Description:
PET blow molder breaking down often? Bottles deformed, uneven wall thickness, or output below rated capacity? This article helps you pinpoint the root causes and provides practical solutions.

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Introduction

The blow molder is the frontend of any PET filling line – and a frequent source of headaches.

If bottles are not blown correctly, even the best filler won’t save you. But many problems blamed on “poor machine quality” actually stem from operation, maintenance, or auxiliary equipment.

This article walks you through the five most common blowmolding problems and how to solve them.

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1. Uneven Wall Thickness – One Side Thicker Than the Other

Symptoms: The finished bottle has uneven wall thickness – some areas are overly transparent (overstretched), others are whitish (understretched).

Possible causes:

Uneven preform heating (old or misaligned infrared lamps)

Stretch rod offcentre

Inconsistent preform wall thickness (raw material issue)

Solutions:

Check the oven lamps – replace any that are old or uneven

Align the stretch rod

Inspect the preform quality – poor preforms are a common root cause

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2. Bottle Bottom Turns White or Cracks

Symptoms: The centre of the bottom is white, shows cracks, or even splits open.

Possible causes:

Insufficient heating at the bottom

Blow pressure too low or applied too early

Excessive gate residue on the preform bottom

Solutions:

Increase bottomzone temperature (the bottom typically needs 10–15°C more than the body)

Check highpressure air supply – should be ≥28 bar

Check highpressure filters for clogging

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3. Output Below Rated Capacity

Symptoms: The machine is rated “1outof1” or “1outof2”, but actual production is far below the design value.

Possible causes:

Heating time set too long

Cooling water temperature too high (reduces mould cooling efficiency)

Insufficient compressed air pressure or flow

Heavy preforms (requiring longer heating time)

Solutions:

Check cooling water – inlet temperature should be 8–12°C, outlet no higher than 18°C

Use deionised or RO water with a suitable corrosion inhibitor

Verify the compressed air system – undersized compressors are a common mistake

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4. Frosty or Crystallised Surface on the Bottle

Symptoms: The body is hazy, frosted, or shows white crystalline spots.

Possible causes:

Insufficient drying of the PET resin (moisture content too high)

Overheating (PET degrading)

Uneven cooling of the mould

Solutions:

Ensure PET resin is dried at 160–180°C for at least 4–6 hours – use a dehumidifying dryer, not just a hotair dryer

Monitor the dryer outlet dew point

Check mould cooling channels for blockages

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5. Bottle Sticks to the Mould After Blowing

Symptoms: The bottle cannot be released after blowing – it has to be removed manually.

Possible causes:

Mould temperature too high

Blow/cooling time too short (bottle not fully set)

Residue on the mould surface

Solutions:

Lower the mould cooling water temperature

Increase blow/cooling time

Clean the mould surface regularly with a suitable cleaner

Check that the release mechanism operates correctly

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6. Preventive Maintenance Checklist (Monthly)

Item	Frequency	Note
Infrared lamps	Weekly	Aging causes uneven heating
Highpressure air filters	Monthly	Clogging reduces pressure
Cooling water temperature & flow	Daily	High temperature reduces capacity
Mould surface cleaning	Each changeover	Residue causes sticking
Stretch rod position	Monthly	Misalignment causes uneven wall thickness
Valve timing	Monthly	Ensure preblow and highblow timing are correct

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80% of blowmolding problems can be avoided with correct operation and regular maintenance. If your machine is acting up, run through this checklist first – often the problem is not the machine itself.

If the problem persists after checking all these points, it may be a design or selection issue – contact your supplier for further diagnosis.

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Keywords: PET blow molder, blow molding problems, bottle deformation, blow molder maintenance, PET bottle production, blow molder repair