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.
Raw Materials
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.
Applications
- Bottles and containers
- Automotive fuel tanks
- Venting ducts
- Watering cans
- Boat fenders etc
In Extrusion Blow Molding (EBM), plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected.
EBM processes may be either continuous (constant extrusion of the parison) or intermittent. Types of EBM equipment may be categorized as follows:
Continuous Extrusion Equipment
- rotary wheel blow molding systems
- shuttle machinery
Intermittent Extrusion Machinery
- reciprocating screw machinery
- accumulator head machinery
Examples of parts made by the EBM process include dairy containers, shampoo bottles, hoses/pipes, and hollow industrial parts such as drums.
Basic polymers, such as PP, HDPE, PVC and PET are increasingly being coextruded with high barrier resins, such as EVOH or Nylon, to provide permeation resistance to water, oxygen, CO2 or other substances. In dairy applications, it is possible to extrude a black light-blocking layer in the center layer of containers, with opaque white resin used in the inner and outer layers.
Compared to injection molding, blow molding is a low pressure process, with typical blow air pressures of 25 to 150 psi. This low pressure process allows the production of economical low-force clamping stations, while parts can still be produced with surface finishes ranging from high gloss to textured. The resulting low stresses in the molded parts also help make the containers resistant to strain and environmental stress cracking.
