Presentation on theme: "Rapid Prototyping Via Photopolymerization"— Presentation transcript:
1 Rapid Prototyping Via Photopolymerization ISE 767Rapid Prototyping
2 IntroductionNumerous 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.
4 3D Systems Stereolithography SLA Viper355 nm solid state Nd:YVO4 laser up to 100mWDual resolution0.25mm or 0.075mm beam diameter
5 CAD-To-SLA Process CAD models are saved as STL files Models are brought into the Lightyear softwareTranslated, rotated, scaled, copied as neededNest as many parts on the platform as possibleSTL files are verified to ensure that the surfaces are water tightSupports are generated beneath downward-facing surfacesThe build is slicedThe 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 alcoholPostcuringSanding
7 SLA Tempering Untempered SLA parts Tempered SLA parts SLA parts are typically more brittle than thermoplastic resinsA 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 partsTempered SLA partsSource:
8 Sony – Solid Creation System Identical in concept to 3D Systems stereolithography processSystems available withTwo lasers for faster builds1,000 mW lasers (our SLA has a 40 mW laser!)Adjustable laser spot size and layer thickness during the buildSource:www.sonysms.com
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 layerTwo resolutions availableSR model: 0.003" resolution in X,Y and " in ZHR model: " resolution in X,Y and " in ZSource:www.3dsystems.com
10 Objet - Eden New multi-material deposition capabilities! Array of 8 inkjet print heads scan back and forth jetting a photopolymer onto the platformUV lamp cures the photopolymer (no laser)Support material is removed with warm waterSuitable for printing parts with extremely fine details600 μm thick walls, 16 μm layer thicknessNew multi-material deposition capabilities!Source:www.2objet.com
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 layersHigh resolution: 60 μm in X, 32 μm in Y, and 25 to 50 thick layersSource:www.envisiontec.de.com
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 PhotopolymersHighly crosslinked or networked polymers that effectively form a giant macromoleculeStrong covalent bondsCannot be melted once they've been curedCrosslinking significantly raises the glass transition temperatureThey are generally very resistant to solventsThey can generally withstand higher temperatures than TP’sSource:
15 Curing of Cross Linked Polymers Light-curingPhotocuring resins that are liquid until exposed to light of a specific wavelengthExamples: 3D Systems stereolithography, 3D Systems Invision, Envisiontec Perfactory, Objet EdenHeat activatedThermoset in powder form is molded to a particular shape, and heat initiates molecular cross linkingNo RP systems use this approach that I'm aware ofCatalyst and mix-based systemsWhen two components are mixed together, the resulting chemical reaction leads to the desired cross linkingEx: polyurethane casting into rubber molds
17 Radical Polymerization Used to photo-polymerize acrylate resinsPhotons are absorbed by the photoinitiator thus producing free radicalsOnly happens when laser power exceeds the threshold curing exposurePhotoinitiators 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 resinsHigher strength and lower shrinkageOxygen will not inhibit reactionWater (humidity) will inhibit reactionDo 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 PhotocuringThe 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 surfaceSource: Laser-Induced Materials and Processes for RP by Fuh and Wong
22 Sample CalculationWhat 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 mmLaser power (PL) = 40 mWW0 = mmVs = 200 mm/secDp = 0.17 mm
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 surfaceScan 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 curingThey 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 curlingPreventing/minimizing curlingRe-orient the part if possibleUse lots of supports that anchor the downward facing surface in place.Source: Rapid Prototyping and Manufacturing by P. Jacobs
27 Beam ShapeA 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 partsParts 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 plasticActivate surface in palladium or tin chloride to deposit conducting metal into etched surfaceStep 2: Very thin electroless nickel platingStep 3: Electroplating with copperStep 4 (Optional): Electroless nickel (or other metal) plating