Additive Manufacturing Sreekanth N V Assistant Professor Mechanical Engg. BMSCE

Syllabus

Syllabus

Books & Scheme

Unit-2

Vat based polymerization process VAT Polymerization Process -Makes use of liquid polymers which are cured by laser radiation or UV light -Chemical reaction from monomers to cross linked or linear chain polymerization Used in various industries since 1960 Generally in coating, dentistry and printing industry

Introduction Mid 1980 Charles Hull (3D Systems founder) fabricated the solid using stereolithography Gamma rays, X Rays, Electron Beams, UV, Visible Light, NdYAG, Helium Cadmium Lasers etc.. Scanning technologies a. Vector Scan(point-wise) b. Mask Projection (layer wise) c. Two-photon approach (high resolution point to point)

VAT Polymerization materials Acrylate based Resins (Free radical photo-polymerization) Epoxy based Resins (Cationic photo-polymerization) Acrylate +epoxides Resins

Materials Acrylates -characteristic high reactivity -Weak parts due to inaccuracies caused by shrinkage and curling -curing upto 46% only through lasers -photochemical reactions on already cured surfaces due to penetration issues -additional cross linking creates extra shrinkage -increased stresses in the layers during and after fabrication causes curling in the part

Materials Epoxide -ring opening polymerisation is carried out -more accurate, harder part as compared to acrylates -less shrinkage of around 1-2% in epoxies as compared to 5-20% in acrylates - excellent adhesion -reduced tendency for flexible substrates to curl while curing -polymerization not inhibited by atmospheric oxygen -lower photo-intiator requirement in concentration -lower residual odor as compared to acrylates -slow photospeed and brittleness in parts -sensitivity to humidity

Materials Acrylate +epoxides Resins Combination of both these resins overcome the weaknesses displayed by the polymers Increased hardness Less susceptible to atmospheric oxygen and humidity Improved accuracy and part build Enhanced build quality and part conformance

Ingredients of Polymer Photoinitiators- act as Catalyst Reactive Diluents Flexibilizers Stabilizers Liquid Monomers

Process Free Radical Photo-polymerization(Acrylates) Photons Free Radical Photo-polymerization(Acrylates) Cationic Photo Polymerization(Epoxy and Vinyl Ether) Photo initiators become reactive Liquid Monomers react with photo initiators for polymer chain C, H= Carbon and Hydrogen Atoms R=Molecular Group one or more Vinyl Group (Carbon carbon Double Bond) help in cross linking Reactions build polymer chain cross link forming covalent bonds

Process Free radical Photo Polymerization -Resins used are Acrylates -Builds molecule linearly forming a polymer chain after photo initiator becomes reactive to acrylate monomers -Cross Linking happens as the chain continues to grow -Has high reaction rate so react quickly to UV radiation Without epoxy or other elements leads to shrinkage and curling

Process Epoxies/Vinyl ethers -Epoxies have monomer rings which opens when reacted to bond chemically -it imparts minimum change in volume thus reduces shrinking effect -minimum volume change because of identical types of bonds before and after reaction -most commercially available SL printers use Epoxies

Process Polymerization -VP monomers is exothermic reaction -85KJ/mol is the heat produced in acrylate -Requires catalyst to initiate the reaction (photo initiator) -Every two Photons of laser produces one free radical enough to react with 1000 monomers(propogation) -Longer polymer molecules yields higher molecular weights which is preferred. P-I= Photo initiator -I*=free radical M= Monomer Terminates- Recombination/Disproportionation/Occlusion Dissproportionation– cancellation of one radical by another without joining Occlusion- trapped free radicals with solidified polymer, later solidifies due to ageing and leads to imperfections

Process Cationic Polymerization -Similar to free radical polymerization broadly -here after laser is focused on the photo initiator , the photo-initiator generates a cation -Cations react with monomers to form a polymer -Catalyst here is Lewis Acid (BF3) -most of advanced microelectronics use this process -Ring opening leads to polymerization

Resin Formulations Polyols/Acrylic Acids/Esters/Diisocyanates (for production of monomers and oligomers) -Polyester acrylate (PEA) -Epoxy Acrylates(EA) -Urethane Acrylates(UA) -Amino Acrylates (photoaccelerator) -Cycloaliphatic epoxies -Photosensitizers+ photoinitiators= longer wavelengths -Dilutent for adjusting viscosity

Photo-initiator System Conversion of physical energy of incident light to chemical energy in the form of reactive intermediates Photointiator should have strong absorption at laser emission wavelength, and undergo fast photolysis to get intiators with good yield Reactive intermediates are either radicals capable of adding to vinylic or acrylic double bonds Benzoyil species is major initiating species.

