1. Rapid Prototyping Via Photopolymerization
ISE 767
Rapid Prototyping

2. Introduction
Numerous commercially available RP systems are based upon the principle of photo-polymerization.
The aims of this module are:
To provide you with an overview of which systems are available, and what their operating principle is.
To introduce the theory behind light-resin interactions as a means of explaining some of the dozens of process parameters you can control when using one of these systems.

3. Part I – Commercially Available Systems
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

4. 3D Systems Stereolithography
SLA Viper
355 nm solid state Nd:YVO4 laser up to 100mW
Dual resolution
0.25mm or 0.075mm beam diameter

5. CAD-To-SLA Process CAD models are saved as STL files
Models are brought into the Lightyear software
Translated, rotated, scaled, copied as needed
Nest as many parts on the platform as possible
STL files are verified to ensure that the surfaces are water tight
Supports are generated beneath downward-facing surfaces
The build is sliced
The slice images to be drawn by the laser are stored in a new slice file format read by the SLA machine

6. SLA Postprocessing Support removal
Cleaning uncured resin with TPM or alcohol
Postcuring
Sanding

7. SLA Tempering Untempered SLA parts Tempered SLA parts
SLA parts are typically more brittle than thermoplastic resins
A patented tempering process (see photos and article above) calls for fabricating parts with small channels.
A composite material is injected into the channels that dramatically increases impact resistance and flexibility.
Untempered SLA parts
Tempered SLA parts
Source:

8. Sony – Solid Creation System
Identical in concept to 3D Systems stereolithography process
Systems available with
Two lasers for faster builds
1,000 mW lasers (our SLA has a 40 mW laser!)
Adjustable laser spot size and layer thickness during the build
Source:

9. 3D Systems - ProJet http://www.youtube.com/watch?v=5hhnXFmdUHQ
Multi-jet inkjet printing of UV curable photo-polymer.
UV flood lamp curing after printing of each layer
Two resolutions available
SR model: 0.003" resolution in X,Y and " in Z
HR model: " resolution in X,Y and " in Z
Source:

10. Objet - Eden New multi-material deposition capabilities!
Array of 8 inkjet print heads scan back and forth jetting a photopolymer onto the platform
UV lamp cures the photopolymer (no laser)
Support material is removed with warm water
Suitable for printing parts with extremely fine details
600 μm thick walls, 16 μm layer thickness
New multi-material deposition capabilities!
Source:

11. Envisiontec - Perfactory
Uses Texas Instruments DLP chip (same as that used in some projection TV's) to project a visible light image onto a visible light curing photo-polymer.
Two resolutions available:
Standard res: 148 μm in X, 93 μm in Y, and 50 to 150 thick layers
High resolution: 60 μm in X, 32 μm in Y, and 25 to 50 thick layers
Source:

12. V-Flash 3D Systems - $9,900 http://www.youtube.com/watch?v=0Rs7RQpO8p0
Resin is printed onto plastic film.
A platform lowers down onto the film, thus transferring resin from the top of the film to the bottom of the plate.
UV light cures the resin, and the process is repeated.
The parts come out completely dry with no postprocessing needed.

13. Part II: The Science Behind Photopolymerization

14. Source: www.pslc.ws/mactest/images/xlink02.gif
Photopolymers
Highly crosslinked or networked polymers that effectively form a giant macromolecule
Strong covalent bonds
Cannot be melted once they've been cured
Crosslinking significantly raises the glass transition temperature
They are generally very resistant to solvents
They can generally withstand higher temperatures than TP’s
Source:

15. Curing of Cross Linked Polymers
Light-curing
Photocuring resins that are liquid until exposed to light of a specific wavelength
Examples: 3D Systems stereolithography, 3D Systems Invision, Envisiontec Perfactory, Objet Eden
Heat activated
Thermoset in powder form is molded to a particular shape, and heat initiates molecular cross linking
No RP systems use this approach that I'm aware of
Catalyst and mix-based systems
When two components are mixed together, the resulting chemical reaction leads to the desired cross linking
Ex: polyurethane casting into rubber molds

16. Photopolymer Chemistry
Monomers, initiators, etc.
Radical photo-polymerization
Cationic photo-polymerization

17. Radical Polymerization
Used to photo-polymerize acrylate resins
Photons are absorbed by the photoinitiator thus producing free radicals
Only happens when laser power exceeds the threshold curing exposure
Photoinitiators are sensitive to a specific range of wavelengths (mostly in the UV range)
Free radicals react with monomer

18. Cationic Polymerization
Used for photo-polymerization of epoxy and vinylether resins
Higher strength and lower shrinkage
Oxygen will not inhibit reaction
Water (humidity) will inhibit reaction
Do not react as quickly, so a more powerful laser is needed to cure at the same rate as with acrylate resins.

