Additive manufacturing MECH3775

What is Photolithography?

Photolithography is a process that uses light to transfer a geometric pattern from a photomask to a light sensitive chemical called a photoresist.

What are some manufactured electronics photolithography can be used to make?

• semiconductors

• MEMS (micro-electrical-mechanical systems)

• PCBs

• LCD and OLED displays

What was the original SLA process?

• used a uv wavelength laser

• the laser scans the liquid surface and causes the liquid to polymerise

• the platform drops and layer and repeats

What is projection based SLA?

• typically uses a DLP but LCD are now emerging 

• image is projected from below and polymerises a layer at a time

• the part is moved up and this repeats

What is LCD SLA?

Uses a LCD screen to mask regions of light

• the systems can be low cost

What is SLA? (method of patterning with light)

What is DLP? (method of patterning with light)

What is LCD? (method of patterning with light)

What are the advantages of top down projection methods?

• resolution of the part is higher

• the current later only adheres to the previous layer

  • simplicity for retrofitting

What are the Disadvantages of top down projection methods?

• surface characteristics of the resin can affect the part quality

• the process is usually longer due to the re coating step

• viscosity of the resin determines the thickness of the part

What are the advantages of bottom up projection methods?

• so specific re coating stage needed

• ultra thin layers can be produced

• reduce of waste materials

What are the disadvantages of bottom up projecting?

• light has to pass through more surfaces causing aberrations and less intensity

• the current layer may adhere to the surface of the vat (damaging the part)

• harder to retrofit components

what are the materials used for Stereolithography?

• light curable resins

• resins consist of a monomer, photo initiator and an inhibitor 

How does photopolymerisation work?

• Photopolymerisation is initiated by excitement of the photoinitiator by UV radiation which produces free radicals

• Photopolymerisation causes the linking of small molecules (monomers) to larger chain like molecules (polymers)

Why do the materials used for Photopolymerisation contain additives and what are they?

to try to simulate common engineering materials

they are all based on epoxy/acrylic formulations of non-polymerised liquid

some can change the colour/improve stability/are used for specialised applications

What is post-processing of materials for Photopolymerisation?

• ensures complete polymerisation 

• ensures complete cross linking of polymer bonds

• more stable and integrity 

• curing time depends on mass and material

What are some positives of the post processing for Photopolymerisation?

• supports can be easier to remove before final cure

• materials can mimic traditional engineering polymers

• process energy input low compared to thermal AM systems

• Very accurate / very good surface finish

• parts can be mechanically/chemically smoothed

What are some negatives of the post processing for Photopolymerisation?

• the machine is expensive

• support structures are always required 

• stereolithogrpahy resins are often toxic (PPE is needed)

• materials must be photo-curable resins

• parts age in the sunlight

• material change requires thorough cleaning

• part orientation affects surface finish