Injection Moulding PET Preforms: Melt, Mould Temperature and Cycle Time

Processing bottle-grade polyethylene terephthalate into injection-moulded preforms is governed by the interaction between melt temperature, mould temperature and total cycle time. The resin is normally a bottle-grade copolymer with an intrinsic viscosity of 0.78–0.84 dL/g measured according to ASTM D4603-18; carbonated-soft-drink grades are specified at 0.82–0.86 dL/g, while still-water grades are specified at 0.76–0.80 dL/g. Drying is performed in desiccant-bed dryers supplying air at a dew point of −40°C to −50°C and a hopper outlet temperature of 160–180°C for 4–6 h, reducing residual moisture to 30–50 ppm or less as measured by ISO 15512:2019. Moisture above 50 ppm at melt temperatures above 270°C promotes hydrolytic chain scission, producing an intrinsic viscosity loss of 0.02–0.04 dL/g in a single heat history and increasing acetaldehyde concentration. Because the preform is an amorphous intermediate intended for stretch-blow moulding, its density should remain in the range 1.335–1.340 g/cm³ according to ISO 1183-1:2019 or ASTM D792-20. Any development of crystallinity during injection moulding reduces the reheat stretch-blow process window and alters the final container sidewall distribution.

Why Does Melt Temperature Control Acetaldehyde Formation and Molecular Weight Retention?

Actual melt temperature, rather than barrel setpoint, determines thermal degradation during preform injection moulding. Barrel zone settings commonly range from 260°C in the feed zone to 285–290°C at the metering zone, but shear heating from a 20:1–25:1 L/D barrier screw rotating at 60–120 min⁻¹ with a back pressure of 10–30 bar can raise the measured melt temperature by 5–15°C. Melt temperature should be verified with a needle-probe pyrometer in an air-shot. A measured melt temperature below 270°C can leave unmelted granules and cause high injection pressure, while a measured melt temperature above 295°C accelerates acetaldehyde formation. For a still-water preform, acetaldehyde concentration can rise from below 3–4 µg/L to above 8 µg/L when the melt is held at 285°C for more than 5 min in a 72-cavity hot-runner system. The practical melt-temperature window for bottle-grade PET is therefore controlled within ±5°C of a centre point between 280°C and 285°C.

Residence time distribution is controlled by the material cushion and barrel capacity. The cushion is maintained between 4 mm and 8 mm, and the shot weight should represent 35–75% of barrel capacity. On a 96-cavity production line, the hot-runner manifold volume and valve-gate tip temperatures are specified for tip-to-tip uniformity within ±1°C, because a nozzle operating at 265°C can freeze the gate and raise injection pressure, while a nozzle at 295°C produces a local acetaldehyde excursion. Injection fill time for thin-wall preforms is normally 0.3–0.8 s, followed by a pack pressure of 35–50 MPa and a hold time of 3–6 s. During the short injection phase, switchover from velocity control to pressure control occurs at 98–99% of final fill volume, using screw position or cavity pressure; cavity pressure at switchover in a 2.0 mm sidewall preform is approximately 30–40 MPa. If the switchover is too late, flash can occur; if too early, short shots or sink marks appear.

Plasticating behaviour in PET preform moulding is linked to screw geometry and recovery time. The reciprocating screw in a dedicated PET line is a 20:1–25:1 L/D barrier or general-purpose design with a compression ratio of 2.5:1–3.0:1 and a back-flow check ring. With a shot weight of 12–20 g, recovery time at 80–120 min⁻¹ is typically 2–4 s, well below the cooling-dominated portion of the cycle. If the total cycle time falls below 15 s, plasticating recovery and hot-runner residence time may become the limiting constraints rather than mould cooling. Melt pressure at the nozzle during filling typically reaches 70–100 MPa, while hydraulic pressure on the injection cylinder depends on the machine intensification ratio, commonly 10:1. Preforms produced under these conditions exhibit body weight variation of ±0.5% or less across 72–96 cavities when the hot runner is thermally balanced.

Mould Temperature Governs Solidification Rate and Preform Haze

Preform moulds operate with turbulent chilled-water circuits maintained at 6–12°C, with some high-speed lines using 4°C and thick-wall lines operating at up to 15°C. A pressure drop of 2–4 bar across the mould circuit is typical, and the flow condition should exceed a Reynolds number of 10 000 to maintain heat-transfer consistency. The low mould temperature freezes the preform in an amorphous state; density measured by ISO 1183-1:2019 or ASTM D792-20 remains in the range 1.335–1.340 g/cm³, corresponding to a crystallinity below 3%. If mould surface temperature exceeds 20–25°C, spherulitic crystallinity can develop in the sidewall and gate region, increasing haze and disturbing stretch-blow processability because the preform can no longer be reheated uniformly above the glass transition temperature of 75–80°C as measured by ISO 11357-2:2020.

