Acetaldehyde (AA) in PET Preforms: What Mould Design Actually Controls
Acetaldehyde is a taste problem buyers measure, and a mould maker is often asked to design it out. The mould cannot choose the resin, dry it, or set the barrel. It does change how long molten PET stays hot after it leaves the machine nozzle, and how much heat the hot runner and the gate add. Specify that path before steel is cut. This page is not a laboratory result.
What acetaldehyde is in a PET preform
Acetaldehyde, usually shortened to AA, forms when PET is molten and polymer chains break. It stays in the preform. After the bottle is blown and filled, it can move into the drink. Buyers describe the taste as sweet or fruity. You cannot see it on a clear preform, and you cannot catch it on the scale you use to compare cavity weights.
An AA level on a PET preform mould is not a cavity count, a steel grade or a cycle time. It is a test result. Resin, drying, barrel heat and residence time all sit in that result. The mould changes what the hot runner and the gate do after the melt leaves the nozzle. Put those facts on the quotation so the rest is not guessed.
A string or a white patch at the base shares some of that heat, and it is a different check. The gate guide separates a hot tip, a worn pin, weak gate water and wet resin. A preform can fail an acetaldehyde test and still look clear. A white gate is worth fixing. It is not an AA number.
Why still water is stricter than a flavoured drink
Still water has little taste of its own. A sweet or fruity note that a cola, a juice or a flavoured water can cover is a complaint in mineral water, so water buyers write a tighter expectation for acetaldehyde in PET preforms. That limit belongs in their specification. This site does not publish one, and a figure from another bottle should not be copied into the mould order.
Carbonated drinks, juice, edible oil and hot-fill bottles are different jobs. The resin, the wall and the heat after injection are not the mineral-water case, even when the gram weight looks familiar. Write the product on the inquiry. A mould discussed only as a water tool is the wrong start for a hot-fill preform.
The two preform projects published in detail on this site are mineral-water tools. Read them for the fields that were recorded. Do not mine them for an AA level. None is there.
| Item | 64-cavity case | 72-cavity case |
|---|---|---|
| Neck finish | 29/25 | 30/25 |
| Preform weight | 14 g, tolerance ±0.2 g | 14 g |
| Injection machine | Yizumi 320T | Yizumi 480T |
| Hot runner | Needle-valve | Needle-valve |
| Target cycle | 12 seconds | 15 seconds |
| Application | Mineral water | Mineral water |
| AA test | Not published | Not published |
A ppm figure added beside this table did not come from the case pages. The buyer or the resin supplier holds the limit and the method. Keep samples when water taste matters. Residence time in the manifold affects the result. A laboratory figure is quoted only when the trial measured it.
Valve gate or open gate, including the note on acetaldehyde samples
Melt temperature, residence time and gate shear
Three things in the melt path move acetaldehyde in PET preforms. After the melt leaves the barrel, the mould and the hot runner influence all three.
Melt temperature
AA rises when PET is hotter, and when it stays hot longer. The barrel setpoint comes from the resin datasheet and the process sheet, not from a mould dimension. In the manifold, every path has to stay fluid without cooking the melt. A manifold or tip set hotter than the resin needs, or a tight gate that shears the shot, adds heat the barrel did not put there.
Ask for the zone count, and whether the manifold and the tips are set apart. One figure called hot-runner temperature hides a tip that is the hottest point in the tool. A drifting zone changes one cavity. A sample blended from the whole shot hides the split. Weigh every cavity and look at every gate. The acceptance checklist already asks for both.
Residence time
Residence time is how long PET stays molten: in the screw and barrel first, then in the manifold. A screw or a runner that is large for the shot, or a corner the flow does not flush, holds melt at temperature longer than a path washed through on every cycle.
Scale it by shot weight: cavities times grams. The cavity guide states the published 72-cavity, 14 g preform as 1,008 g per shot. That is what the press must melt, not an AA result. Manifold volumes are not on the case pages. Ask where melt could sit. A start-up sample measures a stop. A settled sample measures running conditions. Write which one the specification means.
How shot weight follows cavity count
Hot runner layouts, and what a balanced manifold means
Gate shear
The preform is gated in the base. The opening has to fill, pack, and then shut, by freezing or by a pin. A smaller opening heats the melt as it passes. That is also how a string or a white base starts: a hot tip, a pin that does not seat, weak gate water, or melt that was already hot. A clear base only shows that the gate cooled. Photographs with cavity numbers go to the gate guide. AA samples go to the laboratory named in the specification.
How to separate a hot tip from wet resin
What cooling does not undo
Cooling through the core, cavity and neck rings decides when the preform can be ejected, and whether the base stays clear. Faster cooling does not remove AA already formed in the melt. A cold mould and a hot, slow manifold can be the same tool. The steel and cooling guide is where the cycle is tied to a preform, a machine and the water.
How steel and cooling set the cycle
PET preform molds, and the fields the case pages actually record
| Factor | Mould and hot runner | Process and resin |
|---|---|---|
| Melt temperature | Manifold and nozzle setpoints, and shear at the gate | Barrel profile and screw speed |
| Residence time | Runner volume, dead ends, balance to each gate | Barrel size against the shot, and idle time |
| Gate | Opening, tip, pin or thermal shut-off, gate cooling | Injection speed and hold |
| Drying | Not a mould part | Dryer and the resin supplier's instruction |
| Resin grade | Not a mould part | The grade bought, including one chosen for water |
| AA sample | Shots kept at the trial, with zone temperatures | Limit, method and laboratory |
Valve gate or open tip
An open gate, also called a thermal tip, has no pin. The tip stays hot so PET can flow, then the surrounding steel freezes the opening. Raising the tip to stop an early freeze adds heat where shear is already high. After a stop, melt can sag from the tip. Those first shots are a poor sample of the running AA level, and a colour change has to purge through an open tip.
