1. Additive Manufacturing
Sreekanth N V
Assistant Professor
Mechanical Engg.
BMSCE

2. Syllabus

3. Syllabus

4. Books & Scheme

5. Unit-2

9. 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

10. 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)

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

12. 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

13. 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

14. 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

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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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.

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

24. 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)

25. 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

26. 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.

27. 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.

28. 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)..

29. 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

30. 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]

31. 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

32. 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

33. Irradiance If we set z=Dp
We get the irradiance depth Dp at 37% of the irradiance at resin surface( e^-1 = )
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

34. Irradiance

35. Irradiance and Exposure

36. Irradiance and Exposure

37. Irradiance and Exposure

38. Irradiance and Exposure
After Integration Exposure is given by:

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

40. Parabolic shape of line

41. Parabolic shape of line

42. Exposure v/s cure depth

43. Properties of working Curve

44. Photospeed (Scan Velocity)

45. 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..

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

47. 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.

48. Subsystems of SL technology