Blow moulding Flashcards
1
Q
Main steps
A
- Melting the polymeric material
- Forming a preform tube
- Blowing the preform tube into the shape desired
2
Q
Process characteristics
A
- generally aluminium
- low pressure process
- tooling relatively inexpensive
- short lead times
3
Q
Injection blow moulding stages
A
- melted plastic injected into split mould cavity, forms a pre-parison with a screw finish on the top
- parison transferred on a core rod to another blow mould cavity, air injected till it takes the shape of cavity
- still on the rod, container is transferred then ejected from the machine
- parison can be stretched downwards before inflating
4
Q
Extrusion blow moulding
A
- Material fed into heated barrel of extruder
- Screw rotation and heaters homogenise the melted plastic
- Melted material forced through a die to form a parison
- Blow moulding as usual
5
Q
Co-extrusion blow moulding
A
- allows for products to have different properties on each side
- reduced tendency for the products to delaminate
- polymers should have similar rheological characteristics to avoid curl while cooling
- lower viscosity material should be on the outer layers
6
Q
RAM accumulator
A
- recommended for larger parts
- size of extruder is independant of the accumulator size
- first material is last to leave, cant be heat-sensitive materials
7
Q
Mould features
A
- neck ring insert
- mould guide pins
- cooling channels
- mold cavity
- pinch-off insert
- flash pocket
8
Q
Parison geometry
A
- dimensions will be greater than the die due to swelling
- gravity causes the parison to sag, thinning parts of the parison
- bottom becomes heavier, top experiences necking
9
Q
Parison faults & Causes
A
- Too long: excess extrusion output, melt too fluid
- Diameter too large: incorrect die selections, excess die swell
- Wall too thin: incorrect thickness controller, excess sag
- Curl inwards: die temperature too high
- Sharkskin: die head temp too low, melt temp too low
