Blow Moulding: Parison Programming, Die Kinetics & Stretch-Blow Physics

Blow moulding is the primary process used to manufacture hollow plastic containers, ranging from 100 mL cosmetic bottles to 200 L industrial drums and automotive fuel tanks. Extrusion Blow Moulding (EBM) and Stretch Blow Moulding (SBM) are highly complex due to the dynamics of parison inflation, gravity-induced sag, and die swell. In India, massive packaging processors like Supreme Industries and consumer packaging converters manufacture containers for food, dairy, and chemical distribution, requiring optimized parison programming to prevent wall-thinning defects.

• Analyze parison swell (weight and diameter swell) and gravity-induced parison sag kinetics.
• Set parison programming profiles using a 100-point thickness controller to achieve uniform wall thickness.
• Explain the molecular orientation mechanics of the Stretch Blow Moulding (SBM) process for PET bottles.
• Calculate the blow-up ratio (BUR) and estimate final part wall thickness.
• Reference blow moulding quality standards such as IS 12257 and ISO 9001 packaging protocols.

3.1 Parison Swell vs. Parison Sag

In Extrusion Blow Moulding, a molten tube of polymer (parison) is extruded vertically downward. It experiences two competing phenomena:

• Parison Swell: Viscoelastic relaxation causes the parison to expand in diameter ($B_D$) and wall thickness ($B_w$) upon exiting the die.
• Parison Sag: The weight of the extruded parison causes it to stretch and thin under gravity. Sag increases with time and temperature: the top of the parison becomes thinner than the bottom. To counter this, Parison Programming dynamically adjusts the die core pin position (using electro-hydraulic actuators) during extrusion to vary the die gap, matching the parison thick-and-thin profile to the mould cavity.

3.2 Blow-Up Ratio (BUR)

The Blow-Up Ratio is the ratio of the mould cavity diameter to the parison diameter:

The average wall thickness of the blown container ($t_c$) relates to the parison thickness ($t_p$) and BUR:

For complex shapes, thickness variations are significant at corners and deep-draw regions.

3.3 Stretch Blow Moulding (SBM) of PET

SBM is a two-stage process for producing high-clarity carbonated beverage bottles:

1. Preform Injection: Injection moulding of a thick-walled test-tube-like "preform".
2. Reheating and Blowing: Preforms are reheated to just above $T_g$ ($90-110^\circ$C), placed in the blow mould, and stretched axially using a mechanical stretch rod while being blown radially with high-pressure air (3–4 MPa).

• Biaxial Orientation: Mechanical stretching + inflation aligns polymer chains both axially and circumferentially, triggering strain-induced crystallization. This improves tensile strength, gas barrier (OTR/CO₂ retention), and impact resistance.

Problem: A HDPE fuel container is manufactured by extrusion blow moulding. The die outer diameter is $D_{die} = 80$ mm. The parison has a diameter swell ratio $B_D = 1.25$. The mould cavity diameter is $D_{mould} = 220$ mm. If the target average wall thickness of the container is $t_c = 2.5$ mm, calculate:

1. The parison diameter ($D_p$) after swell.
2. The Blow-Up Ratio (BUR) based on the swelled parison.
3. The required parison wall thickness ($t_p$) before inflation.

Solution:

1. Calculate the swelled parison diameter $D_p$:

1. Calculate the Blow-Up Ratio (BUR):

1. Calculate the required parison wall thickness $t_p$:

Interpretation: Sizing calculations indicate that the extruded parison must have a thickness of 5.50 mm to yield a final container wall thickness of 2.50 mm after inflating by a factor of 2.20. Parison programmers must set the die gap profile to yield 5.50 mm after accounting for viscoelastic swell.

Supreme Industries (Jalgaon) produces large agricultural chemical containers using multi-layer co-extrusion blow moulding. They incorporate a barrier layer of polyamide (Nylon 6) or EVOH to prevent hydrocarbon migration, satisfying central pesticide packaging directives.

Indian mineral water and carbonated beverage packaging units utilize high-speed rotary stretch blow moulding lines (e.g., Sidel or ASB machines) to produce PET bottles conforming to IS 12257 guidelines for thermal stability and top-load crush strength.

• Parison: The hot extruded polymer tube used as the preform in extrusion blow moulding.
• Parison Programming: Dynamic die gap adjustments during parison extrusion to control wall thickness.
• Biaxial Orientation: Aligning polymer chains in two directions, improving physical and barrier properties.
• Preform: Injection-moulded intermediate tube that is reheated and blown into a bottle.
• IS 12257: Indian Standard code governing PET bottles for packaging of drinking water.

1. IS 12257 — Bureau of Indian Standards (BIS) specification for polyalkylene terephthalate (PET) bottles for packaging of food and beverages
2. ISO 9001 packaging protocols for rigid containers
3. ASTM D2911 — Standard Specification for Dimensions and Tolerances for Plastic Bottles

1. Derive the equations representing parison diameter swell ($B_D$) and thickness swell ($B_w$). How do they depend on shear rate and die land geometry?
2. Explain the physical mechanism of strain-induced crystallization during the stretch blow moulding of PET. How does this affect bottle top-load strength?
3. Discuss the typical defects in EBM (e.g., parison curtaining, blow-out, wall thin-spots at corners) and their troubleshooting steps.

Q1. Parison programming is used to compensate for:

• C) The thinning of the parison at the top due to gravity-induced sag

Q2. The molecular orientation in stretch blow moulding of PET bottles is introduced in how many directions?

• B) Biaxially (axial stretching by rod + radial expansion by air)

Q3. The Blow-Up Ratio (BUR) is mathematically defined as the ratio of:

• A) Mould diameter to parison diameter

Q4. Which polymer is the standard material for manufacturing carbonated soft drink bottles via stretch blow moulding?

• B) Polyethylene Terephthalate (PET)

Q5. Which Indian standard governs PET bottles used for packaging drinking water?

• C) IS 12257