In extrusion blow molding the parison is formed by forcing molten plastic through an annular orifice in a die that is part of the die head assembly. The orifice is formed by the space between the mandrel and the die. Extrusion may be directly from an extruder, or for large parts for which more material is needed than the extruder can continuously provide an accululator is used.
The parison is extruded and drops to between the mold halves and when the mold closes the parison is sealed. Air injected into the parison inflates it to the shape of the mold cavity. After cooling and solidification the mold is opened and the part removed.
||Step 1. Parison Extrusion. Parison dented by arrow
|Step 2. Mold Halves close onto parison.
When the parison has reached a sufficient length a hollow mould is closed around it. The mould mates closely at its bottom edge thus forming a seal. The parison is cut at the top by a knife prior to the mould being moved sideways to a second position where air is blown into the parison to inflate it to the shape of the mould.
|Step 3. Parison inflated against inflated internal mold walls.
The parison is cut at the top by a knife prior to the mould being moved sideways to a second position where air is blown into the parison to inflate it to the shape of the mould.
||Step 4. Mold halves open, Blow molding part ready for ejection
After a cooling period the mould is opened and the final article is ejected. To speed production several identical moulds may be fed in cycle by the same extruder unit.
This process usually use commodity materials such as:
- Polypropylene PP
- Polyethylene PE
- Polyethylene – Terephthalate PET
- Polyvinyl chloride PVC
Important factors one should consider for extrusion blow molding include the following:
- Polymer viscosity at high & low shear rates
- Melt strength (important for uniform wall thickness, no holes)
- Strain recovery (MW & Distribution)
- Crystallization rate (slow rate desired)
- Thermal properties (thermal diffusitivity, thermal conductivity, specific heat, etc.)
Advantages of Extrusion Blow Molding:
- Low initial mold tooling costs.
- Flexibility of tooling. Molds can accommodate interchangeable neck finishes and body sections.
- Flexibility in production: Neck inner diameters (I.D.) can be easily controlled to varying requirements. Bottle weights are adjustable.
- Container sizes can range from less than 1 oz. to 55 gallons and up. (Custom Bottle’s equipment is most efficient producing containers up to 1 liter in capacity.)
- Container shape is not restricted by blow-up ratios. Bottles can be long and flat or have handles.
- Wide selection of machine sizes: Molds can be geared to volume requirements.
- Bottles and containers
- Automotive fuel tanks
- Venting ducts
- Watering cans
- Boat fenders etc