VAT Polymerisation - Summary of additive manufacturing process PDF

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Summary

Summary of VAT Polymerisation, a type of additive manufacturing process. Includes background VAT Polymerisation information, types of polymerisation and example applications...

Description

VAT photopolymerization There are a set of newly formulated standards (ASTM-F42) that define the range of additive manufacturing processes into 7 categories: 1. Material extrusion- A material is selectively dispensed through a nozzle or orifice. 2. Vat photopolymerization - Liquid photopolymer in a vat is selectively cured by light-activated polymerization. 3. Powder bed fusion - thermal energy selectively fuses regions of a powder bed. 4. Material jetting - Droplets of build material are selectively deposited. 5. Binder jetting - A liquid bonding agent is selectively deposited to join powder materials. 6. Directed energy deposition - Focused thermal energy is used to fuse materials by melting as the material is being deposited. 7. Sheet lamination - Sheets of material are bonded to form an object. Vat Photopolymerisation Background This category produced the first of the AM processes • • • • •

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Chuck Hull invented and patented a process called Stereolithography in 1986. He founded a company called 3D Systems Corporation to refine and sell this technology. First commercial machine launched in 1988. Now one of the biggest AM companies in the world with a multitude of technologies and purchased companies. Stereolithography is not the only vat polymerisation based AM system (MicroStereolithography, Two-photon Polymerization, Continuous Liquid Interface Production) Name coined by Chuck Hull Didn’t trademark name so now a common term for all photo-curable resin based processes Stereo – means “three-dimensional” Lithography – comes from the Greek lithos “stone” + graphein “to-write” – means “method of printing” Originally based on the principle that oil and water do not mix Invented by 1796 by German author and actor Alois Senefelder as a cheap method of publishing theatrical works Lithography is used to print text or artwork onto paper or other suitable material Lithography stone and mirror image print of a map of Munich

What is photolithography? •

VAT polymerisation has it’s roots in photolithography

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

Photolithography • • • • •

Photolithography is used extensively for the manufacture of electronics Semiconductors MEMS (Micro-Electrical-Mechanical Systems) Printed Circuit Boards LCD and OLED displays

Process Description – Original SLA process • •

Laser based photopolymerisation uses a UV wavelength laser (typically around 350 nm) The laser scans the liquid surface and causes the liquid to polymerise. The platform then drops a layer which is recoated and then scanned

Process Description – Projection based SLA • • • • • • •

Projected image based Stereolithography – typically using a DLP Image projected from below and polymerises the given layer. The part is then moved one layer up and process is repeated Digital Micromirror Device Invested by Dr. Larry Hornbeck of Texas Instruments and termed Digital Light Projection (DLP) Produced using microsystem fabrication technology These micromechanical mirrors can be individually deflected to turn on and off the pixel Many devices can be manufactured on a single wafer

Photopolymerisation • • • • • • •

The materials used for Stereolithography are lightcurable resins (typically UV spectrum) The resins consist of a monomer, photoinitiator and an inhibitor Photopolymerisation is initiated by excitement of the photoinitiator by UV radiation which produces free radicals Photopolymerisation causes linking of small molecules (monomers) to larger chainlike molecules (polymers) Materials contain additives to try to simulate common engineering materials such as: ABS, PP, PC. Some have additives to change the colour and also improve stability Certain materials are created for specialised applications

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Exact mixes and contents are usually proprietary as this is a key area for manufacturers: Post-Processing of Materials All parts must be post cured to ensure complete polymerisation. Post cure also ensures complete cross linking of polymer bonds. Insufficient curing may effect mechanical integrity & safety. UV curing time depends on mass and material. Parts should be re-positioned in the chamber so that light reaches all sides.

Advantages • • • • • • • • • • •

The process is very accurate for AM processes: Feature size generally down to 0.1mm (or less) Layer thicknesses down to 0.025mm (note: limitation in coating top-down) The parts have a very good surface finish that is similar to injection moulded components Parts can be mechanically and chemically smoothed as well as painted, coated, etc. Large build volumes possible Support structures can be easier to remove by doing so before final curing in the UV oven The available materials can mimic traditional engineering polymers Still not the actual material though! By using lower density infill the parts can be used for highly accurate investment casting The process energy input is low when compared to thermal AM systems (mW laser powers) QuickCast

Disadvantages • • • • • • • • • • •

The machines are expensive (production machines cost £30,000 – £500,000) Note the cost of the table-top systems has dropped significantly in recent years. (...