Preform Mould Steel Selection: Hardness, Polish and Life

A preform that looks perfect on the day of the mould trial and shows haze, a dull gate scar or a faint scratch pattern eight weeks later is almost never a machine problem. The screw, the hot runner and the cooling circuit did not change. The cavity surface did. Preform mould steel selection decides how long that surface survives, how well it can be polished, and whether a small repair can be made on the bench or requires the cavity to be scrapped.

The commercial damage is quiet rather than dramatic. A cavity that cannot hold a mirror finish forces a higher gate and a slower injection profile, which adds cycle time on every shot for the rest of the mould’s life. A neck insert that wears before its planned life turns into a dimensional drift on a critical sealing surface, which is discovered by a capping failure at the filling line rather than by a measurement in the mould shop. And a cavity steel that was never specified in writing gives nobody anything to argue with when the tool arrives with the wrong polish.

Sailwin has supplied PET injection moulding machines for 15+ years, with 500+ machines delivered into 60+ countries, CE marking, ISO 9001:2015 manufacturing and a 2-year whole-machine warranty. The SW-P series covers 14 models from 170 kN to 5500 kN clamping force and supports valve-gate hot runners up to 64 cavities, with dedicated PET screws, far-infrared nano heating coils and an EUROMAP 67 robot interface. This article sets out which steels belong in which position, what hardness and polish really mean for clarity, and how to write a mould steel specification that a toolmaker can quote against.

1. Why Preform Mould Steel Selection Decides Optical Clarity

A PET preform is stretched and blown into a bottle. Any surface defect on the inside of the preform is amplified by the stretch ratio — typically around 2.5 to 3.5 in the axial direction and 3 to 4 in the hoop direction for a standard carbonated soft drink bottle. A scratch that is invisible on the preform becomes a visible streak on the finished bottle wall. That is the whole argument for spending money on the cavity surface: clarity is not created in the blow moulder, it is preserved from the cavity.

The cavity must therefore satisfy three requirements at the same time. It must polish to a surface finish fine enough that the polymer replicates it faithfully, normally in the region of 0.02 to 0.05 µm Ra for optical-grade preforms as an industry typical target. It must hold that finish through hundreds of thousands of cycles without pitting, scratching or etching from the melt. And it must not release material or shed particles that become inclusions in the preform.

Melt conditions decide which of those requirements dominates. PET is processed with a barrel profile that keeps the melt hot and shear-thin, and Sailwin machines hold barrel zones with PID control to about ±1 °C while monitoring 40+ parameters in real time. A cavity steel with poor hot hardness or a coarse carbide distribution will start to lose its polish at the gate and the last-filled extremity first, because those are the points of highest shear and temperature. This is why the steel grade should be chosen against the preform’s wall thickness and the intended annual volume, not against the mould price alone.

2. Preform Mould Steel Grades: A Comparison Table

The table below groups the steel families commonly specified for PET preform tooling. Hardness figures and finish capabilities are industry typical values for these grades rather than Sailwin product data, and the final choice should always be confirmed with the toolmaker against the actual preform drawing and production plan.

Two patterns are worth noticing. First, the grades with the best mirror polish are generally the ones that are hardest to machine, which is why a cavity insert is usually finished by EDM and then hand or machine polished in several stages. Second, the highest hardness is not automatically the right answer: a 54 HRC cavity is more resistant to wear but is also more likely to chip if the mould is handled carelessly or if a cold slug of PET is fired into it. Matching hardness to the real operating discipline of the plant is part of the decision.

3. Hardness and Polishing: The Two Specifications That Actually Decide Service Life

A mould purchase order that says “hardened steel, polished” is not a specification. Two numbers make it one, and both should appear on the drawing. The first is a hardness with a tolerance and a stated test method — for example 50 HRC ±2 measured on the cavity surface after final heat treatment, not on a test coupon. The second is a surface finish with a stated maximum roughness and a stated measurement direction, because roughness measured across the polish direction is not the same number as roughness measured along it.

Polish quality is where the specification most often gets loose. A toolmaker may polish to a visual standard that looks acceptable under workshop lighting and still leave a pattern along the flow path, which is exactly where the polymer replicates it. For preforms, the surface should be inspected under directional light with the axis of the preform aligned to the light source, and the specification should say so. Any remaining EDM recast layer must be removed completely; a recast layer under a polished surface will spall later and produce the inclusions that show up as black specks in a bottle.

Clarity is bought twice: once when the cavity is polished, and again every time it is serviced. A specification that names the steel and the hardness but not the finish will be interpreted differently by every toolmaker who quotes it.

4. Cores, Neck Inserts and Cooling Inserts: Different Steels for Different Jobs

Treating a preform mould as one piece of steel is the most expensive habit in the mould shop. The five functional areas of a preform tool wear at different rates and fail in different ways, so each one justifies its own material decision:

• Cavity body. Governs the outside surface of the preform and therefore the visible clarity after blowing. A stainless 420 ESR or a well-polished 718H cavity is normal; the deciding factor is the finish the toolmaker can hold repeatedly, not the nominal hardness alone.
• Core pin. Thin, highly loaded and in contact with the hot melt on its full length. It needs higher hot hardness and better thermal conductivity than the cavity, and is normally made from a through-hardened H13-class steel in the 46–52 HRC range.
• Gate and injection point. The highest shear and the highest local temperature in the tool. This is the first place to lose polish, so it should be a separate replaceable insert in a hard grade even when the rest of the cavity is softer.
• Neck finish, thread and tamper band. These are sealing and handling surfaces with tight dimensional tolerances. They wear, and when they wear the cap torque changes and the filling line notices before the mould shop does. Hardened replaceable inserts, typically 48–54 HRC, allow replacement without touching the body cavity.
• Cooling inserts and hot spots. Beryllium copper inserts raise the effective cooling rate at the tip and neck, where PET cycle time is usually limited. They are not wear surfaces and should be treated as consumable thermal components with their own inspection interval.

The machine side of this decision is cavity count. Sailwin’s SW-P series spans 14 models from 170 kN to 5500 kN, and supports valve-gate hot runners up to 64 cavities, which is the configuration where per-cavity steel discipline pays back fastest: at 64 cavities, a single cavity that loses its finish affects 1.5% of every shot for as long as it stays in the tool. The dedicated PET screw and far-infrared nano heating coils on these machines are designed to deliver a consistent, low-shear melt, and the EUROMAP 67 robot interface lets a take-out robot handle the preforms without manual contact. Chilled water for the mould circuits should be supplied at 8–12 °C, and the balance between cavity and core circuits is set as part of the mould design, not adjusted afterwards on the machine.

5. Case Study: Haze and Gate Marks After a Cavity Insert Replacement

A bottler running a 32-cavity preform tool replaced two damaged cavities with inserts from a different steel grade to shorten the repair. Both cavities produced preforms that passed weight and dimensional checks, but the blown bottles showed a faint haze band that was not present on the other thirty cavities.

Scenario based on a Sailwin customer project; site-specific figures available on request during engineering review.

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