The ISBM moulding process — injection stretch blow moulding — converts raw plastic resin into a finished hollow container through a precisely sequenced series of steps. Each step occurs at a dedicated station on the machine’s rotary table, and all stations operate simultaneously on different preforms within the same machine cycle. Understanding what happens at each step, and what controls quality at each point, is essential for anyone operating or specifying ISBM equipment.

Overview: How Many Steps Does ISBM Have?

A one-step ISBM machine performs between three and six steps depending on its station count. Three-station machines perform the minimum required steps — injection, stretch-blow, and ejection. Four-station machines add a dedicated conditioning step. Six-station machines further add post-blow cooling and a second conditioning zone. All configurations produce the same type of finished container; the additional stations enable more precise control over complex bottle geometries and higher cycle speeds.

4-station ISBM rotary table — all four steps run simultaneously on different preforms within each machine cycle.

Step 1: Resin Drying and Preparation

Before any ISBM cycle begins, the resin must be prepared correctly. PET — the dominant resin in ISBM production — is hygroscopic: it absorbs moisture from the air. Moisture in the resin causes hydrolytic degradation during processing, reducing intrinsic viscosity (IV) and producing acetaldehyde (AA), a compound that creates off-flavour in food and beverage containers.

Standard PET drying: desiccant dryer at 160–170 °C, minimum 4–6 hours, target moisture below 50 ppm. The dryer’s dew point should be verified at −40 °C or below before processing begins. This step happens upstream of the machine and is not part of the machine cycle itself, but it determines the quality of every bottle the machine produces.

Step 2: Injection — Forming the Preform

The first in-machine step is injection moulding. Dried resin is fed from the hopper into a heated barrel where a reciprocating screw melts and conveys it. The melt — at 270–295 °C for standard PET — is injected at high pressure into the preform mould cavity, forming the thick-walled intermediate tube known as a preform.

Key parameters controlled at this step:

• Melt temperature — 270–295 °C for PET; too high degrades IV and generates AA; too low causes short shots and surface defects
• Injection speed and pressure — determines fill time and packing; insufficient packing causes gate sink and dimensional inaccuracy at the neck finish
• Cooling time in mould — the preform must cool to ejection temperature (60–75 °C) before the mould opens; insufficient cooling causes preform distortion on ejection
• Clamping force — must hold the mould closed against injection pressure; insufficient clamp causes flash on the preform parting line

The injection step is the longest single operation in the ISBM cycle, which is why it occupies a dedicated station regardless of total station count. On the EP-HGY series, injection clamping forces range from 50 kN (EP-HGY50-V3-EV) to 400 kN (EP-HGY650-V4), covering container volumes from 30 ml to 10 litres.

Step 3: Conditioning — Temperature Profile Adjustment (4-Station and 6-Station Only)

On 4-station and 6-station machines, the preform — still on the mandrel — transfers to a conditioning station after ejection from the preform mould. Here, infrared heaters adjust the preform’s axial temperature profile before it reaches the blow station.

PET can only be biaxially oriented within a narrow temperature window: above its glass transition temperature (Tg ≈ 75 °C) but below its crystallisation onset temperature (≈130–140 °C). The conditioning station brings the preform body to 90–115 °C while keeping the neck zone cool (the neck must not deform during blowing).

Individual infrared heater zones can be adjusted independently to concentrate heat at the base zone, shoulder zone, or body zone — allowing the operator to control where material flows preferentially during the blow step. This is what enables complex bottle geometries: wide bases, pronounced shoulders, contoured grips, and integrated handles.

4-station ISBM process sequence on the EP-HGY150-V4-EV — each station operates simultaneously, so conditioning time equals injection time.

Step 4: Stretch — Axial Extension by the Stretch Rod

At the blow station, the mould closes around the preform. A mechanical stretch rod descends into the preform and contacts its base, then extends axially — pushing the base downward at a controlled speed (typically 1–2 m/s). This extends the preform to 2.5–3.5 times its injection length.

The stretch step is what separates ISBM from standard injection blow moulding (IBM). In IBM, there is no stretch rod and no axial orientation — the preform is simply inflated by air pressure. In ISBM, the mechanical stretch rod produces axial molecular orientation before air expansion begins, which is the primary reason ISBM bottles are stronger, clearer, and lighter than IBM bottles at equivalent volumes.

Step 5: Blow — Radial Expansion Against the Mould

Simultaneously with or immediately after the stretch rod extension, high-pressure air (25–40 bar) is introduced through the stretch rod or a separate blow pin. This air pressure expands the preform radially — outward — against the walls of the closed blow mould.

The combination of axial stretching (Step 4) and radial blowing (Step 5) produces biaxial molecular orientation — polymer chains aligned in both the axial and hoop directions simultaneously. This dual orientation is what gives ISBM bottles their characteristic properties:

• Tensile strength up to 3 times higher than unoriented PET at the same wall thickness
• Glass-like optical clarity, with light transmittance above 90%
• CO₂ barrier performance sufficient for carbonated soft drink applications
• Drop impact resistance to 1.5 m when filled

Complete ISBM process parameter map — from resin drying through each station to the finished bottle, with critical control points at each step.

Step 6: Cooling — Freezing the Orientation in Place

After blowing, the bottle is held against the mould wall while the mould removes heat through its internal cooling circuit. Standard blow moulds run chilled water at 8–12 °C. The bottle must cool below its relaxation temperature — typically below 60 °C at the wall surface — before the mould opens, or it will shrink and distort on ejection.

For hot-fill applications (juice, tea, isotonic beverages filled at 85–95 °C), the blow mould runs at 100–120 °C instead of the standard cold temperature. This elevated mould temperature allows limited crystallisation of the PET at the bottle wall during blowing — a process called heat-setting — which gives the bottle thermal stability at fill temperature.

Step 7: Ejection — Finished Bottle Removal

The mould opens and the finished bottle is released from the mandrel. On 3-station and 4-station machines, ejection occurs at the final indexed position before the table rotates back to inject the next set of preforms. On 6-station machines, a dedicated post-blow cooling station between blow and ejection allows the mould to open sooner (shorter blow hold time) while the bottle completes its cooling on the mandrel before ejection, enabling faster overall cycle times without dimensional instability.

From ejection, bottles pass to a conveyor and accumulation table, and then to downstream filling, capping, labelling, and palletising equipment.

Process Step Summary

For full machine specifications covering each station configuration, see the EP-HGY 3, 4 and 6-station ISBM machine range. For mould design that works with each process step, visit the custom ISBM mould page.

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