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Session 2003-1482INTRODUCTION TO LOW COST MANUFACTURING OF COMPOSITELAMINATESAjit D. Kelkar, Jitendra S. Tate and Ron BolickDepartment of Mechanical EngineeringNorth Carolina A & T State University, Greensboro, NC [email protected]:This new laboratory experiment in MEEN 300 Experimental Methods in Engineering inundergraduate mechanical engineering curriculum, presents new low cost fabrication processknown as Vacuum Assisted resin Transfer Molding (VARTM) to manufacture compositelaminates. This experiment emphasizes mechanical engineering concepts such as ability to applyknowledge of science; ability to design a process to meet desired needs; knowledge ofcontemporary issues; ability to solve engineering problems which are listed in course contentevaluation form of ABET (American Board of Engineering and Technology). The experiment isdivided into three laboratory sessions. Each session runs approximately one hour and fiftyminutes.LABORATORY SESSION 1:In this session students are introduced to composite materials and different manufacturingtechniques. The VARTM process is discussed in detail.Overview of CompositesA composite material is made of two or more chemically different materials with adistinct interface between them. The individual constituents maintain their own properties.“Proceedings of the 2003 American Society for Engineering Education Annual conference & ExpositionCopyright 2003, American Society for Engineering Education”Page 8.785.1However, the combination of materials develops a material that has properties and characteristics
different than those of the original constituents. The properties of the composite material dependon the properties and geometry of the constituent materials and the distribution of the phases.The composites are becoming popular in industry due to their high specific strength andhigh specific modulus. They have improved corrosion and wear resistance, low thermalconductivity, and increased fatigue life. The endurance limit of toughened composites can bemuch higher than for steel and aluminum. Composites have certain disadvantages: they areexpensive; there is lack of high productivity manufacturing methods and clear-cut design rules.Composites have enormous number of applications in the aerospace, automotive, construction,sports and medical industry.Constituent materials in the composite are fibers and matrix. Fibers are major load carryingcomponents. Matrix transfers stresses between the fibers, provides barrier against adverseenvironments, protects the surface from abrasion and provides lateral support. The differentfibers used are glass, carbon, aramid, boron and alumina.There are mainly four different types of composite materials depending upon the matrix used.They are Polymer Matrix Composite (PMC), Metal Matrix Composite (MMC) and CeramicMatrix Composite (CMC) and Carbon/Carbon composites. PMC’s are suitable for relatively lowtemperature applications. Polymer matrix is called resin. The popular resins are polyester, vinylester, cyanate ester and epoxy [1].There are various forms of fiber reinforcements in the composites like unidirectional,multidirectional, woven, knitted, braided and stitched. Each of these forms has certainadvantages. There are various methods used in manufacturing of composites.METHODS USED IN MANUFACTURING OF COMPOSITES:There are various methods that are used to manufacture the composite laminates [2].These methods include wet lay-up, prepreg method, autoclave processing, filament winding,pultrusion, resin transfer molding (RTM) and vacuum assisted resin transfer molding (VARTM).The brief description of these methods is given in the following section. This section alsopresents merits and demerits of these methods. In addition this section also explains VARTMprocess in detail.Copyright 2003, American Society for Engineering Education”Page 8.785.2“Proceedings of the 2003 American Society for Engineering Education Annual conference & Exposition
Wet lay-up methodThis is one of the oldest methods that involve laying the dry reinforcement (most often afabric or a mat) into the mold and applying the resin. The wet composite is rolled by hand toevenly distribute the resin and thereby removing the air pockets. Another layer of reinforcementis laid on top, after which more catalyzed resin is poured, brushed, or sprayed over thereinforcement. This sequence is repeated until the desired thickness is reached. The layeredstructure is then allowed to harden (cure). This method is conceptually simple, does not requirespecial handling of wet fabrics, and allows the resin to be applied only in the mold, thus helpingto maintain a neat surrounding area. But it is very difficult to maintain product uniformity. Voidsare common problem. Mechanical properties are low compared to other compositemanufacturing methods.Prepreg methodIt can be viewed as an extension of the wet lay-up method. The fabrics are usually aunidirectional tape or a woven fabric, impregnated with initiated resin, partially cured and thenrolled up for shipment. But the prepreg method requires a vacuum bagging and is oftenautoclaved. The resin distribution in the prepreg method is usually very even and is controlledduring tape manufacture. But this method is slow and labor intensive compared to the automatedmethods and has a potential high rejection rate because of faulty bagging procedures.Autoclave processingThe autoclave consists of a vessel that can be pressurized internally up to 5 bar ( 75 psi)and then the contents are heated. The vessel must be sufficiently large to accommodate largecomponents. They are pressurized with gas, usually nitrogen that is circulated through theheaters to maintain a uniform temperature throughout the vessel. The basic feedstock for theprocess is preimpregnated warp sheets or prepreg. A raw laminate along with a bleeder pack isplaced under a nonstick gas permeable film and then that is followed by a breather pack. Thiswhole unit is kept in a vacuum bag to maintain vacuum pressure on the laminate. The outermembrane is pressed against the laminate by atmospheric pressure. The whole unit is then placedin the autoclave where the bagged molding may be reconnected with the evacuation system to“Proceedings of the 2003 American Society for Engineering Education Annual conference & ExpositionCopyright 2003, American Society for Engineering Education”Page 8.785.3maintain the vacuum. The autoclave is pressurized which augments the consolidated pressure.
