Quick Answer: The most common problems in a PET preform injection moulding machine are short shots, flash, bubbles, black spots, high AA, neck defects and haze. Most come from wet resin, wrong temperature, poor pressure settings or worn tooling. Dry the resin well, set temperatures right, check the mould and clean the barrel regularly.
One bad preform can ruin a whole bottle. And one wrong setting on your machine can make thousands of them before anyone notices.
If you run a PET preform injection moulding machine, you already know how that feels. A batch comes out with bubbles, black dots or short shots, and the whole shift goes into fixing it. The good news is that most of these problems have simple causes, and many can be fixed without a long shutdown. Let’s get started!
What Is a PET Preform Injection Moulding Machine?
A PET preform injection moulding machine melts PET resin and injects it into a closed mould to make preforms. These PET preforms are later blown into bottles and jars. The machine works in a simple cycle, and a small change at any step can show up as a defect.
Main parts you should know:
- Hopper and dryer: hold and dry the PET resin before use
- Screw and barrel: melt and push the resin forward
- Hot runner and nozzle: carry the melt into the mould
- Mould (cavity and core): shape the neck, body and gate
- Clamp unit: keeps the mould shut during injection
- Chiller: cools the preform fast so it stays clear
Top 7 PET Preform Moulding Machine Problems And Their Solutions
Here are the seven problems that show up most often on a PET preform injection moulding machine. Each comes with causes, fixes and a tip from the shop floor.
1. Short Shots (Incomplete Preforms)
Short shots happen when the melt does not fill the mould fully. You get preforms with missing necks, thin bodies or an incomplete tip.
Possible Causes:
- Melt temperature set too low
- Low injection pressure or speed
- Blocked or partly worn nozzle
- Shot size or cushion too small
- Poor venting in the mould
- Worn check ring leaking melt back
Solutions: Raise melt temperature in small steps and increase injection pressure and speed slowly. Clean the nozzle and gates, check the cushion, and replace the check ring if it is worn out.
Expert Tip: Change only one setting at a time and run a few shots. If you change three things together, you will never know which one worked.
2. Flash Formation (Excess Plastic Burrs)
Flash is thin extra plastic that squeezes out at the mould split line or around the neck. It looks small, but it can cause trouble in blowing and capping.
Possible Causes:
- Clamp force too low for the mould
- Injection or hold pressure too high
- Worn or damaged parting surfaces
- Melt too hot and too thin
- Dirt or old plastic on mould faces
- Mould halves not sitting parallel
Solutions: Raise clamp force and lower hold pressure to what the part needs. Clean the mould faces and repair worn parting lines. If the melt is too thin, bring the temperature down a little.
Expert Tip: Wipe the parting line after every mould change. A tiny speck of dried plastic is enough to stop the mould from closing fully.
3. Bubbles, Voids, and Silver Streaks (Moisture/Gas)
These show up as tiny bubbles, hollow spots inside the wall or silver lines on the surface. They weaken the preform and spoil the look of the final bottle.
Possible Causes:
- Resin not dried enough
- Dryer fault or poor dew point
- Hopper left open to humid air
- High back pressure or screw speed trapping air
- Melt temperature too high, making gas
- Hold pressure too low for thick sections
Solutions: Dry the resin fully before it reaches the screw (moisture below 50 ppm, dryer dew point -40°C or lower) and keep the hopper lid closed. Reduce screw speed, set back pressure to a balanced level, and raise hold pressure so thick sections are packed properly.
Expert Tip: If your PET preform injection moulding machine suddenly gives silver streaks, check the dryer first. It is the culprit more often than the settings.
4. Black Spots and Discoloration (Material Degradation)
Black specks or brown streaks in the preform mean the resin has burnt or picked up dirt somewhere inside the machine. Clear bottles show them straight away.
