Valve gate or open gate: choosing a hot runner for a PET preform mould
On a PET preform mould the hot runner keeps molten PET moving from the machine nozzle to every cavity. The last choice in that path is how the gate closes. An open gate relies on temperature. A valve gate uses a pin. Buyers feel the difference in the mark on the preform base, in stringing after a stop, and in the spare parts they buy later. This is how we compare the two when quotations look alike except for those words.
What the hot runner has to do
A cold runner leaves a thick sprue and puts the gate in the wrong place for a preform base. A hot runner holds the melt in a manifold and feeds each cavity through a nozzle. Every cavity should see a similar temperature and pressure, the gate should close after packing, and the steel around the gate should pull heat out so the base stays clear. Gate type is only one of those. A clean shut-off on a badly balanced manifold still makes preforms that do not weigh the same.
Balance before brand
Balance means a similar path from the machine nozzle to each gate: length, bore and bends. On a high-cavity mould a small difference shows up on the scale, one corner heavy and the other light. Ask for the layout, not only a cavity count. The hot runner page shows 24-, 48- and 96-cavity flow layouts as a starting point. Final geometry still follows your preform.
See the hot runner layout references
Zones and the controller
Each heated section is a zone: a heater, a thermocouple and a controller setpoint. More cavities usually mean more zones. The controller may be included, assumed to be on your machine, or missing from one of the two quotations. Write down which of the three you are being offered. If one zone drifts, that cavity's gate misbehaves while the others look fine. A defect on every cavity is more often the barrel, the resin or a setting for the whole shot. That split is the start of the gate-defect guide.
Open gate: temperature does the closing
An open gate, also called a thermal gate or an open nozzle, has no moving pin. The tip stays hot enough for PET to flow during injection. When injection stops, the surrounding steel cools the PET in the gate until it stiffens and holds the melt back. The next shot opens that passage again. The parts list is short: no pin, no pin seal, and no utility beyond heat and cooling water. For a lower cavity count, or a plant that wants every nozzle to be understandable without a specialist, that can be the right specification.
Where the process window gets narrow
Nothing mechanically stops the melt, so the vestige is harder to keep small. A tip that runs hot leaves a string as the mould opens. A tip that runs cold blushes the gate or fills short. The useful band moves when the resin, the colour or the shop temperature changes. After a stop, melt can also sag from an open tip onto a core. The first shots are scrap, and a drip on a core marks preforms until someone sees it. Colour changes make this worse, because the old colour has to leave the manifold while the tips are still open.
Valve gate: a pin seals each cavity
A valve-gate hot runner, often called a needle-valve system on preform moulds, puts a pin through each nozzle. The pin retracts to fill, then seats in the gate. It stays open through filling and holding, and closes before the mould opens. On preform tools the pin is commonly driven by air. Confirm that, because the air supply is part of the mould. You are paying for a small vestige and for a shut-off during a stop or a colour change. Gates that share one signal also stay in step more easily than tips that each find their own temperature.
The maintenance that comes with the pin
Pins, guides and seals wear. A pin that does not seat leaves a string or a white spot on that cavity. A pin that sticks leaves a short shot. Budget pins and seals for the first year, and dry air if the pin is pneumatic. Then ask whether a technician can change one nozzle or pin without stripping the mould. A hot runner that cannot be serviced cavity by cavity stops the line for one failed zone.
Cap moulds are a different gate
A cap mould gates into a closure, sometimes near a sealing face, and the part is ejected differently. Do not copy a preform gate onto a closure tool, and do not read "hot runner" on a cap quotation as the nozzle you saw on a preform. Send the closure drawing and ask where the gate will sit.
