In this paper, we present a rapid manufacturing process for the layered fabrication of
polymer-based composite parts using short discontinuous fibers as reinforcements. In the recent
past, numerous research efforts, similar to ours, have been made to produce fiber-reinforced
plastic parts via layered manufacturing methods. However, most of these attempts have not
resulted in the development of an effective commercially-viable manufacturing process. Our
proposed fabrication process on the other hand has been experimentally verified to yield
composite parts comparable in quality to pure polymer parts manufactured on a commercial
stereolithography system.Mechanical Engineerin

Benhabib, B.

Park, C.B.

Sela, M.N.

Zak, G.

English

Texas ScholarWorks

A LAYERED-MANUFACTURING PROCESS FOR THEFABRICATION OF GLASS-FffiER-REINFORCED COMPOSITESG. Zak M.N. Sela C.B. Park B. BenhabibComputer Integrated Manufacturing LaboratoryDepartment of Mechanical and Industrial EngineeringUniversity of Toronto5 King's College Road, Toronto, Ontario, M5S 3G8e-mail: beno@mie.utoronto.caABSTRACTIn this paper, we present a rapid manufacturing process for the layered fabrication ofpolymer-based composite parts using short discontinuous fibers as reinforcements. In the recentpast, numerous research efforts, similar to ours, have been made to produce fiber-reinforcedplastic parts via layered manufacturing methods. However, most of these attempts have notresulted in the development of an effective commercially-viable manufacturing process. Ourproposed fabrication process on the other hand has been experimentally verified to yieldcomposite parts comparable in quality to pure polymer parts manufactured on a commercialstereolithography system.This process uses a UV-laser-based system for the selective solidification of the compositeliquid. The primary components of the prototype are: (1) fiber-resin mixing subsystem, (2)composite-liquid deposition subsystem, (3) liquid leveling subsystem, (4) laser-light deliverysubsystem, and (5) contour-milling subsystem.Extensive microscopic examination of composite parts, built by the proposed prototypesystem, has been conducted to evaluate the parts' layer quality. Results indicate that the prototypesystem can yield comparable layer quality, in terms of accuracy and uniformity, to that of pure-resin parts made by a photopolymer-based commercial system.INTRODUCTIONReinforcement of plastics by fibers has been used successfully for over fifty years as meansof improving the mechanical properties of the manufactured products [1]. Combining high-modulus, high-strength fibers with a polymeric matrix produces a composite material with higherstiffness and strength, and lower thermal-expansion coefficient. The reinforcing fibers can beintroduced either in continuous (long) or discontinuous (short) form. While continuous fibersprovide greater relative improvement of the mechanical properties, they also significantlycomplicate composite-material processing. Short-fiber composites on the other hand can beeasily manufactured by automated, and hence more economical, methods.Since the late 1980's, several Rapid Layered Manufacturing (RLM) techniques have beeninvestigated, and some commercially developed. These techniques allow free-form fabrication ofcomplex-geometry parts directly from their CAD models [2]. The most commonly used RLM17technique for the production of plastic parts is Stereolithography (SL). As a building material, itemploys a liquid photosensitive resin, which is selectively solidified by an ultraviolet (UV) laserbeam. The SL technique is used as a basis for the rapid manufacturing of short-fiber reinforcedparts proposed herein.There has been some recent work performed on improving the mechanical properties ofpolymer-based parts produced by SL methods. For example, in [3], long fibers were added to thepolymer matrix by stacking rings with arrays of parallel horizontal fibers stretched across, andthen curing the polYmer via a standard SL procedure. In another approach [4], continuous fiberswere laid out by a dedicated apparatus before curing each layer of the part. Other approachesinclude using solid inserts within the polYmer [5], or building fiber-reinforced shells aroundsolidified resin part [6].Feasibility studies were also reported regarding the use of discontinuous reinforcements inthe form of either 10-15 mm chopped glass fiber bundles or 55 Jlm diameter glass microspheres[7, 8]. Composite samples several-layers thick were produced by manually spreading the glassfibers over the liquid resin on each layer. Fibers were not premixed due to the very high viscosityof the resulting mixture. Improvements of material mechanical properties were reported for fiber-based reinforcements, while no improvement was attained by using microspheres.The above methods of reinforcement (especially those using medium-length or continuousfibers, as in [3, 4, 7, and 8]) are suitable for the rapid layered manufacturing of objects withrelatively simple geometric shapes. However, significant difficulties may arise when applyingthese methods to the production of objects with small-scale features, thin walls or intricateshapes.Thus, herein, we propose reinforcement of SL-made complex-geometry objects by shortfibers introduced directly into the photopolYmer matrix. The fact that the resin remains liquid atroom temperature simplifies the process of adding the fibers, storing and handling the mixture, aswell as controlling the amount of fibers added. Due to its transparency (for photo-curing) andrelatively low cost [1], glass fiber was selected for reinforcement.INCORPORATING FIBERS IN THE SL PROCESSThe benefits of reinforcements are limited by the maximum volume fraction that can beaccommodated by the matrix. This limit is in turn governed by the uniformity of the fiberorientation and the fiber length. For uniformly oriented fibers, the concentration can reach50-60% by volume; for randomly oriented fibers, the limit can be 10-40%, depending on thefiber length.Below, we briefly review the suitability of the SL process for fabrication of fiber-reinforcedparts.(i) Mixture ViscosityAdding fibers to the liquid resin creates a highly viscous liquid. When used in a standard SLprocess, such a liquid is likely to have difficulty in flowing to form an even layer after the part is18submerged. A subsequent wiping operation may also encounter problems because of the presenceof fibers, as was confirmed in our preliminary investigation into the wiping of various fiber-resinmixtures.(ii) Fiber SettlingSince the fiber density is more than twice that of a typical resin (2.54 glcm3 for glass vs.1.13-1.15 glcm3 for resin), the fibers tend to sink if left undisturbed. For spheres in liquid, theterminal sinking velocity is given by [9]:gd 2V =(p s - P1) 18fl (1)where g is the gravitational acceleration, d sphere diameter, Ps density of sphere, PI density ofliquid, and Jl viscosity of liquid. Using a typical resin viscosity of 200 cP, a fiber length of 1.5rom and diameter of 15 flm, and calculating the diameter of a sphere equal to the fiber volume,yields a sinking rate of about 85 mm/hr. Thus, if the mixture was left undisturbed in a vat duringpart building, the fibers would settle continuously, which would lead to lower fiber concentrationnear the surface.(iii) Fibers on the Part SurfacesWhen short fibers are added to polYmers as reinforcements in a typical manufacturingprocess (e.g., injection molding), the solidification of the matrix always takes place within amold with solid boundaries surrounding the composite part. These boundaries guarantee that thefibers will not protrude from the surface of the finished part. On the other hand in our case, whenthe fiber-resin mixture is solidified by the laser beam, the boundary between the solid and theliquid will always fall across some fibers. Preliminary experiments showed that the partsproduced by an SL method had protruding fiber filaments on their vertical surfaces.THE NEW RAPID LAYERED MANUFACTURING PROCESSOne key novel feature distinguishes the proposed RLM process from the standard SL: thecomposite liquid is not stored in a vat but instead deposited from above for each layer (Figure 1).The primary fabrication steps of the current process are as follows:(1) The composite liquid is continuously stirred in a separate container during the part building.(2) A known volume of the composite liquid is deposited from above for each layer (Figure 1b).(3) A wiper levels the liquid at the required height (Figure lc).(4) The layer is selectively cured by a UV laser (Figure Id).(5) A milling tool is used to remove the fibers protruding from the solidified walls of the currentlayer (Figure Ie).(6) The part is lowered into the vat (Figure 1f), and the process steps 2 and 5 are repeated.19Once the part-building process is completed, the platform is raised, and the part is removed,cleaned, and post-cured.This process solves all three primary problems addressed in the earlier section, namely,settling of fibers, spreading of a highly viscous composite liquid, and removing the fibersprotruding from the finished part's vertical surfaces. The fiber-removal solution proposed hereinis the use of a vertical milling tool. At each layer, a small-diameter, numerically controlled end-mill traces the perimeter of the part. The cut could be made to remove just a part of the solidifiedperimeter, thus assuring precise horizontal dimensions of the layer.