What Is a PET Preform Mould? Types, Cavities, Steel & How to Choose (2026 Guide)

A PET preform mould looks simple from outside. Molten PET goes in. Test-tube-shaped preforms come out. But the mould is where your bottle business starts clean or starts with problems you’ll chase for years. Cavity count sets your output. Steel grade sets lifespan. Neck finish decides if the cap actually fits. Cooling design decides if you make money per hour or just run a slow machine.

Buying your first PET preform mould? Trying to figure out why one supplier quotes double another? Don’t compare moulds by cavity count and price alone. That’s how buyers end up with a shiny tool that runs slow, flashes early, or makes preforms the blow mould can’t rescue. This guide walks through what actually matters. How the mould works. Which types are worth paying for. How many cavities you really need. Which steel grades to insist on. What drives price. And how to spot a supplier who knows preforms instead of just selling steel. Note on spelling: “mould” is British and international; “mold” is American. Same thing. Both are used across the industry.

What Is a PET Preform Mould?

A PET preform mould is a precision injection tool. It turns molten PET — polyethylene terephthalate — into preforms. Preforms are short, thick-walled, test-tube-shaped parts with a finished threaded neck. The preform is not the bottle. It’s the part that later becomes the bottle.

Bottle production runs in two steps:

1. Injection moulding — the preform mould shapes hot PET into preforms.
2. Stretch blow moulding — preforms are reheated, stretched, and blown with high-pressure air into the final bottle inside a separate blow mould.

This two-step setup isn’t a textbook trick. It’s how the industry stays flexible. You can mould preforms in one place. Ship them compactly. Then blow bottles close to the filling line. That’s why PET preforms dominate water, beverage, edible-oil, pharma, and personal-care packaging. PET gives clarity, light weight, recyclability, and an efficient production route.

Here’s the buyer’s point. Once the preform is wrong, the bottle is already in trouble. Poor core alignment becomes uneven wall thickness. A bad neck finish becomes leaking caps. Slow cooling becomes higher part cost every cycle. Get the PET preform mould right and clarity, wall distribution, weight, and cycle speed all become easier to control. Get it wrong and no blow mould will save you. For a broader intro to injection tooling, see our guide on what is a plastic mold.

How a PET Preform Mould Works

A PET preform mould runs thousands of cycles a day inside an injection machine. The cycle looks simple. But every step leaves its fingerprints on the final preform.

1. Drying — PET resin absorbs moisture. Skip the drying step and you’ll see bubbles, haze, and weak preforms.
2. Melting — dried resin is heated until it becomes a flowable melt.
3. Injection — the screw drives the melt through the nozzle into the mould.
4. Distribution — the hot runner splits the melt and feeds every cavity at the same time.
5. Packing and cooling — pressure packs PET into the neck threads while cooling channels pull heat out.
6. Ejection — the mould opens, preforms drop out, cycle starts again.

The CNC work and polishing get all the attention during quoting. But in production, the boring stuff decides the money. Runner balance. Venting. Cooling water flow. And whether every cavity behaves like the one next to it.

Core components buyers should understand

When a supplier sends you a mould drawing, don’t just look at cavity count. Ask them to point out the core, cavity, neck ring, hot runner manifold, and cooling layout. A shop that explains these clearly is thinking about production. A shop that only says “high quality steel, good price” is still selling at catalogue level.

The hot runner system

For PET preforms, the hot runner isn’t a luxury add-on. It’s the heart of the tool. It keeps PET molten right up to the gate. It cuts wasted material between cycles. And it makes multi-cavity production practical.

Two gate styles are common:

• Open-gate (open nozzle) — simpler and cheaper. Leaves a small gate mark. More prone to stringing.
• Valve-gate — a pneumatic or hydraulic pin shuts the gate cleanly. Costs more, but gives a better finish, no stringing, and tighter control.

Here’s the practical difference. Open-gate saves money on the quote. Valve-gate saves trouble when the line is running. For most preform programs, that trade-off favors valve-gate. Point-gate diameters for small parts typically start at 0.8–1.5 mm, scaling to 1.5–2.5 mm for larger, heavier parts. Gate land length stays short — 0.5–1.5 mm — to cut pressure loss and get a clean shear-off. For a deeper look at hot runner configurations, see our comparison of 2-plate, 3-plate and hot runner system molds.

