Blow Molding: Inflating a Plastic Bottle in the Blink of an Eye

How a Preform Becomes a Bottle, Step by Step

Nearly every PET drinks bottle is made by two-stage injection stretch blow molding (ISBM).

• Injection-mold the preform. Dried PET is melted at ~280 °C and injected into a steel mold to make a thick, test-tube-shaped preform. Its screw thread and support ring are already final. The preform is chilled fast so it stays amorphous and glass-clear. Preforms ship compactly and are often blown beside the filling line.
• Reheat. Spinning on mandrels, preforms pass banks of near-infrared quartz lamps that heat the body to ~95–115 °C, above PET's ~70–80 °C glass transition, so the chains can slide past one another. Lamps at each height are set separately to build a vertical temperature profile, fans cool the skin so the surface does not overheat, and a cooled shield protects the thread. Too hot, and PET starts crystallizing milky-white.
• Close and seal. A split aluminum mold (two side halves plus a base insert, all water-cooled) closes around the preform, and a blow nozzle seals on the neck.
• Stretch and pre-blow. A stretch rod drives down at ~1 m/s, pinning the preform's tip to the center of the base and stretching it ~2.5–3× lengthwise. Pre-blow air at ~5–15 bar follows within moments. A bubble typically bursts out just below the neck and runs down toward the base.
• High-pressure blow and hold. Air at ~25–40 bar expands the film onto the cold mold wall and into the petaloid feet in ~0.2 s, then holds it there until the plastic is rigid.
• Exhaust and eject. The air is vented, part of it recovered for reuse, the mold opens and the bottle leaves.

The Principle: Strain Hardening Makes the Wall Level Itself

Inflating a thin tube is naturally unstable. For a thin cylinder of radius r and wall thickness t, pressure P produces a hoop stress σ = P·r / t. Plastic keeps its volume as it stretches, so a tube stretched λa times lengthwise (axial) and λh times around (hoop) ends with wall t = t₀ / (λa·λh) and radius r = r₀·λh. Rearranged, the pressure a patch of wall can resist is

P = σ · (t₀ / r₀) / (λh² · λa)

If the material's stress stayed constant while stretching, a spot that bulged slightly would need less pressure to keep bulging: it would thin, bulge further and burst. A party balloon shows the same effect, hardest to start and then suddenly easy. Hot PET escapes this runaway through strain hardening. Its long chains start as random coils; stretching uncoils them easily at first, but once they are pulled nearly straight, and begin packing into tiny stretch-aligned crystals, the stress needed to stretch further climbs steeply. A bulged region stiffens and stops thinning, so the pressure deforms softer, unstretched neighbours instead, and the deformation front travels along the preform until the whole wall has reached a similar stretch.

Engineers call the stretch where hardening takes over the natural stretch ratio, and they design preforms so every part of the bottle is stretched beyond it; understretched walls are left with thick and thin bands. The payoff is biaxial orientation: chains aligned both along and around the bottle, with roughly 20–30% strain-induced crystallinity in crystals too small to scatter light. The wall stays clear, becomes several times stronger than unoriented PET and lets CO₂ through more slowly.

Worked Numbers: Thinning, Hoop Stress, Mold Force and Cooling

Wall thinning. Take an illustrative preform with a ~3 mm wall and a mean radius of ~10 mm. Stretch it 2.5× lengthwise and 4× around: the area grows 2.5 × 4 = 10×, so by volume conservation the wall becomes 3 mm ÷ 10 = 0.3 mm. At 3× and 4.5× the area grows 13.5× and the wall falls to ~0.22 mm. In the pressure relation, λh²·λa = 4² × 2.5 = 40: at the same pressure the finished film carries 40 times the stress the preform wall did. Only a steeply hardening material survives that without a blow-out.