Mould temperature below 6°C does not reduce cycle time proportionally. The limiting resistance is thermal diffusivity of the PET, not only the coolant temperature. A one-dimensional conduction model for an amorphous PET plaque with thermal diffusivity 0.08 mm²/s shows that reducing the mould temperature from 12°C to 6°C decreases theoretical cooling time by less than 10%, while condensation risk on the cavity surface and chiller energy load increase. The practical optimum for injection-moulded PET preforms is a mould supply temperature of 8–12°C with a deviation of ±1°C across all cavities. Cavity vents of 0.01–0.03 mm depth and 2–4 mm width at the end of fill prevent air traps and short shots; insufficient venting can cause burns and higher acetaldehyde due to adiabatic compression.

Cooling circuit balance across a multi-cavity mould is verified with thermal imaging or in-cavity thermocouples. Ejector-side and core-side circuits are often run at different temperatures, with the core side 2–4°C warmer than the cavity side to prevent the preform from sticking on the core during ejection. Air eject pressure is normally 4–6 bar, and the preform surface temperature at ejection is kept between 60°C and 70°C to avoid deformation while minimising residual cooling outside the mould. Mould cooling channel design should be balanced so that the cavity-to-cavity surface temperature difference is no greater than 2°C; otherwise preform weight and shrinkage variation increases.

When Wall Thickness Doubles, Cooling Time Increases by Approximately a Factor of Four

Total cycle time in PET preform injection moulding is dominated by cooling time, which scales with the square of wall thickness. Using a transient conduction estimate t_cool ≈ (s²/(π² α)) ln((4/π)(T_melt − T_mould)/(T_eject − T_mould)), with α = 0.08 mm²/s, T_melt = 280°C, T_mould = 10°C and T_eject = 70°C, a 2.0 mm sidewall requires approximately 10–12 s of cooling time and a total cycle of 18–22 s. For a 4.0 mm sidewall, the calculated cooling time rises to 40–50 s, and total cycle time rises to 55–70 s after injection, packing, plastication and ejection. The exact value depends on gate geometry, core steel conductivity and coolant Reynolds number, but the square-of-thickness relationship remains the central process constraint. Reducing sidewall thickness from 3.5 mm to 3.0 mm can therefore lower cycle time by 20–30% when cooling dominates, while maintaining the required top-load performance of the final container as measured by column crush resistance testing according to ISO 8113 or equivalent.

The ejection temperature must remain below the glass transition temperature of bottle-grade PET, which is 75–80°C by ISO 11357-2:2020. If ejection occurs above 80°C, the preform can deform during handling, and post-mould crystallisation can occur. If ejection occurs below 50°C, the cycle time is usually longer than necessary unless the preform is very thick. In practice, preform surface temperature at ejection is maintained between 60°C and 70°C as measured by a contact pyrometer. The cooling-time calculation assumes one-dimensional heat conduction through the sidewall; for thick preforms with neck and thread inserts, the gate region and neck finish frequently require separate cooling circuits because thread inserts retain heat and can extend the cycle by 2–5 s when not thermally isolated.

Gate design interacts with hold pressure and cooling time. The gate must remain molten long enough for pack pressure to transmit to the body; if gate freeze time is shorter than the holding phase, sink marks and voids can form in the sidewall. For a 28 mm neck preform, the gate diameter is typically 1.0–1.5 mm, and the valve pin is held open for 2–4 s during packing and then closed to isolate the cavity from the hot runner. Gate crystallinity is measured as a local density increase above 1.340 g/cm³ and is controlled by a separate gate-insert cooling circuit running 2–4°C colder than the body cavity. If the gate insert is too cold, the gate freezes before packing completes and causes short shots; if too hot, the preform can stick to the gate and string on ejection.

Operational Limits for Drying, Melt Delivery and Cooling Water Circuits

Compliance and process control for PET preform production rely on a small set of standardised measurements. A production line should monitor residual moisture, intrinsic viscosity, density and food-contact migration. The table below summarises the minimum measurement matrix for injection moulding bottle-grade PET preforms.

Operational boundaries are non-negotiable in high-volume preform production. Dried resin should not be left in open hoppers for more than 30–60 min when ambient relative humidity exceeds 60%, because virgin flakes and regrind can regain moisture rapidly. Amine-based additives and certain colour concentrates should be avoided unless explicitly tested for acetaldehyde generation and intrinsic viscosity loss, because basic species can catalyse ester cleavage. The hot-runner system must be purged before shutdown at 270°C and kept under an inert gas sweep if residence time exceeds 15 min. Cooling water circuits should be treated with a corrosion inhibitor and filtered to 100 µm or finer to avoid mineral fouling that increases cavity temperature variability.