Photo Initiators Benzoin ether derivatives Benzyl Ketals Hydroxyalkylphenones Alpha Amino Ketones Acylphosphine Oxides

Other initiators Onium salts -Triaryl Sulfonium hexaflourophosphate solutions in propylene carbonate Eg: Degacure KI85 SP-55 Sarcat KI-85 113-8(Aldrich) SR1010 Salts UVI 6976(B-V) UVI 6992(B-VI)

Reaction Rates Reaction rates Controlled by concentrations of photoinitiators and monomers K= constant function of radical generation efficiency, rate of radical initiation, rate of radical termination

Reaction Rates Average molecular weight of polymers is the ratio of the rate of propogation and the rate of initiation, Vo is the kinetic average chain length.

Reaction rates The higher the rate of ploymerization the faster the parts can be built Monomer concentration cannot be varied but Initiator conc can be varied to control the rate of polymerization But using the equations even twice the concentration of initiator will only increase the rate by 1.4 Thus decreasing the molecular weight only applicable for Acrylates but will not work for epoxies.

Laser Scan Vat Polymerization Selective solidification of resin by UV laser Platform is dipped and lowered and recoated and exposed to laser Input Data(STL file)— Part Preparation(supports)— Layer Preparation(slicing)– Laser Scanning(solidification)..

Polymerization Process Modeling Laser Energy with Photopolymer resins Beer-Lambert Law: linear relationship between absorbance and concentration of an absorbing species. Useful in specifying laser scan speeds

Variables Cd=cure depth= depth of resin cure as a result of laser irradiation[mm] Dp=depth of penetration of laser into resin until a reduction in irradiance of 1/e is reached= key resin characteristics[mm] E= exposure. Possibility as a function of spatial co-ordinates[energy/unit area] [mJ/sq.mm] Ec=critical exposure= exposure at which resin solidification starts to occur[mJ/sq.mm] Emax=peak exposure of laser shining on the resin surface(center of laser spot) [mJ/sq.mm] H(x,y,z)= irradiance(radiant power per unit area) at a arbitary point in resin= time derivative of E(x,y,z)[W/sq.mm] PL=output power of laser[W] Vs=scan speed of laser[mm/s] W0=radius of laser beam focussed on resin surface[mm]

Irradiance and exposure Scanning happens through a line and cures to a depth Width of cure line is a important factor Shape of curved line is dependent on -resin characteristics -laser energy -scan speed

Irradiance and exposure Radiant power of laser/unit area H(x,y,z) Consider the following fig. The general form of the irradiance equation for a Gaussian laser beam is given as

Irradiance If we set z=Dp We get the irradiance depth Dp at 37% of the irradiance at resin surface( e^-1 =0.36788) Due to Beer-lambert law, Dp is a function of the resin used Assume that the laser scans along x-axis from the origin to a point b Irradiance at co-ordinate x along scan line is given by

Irradiance

Irradiance and Exposure

Irradiance and Exposure

Irradiance and Exposure

Irradiance and Exposure After Integration Exposure is given by:

Laser Resin Interaction Using Irradiance and Exposure relationships The shape of scanned vector line and its width can be determined

Parabolic shape of line

Parabolic shape of line

Exposure v/s cure depth

Properties of working Curve

Photospeed (Scan Velocity)

Vector Scan VP machines 3D Systems Fockele&Schwarze Germany micro Vat Denken Engg CMET(Mitsubishi) Sony Meiko Corp. Mitsui Zosen Teijen Seiki (Dupont) Form Labs Titan– Kudos 3d Carbon 3D Printers etc..

Vector Scan VP Systems Five major components are Recoating Systems Platform System Vat System Laser and Optics System Control system

Vector Scan VP System Process -A layer is cured and platform dips down by a layer thickness -The recoater blade(zephyr blade) slides over the whole build depositing a new layer of resin and smoothening the surface of vat.

Subsystems of SL technology