Cooling Channels Decide Whether the Mould Is Fast or Just Expensive

Cooling is where cheap preform moulds lose the fight. Cooling takes 50–70% of the total cycle. That makes cooling the biggest lever on output — not a side detail. Two moulds can both be 32-cavity. One makes money because it cools evenly and fast. The other looked cheap on the invoice and stays expensive for the rest of its life.

Well-engineered moulds use spiral or independent rapid-circulation channels around each cavity. They pull heat out evenly. The numbers to ask about:

• Channel diameter: φ8–12 mm for standard preform moulds. φ8–10 mm for smaller moulds, φ10–14 mm for larger tools.
• Channel center-to-surface distance: 1.5–2 × channel diameter. So 15–20 mm for a φ10 mm channel.
• Channel center-to-center spacing: 3–5 × channel diameter. So 30–50 mm for a φ10 mm channel.
• Inlet-to-outlet water temperature rise (ΔT): aim for 2–4 °C, never above 5 °C.
• Minimum clearance from ejector pins or other bores: ≥ 5 mm.

That ΔT number is worth asking about directly. If water comes in cold and leaves much hotter, one side of the preform is shrinking under different conditions than the other. That’s how you get warped preforms and inconsistent wall thickness. Operators call it a “process problem.” It was really designed into the mould. For a full breakdown of cooling system design, see our page on injection mold cooling.

Venting

Buyers rarely ask about venting until they see burn marks or short shots. In high-speed PET preform moulds, trapped gas has very little time to escape. Shallow vents, blocked vents, or vents in the wrong spot send the operator chasing pressure and temperature. Meanwhile the real problem sits in the steel.

Standard vent parameters:

• Vent depth (cavity vent clearance): 0.02–0.05 mm
• Vent width: 3–12 mm
• Vent land (flat section next to the cavity): about 1.5 mm. Perimeter lands run 3.2–6.4 mm.

Vents go at the flow end opposite the gate. At runner terminations. And wherever thin sections or converging flow fronts show up. Ask the supplier where the air leaves the cavity. If they can’t answer that simply, they haven’t thought hard enough about filling.

Types of PET Preform Moulds

Suppliers describe PET preform moulds many ways. But most buying decisions come down to three choices: single-cavity or multi-cavity, hot runner or cold runner, and the mould structure.

A few structure terms are worth knowing too. Suppliers use them to separate basic tools from serious production tools:

• Two-plate vs three-plate moulds — two-plate is simpler and more common. Three-plate allows more complex gating.
• Long-tail vs short-tail vs self-lock — long-tail preforms have a tail that needs trimming by hand or machine. The mould is cheaper but adds a production step. Short-tail and self-lock cost more but skip the trimming.

For most commercial bottle programs, the right answer is a multi-cavity, hot runner mould. Single-cavity and cold runner tools only make sense when you’re proving a design, running specialty bottles, or genuinely producing small batches. The trap is buying “low cost” and then paying for it every cycle through waste, slow production, and manual trimming.

Cavity Count: How Many Do You Actually Need?

Cavity count is the first decision that sounds simple and gets expensive fast. Common configurations: 4, 8, 16, 32, 48, 72, 96, 128, and 144 cavities. More cavities mean more preforms per cycle and a lower cost per unit. They also mean a higher mould price, a bigger machine, more cooling demand, more hot runner complexity, and more ways for one cavity to drift away from the rest.

Most buyers miss this part. The cheapest preform isn’t always made in the highest-cavity mould. It’s made in the mould that matches your real annual demand and your machine capacity.

How to size cavity count in practice:

1. Calculate required preforms per hour based on target bottle volume.
2. Match that to a cavity count that hits the number at a realistic cycle time.
3. Leave some headroom for growth. Don’t massively over-buy.

A useful rule of thumb. Below about 5,000 bottles/day, a dedicated high-cavity mould often isn’t the most economical route. Above that, a well-chosen multi-cavity mould is hard to beat. Above ~10,000 units/day, hot runner or stack configurations start to make clear sense.

