MMME4102 Vat Polymerization PDF

MMME4102 Vat polymerization Dr Lyudmila Turyanska 1 Attendance monitoring 2 Learning objectives Understand the fundamentals of vat polymerisation Evaluate the material requirement for vat polymerisation Familiarize with stereo...

MMME4102 Vat polymerization Dr Lyudmila Turyanska 1 Attendance monitoring 2 Learning objectives Understand the fundamentals of vat polymerisation Evaluate the material requirement for vat polymerisation Familiarize with stereo-lithography techniques Understand principles of 2-photon lithography Overview of applications of vat polymerisation Understand the advantages and disadvantages Have an overview current research directions Additive manufacturing technologies www.sulzer.com 4  Vat polymerization process Liquid photopolymer in a vat is selectively cured by light-activated polymerization VAT photopolymerization was invented in 1986 for 3D Systems Steps 1. The container (VAT) is filled with photo polymer resin. 2. Platform Descends at given speed. 3. Resin is exposed to light and is cured to solidify. 4. Wiper moves across cross section and re coats the material freshly. Vat polymerization process Stereolithography (SL) -Laser beam, scanning -Resolution ~ 100 µm, Microstereolitography (MSL) -Scanning MSL (point-by point) -Projection MSL (Integral curing) -Resolution ~ 1-10 µm Multi-photon polymerization -Resolution < 1 µm Materials for vat polymerization Composition - Main components: Monomers - acrylate, epoxy Photoinitiators – produce radicals Neutral absorber – to control light penetration depth Additives - fillers, surfactants, diluents Required properties: Photosensitivity at operating wavelength. Low viscosity to produce smooth surface High curing speed Low shrinkage during polymerization https://doi.org/10.1590/1980-5373-mr-2020-0010 High absorption for low penetration flight. Materials for vat polymerization MONOMERS Acrylates: High reactivity Inaccuracy (shrinkage and curling) Oxygen inhibition Free-radical polymerization Epoxy Slow photopolymerization Brittleness Better accuracy and lower dimensional changes No oxygen inhibition Lower photoinitiator concentration Sensitivity to humidity Cationic polymerization EXAMPLES: SL commercial resins polyester acrylate (PEA); epoxy acrylates (EA); urethane acrylates (UA); amino acrylates and cycloaliphatic epoxies Materials and polymerization process Polymer chains Photopolymers Radical polymerisation mechanism Free radical formation: P-I → -I Initiation: I + M → I-M Propagation: I-M → → I-M-M-M-M…-M Termination: → I-M-M-M-M…-M-I Crosslinked network Example Interaction of light and matter Interaction with light is required for material curing Gaussian laser beam Absorption: Beer-Lambert law of absorption Each laser pass cures a parabolic cross-section of the resin Interaction of light and matter Resin properties determine the relationship between light exposure and cure depth Working curve equation : Cd = Dp ln(𝐸/𝐸𝑐) Cd - cure depth Dp - penetration depth E - exposure energy Ec - critical exposure The system is optimized by: - changing laser power - initiator concentration Stereolithography (SL) Curing Mechanism Point by point curing of the resin by UV laser Laser intensity and velocity optimised for sufficient curing Light penetration through the resin is required For the first layer the platform is positioned to the depth of the first layer below the resin surface SL: support generation and post processing SUPPORT STRUCTURES: Support overhanging features and temporary disconnected parts Prevent part from falling over during the build Prevent de-lamination or cantilever curl Prevent wiper blade collision POST PROCESSING: Support removal Post cure ( UV chamber) Part finishing: surface smoothing, painting Factors affecting resolution Motion and quality of the beam Laser beam spot diameter Layer thickness Photopolymer/ monomer used Exposure time/power Laser beam focus depth SL: advantages and disadvantages ADVANTAGES ✓ Robust material and good surface finish ✓ Large build volume ✓ High accuracy (+/-.05mm) ✓ Established process (hardware, software & materials) DISADVANTAGES o Expensive to have a large vat of resin o Brittle parts o Limited materials o Liquid SL resin is a potential hazard o Extra curing required after build SL: limitations Photocuring Curl Distortion Control in Projection-Based Stereolithography LIMITATIONS: Volumetric shrinkage (6 –7%) Curl distortion Swelling Over-curing DOI:10.1115/1.4034305 Micro-stereolithography (MSL) Additive method to fabricate arbitrary 3D micro structures Spatially controlled solidification of a liquid resin by UV light; Resolution: ~ 1µm EQUIPMENT: 1) Laser source. 