19. Representative Material Properties Stereolithography
Source:

20. Photocuring
The process of hardening a liquid resin via the selective application of energy (UV, IR, etc).
Penetration Depth (Dp) – the depth at which the energy intensity has been reduced to approximately 1/3 the intensity at the surface.
Scan Velocity (Vs) – the speed (mm/sec) at which the laser beam is scanned over the liquid resin.
Critical Exposure (Ec) – the energy per unit area needed to produce gelation.
Cure depth (Cd) – is a function of penetration depth, critical exposure, energy intensity, exposure area, and exposure time.

21. Laser Exposure In Resin
Tells you the laser exposure (mJ/cm2 or equivalent) as a function of depth beneath the surface of the resin (z) and distance from the center of the beam (y).
PL = laser power (mW)
W0 = 1/e2 Gaussian half width of the beam (mm)
Vs = velocity of the beam (mm/sec)
Dp = penetration depth (mm) which is depth at which energy is 1/e that of energy at the surface
Source: Laser-Induced Materials and Processes for RP by Fuh and Wong

22. Sample Calculation
What is the laser exposure (mJ/cm2) at a depth of 0.05 mm and a distance of 0.03 mm from the center of the beam?
Given:
Z = 0.05 mm and y = 0.03 mm
Laser power (PL) = 40 mW
W0 = mm
Vs = 200 mm/sec
Dp = 0.17 mm

23. Solution

24. Laser Exposure In Resin
Ec is the critical exposure level needed to initiate curing.
If energy density is less than Ec, then no curing takes place.
If you know Ec, then you can determine the maximum value of y where curing takes place (i.e. you can figure out the width of the cured line at the surface
Scan pitch is the step over distance between adjacent laser tracks when filling in an area.
Many different fill strategies exist.
In general, you don't want track lines from one layer exactly on top of track lines with previous layers as shown in the illustration.
They are staggered to promote more complete curing
They are often shifted 90 degrees in orientation between subsequent layers to balance shrinkage stresses that lead to curling.
Source: Laser-Induced Materials and Processes for RP by Fuh and Wong

25. Cure Depth (Cd) Maximum cure depth Maximum exposure energy (Emax)
Laser velocity (Vs) to produce a desired cure depth ( )

26. Curling and Distortion
Curling of large flat horizontal surfaces is a significant problem.
Each layer shrinks during solidification.
When one layer shrinks on top of a previously solidified (pre-shrunk) layer, then there is stress between the two layers.
The result is curling
Preventing/minimizing curling
Re-orient the part if possible
Use lots of supports that anchor the downward facing surface in place.
Source: Rapid Prototyping and Manufacturing by P. Jacobs

27. Beam Shape
A round laser beam that is projected straight down onto a perpendicular surface will produce a round spot.
When the beam is swept at an angle to other (non-perpendicular) spots on the vat of resin, the spot will have the shape of an oval.
Newer SLA machines (very expensive) have active optics that can reshape the spot on the fly in order to maintain a round spot anywhere on the surface of the resin.
Do print-based systems have this problem?

28. Electroplating of SLA Components
A handful of companies in the U.S. are able to electroplate SLA parts
Parts shown in the photos are nickel-plated SLA parts assembled into a functioning handheld air compressor (courtesy of Fineline Prototyping)
Source: Fineline Prototyping

29. Plating of Plastics Step 1: Make the surface electrically conducting
Brush on silver paint (typically shows poor adhesion)
Chromic acid will etch ABS plastic
Activate surface in palladium or tin chloride to deposit conducting metal into etched surface
Step 2: Very thin electroless nickel plating
Step 3: Electroplating with copper
Step 4 (Optional): Electroless nickel (or other metal) plating

30. Case Study: Invisalign Braces
Digital impression is made
Software creates steps of tooth movement
12-48 aligners, each of which is worn for about 6 weeks each
Each SLA machine makes ~100 unique aligner patterns per build
Polycarbonate/Polyurethane sheet ” thick is thermoformed over the SLA pattern
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

31. Case Study: Hearing Aids
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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e