The word “well-chosen” matters. A 96- or 144-cavity mould isn’t just a bigger version of a small tool. It needs a large machine, high uptime, accurate runner balance, stable cooling, and real high-cavity experience. If demand can’t justify it, you’ve tied up capital in a mould and machine you can’t keep busy. Worse, if one major high-cavity tool goes down, you don’t lose one cavity. You can lose the whole program until maintenance brings it back.

Before ordering, ask the supplier to show estimated part cost at 4, 8, 16, 32, 48, 72, 96, 128, and 144 cavities. Build the comparison from real supplier spec sheets, not generic internet numbers. A serious shop will help you see where the tooling stops paying for itself.

A quick machine-sizing reference: clamp force ≈ melt pressure × projected part area × safety factor (1.1–1.3). For multi-cavity preform moulds running high injection pressures, this number climbs fast. Confirm the mould spec against the machine you plan to run before ordering. Not after the mould arrives. Our guide to mastering injection molding costs covers how cavity count and tonnage interact with project economics.

Mould Steel & Build Quality: Where Cheap Tools Hide Their Cost

Steel is where PET preform mould quotes look similar on paper and behave very differently in production. A PET preform mould runs under high pressure and temperature, cycle after cycle, often past a million shots. The steel has to hold polish, resist corrosion, keep the neck finish accurate, and survive heat and wear without turning maintenance into a monthly event.

Common grades you should recognize:

The buyer question isn’t “Do you use good steel?” Every supplier says yes. The real question is this: “Which steel is used for the cavity, core, neck ring, hot runner seat, and mould base — and can you send certificates and heat-treatment reports?” That one question separates a real mould builder from a reseller fast.

You may also see “nitrided steel” in datasheets. Be careful with that wording. Nitriding is a surface hardening treatment. It typically reaches 60+ HRC at the surface. It’s not a steel grade by itself. It’s commonly applied to neck-ring or thread parts because those areas see heavy wear. When a supplier says “nitrided,” ask for the base steel. For a full overview of mould steel grades and selection, see our detailed guide on mould steel.

PET preform wall thickness usually runs 2–4 mm. That’s thicker than many general injection parts. Draft angles on exterior surfaces are normally 0.5–1°. Deep interior features may need 1–2°. Textured surfaces need more draft — 1–3° depending on texture depth.

Tolerance claims deserve special attention. General cavity dimensions are held to about ±0.05–±0.25 mm. Precision inserts and critical fit features — like core-to-cavity alignment and neck-ring sealing faces — usually run ±0.01–±0.05 mm. If a supplier claims “0.01 mm tolerance” across the whole mould, ask which features that applies to. A real engineer answers by feature. A salesman repeats the number.

The rule that never bends: always request steel certificates and heat-treatment reports. A reputable supplier provides them without drama. A supplier who can’t or won’t identify the steel grade isn’t giving you a small warning. They’re showing you the biggest red flag in the whole buying process.

Before you sign, your quality checklist should include: certified steel grade, heat-treatment report, cavity-to-cavity deviation spec, polishing standard, and cooling channel design documentation.

Neck Finish & Thread Standards

The neck finish is the threaded top of the preform. It has to match the closure, the cap, and the filling line exactly. This is one of the easiest mistakes to avoid. And one of the most expensive to discover late. A preform can have perfect clarity, weight, and cycle time — and still be useless if the neck doesn’t match the closure.

Common neck-finish diameters include 28mm, 30mm, 38mm, and 48mm. Each has specific thread profiles. The thread standard matters as much as the diameter:

PCO1881 and PCO1810 are standard for water and CSD. They’re not the right answer for lotion bottles, food jars, detergent packaging, or other closures. Before you cut steel, confirm the neck finish against your cap supplier, filler, and customer spec. Don’t rely on “28mm” alone. The diameter isn’t the full specification. The ASTM plastics standards are the authoritative reference for thread-finish dimensions used worldwide.

What Drives PET Preform Mould Price?

Every buyer asks for the price first. The better question is what’s hidden inside the price. A PET preform mould isn’t expensive because the supplier feels like charging more. The quote is built from cavity count, steel, hot runner type, precision, polishing, cooling, and how much risk the builder is actually engineering out of the tool.

