephthalate. Bottle-grade PET is commonly processed at an intrinsic viscosity of 0.80–0.84 dL/g per ASTM D4603 and is converted into preforms by injection moulding followed by stretch blow moulding. Polyethylene terephthalate glycol-modified (PETG) is an amorphous copolyester in which a portion of the ethylene glycol is replaced by cyclohexanedimethanol. That modification removes the crystallisation plateau and reduces density to approximately 1.27 g/cm³ per ASTM D792-20. Switching a bottle line from PET to PETG: what has to change includes the dryer setpoint, melt temperature profile, screw shear input, preform cooling, stretch blow moulding recipe, heat-set capability, purging protocol, mechanical acceptance criteria, and food-contact documentation.

Drying is the first critical change. PET bottle resin is dried at 160–170 °C in a desiccant dryer with a dew point of −40 °C to reduce moisture to 50 ppm (0.005%) before extrusion. PETG must not be dried at these temperatures. Manufacturer processing guidelines specify 65–70 °C for 4 hours. A retained 160 °C drying setpoint causes pellet softening, hopper bottom bridging, and feed throat blockages. Because PETG has a glass transition temperature near 81 °C per ASTM D3418-21, exposure to residual hopper temperatures above this level can cause pellet agglomeration. The dryer hopper, conveying lines, and feed throat should be inspected for dead spots where amorphous pellets can stick. Dryer desiccant wheel regeneration at 180 °C or higher is acceptable for PETG, but the process air delivered to the hopper must be reduced to the lower setpoint. Moisture after drying should be verified by Karl Fischer coulometric titration; PETG moisture should remain below 0.01% before melt processing. Hydrolysis in PETG at processing temperature reduces melt strength and produces splay, bubbling, and preform haze. If the line returns to PET, the dryer temperature must be raised back to 160–170 °C and the hopper purged to remove amorphous PETG pellets, because small amounts of PETG in PET preforms can create haze and delamination.

Thermal Stability Boundaries in Injection Moulding Hot-Runner Manifolds

The injection moulder barrel temperatures for PETG are lower than PET homopolymer settings. Preform equipment designed for PET typically operates with a barrel profile from 270 °C to 290 °C. PETG requires a profile from 230 °C to 260 °C. Hot-runner manifold and nozzle tip temperatures should follow the same downward shift, but the manifold should not be allowed to drop below 220 °C if the selected PETG grade has sufficiently high melt viscosity to create short shots. Because PETG is amorphous, it does not require the same rapid quench in the preform mould. Mould coolant at 10–20 °C is sufficient; colder 8–12 °C settings used for PET may cause surface condensation on the cores and reduce cycle capacity. Screw speed should be reduced to limit shear heating, and back pressure should be re-tuned rather than transferred from the PET recipe. A high-shear mixing screw with compression ratio above 3.0:1 may overheat PETG; a low-compression screw of 2.0:1 to 2.5:1 is preferred, although published data for universal screw changeout thresholds across all machine brands is limited. The check ring, shut-off nozzle, and valve-gate pin should be inspected for degraded PET residues before the switch because PET processed at 270–290 °C can mask wear that becomes visible with lower-viscosity PETG. Melt viscosity differences can be quantified by capillary rheometry per ISO 11443.

How Does the Absence of a Crystallisation Plateau Redefine Stretch Blow Moulding?

PET blow moulding relies on strain-induced crystallisation to stabilise the expanding preform and generate sidewall strength. The preform is conditioned to 105–115 °C and stretched at axial ratios around 2.5:1 to 3.0:1 and hoop ratios around 3.5:1 to 4.0:1. PETG remains amorphous during stretching; no crystallisation plateau arrests bubble growth. The reheat oven must therefore be set to 88–96 °C, and the blow delay and preblow pressure must be rebalanced to avoid stress whitening and uneven wall distribution. The reheat oven should also be examined for infrared absorption differences. PET preform reheat systems are tuned to the crystallisation-sensitive heating behaviour of PET; PETG has a different IR absorption response and no crystallisation exotherm, so the oven zone percentages may need to be shifted from the centre to the base of the preform to prevent neck overheating. If the preform is overheated above 100 °C, PETG can exhibit surface tack and uneven stretch; below 85 °C, stress whitening and rupture occur. Heat-set capability is lost: PET heat-set moulds operate at 120–150 °C to induce crystallinity and hot-fill stability, whereas PETG deflects at approximately 70 °C at 0.45 MPa per ASTM D648-18. PETG bottles are therefore limited to ambient-fill or cold-fill products.

Barrier performance also declines because the amorphous PETG matrix lacks the crystallite network that reduces gas permeation in oriented PET. Oxygen and carbon dioxide permeability are higher than oriented PET. A bottle line converting to PETG for non-carbonated beverages may need a thicker sidewall or additional barrier structure. Published data for specific bottle geometries is limited; therefore, oxygen permeation testing per ASTM D3985-17, carbon dioxide transmission per ASTM F2476-13, and moisture vapour transmission per ASTM F1249-20 should be performed on first production samples. Acetaldehyde generation is reduced at PETG processing temperatures, but if the line returns to PET for water bottles, acetaldehyde control, preform cooling, and blowing parameters must be restored because PET requires rapid quenching to keep acetaldehyde below sensory thresholds.

When the Line Returns to PET: Purging and Reclaim Compatibility

A shared bottle line requires purging when moving from PET to PETG and back because the two resins are not fully miscible in the melt. PETG residue in a PET preform injection unit can create haze, delamination, and preform cracking. The purging protocol should begin at PETG melt temperature, followed by a polyester-compatible purge material, then transition to PET at 270–290 °C. The hot-runner system and nozzle tips should be purged until the melt stream is clear and free of speck contamination. Regrind segregation is mandatory. PETG is classified by the Association of Plastic Recyclers as a non-PET resin that contaminates the PET bottle reclaim stream. Mixed flake containing PETG can produce haze and reduced crystallinity in recycled PET. Segregation and dedicated gaylord or hopper labelling are required. If a plant uses closed-loop post-industrial regrind, PETG regrind must be kept separate because it cannot be re-introduced into the PET homopolymer stream at measurable levels without affecting intrinsic viscosity and bottle physical properties.

Tooling changes are less extensive than dryer and screw changes. Existing preform moulds can often run PETG after adjusting valve-gate timing and hot-runner temperature, but gate vestige may be larger because amorphous PETG freezes differently in the gate area. Material data for PETG show tensile yield strength near 50 MPa per ASTM D638-14, flexural modulus near 2,000 MPa per ASTM D790-17, and elongation at break above 100%. Oriented PET sidewall can exceed 150 MPa in the hoop direction after strain hardening. Top-load and burst-pressure acceptance criteria must therefore be revalidated per ASTM D2659-16 and ASTM F1140-17; a PETG bottle of identical geometry will not retain the same stiffness or carbonation pressure. If the line produces dairy or personal-care containers, a lower top-load limit may be acceptable, but the specification must be changed before release.

Food-contact documentation is a separate switch item. PET bottle grades are commonly covered under FDA 21 CFR and EU Regulation (EU) No 10/2011. PETG copolyester grades may have separate food-contact letters of compliance, but migration test data are grade-specific. The overall migration limit of 10 mg/dm² under EU 10/2011 and any specific migration limits for cyclohexanedimethanol-derived moieties must be verified. When the line produces food-contact bottles, first-production samples should be tested under EN 1186 migration protocols, and organoleptic evaluation should be repeated because copolyester stabilisers and lower processing temperatures alter taste and odour carryover relative to PET. REACH SVHC screening of PETG additives is required if the bottle enters the European Union market.