2) Beam delivery system: - scanning MSL (or vector-by-vector MSL) - projection MSL (or integral MSL) have been developed. 2) CAD design tool and computer-controlled precision x-y-z stages. 3) Vat containing UV curable resin. Projection stereolithography LCD (liquid crystal display) or DMD (digital micromirror device) masks. Cure the whole area (not point by point). Illumination from below or onto surface of vat Rev.Sci.Instrum.83, 125001 (2012) Two-photon (2P) lithography High geometrical degree of freedom to create 3D nano/micro structures Femtosecond (fs) laser induces multi-photon polymerization Direct fabrication without a mask Materials: polymers, ceramics, metals, hybrid Resolution: ~100 nm Wei Long Ng et al 2020 Biofabrication 12 022001 One photon vs two photon excitation 2PP: Examples of printed structures Consider and discuss resolution and surface finish of the parts shown Materials for multi-photon process Composition Requirements Photoinitiators Transparent in the visible and near infrared regions Fast curing speed Reactive monomer Resistant to the solvent used in post-processing Mechanical and thermal stability to maintain shape https://doi.org/10.1002/smll.201902687 Materials: examples HYBRID MATERIALS 2PP structures with Au NP produced in situ by salt reduction process FLEXIBLE MATERIALS Fabricated Hydrogel on flexible substrate 2P: Advantages and disadvantages ADVANTAGES: ✓ Ultrashort pulses initiate intense nonlinear processes at relatively low average power, without thermal damage to the sample. ✓ Easy to operate and maintain. ✓ No mask, mold or stamp is needed for fabrication. o DISADVANTAGES: o Reduced mechanical strength o Limited material and substrate choice o Not suitable for large scale deposition Applications ✓ Functional parts ✓ Patterns for prototype tooling ✓ Patterns for metal castings ✓ Education/research ✓ Tooling components Applications of SL: parts and devices AUTOMOTIVE http://rossnicholls.net/rapid-prototyping-services/stereolithography- sla/applications-of-stereolithography-sla/#optical MICRO-OPTICS Applications: parts and devices WAVEGUIDES PHOTONIC CRYSTALS Applications: medicine MICRO NEEDLES FOR DRUG DELIVERY HEARING AIDS Summary Vat polymerization process: - Polymerization process - Light-matter interaction Materials: - Resin composition: Monomer, photoinititor, absorbents, fillers Processes: - Stereolithography (SL) - resolution ~ 100µm - Microstereolitography(MSL) - resolution ~ 1-10µm Scanning MSL (point-by point) Projection MSL (Integral curing) - Multiphoton SL - resolution < 1 µm  Key points… ✓ Vat process and materials used; ✓ Resolution of different processed and factors affecting resolution; ✓ Advantages and limitations of vat polymerization additive manufacturing ✓ Applications and current developments. Recommended textbooks Advances in 3D printing & additive manufacturing technologies David Ian Wimpenny, Pulak M. Pandey, L. Jyothish Kumar, editors. Springer 2017 3D printing and additive manufacturing : principles and applications Chua, Chee Kai. 5th Ed: World Scientific 2017 Activity View the video of SLA process (by Solid Concepts) https://www.youtube.com/watch?v=NM55ct5 KwiI&feature=youtu.be Consider benefits and challenges association with deposition from the bottom up compared to top down Extra reading - References: https://doi.org/10.1590/1980-5373-mr-2020-0010 https://doi.org/10.1016/j.procir.2018.04.049 Consider VP produced gears assembled in the reducer, shown on the right. Based on lecture material and video viewed, list post processing requirements and approaches used for parts produced using vat polymerization. Mater Research 2020, V23, N4 http://dx.doi.org/10.1590/1980-5373-mr-2020-0010