Price gaps between suppliers usually come down to materials, design complexity, and machining precision. Not random markup. When two quotes are far apart, ask what steel is used in each working area. What hot runner is included. What tolerance applies to critical features. And what cooling layout is being built.

The purchase price is only the first line of the cost. The real number is total cost of ownership. Maintenance. Spare parts. Downtime. Scrap. Cycle time. And how many cycles the mould lasts before major work. A cheap mould built from uncertified steel with poor cooling can cost more than a good mould long before its life is up. The bad tool doesn’t send you one invoice. It quietly charges you every shift through defects, slow cycles, and early replacement. For a structured framework on total injection moulding investment, see our article on how much does it cost to get a plastic mold.

One final pricing note. Be careful publishing hard dollar amounts unless you can stand behind them for your market, cavity count, hot runner, and steel package. Cost drivers and relative ranges are more credible than invented price tags.

Mould Lifespan, Maintenance & Cycle Life

A good PET preform mould isn’t a consumable. It’s a capital asset. Well-built moulds are commonly rated for 1 million or more cycles. Premium moulds run well past that — some manufacturers cite 2.5 million-plus shots at consistent quality. But lifespan isn’t printed into the mould by marketing. It comes from steel grade, heat treatment, machining, polishing, cooling, and how the tool is run every day.

To reach that lifespan, treat maintenance as production insurance — not cleanup after defects appear:

• Regular cleaning. Stop residue buildup in cavities and cooling channels.
• Proper lubrication of moving parts — slides, ejection, and neck-ring splits.
• Scheduled inspection of cavities, cores, and cooling channels for wear, pitting, and blockage.
• Standardized operation — run within rated tonnage and pressure, with properly dried resin.
• Clean raw material — properly dried PET. PET typically needs drying at 160–180 °C for 4–6 hours. That cuts abrasion and contamination inside the mould.

One factor is getting harder to ignore: recycled PET (rPET). As more producers blend rPET into their resin, viscosity can vary batch to batch. Harder inclusions can speed up cavity wear. If you plan to run rPET, tell the supplier before steel is specified. That conversation affects steel grade, surface hardness, and maintenance expectations. For a complete maintenance framework for all injection tooling, see our ultimate mold maintenance guide. The Society of Plastics Engineers (SPE) also publishes technical resources on mould maintenance.

How to Choose the Right Supplier

The mould is only as good as the people who build and support it. This matters more with PET preforms than with simple injection parts. High-cavity balance, neck finish accuracy, hot runner stability, and cooling all have to work together.

Most suppliers fall into three groups:

• Specialized mould manufacturers — focus on mould design and production. Usually the deepest tooling expertise.
• Machine OEMs — sell complete injection or blow systems. May offer moulds as part of the package.
• Traders / intermediaries — resell moulds. Convenient sometimes, but they add a layer between you and the actual maker.

A practical vetting checklist:

• ✅ Certifications — ISO 9001, and FDA/food-grade where relevant.
• ✅ Precision evidence — ask about cavity-to-cavity deviation and tolerance control. General cavity tolerances should sit at ±0.05–±0.25 mm. Critical fit features at ±0.01–±0.05 mm. Ask which features their tolerance claims actually apply to.
• ✅ Engineering capability — Moldflow / thermal simulation, CNC + EDM machining, automated polishing.
• ✅ Material transparency — steel certificates and heat-treatment reports, provided without pushback.
• ✅ Scale match — high-cavity expertise is different from low-cavity. Match the supplier to your configuration.
• ✅ After-sales support — spare parts, maintenance programs, and response time. The mould will need support over years, not just a one-time sale.

Here’s the question that quickly reveals the supplier’s level. “Which parts of this mould worry you most for my target volume and material?” A real preform mould builder will talk about hot runner balance, cooling, neck-ring wear, cavity-to-cavity deviation, or machine tonnage. A weak supplier says “no problem” before they’ve even studied the project. “No problem” sounds comforting. In tooling, it usually means they haven’t found the problems yet.