Possible Causes:
- Carbon build-up on screw and barrel
- Dead spots in the hot runner
- Resin sitting too long in the barrel
- Dirt or other plastic mixed in
- Worn check ring or screw tip
- Barrel heaters running too hot
Solutions: Purge the barrel with a proper purging compound and clean the hot runner and nozzle. Match the shot size to the barrel, lower barrel temperatures, and keep resin bins and regrind covered.
Expert Tip: Purge before every long shutdown, not after it. Burnt resin left in a cold barrel turns into hard carbon that is painful to remove.
5. High AA (Acetaldehyde) Values and Yellowing
AA is a gas formed when PET breaks down from too much heat. High levels spoil the taste of water and drinks, and often come with a yellow tint.
Possible Causes:
- Melt temperature set too high
- Long residence time in the barrel
- Too much shear from screw speed
- Wet resin breaking down in the barrel
- Hot spots in the hot runner
- Resin with high starting AA
Solutions: Bring melt temperature down to the lowest level that still fills the mould. Cut the time resin spends in the barrel, reduce screw speed, and use a low-AA grade resin with proper drying.
Expert Tip: Do not wait for a lab report. If the preforms look slightly yellow, treat it as an early AA warning and check temperatures at once.
6. Thread and Neck Finish Defects
Neck and thread defects are common on a PET preform injection moulding machine and cause leaks at the cap. Watch for rough threads, ovality, sink marks and flash.
Possible Causes:
- Poor cooling around the neck area
- Worn neck inserts or split rings
- Hold pressure too low or too short
- Flash at the neck split line
- Neck ring and core not aligned
- Uneven mould temperature
Solutions: Check cooling flow around the neck and clear any blocked channels. Raise hold pressure and time a little, then inspect neck inserts for wear. Replace worn parts before they cause leaks.
Expert Tip: Test every new batch on your own capping line, especially for PCO 1881 short neck preforms and Alaska neck mineral water preforms. A neck that measures fine on the gauge can still leak at the cap.
7. Haze, Opacity, or Crystallinity Issues
Preforms should come out clear and glassy. If you see cloudy patches or a white ring near the gate, the plastic has started to crystallise.
Possible Causes:
- Cooling water too warm
- Blocked or scaled cooling channels
- Cooling time too short for wall thickness
- Thick gate area holding heat
- Other plastics like PP or PVC mixed in
- Wet resin or high melt temperature
Solutions: Lower chiller water temperature and flush the cooling channels. Check flow at each cavity, give the preform enough cooling time, and keep other plastics out of the resin bin.
Expert Tip: Look at the gate area first. A white ring or cloudy tip is usually the first sign that cooling is falling behind.
Conclusion
Most preform defects are not mystery problems. They come from wet resin, wrong temperatures, dirty barrels, weak cooling or worn tooling, and they show up in the same few ways. Keep a simple checklist for your PET preform injection moulding machine, change one setting at a time and clean before small issues grow. A well-run machine saves you scrap, time and stress.
If you buy preforms instead of making them, Mahalaxmi Polymer makes virgin, food-gradebottle preforms in-house at Bawana, New Delhi. Good preforms start with good habits, so fix the small things early.
Frequently Asked Questions
Bubbles mostly come from wet resin. If the dryer is not working properly, moisture turns to steam in the barrel and gets trapped in the melt. Air trapped by high screw speed or back pressure can also cause them.
Keep melt temperature as low as the mould allows, cut the time resin sits in the barrel and avoid high screw speed. Dry the resin properly and use a low-AA grade for water and drink bottles.
Cloudiness is usually crystallisation from slow cooling. The gate area stays hottest, so it turns cloudy first. Cold chiller water, clear cooling channels and enough cooling time usually fix it. Mixed-in plastics like PP can also cause it.
Flash is thin extra plastic at the mould split line. It happens when clamp force is too low, pressure is too high or the parting surfaces are worn. Clean the mould faces and repair any wear.
Black spots are burnt or carbonised resin. They come from carbon build-up in the barrel or hot runner, resin sitting too long at high heat, or dirt in the material. Regular purging and cleaning keeps them away.