PET Preform Temperature Measurement: What to Measure and Record

Temperature is the single most critical process variable in PET reheat stretch blow moulding. A deviation of just $\pm 2^\circ\text{C}$ can shift a bottle from optimal biaxial orientation into micro-cracked pearlization (if too cold) or hazy thermal crystallization and poor top-load strength (if too hot). However, recording “preform temperature” is fraught with metrological pitfalls. Handheld infrared thermometers, inline pyrometers, and thermal imaging cameras frequently report conflicting values for the exact same preform. To collect trustworthy data, process teams must understand optical filtering, measurement timing, and the physical distinction between surface temperature and core temperature.

1. Instrument Selection: Why Broad-Band Infrared Guns Fail on PET

A standard commercial infrared gun (typically sensitive in the $8\text{–}14\ \mu\text{m}$ broad-band spectrum) cannot measure clear PET accurately. Clear PET resin is semi-transparent across broad infrared wavelengths. An unfiltered sensor “sees through” the preform wall, capturing reflected background radiation from hot oven reflectors, glowing quartz filaments, or machine frames.


[Broad-Band IR Gun (8-14 µm)] ───────> Transmits through clear PET ──> Measures hot background frame! (INCORRECT)
[Narrowband Pyrometer (3.43 / 5.0 µm)] ──> Absorbed at PET surface ────> Measures true polymer surface! (ACCURATE)

To measure preforms reliably:

Narrow-Band Spectral Filtering: The pyrometer or sensor must be optically filtered to a specific absorption peak where PET is completely opaque. Industry standard wavelengths are:
- **$3.43\ \mu\text{m}$:**Corresponding to the fundamental Carbon-Hydrogen ($\text{C-H}$) molecular stretching band.
- **$5.0\ \mu\text{m}$:**Corresponding to the Ester carbonyl absorption band.
Emissivity Setting ($\epsilon$): At these narrow wavelengths, clear amorphous PET behaves as an opaque blackbody radiator with an emissivity coefficient of**$\epsilon = 0.95\text{–}0.96$**. Never use the generic $\epsilon = 0.85$ default found on multi-purpose sensors.

2. Measurement Timing and Location

Because heat constantly diffuses through the preform wall and convective air cools the outer skin, temperature changes dynamically after the preform exits the oven.

Measurement Station	Measurement Target	Primary Engineering Value
Oven Tunnel Exit	Peak outer surface temperature	Assesses maximum thermal dose; prevents surface haze/crystallization
Soak Zone (Mid-Transfer)	Equalizing skin temperature	Tracks conduction from outer skin toward the core
Blow Mould Infeed	Equalized blow temperature ($95^\circ\text{C}\text{–}115^\circ\text{C}$)	True starting condition for mechanical stretch blow moulding

Consistency Rule:* Never compare an oven-exit reading with a mould-infeed reading. By the time a preform travels the transfer wheel into the blow mould (typically 1.5 to 3 seconds), outer surface temperature drops by $3^\circ\text{C}\text{–}6^\circ\text{C}$ due to air convection, while the inner core heats up due to conduction.

3. The Skin-to-Core Temperature Gradient

A pyrometer measures only the outer optical skin (approximately the first $50\text{–}100\ \mu\text{m}$ of depth). However, blowing behavior is governed by the bulk temperature across the entire wall cross-section.


Outer Surface (Measured by IR Pyrometer):  ~112°C   (Cooled by ventilation air)
Mid-Wall Core (True Bulk Polymer):          ~104°C   (Absorbed short-wave photons + conduction)
Inner Surface (Internal Core):              ~98°C    (Heated slowly by thermal conduction)

Because virgin PET has a low thermal conductivity ($k \approx 0.15\text{–}0.24\ \text{W}/(\text{m}\cdot\text{K})$), a significant thermal gradient exists across thick-wall preforms (wall thickness $>3.5\ \text{mm}$). If the inner wall remains below the stretch transition temperature while the outer skin reaches $115^\circ\text{C}$, the inner surface will tear during stretch rod penetration. To measure internal wall temperatures for process audits, technicians should cut sacrificial preforms immediately at the oven exit and use a calibrated needle thermocouple or high-speed micro-thermal camera on the cross-section.

4. Axial Measurement Points on the Preform

A single temperature reading is never sufficient. The preform must be measured at specific vertical elevations corresponding to container stress zones:

Sub-Neck / Shoulder Transition: $5\text{–}10\ \text{mm}$ below the neck support ring.
**Mid-Body (Label Panel):**Center of the cylindrical body wall.
**Lower Body / Base Transition:**Immediately above the base radius.
**Injection Gate:**Direct center of the hemispherical bottom.

5. Standardized Preform Temperature Logging Template

To build a defensible quality record, process engineers should record thermal data using this standard tabular format:


===================================================================================
PET PREFORM THERMAL VERIFICATION LOG
===================================================================================
Date / Shift:              [                ]   Machine ID / Line: [              ]
Preform Resin / Color:     [ Virgin / rPET %]   Preform Weight / Wall: [   g / mm ]
Pyrometer Model & Wavelength: [ e.g., 3.43 µm ] Emissivity Setting: [ 0.95        ]
Oven Line Speed:           [          bph   ]   Ambient Temp / Humidity: [  °C / %]

---

Measurement Station: [ ] Oven Exit    [ ] Transfer Wheel    [ ] Mould Entrance

---

Vertical Elevation          Target Temp (°C)    Measured Temp (°C)   Delta (Actual - Target)
1. Sub-Neck Transition      [     °C ]          [     °C ]           [      °C ]
2. Mid-Body (Panel)         [     °C ]          [     °C ]           [      °C ]
3. Base Transition          [     °C ]          [     °C ]           [      °C ]
4. Injection Gate           [     °C ]          [     °C ]           [      °C ]

---

Circumferential Delta:
Quadrant 0°: [   °C ]  Quadrant 90°: [   °C ]  Quadrant 180°: [   °C ]  Quadrant 270°: [   °C ]
Max Circumferential Delta (Max - Min): [      °C ]  (Must be < 3.0°C)
===================================================================================

Precise preform temperature measurement requires stable, repeatable infrared heat sources that respond predictably to voltage modulation:

**High-Stability Short-Wave Emitters (GIG-010):**Fast-response quartz halogen technology providing rapid thermal equilibrium during setpoint changes. View Specifications on Goldisgood
**Consistent Directional Radiation (GIG-011):**Sintered ceramic white reflectors eliminate temperature variations caused by corroded machine back-plates. View Product Details
**Thermal Audit Consultation:**Share your preform pyrometer data and oven recipe logs with Goldisgood engineering at info@goldisgood.com to optimize zone repeatability.

Sources & References

International Organization for Standardization (ISO) 18434-1: Condition Monitoring and Diagnostics of Machines - Thermography - General Procedures.
National Institute of Standards and Technology (NIST) Special Publication 250-93: Calibration of Industrial Radiation Thermometers and Pyrometers.
Society of Plastics Engineers (SPE) Plastics Research: Spectral Emissivity and Narrowband Pyrometry of Semi-Crystalline Polyethylene Terephthalate.
Goldisgood Process Engineering Standard: Pyrometer Calibration and Preform Temperature Logging Procedures.