Appearance
A manufacturer of 28-cavity PET preform moulds (H13, core pin 8 mm OD x 350 mm, 4 mm cooling channel coaxial within 0.10 mm TIR) used two-pass gun drilling: rough drill 3.5 mm annealed, heat treat (1020 C quench, 550 C double temper) to 48-52 HRC, finish gun drill to 4.0 mm hardened (AlCrN-coated carbide, Vc = 25 m/min, f = 0.015 mm/rev, oil at 60 bar). Ultrasonic wall thickness verification: max variation 0.08 mm (within 0.15 mm spec).
PET Preform Core Pin Cooling Channel Drilling Process
PET preform injection mould cores are long, slender pins that form the inside surface of the bottle preform. The core pin must have an internal cooling channel that is coaxial with the pin OD to ensure uniform cooling of the preform wall. If the cooling channel is off-centre, one side of the preform cools faster than the other, causing the preform to shrink unevenly — the preform becomes oval, and the final bottle has a non-uniform wall thickness. The coaxiality requirement for a PET preform core pin cooling channel is typically 0.10-0.15 mm TIR between the channel axis and the pin OD.
| Parameter | Standard Preform Core (H13) | High-Cavity Preform Core (H13) | Small Bottle Core (P20) | Large Bottle Core (H13) |
|---|---|---|---|---|
| Pin OD | 6-12 mm | 4-8 mm | 6-10 mm | 10-20 mm |
| Pin length | 200-350 mm | 150-250 mm | 150-250 mm | 300-450 mm |
| Cooling channel diameter | 3-6 mm | 2-4 mm | 3-5 mm | 5-10 mm |
| Channel depth | 180-340 mm | 140-240 mm | 140-240 mm | 280-440 mm |
| L/D ratio | 60-100:1 | 70-120:1 | 50-80:1 | 50-80:1 |
| Coaxiality requirement | < 0.10 mm TIR | < 0.08 mm TIR | < 0.15 mm TIR | < 0.10 mm TIR |
| Rough drill diameter (annealed) | 3.5 mm | 2.5 mm | 3.5 mm | 5.5 mm |
| Heat treat hardness | 48-52 HRC | 48-52 HRC | 30-35 HRC | 48-52 HRC |
| Finish drill diameter (hardened) | 4.0 mm | 3.0 mm | 4.0 mm | 6.0 mm |
| Finish drill Vc (hardened) | 20-30 m/min | 20-30 m/min | 40-60 m/min | 20-30 m/min |
| Finish drill feed (hardened) | 0.01-0.02 mm/rev | 0.01-0.02 mm/rev | 0.02-0.04 mm/rev | 0.01-0.02 mm/rev |
| Coolant pressure (finish) | 50-80 bar | 60-100 bar | 40-60 bar | 50-80 bar |
The manufacturing sequence for a coaxial cooling channel is: (1) rough gun drill the channel to within 0.5 mm of the final diameter in the annealed condition; (2) heat treat the core pin (quench + temper); (3) measure the channel position relative to the pin OD by ultrasonic wall thickness measurement; (4) finish gun drill the channel to the final diameter in the hardened condition, with the drill axis offset by half the measured eccentricity.
Heat Treatment Sequencing and Coaxiality Correction
The heat treatment can distort the rough-drilled channel by 0.05-0.20 mm due to non-uniform cooling of the thick and thin sections of the pin cross-section during quenching. The channel position is measured by ultrasonic wall thickness measurement at 4 positions around the circumference at 5-10 points along the pin length.
| Parameter | Annealed Condition (Pre-HT) | Hardened Condition (Post-HT) | Correction Method |
|---|---|---|---|
| Material | H13 (annealed, ~20 HRC) | H13 (hardened, 48-52 HRC) | — |
| Rough drill diameter | 3.5 mm | N/A | — |
| Finish drill diameter | N/A | 4.0 mm | — |
| Heat treat distortion | N/A | 0.05-0.20 mm eccentricity | Ultrasonic measurement |
| Cutting speed (Vc) | 40-60 m/min | 20-30 m/min | Reduced for hardened steel |
| Feed rate (f) | 0.02-0.04 mm/rev | 0.01-0.02 mm/rev | Reduced for hardened steel |
| Tool coating | TiAlN | AlCrN | AlCrN for high-temp hardness |
| Coolant | Oil (40 bar) | Oil (60 bar) | Higher pressure for chip evac |
| Material removal per side | N/A | 0.25-0.50 mm | Corrects 0.05-0.20 mm offset |
The finish drill follows the existing rough channel, but the drill axis is offset by half the measured eccentricity to bring the final channel back to the centre of the pin. The finish drill removes 0.25-0.50 mm per side, which is sufficient to correct a 0.05-0.20 mm eccentricity.
