Polymers- Additive Manufacturing, Compression and Transfer Moulding, Other Processing

Question 1

How does additive manufacturing generally work?

Answer 1

Also known as 3D printing. Successive layers of plastic are laid down to build a 3D object from a CAD model

Question 2

What is additive manufacturing used for?

Answer 2

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

Question 3

Features of things made using additive manufacturing?

Answer 3

Medium sized parts (max 2x1x1m). Aim for properties similar to injection moulded parts. Accuracy and surface finish vary.

Question 4

Features of general additive manufacturing process

Answer 4

Average speed but increasing. Machine and material prices falling

Question 5

Stereolithography (SLA)

Answer 5

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

Question 6

Fused deposition modelling (FDM)

Answer 6

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

Question 7

Selective laser sintering (SLS)

Answer 7

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.

Question 8

Jetted photopolymer

Answer 8

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

Question 9

What is compression moulding?

Answer 9

Where pre-heated plastic charge is forced into shape using two matched dies in a hot press

Question 10

Features of compression moulding and what it’s used for

Answer 10

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

Question 11

Flash with compression moulding

Answer 11

Overcharging of mould is common so flash is inevitable. More flash in more complicated moulds. Pressure applied to ensure complete filling of mould

Question 12

3 different mould designs to compression moulding

Answer 12

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

Question 13

Why might transfer moulding be preferred over compression moulding?

Answer 13

Uniform heating and cure difficult to achieve in compression moulding

Question 14

How does transfer moulding work?

Answer 14

The plastic charge is softened in a pre-heating chamber before being forced into the mould by a ram. Like injection moulding

Question 15

Advantages of transfer over compression moulding

Answer 15

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

Question 16

Disadvantages of transfer over compression moulding

Answer 16

Moulds are more complicated so more expensive to machine. Wastage from the sprue and runners can’t be recycled with thermoset polymers

Question 17

How do different types of liquid polymer processing work?

Answer 17

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

Question 18

Advantages of liquid polymer processing

Answer 18

Lower viscosity so lower processing pressures required and better filling of moulds and wetting of substrates or reinforcements

Question 19

How does powder polymer processing work?

Answer 19

Finely divided powders can be fluidised in a carrier gas and processed as a fluid

Question 20

Polymer casting for thermosets and thermoplastics

Answer 20

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

Question 21

What casting is used for

Answer 21

Prototypes and large, thick sections. There are low volume runs as less automated. Thermosets, thermoplastics or elastomers

Question 22

Casting PU foams

Answer 22

Continuous reactive casting or slabstock process. Creates foams in a huge trough where reactants expand to up to 50 times their original volume

Question 23

Why might you cast nylon?

Answer 23

For large or thick parts from caprolactam flakes. Better stiffness and solvent resistance, etc

Question 24

Why might you cast acrylics?

Answer 24

PMMA sheets and tubes. Superior optical properties to those produced by extrusion

Question 25

How are soft contact lenses made?

Answer 25

Acrylic elastomer (transparent and rubbery) spun on centrifugal cast. Higher speed means thinner lens

Question 26

Slush moulding

Answer 26

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

Question 27

Dip moulding

Answer 27

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

Question 28

Polymer sintering

Answer 28

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.

Question 29

How does polymer coating work?

Answer 29

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)

Question 30

Substrates for and functions of polymer coating

Answer 30

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)

Question 31

Spray coating

Answer 31

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

Question 32

Electrostatic spray coating

Answer 32

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

Question 33

Uses of plastic welding

Answer 33

Joing and repairing

Question 34

Methods of plastic welding

Answer 34

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

Question 35

Guide to polymer processing choices

Answer 35

See slide 19 lecture 6