How to Optimise Stretch Ratio and Blow Pressure on an ISBM Machine?
Stretch ratio and blow pressure are the two most directly influential process parameters in stretch blow molding. They determine the degree of biaxial orientation achieved in the bottle wall, which in turn controls wall thickness distribution, mechanical strength, gas barrier performance, optical clarity, and resistance to top-load collapse. Getting these parameters right — and maintaining them consistently — is the difference between a process that reliably produces premium containers and one that generates chronic quality defects and excessive scrap.
This article is written for process engineers and technical operators who already understand the basics of ISBM and want to systematically optimise their stretch ratio and blow pressure settings to achieve target bottle specifications. Both the theory and the practical trial methodology are covered in depth.
Understanding Stretch Ratio: The Foundation
Stretch ratio describes the degree of deformation the preform undergoes during the stretch-blow process. It is expressed in two directions and their product:
| Ratio | Formula | Typical Range (PET) | Effect on Bottle |
|---|---|---|---|
| Axial Stretch Ratio (ASR) | Bottle body length ÷ Preform body length | 2.5–3.5× | Controls orientation in height direction; wall thickness in sidewall |
| Hoop Stretch Ratio (HSR) | Bottle body diameter ÷ Preform body diameter | 3.0–4.0× | Controls circumferential orientation; hoop strength; barrier |
| Biaxial Stretch Ratio (BSR) | ASR × HSR | 8–12× | Overall orientation intensity; strain-induced crystallinity in PET; barrier performance |
The ASR is primarily determined at the preform design stage — the preform body length relative to the target bottle body length sets the axial stretch ratio before the machine is even started. However, the effective ASR can be adjusted within limits by modifying the stretch rod stroke on the machine, or by changing the preform design. The HSR is similarly set by the relationship between preform and bottle body diameters. This means that stretch ratio optimisation begins with preform design, not just with machine parameter adjustment.
If the preform design gives ASR 2.8× and HSR 3.5×, the BSR is 9.8× — within the PET target range. If your bottle is showing poor barrier or low top-load strength despite correct conditioning temperatures, the first thing to check is whether the preform dimensions are giving the target stretch ratios, not just the blow parameters.
What Happens When Stretch Ratio is Wrong
- Poor top-load crush strength — bottles fail in pallet stack
- Higher oxygen and CO2 permeability — shorter product shelf life
- Greater creep under sustained pressure load
- Heavier base material (thick gate area not stretched away)
- Lower drop impact resistance — shatters rather than deforms
- Maximum tensile strength per unit weight
- Minimum gas permeability for the material
- Crystal clarity with minimal haze
- Uniform wall thickness distribution
- Excellent drop impact resistance
- Stress whitening and crazing at gate area
- Very thin base at gate — risk of field failure under abuse conditions
- Fibrillation — wall splits parallel to stretch direction on impact
- Gate puncture or tearing during stretch-blow
Blow Pressure — Understanding the Role of Pre-Blow and Main Blow
Blow pressure in ISBM is delivered in two sequential stages, each with a distinct mechanical function:
Stage 1 — Pre-Blow (Low Pressure)
Pressure range: 4–8 bar
Timing: Introduced simultaneously with or just before stretch rod descent
Function: Contacts the inner preform wall, creating a pressure cushion that prevents the stretch rod tip from puncturing the gate area. Also begins the controlled radial expansion of the preform body against the blow mold before high pressure is applied. Pre-blow pressure must be precisely timed relative to rod movement — too early inflates the preform before the rod reaches the gate; too late allows the rod to contact the gate without air cushion.
Stage 2 — Main Blow (High Pressure)
Pressure range: 15–40 bar
Timing: Applied after stretch rod has reached full stroke (or near-full); often with a brief overlap with pre-blow switch
Function: Provides the force to fully expand the preform outward against all mold surfaces, including fine panel details, shoulders, and base profile. Must be high enough to fully fill the mold cavity — insufficient pressure leaves flat spots and incomplete shoulder radius. Must be held for sufficient time (typically 0.3–1.5 seconds) for the walls to contact the chilled mold and begin cooling before exhausting.