Quality Control for Core Pin Cooling Channels
| Inspection Parameter | Method | Acceptance Criteria | Frequency |
|---|---|---|---|
| Channel coaxiality (pre-finish) | Ultrasonic wall thickness (4 positions x 10 points) | Record eccentricity for offset correction | 100% of pins |
| Channel coaxiality (final) | Ultrasonic wall thickness | Max variation < 0.15 mm | 100% of pins |
| Channel diameter | Air gauge or plug gauge | +/- 0.05 mm | Sample (20%) |
| Channel surface finish | Profilometer (replica tape) | Ra < 0.8 microns | Sample (per batch) |
| Pin OD runout vs channel | Dial indicator + gauge rod | < 0.10 mm TIR | Sample (10%) |
| Coolant flow rate | Flow meter at operating pressure | Within spec | 100% of moulds |
| Preform body ovality | Optical micrometer on preform | < 0.1 mm ovality | Sample (per cavity) |
| Bottle capacity variation | Fill test on finished bottles | Within +/- 1% nominal | Sample (per cavity) |
Frequently Asked Questions
Why is a two-pass gun drilling process used for PET preform core pins?
A two-pass gun drilling process is used because the heat treatment (quench and temper) distorts the rough-drilled cooling channel, making it non-coaxial with the pin OD. If the channel were finished to the final diameter before heat treatment, the distortion would make the final channel off-centre. By rough-drilling first, heat-treating, measuring the distortion, and then finish-drilling with an offset correction, the final channel can be brought back to coaxiality within 0.10 mm TIR. This two-pass approach is essential for core pins with L/D ratios exceeding 100:1.
How is the coaxiality of the cooling channel measured by ultrasonic wall thickness?
The coaxiality is measured by an ultrasonic thickness gauge that sends a high-frequency sound pulse through the pin material and measures the time for the echo to return from the channel wall. The probe is placed on the pin OD at 4 positions around the circumference (0, 90, 180, 270 degrees) at 5-10 intervals along the pin length. The wall thickness at each position is calculated from the sound velocity in the tool steel (approximately 5900 m/s for H13). The difference between the maximum and minimum wall thickness at any cross-section is the channel eccentricity at that point.
What is the effect of an off-centre cooling channel on bottle quality?
An off-centre cooling channel causes the preform wall to cool unevenly — the thin side of the preform cools faster and shrinks more than the thick side, producing an oval preform cross-section. When this oval preform is blow-moulded into a bottle, the bottle wall thickness varies around the circumference, with the thinner side having lower mechanical strength and the thicker side using more material than necessary. The bottle capacity also varies because the oval preform does not expand uniformly in the blow mould. Published industry data shows that a 0.15 mm channel eccentricity can cause up to 2% capacity variation in 500 ml bottles.
What coating is used for gun drills in hardened H13 tool steel?
AlCrN (aluminium chromium nitride) coating is used for gun drills in hardened H13 (48-52 HRC) because it maintains its hardness at the high cutting temperatures generated when machining hardened steel. AlCrN has a hot hardness of approximately 3000 HV at 800 C, compared to TiAlN's 2500 HV at the same temperature. The coating is applied by PVD (physical vapour deposition) to a thickness of 2-4 microns. The gun drill geometry for hardened H13 uses a point angle of 130-140 degrees with a -5 degree rake angle (negative rake provides stronger cutting edge for interrupted cuts in hardened material).
Can the cooling channel be gun-drilled after the pin OD is final-ground?
No, the cooling channel must be gun-drilled before the pin OD is final-ground. The sequence is: rough gun drill the channel, heat treat, finish gun drill the channel, then grind the pin OD to the final dimension. If the OD were ground first, the heat treatment distortion would change the OD geometry, and the finish drilling operation would not have an OD reference for the offset correction. The ultrasonic wall thickness measurement between finish drilling and OD grinding also requires a consistent OD surface for the ultrasonic probe.
Data are based on published research and industry experience as of 2026. Always consult your mould manufacturer or tooling supplier for application-specific parameters.