Why the stretch is larger around than along. In a closed pressurized cylinder the hoop stress P·r/t is twice the axial stress P·r/(2t). For a 500 mL bottle (r ≈ 32.5 mm, t ≈ 0.3 mm) at a room-temperature soda pressure of ~4 bar (0.4 MPa), σ ≈ 0.4 × 32.5 ÷ 0.3 ≈ 43 MPa around and ~22 MPa along. At 100 psig (≈6.9 bar) the hoop stress reaches ≈75 MPa, above the ~55 MPa yield strength of unoriented amorphous PET but well below the ~200 MPa that biaxially oriented PET film reaches. Stretching ~4–5× around versus ~2.5–3× along puts more chain alignment where the load is highest.

Mold clamping. A 500 mL cavity has a projected area on its parting line of roughly 65 mm × 200 mm ≈ 0.013 m². At 40 bar (4 MPa) the air pushes the halves apart with 4 × 10⁶ Pa × 0.013 m² ≈ 52 kN, over 5 tonnes-force, every cycle.

Cooling. Heat crosses a slab on a time scale t ≈ L²/α. With PET's thermal diffusivity α ≈ 1.2 × 10⁻⁷ m²/s, a 0.3 mm wall gives (3 × 10⁻⁴ m)² ÷ (1.2 × 10⁻⁷ m²/s) ≈ 0.75 s, the same order as the ~1.3 s cycle. The 3 mm preform wall, ten times thicker, needs a hundred times longer, ≈75 s. That is why ovens use penetrating near-infrared rather than waiting for heat to soak in from the surface.

Real Machines, Molds and Specifications

High-output bottles come from rotary stretch blow molders such as the Krones Contiform, Sidel EvoBLOW and KHS InnoPET Blomax. Krones' Contiform 3 Speed (2018) makes 2,750 bottles per mold cavity per hour, one every ~1.3 s, and with 30 cavities up to 82,500 bottles per hour.

• Resin. Bottle-grade PET is specified by intrinsic viscosity (IV), a proxy for chain length: roughly 0.8 dL/g for carbonated soft drinks and somewhat lower for still water. Higher-IV resin strain-hardens at a lower stretch. PET must be dried to below ~50 ppm water before injection, or the melt hydrolyses and loses chain length.
• Preform and finish. The preform's length, diameter and wall set the stretch ratios. A 500 mL soda bottle typically uses a ~20 g preform; a lightweight still-water bottle can be under ~10 g. The common soda thread is the 28 mm PCO 1881 finish.
• Molds. Blow molds are usually aluminum alloy with chilled-water channels and fine vents that let trapped air escape as the film arrives. Many machines feed blow air into a compensation chamber behind the mold shell, so the pressure trying to open the mold also holds it shut.
• Air. A 500 mL bottle at 40 bar gauge holds about 0.5 L × 41 ≈ 20 L of free air. At 82,500 bottles per hour that is ~1,700 m³ of free air per hour, so blowers recycle blow air into pre-blow and factory air lines.
• Hot-fill bottles. Juice filled at ~85 °C needs heat-set bottles blown in heated molds, usually with a crystallized, white neck.

How Bottles Are Tested and Measured

Brand owners and the International Society of Beverage Technologists (ISBT) publish test methods; the core checks are:

• Material distribution. Section-weight testing cuts a bottle into horizontal bands at set heights with a hot wire and weighs each band; a heavy shoulder means a starved base. Hall-effect gauges, with a steel ball inside the bottle and a magnetic probe outside, map wall thickness point by point, and in-line infrared systems check every bottle.
• Burst and top load. Hydrostatic testers ramp water pressure until the bottle bursts; carbonated-drink bottles commonly survive above ~10 bar (~150 psi). Top-load testers crush bottles vertically to check capping and pallet stacking.
• Thermal stability. Bottles filled with carbonated water at ~4 volumes of CO₂ are stored warm, often 38 °C for 24 h, then height, diameter, base clearance and fill level are re-measured to catch creep and base roll-out.
• Stress cracking. Pressurized bottles stand with their bases in dilute sodium hydroxide, mimicking alkaline conveyor lubricants, and the time to crack is logged.
• Orientation and barrier. Between crossed polarizers, orientation and residual stress show as colored fringes. CO₂ shelf life is the time to lose a set share of carbonation, commonly ~15–17.5%; small bottles get there sooner because they have more wall per litre.

