Processing Plastics Flashcards
(20 cards)
Describe injection moulding
1 - plastic granules - (additives and colouring) - in hopper - mix falls through - on Archimedean screw
2 - screw rotated - motor and gearbox - polymer forced forward towards heaters - softened - ready to be injected into mould
3 - hydraulic ram - force softened polymer through feed hole - to mould - pressure ensures cavity is filled
4 - time passed - polymer cools and solidifies - seconds - mould halves opened - ejector pins activated - take product out
List plastic processing techniques (9)
Injection moulding Blow moulding Rotational moulding Thermoforming Vacuum forming Extrusion Calendaring Line bending Compression moulding
Advantages of injection moulding (3)
- Make very complex 3 D shapes
- high volumes produced - consistent quality
- metal inserts included in item
Disadvantages of injection moulding (2)
- initial set up - costs high
- moulds - expensive
Define injection moulding
Method of processing thermoplastic material - produce complex 3D shapes
Define blow moulding
- Process of manufacturing thermoplastic materials into re-entrant shapes - single entrance
- e.g soft drinks bottles
- objects usually hollow , narrow neck
Describe blow moulding
1 - tube of heated and softened polymer - extruded vertically downwards - tube called parison
2 - mould halves close - trap upper end of parison - seal it
3 - hot air blown into parison- forces it out to shape of mould
4 - mould cools polymer - can be released from mould - halves opened - product removed
Advantages of blow moulding (2)
- once set up - rapid method - producing hollow objects - narrow necks
- non circular shapes - produced
Disadvantages of blow moulding (3)
- moulds expensive
- difficult to produce re - entrant shapes i.e. shapes do not allow easy extraction from mould - dovetail joint
- triangular shaped bottles - difficult to produce
What is a parison?
- Extruded tube of thermoplastic material
- used in process of blow moulding
Define rotational moulding
- Method of moulding thermoplastic polymers
- Involve plastics in powder form - take shape of fully enclosed mould as mould passes through heated section
- Material becomes rigid as mould cooled
- machine - number of arms - rotate around fixed central point - moulds on each arm - rotated continuously - do not rotate at starting point and end point
Describe rotational moulding
1- moulds loaded - precise weight of thermoplastic powder - eg polyethylene - mould halves clamped
2- moulds rotated about arm spindle - whole arm rotated towards heated chamber - thermoplastic material heated to melting point - continuous rotation ensures thermoplastic covers mould
3- cooling chamber - material cooled - extracted from mould
4- mould returned to starting point - halves separated - product removed
Advantages of Rotational Moulding (6)
- One piece mouldings can be produced
- Ideal for rigid, tough, and flexible shapes
- Large range of sizes possible - small medical components to storage tanks
- Surface textures applied - finished products or textures in mould
- Moulds cheaper - than injection or blow moulding as high pressures not needed - also means lower production runs
Disadvantages of Rotational Moulding (1)
- Only hollow shapes made - complex 3D shapes blow or injection moulded
What does vacuum forming do in comparison to thermoforming ?
A vacuum ‘pulls’ the softened polymer down and around a mould
- Thermoforming uses an outer mould in the process
- greater level of detail e.g. Lettering, symbols etc
Describe thermoforming
- material heated - just above softening point
- held securely in frame between two mould halves
- halves close
- vacuum applied through lower mould
- upper mould - adds detail
Advantages and disadvantages of thermoforming
5
- low cost process
- good - smooth shapes + detail
- deep moulds - thinning of wall thickness - stretched
- limited simple designs
- trimming needed
Describe extrusion
1 - thermoplastic powder - in hopper - falls - Archimedean screw - pushes material towards heated end of extruded
2 - softens plastic - forced through die - by rotating screw
3 - cooled using water jet
4 - further down transfer table - cut to length
What is extrusion used for ? How are wires insulated using this technique?
Used to create products with continuous cross sections - required
Wires can be insulated - special mandrel arrangement - allows wire to pass through
Advantages and disadvantages of extrusion (2)
- low cost process - requires simple dies
- only produces continuous cross sectional shapes
