Blow molding forms a hollow container by inflating a soft, hot hollow blank inside a mould until it takes the shape of the mould wall, and the pressure needed is low but the force it puts on the mould is not: an assumed 0.5 bar gauge acting on the 0.016 m² parting area of an 80 mm x 200 mm bottle mould pushes the halves apart with 800 N. Use a regulator and a relief valve on the air, and clamp the mould shut.
Part of Pneumatic Systems
Blow molding makes bottles, jars and hollow containers from glass or plastic by blowing air into a heated, softened blank held inside a mould.
A Soft Blank, a Mould and a Little Air
The method is the same for glass and plastic:
- Heat the material until it is soft enough to stretch but still holds its shape; this is a parison, a closed tube or bubble.
- Close a mould around it and seal the open end.
- Blow air in. The parison swells until it presses against the cold mould wall and freezes in the mould’s shape.
- Open the mould, release the bottle, let it cool, and for glass anneal it.
Hot glass works at 900-1,050 °C for soda-lime glass (Glass Blowing and Forming); the plastics article gives melting ranges for polyethylene, 120-180 °C, and polypropylene, 160-220 °C (Plastics Manufacturing). The corpus does not give the pressure for a given material, so find it by trial, starting low.
Low Pressure Still Makes a Large Force
Pressure times projected area is the force on the mould halves. Worked example, with an assumed pressure for illustration:
- Bottle mould 80 mm wide and 200 mm tall: projected area on the parting plane = 0.08 x 0.2 = 0.016 m²
- Assumed blow pressure 0.5 bar gauge = 50,000 Pa
- Force = 50,000 x 0.016 = 800 N, the weight of about 82 kg
The 0.5 bar figure is not a recommendation; it shows that a modest pressure on a big surface needs a firm clamp. Doubling the pressure doubles the force, and a larger bottle raises it in proportion.
The Air Supply Is Simple but Must Be Controlled
- A regulator or a restricted supply so the pressure rises slowly and cannot go above what the blank and mould can stand.
- A relief valve on the blowing line.
- Clean, dry air so water or oil does not spit into a hot parison; see Moisture Removal.
- A vent in the mould, where air trapped between blank and mould can escape, or the part will have dimples.
Compressed air does not need much volume. A 0.5 L bottle inflated to 0.5 bar gauge takes 0.5 x 1.5 = 0.75 L of free air, which a mouth, a bellows or a small tank can supply. The gain from compressed air is control and a cleaner, steadier supply, not extra power. See Glass Blowing with Compressed Air and Plastic Blow Molding.
Common Mistakes
- Too little clamp force. The mould halves open at the seam and the blank flashes out.
- Pressure applied in one jerk. The parison thins at one spot and bursts.
- Cold or uneven blank. It tears instead of stretching; heat evenly.
Safety
A burst parison throws hot material
Overpressure in a hot blank can burst it and spray molten glass or hot plastic. Wear eye and face protection and heat-proof gloves, work behind a shield if you can, and limit the air supply with a regulator and relief valve. Hot glass and hot plastic burn on contact. Burning plastic gives off toxic smoke; the plastics article says never to burn PVC and to keep processing ventilated.
What’s Next
- Learn next: Glass Blowing with Compressed Air — a bottle shop with an air line
- Combine with: Plastic Blow Molding — containers from thermoplastic