Polymers- Additive Manufacturing, Compression and Transfer Moulding, Other Processing Flashcards
How does additive manufacturing generally work?
Also known as 3D printing. Successive layers of plastic are laid down to build a 3D object from a CAD model
What is additive manufacturing used for?
Making parts that can’t be made using moulding or subtractive manufacturing. Mostly used for prototypes or niche applications. Developing technology and future very promising
Features of things made using additive manufacturing?
Medium sized parts (max 2x1x1m). Aim for properties similar to injection moulded parts. Accuracy and surface finish vary.
Features of general additive manufacturing process
Average speed but increasing. Machine and material prices falling
Stereolithography (SLA)
Type of AM. Based on photo-polymerisation. Layers formed by using a UV laser to cure a liquid photopolymer. Good mechanical properties and resolution. Support systems for overhanging parts are difficult. Used for acrylics and other thermoplastics
Fused deposition modelling (FDM)
Type of AM. Based on extrusion deposition. Layers formed by depositing a molten plastic filament. Relatively cheap and can co-extrude support systems. Poor z direction properties and surface finish. Used for ABS and other thermoplastics. Very cheap machines
Selective laser sintering (SLS)
Type of AM. Based on binding of granular materials. Layers formed by using a laser to sinter polymer powder. Good mechanical properties. Support systems not needed due to surrounding powder bed. Poor surface finish. Used for nylon-12 and other thermoplastics.
Jetted photopolymer
Type of AM. Based on inkjet printing and photo-polymerisation. Layers formed by printing layers and then curing with a UV lamp. Good mechanical properties and resolution. Support systems can be printed. Relatively expensive. Used for acrylics and waxes
What is compression moulding?
Where pre-heated plastic charge is forced into shape using two matched dies in a hot press
Features of compression moulding and what it’s used for
Can be applied to thermoset, thermoplastic and elastomeric polymers. Similar to injection-compression moulding. Mostly for making thicker gauge parts. Charge is preformed thermoset/elastomer or plasticised thermoplastic. Thermosets used like phenolics and epoxies. Elastomers used like rubbers and silicones. Difficult thermoplastics like PVC and PTFE
Flash with compression moulding
Overcharging of mould is common so flash is inevitable. More flash in more complicated moulds. Pressure applied to ensure complete filling of mould
3 different mould designs to compression moulding
Open flash: pressure applied via top plate onto charge, pressure lost when plate contacts the flash lands leading to poor definition and shrinkage and porosity issues.
Positive: no flash accommodation so need charge volume exactly right, thickness dependent on charge volume and pressure.
Semi-positive: has a spew angle above where final part will finish, allows application of pressure and flash to form, costs more
Why might transfer moulding be preferred over compression moulding?
Uniform heating and cure difficult to achieve in compression moulding
How does transfer moulding work?
The plastic charge is softened in a pre-heating chamber before being forced into the mould by a ram. Like injection moulding
Advantages of transfer over compression moulding
Forcing polymer through restrictions (runners and gates) improves heat and cure uniformity through shear heating allowing larger parts to be moulded. Relatively high pressures exerted lead to increased flow rates and good moulding definition. No flash is required
Disadvantages of transfer over compression moulding
Moulds are more complicated so more expensive to machine. Wastage from the sprue and runners can’t be recycled with thermoset polymers
How do different types of liquid polymer processing work?
Low Mr monomers or pre-polymers: react to form high Mr solid polymers
High Mr solutions or dispersions: evaporate solvent to leave solid polymer
PVC or plastisol: PVC powder dispersed in a liquid plasticiser, heat caused powder and plasticiser to fuse to form solid plasticised PVC
Rubber latex: rubber powder in aqueous dispersion, thermal or chemical activation creates fused vulcanised solid rubber
Advantages of liquid polymer processing
Lower viscosity so lower processing pressures required and better filling of moulds and wetting of substrates or reinforcements
How does powder polymer processing work?
Finely divided powders can be fluidised in a carrier gas and processed as a fluid
Polymer casting for thermosets and thermoplastics
Thermosets: pour pre-mixed reactive monomers or oligomers into a mould to complete polymerisation, may require chemical and/or thermal activation.
Thermoplastics: pour molten polymer into mould and cool, some parts can’t be made another way
What casting is used for
Prototypes and large, thick sections. There are low volume runs as less automated. Thermosets, thermoplastics or elastomers
Casting PU foams
Continuous reactive casting or slabstock process. Creates foams in a huge trough where reactants expand to up to 50 times their original volume
Why might you cast nylon?
For large or thick parts from caprolactam flakes. Better stiffness and solvent resistance, etc
Why might you cast acrylics?
PMMA sheets and tubes. Superior optical properties to those produced by extrusion
How are soft contact lenses made?
Acrylic elastomer (transparent and rubbery) spun on centrifugal cast. Higher speed means thinner lens
Slush moulding
Fill heated hollow mould with liquid polymer, allow skin to form on internal surface of mould and pour out excess. Used for plasticised PVC and rubber toys, balls. Similar to rotational moulding but mould not completely enclosed
Dip moulding
Dip a heated, positive former into liquid polymer, solidify and remove. For balloons and gloves. Don’t have to remove so can have plastic coating like for tool grips
Polymer sintering
Fusing together solid polymer powder under heat and high pressure. Used for PTFE or UHMWPE which have very high melt viscosity and can’t otherwise be processed.
How does polymer coating work?
Add a layer of liquid polymer to a substrate which solidifies to form a coating.
Roll coating applies a layer of liquid coating from a bank by means of rubber backed roller.
Knife coating applies a layer of liquid coating by means of doctor blade setting the thickness of the coating (slides under it)
Substrates for and functions of polymer coating
Substrates: metal foil, paper, cardboard, polymer films and sheets from sheet extrusion or calendering
Functions: protection (water proof, anti-scratch, anti-corrosion), adhesives (stick and non-stick), optical (reflective or anti-reflective)
Spray coating
Spray atomised plastisols and solutions or fluidised powders onto substrate. Paints often epoxy based for good properties. Heat or dry to solidify. Non-uniform thickness
Electrostatic spray coating
Spray charged thermoplastic or thermoset powders towards earthed substrate. Heat to form continuous coating then solidify (heat for thermoset, cool for thermoplastics). Uniform thickness and very efficient. Used for white goods (fridges, washing machines), bicycles, furniture, fencing
Uses of plastic welding
Joing and repairing
Methods of plastic welding
Hot gas welding: melt two ends to be joined along with filler material
Speed tip welding: similar to soldering iron with a feeder for the filler material
Heat sealing: direct application of heated element
High frequency or ultrasonic welding: vibrate polymers to cause friction and melting
Laser: hold parts together and pass laser along the join
Solvent: dissolve edges of each part and press together
Guide to polymer processing choices
See slide 19 lecture 6