The strongest supplier relationships align technical capability with your production goal. You’re not just buying a tool. You’re buying the ability to keep that tool running. Our guide on choosing the right injection molding manufacturer in China covers the full due-diligence process. For independent third-party auditing standards, the ISO 9001 quality management standard is the global benchmark to verify against.

Send us your bottle drawing and target volume if you want a practical mould design and quote based on your actual production.

Common Defects & Troubleshooting

Preform defects often get blamed on the operator first. Sometimes that’s fair. But many “process problems” are really mould problems showing up after the tool is in production. The trick is knowing where to look.

The pattern is simple. Bubbles and haze usually start with resin drying. Short shots can come from pressure, venting, or both. Weight inconsistency and uneven wall thickness point back to runner balance, core alignment, or cooling. A good supplier prevents most of the second category before the mould ever ships. Thermal simulation, correct machining, and a cooling layout that isn’t an afterthought. For a defect reference covering all injection-moulded parts, see our full guide to troubleshooting product defects.

Frequently Asked Questions

What is a PET preform mould and what does it do?
A PET preform mould is an injection tool. It shapes molten PET into test-tube-shaped preforms with a threaded neck. Those preforms are later reheated and blown into finished bottles.

What’s the difference between a preform mould and a blow mould?
The preform mould makes the intermediate preform in injection moulding. The blow mould shapes that preform into the final bottle in stretch blow moulding. Two separate tools in a two-step process.

How many cavities do I need?
It depends on your target output. Calculate required preforms per hour. Then pick a cavity count that hits the number at a realistic cycle time. Below ~5,000 bottles/day, a small configuration may be enough. Above ~10,000/day, high-cavity hot runner moulds start to make sense.

What steel is best for a PET preform mould?
S136 at 48–52 HRC is the standard for cavities and cores where mirror polish and corrosion resistance matter. NAK80 at 38–42 HRC works well where weld repair is expected. H13 / 1.2344 at 44–50 HRC suits hot runner interface zones. P20 or 2738 covers structural plates and mould bases. Always request steel certification and heat-treatment records.

How long does a PET preform mould last?
Quality moulds are typically rated for 1 million or more cycles. Premium moulds run well beyond that — provided they’re properly maintained and run within spec.

Hot runner or cold runner?
Hot runner is the standard for automated, high-speed, low-waste commercial production. Cold runner only makes sense for simpler, lower-volume work. Choose cold runner when upfront cost matters more than efficiency.

What affects the price?
Cavity count, steel grade, hot runner type, design complexity, machining precision, and supplier reputation. Valve-gate systems cost more than open-gate. Certified S136 or NAK80 costs more than P20. The difference shows up in lifespan, surface quality, and production stability.

Which neck finish should I choose?
Match the neck finish to your closures and filling line. PCO1881 and PCO1810 are standard for water and soft drinks. Food jars, oils, personal-care products, and detergent use different finishes. Confirm compatibility before ordering.

What cooling water temperature difference should I target?
Keep ΔT (inlet-to-outlet rise) at 2–4 °C. Treat 5 °C as the hard upper limit. A larger ΔT usually means low flow or a poorly balanced cooling circuit. That causes uneven shrinkage and warped preforms.

The Bottom Line

A PET preform mould is the foundation of bottle manufacturing. It sets your quality, speed, and cost per unit before the bottle even exists. The right tool isn’t simply the mould with the most cavities or the lowest quote. It’s the mould that matches your real demand. Uses certified steel. Controls the neck finish. Cools evenly. Holds cavity-to-cavity consistency. And comes from a supplier who can support the tool after delivery.

Match cavity count to production volume. Insist on certified S136 or NAK80 for working surfaces. H13 for hot runner zones. P20 or 2738 for structural areas. Check hardness values. Confirm cooling design can hold ΔT within 2–4 °C. Make sure the neck finish fits your closure system before steel is cut. Ask for steel certificates, heat-treatment reports, and maintenance support in writing.

Buy on total cost of ownership. Not the lowest initial quote. A properly specified PET preform mould can pay for itself over 1 million cycles or more. A cheap mould can start charging you back from the first production run.

Ready to spec a mould for your application? Send your bottle drawing and target volume for a free mould design and quote.