The temperature of the autoclave is reduced when the resin is adequately cured. The main aim ofthis process is to manufacture the laminate with uniform thickness and to ensure minimumporosity. The major difficulty in the autoclave process is the high capitalization cost and thestringent pressure code regulations.Filament windingA continuous tape of fibers impregnated with resin is wrapped over a mandrel to form apart. Successive layers are added at the same or different winding angles until the requiredthickness is attained. The mandrel or the application head can rotate to give the fiber coverageover the mandrel. Cylindrical parts can be manufactured with filament winding procedure. Thepressure vessels, fuel and water tanks for storage and transportation and pipes can bemanufactured by this method. Use of pressure during the cure is another method of making noncylindrical parts. The process can be used to make parts with strength in several directions. Thisprocess can easily manufacture parts with high-pressure ratings. The difficulty of this process isprogramming the winding.PultrusionContinuous reinforcement fibers are impregnated with resin and passed through a die.Then the part is cured and available for use. As this is a continuous process the production rate isvery high. The cross section of the part has to be constant for using this process. But thethickness of the part produced can be varied by having a movable dies. The part usually gels inthe die itself and is then fully cured when the part travels through a curing oven. The mainadvantage of this process is the high usage of fabrication material. But the problem can comewhen the resin or fibers accumulate and build up at the die opening and the equipment can jam.Voids can also result if the dies are run with too much opening for the fiber volume.Resin transfer molding (RTM)In this process a mold is loaded with the reinforcement material and then it is closed. Theresin is injected into it. The mold with the preform is often put under vacuum so that the vacuumremoves all the entrapped air in the preform and speeds up the RTM process. Typically, the resin“Proceedings of the 2003 American Society for Engineering Education Annual conference & ExpositionCopyright 2003, American Society for Engineering Education”Page 8.785.4is injected at the center of the top surface of the mold and the flow of resin occurs radially
outwards till it reaches the vent lines. In this process the flow of the resin occurs in the plane aswell as in the transverse direction of the preform. The fiber architecture, permeability of thepreform and the fabric crimps has an influence on the wetting of the fabric.Vacuum assisted resin transfer molding (VARTM)VARTM is a single sided tooling process where the dry preform is placed into the tooland vacuum bagged in conjunction with resin distribution and vacuum distribution lines. A lowviscosity resin is drawn into the preform through the aid of vacuum. In VARTM process, theflow of resin occurs in plane as well as in the transverse directions to the preform. Thepermeability of the preform, fiber architecture and fabric crimp has an influence on the wettingof the fabric.This process has certain advantages as listed below. Relatively low cost for high volume production Simple low cost tooling Very large and complex parts are practical High fiber volume fraction than hand lay-up On site manufacturing and repairing is possible Reduced environmental concerns than hand lay-up as it is closed systemThis process is being currently used in many of the applications in the general aviationindustry, defense sector and in the transport industry. The schematic for the fabrication is shownin the Figure 1 and is discussed in detail in the following section.VACUUM ASSISTED RESIN TRANSFER MOLDINGMaterial systemIn the present study plain-woven carbon fabric in conjunction with the epoxy vinyl ester isused to fabricate the composite panels. This plain-woven fabric (3K-70-P) is supplied by BGFIndustries, Inc. [3]. The carbon fiber used in the fabric is AS4TM manufactured by HexcelCorporation [4]. The resin used is DerakaneTM MomentumTM 411-350 epoxy vinyl ester suppliedby The Dow Chemical Company, Inc. [5]. The viscosity of the resin is 350 cps at 770Ftemperature. This is suitable to do VARTM at room temperature.Copyright 2003, American Society for Engineering Education”Page 8.785.5“Proceedings of the 2003 American Society for Engineering Education Annual conference & Exposition
