Rapid Prototyping Via
Photopolymerization
ISE 767
Rapid Prototyping
www.finelineprototyping.com
Introduction
 Numerous commercially available RP systems
are based upon the principle of photopolymerization.
 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.
Part I – Commercially Available Systems
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
3D Systems Stereolithography
 http://www.youtube.com/watc
h?v=NRc8yP-YM1A
 SLA Viper
 355 nm solid state Nd:YVO4
laser up to 100mW
 Dual resolution

0.25mm or 0.075mm beam
diameter
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
SLA Postprocessing
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Support removal
Cleaning uncured resin with TPM or alcohol
Postcuring
Sanding
SLA Tempering
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
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http://home.att.net/~edgrenda/pow/pow21.htm
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: http://home.att.net/~edgrenda/pow/pow21.htm
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:www.sonysms.com
3D Systems - ProJet
 http://www.youtube.com/wat
ch?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 0.0016" in Z
 HR model: 0.0015" resolution
in X,Y and 0.0016" in Z
Source:www.3dsystems.com
Objet - Eden
 http://www.youtube.com/wat
ch?v=r_2-4SFlsHk
 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:www.2objet.com
Envisiontec - Perfactory
 http://www.youtube.com/watch?
v=LZIy4LU-Qz0
 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:www.envisiontec.de.com
V-Flash
 3D Systems - $9,900
 http://www.youtube.com/wat
ch?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.
Part II: The Science Behind
Photopolymerization
Photopolymers

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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: www.pslc.ws/mactest/images/xlink02.gif
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
Photopolymer Chemistry
 Monomers, initiators, etc.
 Radical photo-polymerization
 Cationic photo-polymerization
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
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.
Representative Material Properties
Stereolithography
Source: www.finelineprototyping.com
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.
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
E (y , z) 
 2y 2

Z

 
2
Dp 
W0
 

2  PL



  W 0V s 
Source: Laser-Induced Materials and Processes for RP by Fuh and Wong
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:

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Z = 0.05 mm and y = 0.03 mm
Laser power (PL) = 40 mW
W0 = 0.125 mm
Vs = 200 mm/sec
Dp = 0.17 mm
Solution
E ( 0 . 03 , 0 . 05 ) 
 0 . 6515
mJ
mm
 65 . 15
mJ
cm
2
2
2
40 mW



  0 . 125 mm  200 mm / sec 
 2 ( 0 . 03 mm ) 2 0 . 05 mm


  0 . 125 mm 2 0 . 17 mm





Laser Exposure In Resin

Ec is the critical exposure level
needed to initiate curing.

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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.
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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
Cure Depth (Cd)
 Maximum cure depth
 Maximum exposure energy (Emax)
 Laser velocity (Vs) to produce a desired cure
depth ( )
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 (preshrunk) 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
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?
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
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
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
0.030-0.040” thick is thermoformed
over the SLA pattern
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Case Study: Hearing Aids
 http://www.materialise.com/materialise/view/en/
2562804Rapid+Shell+Modelling+%28RSM%29.html
 Download brochure
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e