A water bottler calls on Monday morning. Bottles from last week's batch taste faintly sweet, and some preforms in the retained samples show a white patch at the gate. Those are the two faults this article covers. Haze and acetaldehyde in PET preforms start in different places but share causes: wet resin, too much heat, too much time in the barrel and poor cooling. Fix those and both faults can settle together.
The checks below follow the order a shift engineer can work through: resin and dryer, melt temperature and residence time, then the hot runner and mould cooling.
Key Takeaways
- Acetaldehyde (AA) rises in a straight line with residence time and much faster with temperature, according to Ottmar Brandau.
- A lighter preform with the same wall can spend almost twice as long in the barrel, and so make more AA.
- White haze at the gate can mean the nozzle tip is set too cold; milky specks can mean unmelted pellets.
- Moisture in the resin creates AA as well as cutting IV, so the dryer is the first check for both faults.
Where Does Acetaldehyde in a Preform Come From?
From heating PET to melt temperature. Resin leaves the maker with very little AA; Brandau notes that solid-state processing brings it below 1 ppm. The AA in your preforms is mostly made at your machine, by heat, time in the barrel, moisture and shear from worn screws.
Reliance's data sheet for Relpet G5761, a still-water grade, lists AA at 1.0 ppm maximum in the chips. Any rise between chip and preform is therefore a process figure you control, not a resin defect.
Brandau explains why water bottlers care most. Still water taste reacts to even small AA levels, while the sugar in soft drinks and juices masks it. He adds that other preform makers still track AA, because rising AA on an unchanged job can show screw and barrel wear.
He also gives two relationships worth keeping in mind. AA rises linearly with residence time and exponentially with temperature, so a few degrees too hot can cost more than several seconds too long. A resin temperature of 165 °C at the feed throat has proven optimal for low AA, in his account.
Why Do Lighter Preforms Raise AA?
Because they stay longer in the barrel. Preform cycle time follows wall thickness, not weight, so a lighter preform with the same wall runs almost the same cycle. Each shot then takes less melt from the barrel, and the rest waits longer at melt temperature, making more AA.
Brandau makes the point with two preforms of equal wall, one twice the weight of the other: their cycles may differ by only 1 or 2 seconds, so the lighter one's residence is almost double. He observes that lightweighted water preforms run on the same extruders as before carry this penalty.
His conclusion is to match extruder size to preform weight. The edible oil article in this series covers how wall sets the cycle; the worked example below shows what the halved shot does to residence.
How Do You Control Haze and Acetaldehyde in PET Preforms?
Work through dryer, melt temperature, residence, hot runner and cooling in that order. The table links each symptom to its likely cause and first check. The first fixes are settings, but worn screws and leaking feed lines need hardware attention, and only lab AA and IV tests confirm the result.
Haze and AA in PET preforms: symptom, likely cause and first check
| Symptom | Likely cause | First check |
|---|---|---|
| High AA | Melt too hot, long residence, wet resin, worn screw | Melt near 280 °C ± 10 °C, shot share of barrel, dryer dew point, screw wear |
| White haze at the gate | Nozzle tip set too cold, so the gate crystallises | Raise tip heat until the haze clears, while keeping the gate nub solid |
| Milky specks or unmelts | Throat too cool, throughput above the screw's rating, low back pressure | Resin at least 160 °C at the throat, kg/h against the screw rating, back pressure 15 to 20 bar |
| Gate stringing or a hot dome | Nozzle tip set too hot, cooling too short | Mould water at 7 to 10 °C, cooling time about 1 to 3 s |
Rick Unterlander and Brandau's troubleshooting guide in PETplanet Insider supplies those settings. On the gate, it says nozzle tip heat should be low enough to solidify the gate nub but not so low that crystallinity forms, so white gates and stringing gates are two ends of the same adjustment.
On unmelts, the same guide asks that the processed kilograms per hour stay below the screw maker's rating, and that throat cooling not chill pellets below 160 °C. It adds that degraded pellets can carry higher crystallinity and a higher melting point, which also leaves unmelted specks.
Why Is Drying Part of Both Problems?
Water in the melt does double damage. Brandau notes that moisture both breaks the molecular chains and creates AA. The troubleshooting guide asks for a dew point of -40 °C or lower and about 4 hours at 175 °C, or up to 6 hours at 160 °C, in the hopper.
Our view: test AA on a fixed schedule even when your customer does not ask for it. A slow rise on a job that has not changed is the earliest cheap warning of a wet dryer or a worn screw, and it comes before the bottler's complaint.
Keep the dryer, the throat temperature and the melt temperature on the same shift log as the AA results. When AA moves, you can then see which of the three moved first.
What Should You Send to Diagnose Haze or AA?
Send the preform drawing and weight, the resin grade, your melt, throat and hot runner temperatures, the cycle, the dryer settings and recent AA or IV results. With those, residence time and the likely cause can be worked out before any parts are changed. See the 125 ton PET injection moulding machine page.
Frequently Asked Questions About Haze and Acetaldehyde in PET Preforms
What AA level should a water preform meet?
There is no single figure in the sources used here; bottlers set their own limits. Ask your customer for their preform AA specification and test method, and agree how often samples are taken. Then run AA on trial preforms from every cavity before production starts.
Can additives reduce AA?
Additives that reduce AA exist. They add cost and must meet food-contact migration rules, so treat them as a decision for your customer. Fix residence time, temperature and drying first; an additive should not hide a worn screw or a wet dryer.
Why do only some cavities show haze?
Look at differences between cavities: a nozzle tip running colder than its neighbours, a blocked water channel, or a valve stem not opening fully. Compare tip temperatures and water flows cavity by cavity. Haze in every cavity points back to the melt or dryer instead.
Does a hotter melt cure unmelts without raising AA?
Not freely. Raising melt temperature melts the pellets but raises AA quickly, since AA climbs exponentially with temperature. Check throat temperature, back pressure and throughput against the screw rating first, and raise melt temperature only in small steps while testing AA.
How do I know if my screw is wearing?
Watch for AA rising on a job whose settings have not changed, longer recovery times and drifting shot weights. Any of these is a reason to pull and measure the screw and check ring against their original dimensions at the next planned stop.
Related Articles
- What PET Products Can You Make on a 125 Ton Machine?
- Edible Oil Bottle Preforms on a 125 Ton Machine
- PET 125 Ton vs 150 Ton: Sizing by Shot Weight and Mould Size
Sources & References
- PETplanet Insider, O. Brandau: Bottles, Preforms and Closures extract, 3.6 Acetaldehyde in PET Bottles (Vol. 16, 03/2015)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 32: Unmelts (Vol. 13, 09/2012)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 28: Stringing (Vol. 13, 04/2012)
- Reliance Industries: Relpet G5761 Technical Data Sheet (Aug 2022)
Technical Disclaimer
Disclaimer: The information provided in this document is for general informational and educational purposes only. It is based on technical references and industry information available at the time of preparation and may be subject to change. It does not constitute engineering, technical, commercial or professional advice, nor does it constitute a quotation, specification, warranty or contractual commitment by Pratishna Engineers.
Actual injection moulding machine and process requirements vary according to the component design, material, mould, production volume, machine configuration and operating conditions. The information, examples, calculations and recommendations provided should be independently evaluated and verified for the specific application before making any equipment, process or production decision. Information obtained from third-party sources remains subject to the accuracy and limitations of those sources.
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