Blow Moulding: Extrusion Blow (EBM), Injection Blow (IBM) & ISBM Systems

Blow moulding is the primary manufacturing method for hollow thermoplastic containers, ranging from small pharmaceutical bottles ($10 ext{ mL}$) to fuel tanks and large industrial drums ($220 ext{ L}$). The three main variants are Extrusion Blow Moulding (EBM), Injection Blow Moulding (IBM), and Injection Stretch Blow Moulding (ISBM). Understanding parison programming, blow ratio ($BR$), wall thickness distribution, and biaxial orientation is critical for light-weighting and burst-strength optimization.

By completing this lesson, you will be able to:

• Differentiate EBM, IBM, and 2-stage ISBM processes by container precision and material suitability.
• Calculate blow expansion ratio ($BR$) and average container wall thickness.
• Explain parison sag kinetics and electronic parison programming.
• Diagnose pinch-off weld failure, thin corners, and rocker bottom defects.

graph TD
    A["Process Selection"] --> B["Extrusion Blow Moulding (EBM)"]
    A --> C["Injection Stretch Blow Moulding (ISBM)"]
    B --> D["Extrude Tubular Parison -> Clamp Mould -> Pinch Bottom -> Air Blow"]
    C --> E["Injection Mould Preform -> Reheat -> Stretch Rod + Biaxial Air Blow"]
    D --> F["HDPE / PP Containers (Handles, Flash Trimming Required)"]
    E --> G["PET Bottles (High Clarity, Biaxial Strength, Zero Flash)"]

4.1 Blow Expansion Ratio & Wall Thickness Qualification

The Blow Expansion Ratio ($BR$) measures circumferential stretching from parison diameter $d_p$ to mold cavity diameter $D_c$:

BR = rac{D_c}{d_p}

Assuming constant polymer volume, the average final container wall thickness $t_{avg}$ is estimated from initial parison wall thickness $t_p$:

t_{avg} approx rac{t_p}{BR} = t_p left( rac{d_p}{D_c} ight)

4.2 Biaxial Stretch Ratios in ISBM (PET Bottles)

For Injection Stretch Blow Moulding (ISBM), stretching is biaxial:

• Hoop Stretch Ratio ($SR_{hoop}$): SR_{hoop} = rac{D_{bottle}}{d_{preform}} ($3.5 - 4.5 imes$)
• Axial Stretch Ratio ($SR_{axial}$): SR_{axial} = rac{L_{bottle}}{l_{preform}} ($2.0 - 3.0 imes$)
• Total Area Stretch Ratio ($SR_{area}$): $SR_{area} = SR_{hoop} imes SR_{axial}$ ($8.0 - 12.0 imes$)

Worked Numerical Example:

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Problem: An HDPE milk jug is produced via EBM using an extruded parison of outer diameter $d_p = 40.0 ext{ mm}$ and wall thickness $t_p = 3.20 ext{ mm}$. The cylindrical mold cavity has a diameter of $D_c = 120.0 ext{ mm}$. Calculate:

1. Blow expansion ratio ($BR$)
2. Estimated average container wall thickness ($t_{avg}$)

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Solution:

1. Blow Expansion Ratio ($BR$):

BR = rac{120.0 ext{ mm}}{40.0 ext{ mm}} = 3.00

1. Estimated Average Wall Thickness ($t_{avg}$):

t_{avg} = rac{3.20 ext{ mm}}{3.00} = 1.067 ext{ mm}

• PET Carbonated Soft Drink Bottles: 2-stage stretch blow moulding in Silvassa plant. (Illustrative Indian industry scenario based on beverage packaging operations).

• Parison: Extruded hollow molten tube used in EBM.
• Preform: Injection-moulded test-tube shaped entry piece used in IBM/ISBM.

1. ISO 23559:2011 — Plastics — Guidance for the specification and testing of blow-moulded containers, ISO.
2. Lee, N. C. (2006). Practical Guide to Blow Molding, Hanser Publishers.