Presentation on theme: "Rapid Prototyping Via Photopolymerization ISE 767 Rapid Prototyping www.finelineprototyping.com."— Presentation transcript:
Rapid Prototyping Via Photopolymerization ISE 767 Rapid Prototyping
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.
h?v=NRc8yP-YM1A 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 3D Systems Stereolithography
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 Support removal Cleaning uncured resin with TPM or alcohol Postcuring Sanding
SLA Tempering 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. Tempered SLA parts Untempered SLA parts Source:
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 ch?v=5hhnXFmdUHQ 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 " in Z HR model: " resolution in X,Y and " in Z Source:www.3dsystems.com
Objet - Eden ch?v=r_2-4SFlsHk 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 v=LZIy4LU-Qz0 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 ch?v=0Rs7RQpO8p0 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 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:
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
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:
Photocuring The process of hardening a liquid resin via the selective application of energy (UV, IR, etc). Penetration Depth (D p ) – the depth at which the energy intensity has been reduced to approximately 1/3 the intensity at the surface. Scan Velocity (V s ) – the speed (mm/sec) at which the laser beam is scanned over the liquid resin. Critical Exposure (E c ) – the energy per unit area needed to produce gelation. Cure depth (C d ) – 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/cm 2 or equivalent) as a function of depth beneath the surface of the resin (z) and distance from the center of the beam (y). P L = laser power (mW) W 0 = 1/e 2 Gaussian half width of the beam (mm) V s = velocity of the beam (mm/sec) D p = 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
Sample Calculation What is the laser exposure (mJ/cm 2 ) 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 W 0 = mm Vs = 200 mm/sec Dp = 0.17 mm
Laser Exposure In Resin E c is the critical exposure level needed to initiate curing. If energy density is less than E c, then no curing takes place. If you know E c, 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
Maximum cure depth Maximum exposure energy (E max ) Laser velocity (V s ) to produce a desired cure depth ( ) Cure Depth (C d )
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
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