How to compare two quotations
Price per cavity hides this choice. A valve-gate system costs more to build and more to spare. An open-gate system can cost more in scrap and technician time when the cavity count, the colour changes or the gate appearance is demanding. Line the offers up before you compare the price.
| Question | Open gate | Valve gate |
|---|---|---|
| What closes the gate | Tip temperature and local cooling | A pin seats in the gate |
| Vestige | Larger, and sensitive to tip heat | Small when the pin seats fully |
| Stringing after a stop | More likely | Low when the pin, seal and timing are right |
| Moving parts | Few | Pin, guide and seals |
| Extra utility | None specific to the gate | Dry air, if the pin is pneumatic |
| A sensible fit | Lower cavity counts, simpler upkeep | High-cavity preforms, tight gate appearance |
| Spares to budget | Tips, heaters, thermocouples | Those, plus pins and seals |
Four facts apply to both types and still separate a complete quotation from a vague one. How many zones are there? Is the controller included? Is the manifold balanced for this cavity count? Which spares are in the crate? Two offers that both say "hot runner" can still be different machines.
What the published preform projects use
The two preform moulds published in detail on this site both use a needle-valve hot runner. That is a record of those jobs, not a rule for every mould. A heavier preform, a larger neck or a much lower cavity count is a reason to choose again rather than copy the case page.
- 64 cavities, 29/25 neck, 14 g with a tolerance of ±0.2 g, Yizumi 320T, 12-second target, needle-valve hot runner. Mineral water.
- 72 cavities, 30/25 neck, 14 g, Yizumi 480T, 15-second target, needle-valve hot runner. Mineral water.
Both records also list 4Cr13H for the mold base and S636 for the cores and cavities. A pin does not correct a gate that the cooling cannot keep clear. Steel and cycle time are the subject of the companion guide.
What the trial should prove
Accept the gate from a run, not from a picture. Agree the drawing, the cycle and the machine first, then keep one full shot with the cavity numbers marked.
- Weigh every cavity and compare the spread with the tolerance in the order. The 64-cavity record uses 14 g ±0.2 g. Your number may be different. Write it down before the trial.
- Look at every gate: no string, no drool onto the next shot, and a vestige you will accept on a blown bottle.
- On a valve-gate mould, confirm every pin opens and closes. A pin that does not move is not a process tweak.
- Record the manifold and nozzle temperatures with that shot.
- If water taste matters, keep samples for your own acetaldehyde test. Residence time in the manifold affects it. We will not quote a laboratory figure the trial did not measure.
Dimensions, cooling flow, documents and packing sit in the acceptance checklist. Use that list with this one.
What to send with the inquiry
A gate recommendation without a preform and a machine is a guess. If the cavity count is not settled, use the cavity guide and the calculator first. This page does not repeat that arithmetic.
- Preform or closure drawing, weight and neck finish. A sample helps.
- Cavity count, or the hourly output and the injection machine brand and model.
- Resin grade, colour, and how often you change colour.
- Any gate preference, and the reason: scrap, maintenance skill or air quality.
- Whether a temperature controller must be supplied.
- The spare parts you expect in the crate for the first year.
OSLON Precision Mold designs the mould, including hot-runner coordination, in Huangyan, Taizhou. Send the drawing to Frank Deng on the inquiry form. We will say which gate fits that preform and that machine, and which one we would not build.
How many cavities the mould needs
Check the machine in the cavity calculator
Questions buyers ask
Is a valve gate always better for a PET preform?
No. It gives a cleaner gate, and the published 64- and 72-cavity projects use a needle valve. It also adds pins, seals and, when the pin is pneumatic, an air supply. A lower cavity count can still be the right job for an open gate.
Can an open gate be converted to a valve gate later?
Treat that as a new hot runner. The nozzle, gate insert, actuation and often the plate machining all change. If you might want pins later, decide before steel is cut.
Why do only two cavities string?
On a valve-gate mould, check those two pins, their timing, and the heater and thermocouple. On an open-gate mould, check tip temperature and the cooling at those gates. If every cavity strings, look at melt temperature, drying and the settings for the whole shot. Mark the cavity numbers on a full shot first.
Does the temperature controller come with the mould?
Only when the quotation says so. Ask for the zone count, the connector, and whether the controller is included or already on your machine.
What should the trial show before I accept the gate?
A stable run at the agreed cycle, weights for every cavity, and gates with no string and no drool. On a valve-gate mould every pin should move. Record the hot-runner temperatures with that shot. The acceptance checklist covers cooling, dimensions and the documents in the crate.