[a] [b] [c] [d] [e] [f]Figure 1: Steps of the Proposed RLM Process.THE CURRENT EXPERIMENTAL SYSTEMAn experimental system was designed and built to serve as a tool for the processdevelopment. The first prototype was described briefly in [10]. The current prototype is capableof automatically building composite layered parts from the data provided by a CAD software.The main components of the system are (Figure 2):(1) Fiber-resin mixing subsystem,(2) Composite-liquid deposition subsystem,(3) Liquid leveling subsystem,(4) Laser-light delivery subsystem,(5) Z-platform translation subsystem, and(6) Milling subsystem.The system is controlled by custom-written software running on an Intel-processor-basedPC. The PC is interfaced to a motion controller, which drives four stepper motors, activatessolenoid air valves, and receives feedback signals from several limit switches.The fiber-resin mixing subsystem keeps the fibers in suspension throughout the buildingprocess. It consists of an open-top container and a helical-screw stirring device inserted into thecontainer from above. The helical-screw arrangement achieves adequate mixing, when turningslowly, without causing agitation or foaming of the liquid.20The composite-liquid deposition subsystem delivers repeatable desired amounts of resin intothe vat. This function is performed via a peristaltic pump, which uses rollers to squeeze the liquidthrough a flexible plastic tube. One end of the tubing is inserted into the container of the mixingsubsystem; the other end is attached to the platform of the X-Y translator. The pumping action iscombined with the simultaneous translation of the dispensing nozzle in front of the wiper toassure even spreading of the composite liquid. The advantages of delivery via a peristaltic pumpare: (i) accurate metering of the liquid volume delivered, since the pump is of a positivedisplacement type and (ii) simplified system maintenance, since there is no contact between thepump mechanism and the liquid, and since the tubing is easily replaceable. Evaluation of thecurrent version has shown significantly improved consistency of the liquid volume delivered,relative to our earlier system described in [10].Mixer Pump VatFigure 2: Current Version of Composite-Liquid Deposition and Fiber-Resin Mixing Subsystems.The liquid leveling subsystem assures uniform spreading of the liquid over the vat's topsurface to create a layer of consistent thickness. This is achieved by translating a wiper with atriangular edge profile. The wiper movement is actuated by a pneumatic cylinder. Two forward-and-return wiping stroke sequences are performed to form each layer: the first stroke sequence iscarried out after the platform has been lowered by a depth of several layers, and the second strokesequence when the platform has been raised back to the height one-layer lower than the last layerbuilt.The laser-light delivery subsystem delivers a focused beam of UV light to the surface of thecomposite liquid and moves the beam spot in the X-Y plane. The laser-light source is a 20 mWHe-Cd UV laser, Omnichrome 3056-10M. The light is delivered via a fiber-optic cable. At theoutput end of the cable, a lens focuses the light beam. The X-Y motion of the beam is achievedby attaching the focusing lens and fiber-optic cable to an X-Y translator.The Z-platJorm subsystem moves the supporting platform vertically. Since the height of theplatform directly affects the layer thickness, its vertical displacement must be accuratelycontrolled. To achieve the required accuracy, the Z platform is actuated by a stepper motordriving a micrometer attached to a vertical translation stage.The milling subsystem removes surface fibers by a portable milling tool attached to theplatform of the X-Y translator. The milling tool is driven remotely through a flexible shaftattached to an externally-mounted motor.21EXPERIMENTAL RESULTSSince parts produced by an RLM process are fonned out of individual layers, good surfacequality and dimensional accuracy can be achieved only by building high-quality individuallayers. The experimental system described above was therefore used to build a set of pure andcomposite rectangular parts (25x30x4.8 mm), and the layer cross-sectional profiles of these partswere then examined microscopically. The composite liquid comprised the 2202SF photopolymerby Allied Signal and 15% by volume 737BD 1.6 mm milled glass fibers by Owens Coming. Itwas noted that for 1.6 mm long fibers the maximum achievable volume fraction was about 15-20%. An additional pure-resin test part was built on a commercial SL machine (Sony JSC-2000Solid Creator) from DeSolite SCR310 photopolymer. The measurements obtained from this partserved as a benchmark for the layer quality of our own parts.Several vertical sections of each part were made. Figure 3 shows the layer profiles for onesection of the pure-resin part built on a commercial SL machine ("SL_PURE"). Figure 4 showsthe layer profiles for sections of a pure-resin part ("RLM_PURE") and a composite part("RLM_COMP") built on our prototype system. Ideally, the