History: From a Swollen Detergent Bottle to the Petaloid Foot

Glassblowers inflated molten glass about two thousand years ago, and machines automated glass bottles in the early 1900s. Plastic extrusion blow molding arrived in the 1930s and 1940s, later giving HDPE milk jugs. None of these plastic bottles could hold soda: unoriented walls crept, bulged and leaked CO₂.

At DuPont, engineer Nathaniel C. Wyeth, brother of the painter Andrew Wyeth, took on the problem in the late 1960s. By his own account, a plastic detergent bottle he filled with ginger ale swelled overnight in his refrigerator until it was wedged between the shelves. Trials showed that stretching PET in both directions gave a clear, strong, fizz-tight wall. US Patent 3,733,309, “Biaxially oriented poly(ethylene terephthalate) bottle,” with co-inventor Ronald Roseveare, was granted on 15 May 1973 and described a bottle able to hold liquids at up to ~100 psig at about 50 °C.

PET's main rival stumbled: Monsanto's acrylonitrile-copolymer soda bottle, used by Coca-Cola in the mid-1970s, was knocked out when the US FDA banned acrylonitrile beverage bottles in 1977 over concerns that residual monomer could migrate into drinks. Early 2-litre PET bottles had a round, pressure-friendly bottom plus a glued-on polyethylene base cup to stand on. The one-piece petaloid base, usually with five feet, later let the bottle stand by itself.

Failure Modes, Misconceptions and Look-Alikes

• Pearlescence. A preform blown too cold is stretched faster than its chains can follow; microscopic voids open and the wall turns silvery-white and weak.
• Haze. Too hot or held hot too long, PET crystallizes into spherulites big enough to scatter light, leaving a milky zone that resists stretching.
• Poor distribution. Wrong pre-blow timing or a misaligned rod leaves thick shoulders, a thin heel, an off-center gate or half-formed feet.
• Base stress cracking. The center of the base, pinned by the rod, stretches least and stays thick and weakly oriented. Under pressure and alkaline lubricants, cracks start there and bottles leak or burst in storage.
• Creep. A carbonated bottle grows a few percent in volume over its first days under pressure, lowering the fill line and the fizz.

A common misconception is that the bottle is blown from molten plastic. In ISBM the preform is a rubbery solid far below PET's ~250 °C melting point, which is exactly why it can be oriented. Pour boiling water into an ordinary PET bottle and that frozen-in stretch relaxes, so the bottle shrinks and warps.

Blow molding is most often confused with injection molding, which in bottle-making only makes the preform, and with extrusion blow molding, which inflates a pinched, hanging melt tube (a parison) at low pressure into HDPE milk jugs with little orientation.

Blow molding compared with the processes it is most often confused with

Process	What goes into the mold	Typical products	Key difference
Two-stage injection stretch blow molding	Injection-molded PET preform reheated to ~95–115 °C, rod-stretched and blown at ~25–40 bar	Soda and water bottles	Biaxial orientation gives strength and CO₂ barrier; thread is finished beforehand
Single-stage injection stretch blow molding	Preform blown while still warm from its own injection mold	Cosmetic jars, wide-mouth and short-run containers	One machine and no reheat oven; slower, but no preform storage
Extrusion blow molding	Hanging tube of molten plastic (parison) at ~200 °C, pinched shut and blown at low pressure	HDPE milk jugs, detergent bottles, fuel tanks	Handles possible; pinch-off seam and trimmed flash; little orientation
Injection molding	Molten plastic forced into a closed steel mold at hundreds of bar or more	Preforms, caps, solid parts	Makes solid or open-ended shapes whose core can be pulled out; cannot make a narrow-neck hollow bottle in one piece
Thermoforming (vacuum forming)	Heated plastic sheet pulled onto a one-sided mold by vacuum	Cups, trays, blister packs	Open shapes only; pressure difference under 1 bar
Glass bottle forming (IS machine)	Gob of molten glass at ~1,100 °C, shaped in a blank mold, then blown	Glass bottles and jars	Viscous melt formed hot; walls typically a few millimetres thick