A valve gate puts a pin in each nozzle. The pin stays open for fill and hold, then seats before the mould opens. Shut-off does not depend on a cold tip, and a seated pin limits drool on a stop. Both published mineral-water moulds use a needle valve. That record is hardware, not an AA comparison with an open tip. A lower cavity count can still use an open tip if start-up shots stay out of the sample.
Valve gate or open gate on a PET preform mould
What the pin changes
- It can close the gate during a stop, which cuts drool and the time melt sits in an open tip.
- Tip temperature can be set for the melt, not only as the way to stop a string.
- Gates on one close signal stay in step more easily than tips that each find their own temperature.
What the pin leaves alone
- It does not shrink a manifold that is large for the shot.
- It does not dry the resin, and it does not lower a barrel that is already hot.
Pins and seals wear. A pin that does not seat leaves a string on that cavity. Budget the spares, and dry air if the pin is pneumatic, because that air supply is part of the mould. A line that only says valve gate for low AA is not a specification. Name the drink, the resin, the shot and the stops. A valve system costs more to build and to spare. An open tip can cost more in start-up scrap. Do not hide the choice in price per cavity.
What to put in the RFQ before steel is cut
Two offers can both mention acetaldehyde and still describe different tools. Line them up on the facts that change the steel. Leave a blank where you do not know. A guessed resin, written as if it had been chosen, becomes the melt behaviour everyone designs around.
| Item | Write this | If the line is blank |
|---|---|---|
| Product | Still water, carbonated, juice, oil or hot-fill | Taste risk is guessed |
| AA limit | Limit, method and laboratory, if you have them | No number is invented |
| Sample | Running shots, or shots after a stop | The wrong condition can pass |
| Resin | Grade, and whether it was chosen for this drink | Melt range stays open |
| Preform | Weight, wall, neck finish. A sample helps | Gate and cooling have no part |
| Shot | Cavity count, or output and the machine | Residence time stays open |
| Hot runner | Valve gate or open tip, and the reason | Gate type stays a price line |
| Stops | Colour changes, and how long a stop lasts | Start-up melt is ignored |
| Press | Brand and model, or shot capacity and tie bars | Plasticising is unknown |
| Cooling | Water treatment, and any take-out cooling | The cycle has no conditions |
An AA level for a PET preform mould is a test, not a dimension on the drawing. Send the limit, the method and the laboratory if you have them. Otherwise leave the cell empty. Running shots and shots after a stop are not interchangeable. Write the hot-runner temperatures beside the shot you keep.
Preform mould acceptance checklist
OSLON Precision Mold designs the mould, including hot-runner coordination, in Huangyan, Taizhou. Send Frank Deng the drawing, the drink, the resin status, the machine, the gate preference and who will test. Technical questions are answered from Taizhou within 8 hours. A missing resin, product or test method stays a question. It is not a promise of a number the trial never measured.
Send the drawing and the AA notes with your inquiry
Questions buyers ask
Can a mould guarantee an acetaldehyde level in the preform?
No. No AA result is published for the 64- or 72-cavity moulds, and none is added to a quotation to make it look finished. Acetaldehyde in PET preforms depends on the resin, drying, barrel temperature, residence time and the hot runner. Keep samples and use your own test. A figure is real only when that test was run.
Does a valve gate give a lower AA level than an open tip?
It changes the shut-off. A pin can close the gate during a stop, and the tip does not have to stay hot just to keep the gate from freezing. It does not fix a long residence time, a hot barrel or wet resin. The published mineral-water moulds use a needle valve. That is the hardware on those jobs, not an AA comparison.
Why do water buyers ask about AA more often than soft-drink buyers?
Water has little flavour to cover a sweet or fruity note. A carbonated or flavoured drink can hide the same note. Write the product on the request for quotation. Do not copy a limit from a different bottle.
Will a faster cycle reduce AA?
Not by itself. AA is generated while PET is molten, in the barrel and in the manifold, before cooling in the cavity has finished the job. A shorter cooling time does not undo that history. The 12-second and 15-second targets on the published projects are cycle records for those preforms and machines. They are not AA records.
Is a clear gate proof that the AA level is acceptable?
No. A clear base means the gate area cooled without the white crystallinity described in the gate guide. Acetaldehyde is not visible. A string or a white gate is still worth fixing, because both point at heat. Neither one is the laboratory test.
What should the trial keep if water taste matters?
A defined shot: steady running, or after a stop, with cavity numbers marked, and the hot-runner temperatures written down beside it. You test that shot. We will not quote a laboratory figure the trial did not measure. Weight, gate appearance and the cooling checks stay on the acceptance list either way.
Can you design the mould if the resin is not chosen yet?
The layout can start from the preform and the machine. Melt temperature and residence time cannot be closed without a grade, because drying and the melt range come from that datasheet. Say the resin is open. Do not accept a quotation that assumes a grade you have not approved.