Step-by-Step Optimisation Protocol
Follow this systematic sequence when optimising stretch ratio and blow pressure for a new bottle program or after a fault recovery:
| Step | Action | What to Measure |
|---|---|---|
| 1 | Calculate target ASR, HSR, BSR from preform and bottle drawings. Verify rod stroke setting achieves target ASR. | Rod stroke setting (mm); preform and bottle dimension drawings |
| 2 | Set conditioning temperature to mid-range of material window (e.g. 95°C for PET). Allow machine to reach thermal equilibrium (10+ cycles minimum). | IR gun reading at multiple preform body points |
| 3 | Set pre-blow to 5 bar. Set main blow to 20 bar. Blow first articles. Inspect for gate puncture, incomplete fill, and overall shape. | Visual inspection; bottle height; overall weight |
| 4 | If incomplete panel fill: increase main blow in 2-bar increments, blowing 3 cycles per setting, until full panel definition achieved. | Panel definition (visual); sidewall flat-spot absence |
| 5 | Measure wall thickness at 5 points on cross-section. Compare to target wall distribution. Identify heaviest zone. | Ultrasonic wall thickness gauge or physical cut-section measurement |
| 6 | If base is too heavy (insufficient ASR): increase rod stroke by 2mm increments. If sidewall is too heavy (insufficient HSR): adjust conditioning temperature profile or preform design. | Wall thickness after each adjustment; gate condition |
| 7 | Once wall distribution is acceptable, perform functional tests: top-load, drop test, pressure hold (CSD only). | Top-load force (N); drop height; CO2 pressure hold time |
| 8 | If top-load fails: confirm BSR is in target range. If BSR is correct but top-load fails, increase conditioning temperature by 2°C to improve orientation mobility. | Top-load force before and after conditioning temperature change |
| 9 | Record all confirmed optimal parameters in a Master Process Record. Validate stability over 100 consecutive bottles before releasing to production. | Sample every 10 bottles: weight, visual, top-load spot-check |
Material-Specific Stretch Ratio and Blow Pressure Targets
| Параметр | PET | PP (rCopolymer) | PETG |
|---|---|---|---|
| Target ASR | 2.5–3.5× | 2.0–3.0× | 2.0–3.0× |
| Target HSR | 3.0–4.0× | 2.5–3.5× | 2.5–4.0× |
| Target BSR | 8–12× | 5–9× | 6–10× |
| Pre-blow pressure | 4–8 bar | 5–10 bar | 4–8 bar |
| Main blow pressure | 15–35 bar | 20–40 bar | 15–30 bar |
| Main blow hold time | 0.5–1.5 s | 0.5–2.0 s | 0.5–1.5 s |
| Preform conditioning temp | 90–105°C | 125–140°C | 90–100°C |
Common Optimisation Mistakes and How to Avoid Them
When troubleshooting wall thickness problems, changing pre-blow pressure, main blow pressure, and conditioning temperature all at once makes it impossible to identify which variable caused any observed change. Change one parameter per trial run; document each result before moving to the next change.
A bottle can look acceptable visually but have non-uniform wall thickness that will cause top-load failure in distribution. Always measure wall thickness during optimisation, not just visual assessment. At minimum, cut 3 representative bottles for physical wall measurement.
Pre-blow pressure value and pre-blow initiation timing are independent variables. An incorrect pre-blow timing (too late) will cause gate marks even if the pressure is correct. Always optimise pre-blow timing and pressure together, not just pressure in isolation.
Parameters that produce acceptable bottles at cold startup may produce defects after 30–60 minutes when the machine reaches full thermal equilibrium. Always wait for thermal stabilisation (typically 20+ cycles after reaching setpoint) before locking parameters.
For process-specific guidance on stretch ratio and blow pressure optimisation for your bottle and material, or to request a remote process review, contact our process engineering team. We offer structured process optimisation support for all ISBM machines in our range.
Frequently Asked Questions
How do I know if I am achieving the target BSR without specialised lab equipment?
Can I increase BSR by raising blow pressure alone?
What is the minimum blow pressure I need to fully form a standard 500ml PET bottle?
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