plots should consist of straighthorizontal lines representing layer boundaries separated by the nominal layer thickness of0.3 mm.Statistical analysis of the observations from all sections of the above three parts was carriedout after discarding the data for the first 8-10 layers in order to allow for process stabilization(Table 1). Three parameters are shown for each part: the average layer thickness, which is themean of layer boundary separation values observed across all the part sections; the standarddeviation within layers, which is a measure of the unevenness of the layers; and the standarddeviation between layers, which is a measure of the layer-to-Iayer variability of the average layerthickness. The results show that our prototype is able to build layers with a mean thickness veryclose to the nominal value. Also, the variabilities in the building process of the layers are quiteclose to those obtained on the Sony machine, especially when one considers that our system isstill in its prototype stage. The composite part layer variability is only slightly higher than that forthe pure-resin part. Process refinements should reduce the difference even further.Parts built on our prototype system have somewhat higher variability of the layer thicknessthan those built on a commercial SL machine. However, the standard deviation values are withinthe same order of magnitude. Also, note that the composite part layer variability is only slightlyhigher than that for the pure-resin part built on our prototype system. Process refinements shouldreduce the variability even further.Table 1. Statistical Data for Layer Profiles of Multiple Sections.Ayerage Layer St. Dey. Within St. Dey. BetweenThickness (mm) Layers (mm) Layers (mm)SL PURE 0.313 0.008 0.002RLM_PURE 0.306 0.016 0.007RLM_COMP 0.315 0.021 0.01022252010 15X (mm)5I-..I--I---. -'--- -I---..r--... -r---. -r--. -t--. -!""--.. -- ----------- -I--.. -r--. -0.60.0 o1.21.83.613.0N 2.44.24.85.4Figure 3. Layer ProfIles for a Pure-Resin Part Made on a Commercial SL Machine.30252015X (mm)1050.6'L- -- ~1.2.J----------------t5.41'- -----ii;...J1.81.-----------------40.04----+----;.----+---;-_--1""--_-1o-i4.8r---------- -....-- li;---i4.2 f-------------------:J ;3.6 r------------------"!i13.01-- ---t!N 2.4 '------------------130252015X (mm)10-- -5.44.84.21.80.60.0o1.23.613,0N 2.4(a) (b)Figure 4: Layer ProfIles for (a) a Pure-Resin and (b) a Composite Test Part.CONCLUSIONSThis paper presented a new RLM process for the fabrication of glass-fiber-reinforced plasticcomposites. This process is based on a layer-by-Iayer selective solidification of composite liquidscomprising photopolymers and glass-fibers. An experimental system was developed to verify theproposed RLM process and is currently capable of producing high quality glass-fiber-reinforcedcomposite plastic parts.23REFERENCES[1] M.R. Piggott, Load-Bearing Fibre Composites, Pergamon Press, New York, 1981.[2] L.D. Schmidt, "How Chrysler is using stereolithography rapid prototyping survey results,"Proc., Fifth Int. Con! on Rapid Prototyping, June 1994, Dayton, Ohio, pp. 359-370.[3] W.B. Barlage, C.C. Jara-Almonte, A. Bagchi, A.A. Ogale, R.L. Dodey, "Fiber/resincomposite manufacturing using solid freeform fabrication," Proc., Third Int. Con! on RapidPrototyping, June 1992, Dayton, Ohio, pp. 15-24.[4] R. Charan, T. Renault, A.A. Ogale, and A. Bagchi, "Automated fiber-reinforced compositeprototypes," Proc. Fifth Int. Con! Rapid Prototyping, June 1994, Dayton, Ohio, pp. 91-97.[5] D.E. Reiff, D.W. Dorinski, and S.D. Hunt, "Method of manufacturing a three-dimensionalplastic article," Motorola Inc., U.S. Patent 5173220, December 1992.[6] S.W. Thomas, R.D. Key, K.G. Fluegel, W.K. Jackson, K.D. Elwell, and G.L. White,"Method for manufacturing fiber-reinforced parts utilizing stereolithography tooling," E-Systems Inc., U.S. Patent 5296335, March 1994.[7] A.A. Ogale, T. Renault, R.L. Dooley, A. Bagchi, C.C. Jara-Almonte, "3-D photolithographyfor composite development: discontinuous reinforcements," SAMPE Quarterly, October1991, pp. 28-38.[8] T. Renault, and A.A. Ogale, "3-D photolithography: mechanical properties of glass andquartz fiber composites," Proc., ANTEC 1992 - Con! SPE, Vol. 1, May 1992, pp. 745-747.[9] G.K. Batchelor, Introduction to Fluid Dynamics, Cambridge University Press, 1967.[10] G. Zak, N.M. Sela, C.B. Park, B. Benhabib, "A stereolithography method for the rapidmanufacture of glass-fiber-reinforced composites," Proc., 1996 ASME Winter AnnualMeeting, Atlanta, Georgia, November 17-22, 1996, DE-VoI.89, pp. 29-36.24

A Layered-Manufacturing Process For the Fabrication of Glass-Fiber-Reinforced Composites

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A Layered-Manufacturing Process For the Fabrication of Glass-Fiber-Reinforced Composites

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