Material Cost:1. Glass fabric plain weave 300.00 per roll (approximately 0.2 per sq. ft.). Each roll is300 ft. long and 5 ft. in width.2. Vinyl ester resin 5 gallon for 50.00 (approximately 2.3 per Kg.) 5 gallon 21.5 Kg3. Other related fabrics for VATRM approximately 10.00 per panel of size 2’ x 2’Eight layers 2’ x 2’ glass composite panel consumes approximately 1 Kg resin. Thusthe total cost will be 6.4 for glass fabric 2.30 for resin 10.00 for other fabrics 18.70 per panel.The VARTM process involves the following steps [6]:1.Mold preparation and fabric lay up.2.Sealing off the mold and running vacuum.3.Resin preparation and degassing.4.Resin impregnation.5.Post cure of fabricated panels.Top release fabricFabricVacuum bag(nylon film)Distribution medium(Plastics mesh)VacuumpumpResinsuctionSealant tapeBottom release fabricMold surfaceFigure 1. Schematic for the Fabrication of the Composite PanelMold preparation and fabric lay up:1. Mold Surface: The mold used for the fabrication is a metal plate. The metal plate (usuallyaluminum) has a provision of heating while the fabrication is in progress. The heating is notrequired for 411-350 resin as its viscosity is 350 cps at 770F.2. Mold Surface Protection: The 2 mil (25micron) polyester film is used to protect the moldsurface. This film facilitates the easy removal of the panel from the mold surface after thefabrication process is over.Copyright 2003, American Society for Engineering Education”Page 8.785.6“Proceedings of the 2003 American Society for Engineering Education Annual conference & Exposition
3. Bottom Release Fabric: This is a porous release material which leaves an impression onthe part suitable for secondary adhesive bonding (like tabbing) without further surfacepreparation.4. Fabric Lay-up: The size of the composite panel is 2’ x 2’. There are 8 layers or plies ofplain-woven fabric stacked one above the other. Such layered composite is also called laminate.5. Top Release Fabric: This is a porous release material which facilitates the resin flowthrough and leaves an impression on the part suitable for secondary bonding without furthersurface preparation.6. Distribution Medium: The distribution medium is a mesh, laid on top of the top releasefabric. This helps to maintain an even distribution of resin on the top of the panel and alsofacilitates the flow of resin through the thickness of the panel7. Resin and Vacuum Distribution Line: Spirally cut HDPE tubes are used for this purpose.These lines are laid above the distribution media at two sides of the fabric lay-up and go alongthe length. The resin line is closed at one end and connected to resin supply through theperistaltic pump at other end. The vacuum line is closed at one end and connected to vacuumpump through the vacuum gage. It is standard practice to place the closed ends of these linesopposite to each other.8. Breather: The breather material acts as a distributor medium for the air and escapingvolatiles and gasses. It is placed over the resin distribution media and resin and vacuum lines. Italso acts as a buffer between the vacuum bag wrinkles and part surface. It is a highly porousmaterial and mostly made of fiberglass, polyester felt and cotton. The use of breather isdependent on the fabric-resin system and the thickness of the panel.9. Vacuum Bag: 2mil (25micron) Nylon film is used as vacuum bag. This film is placed allover the mold area and is sealed firmly using special sealant tape. The sealant seals off thevacuum bag and helps to maintain a uniform vacuum throughout the molding process.10. Peristaltic Pump: This pump delivers fixed amount of resin in the mold in particular timeduration. It assures the in-plane and through-the- thickness soaking of the fabric in the resin. Thequantity of resin (say cc/min) is dependent on the pump speed. The pump speed again is functionof fabric-resin system and thickness of the panel. The speed is decided according to the previousexperience. Additionally, ON-OFF timer is connected to pump. It keeps pump running for“Proceedings of the 2003 American Society for Engineering Education Annual conference & ExpositionCopyright 2003, American Society for Engineering Education”Page 8.785.7certain period and shuts it off for certain period. In the present study, equal cycle of 30 seconds
for ON-OFF timer is used based on the previous experience with the resin and woven carbonfabric.11. Vacuum Gage: This gage monitors the vacuum in the entire vacuum bagging and resinimpregnation process. It also detects any leaks in the bagging. The vacuum less than 0.5 torr wasmaintained while bagging. Figures 2 and 3 shows vacuum bagging process.Sealing off the mold and running vacuumOnce all the fabrics and other relevant materials are laid over in particular sequence, theentire mold is sealed with sealant and vacuum bag. Then the vacuum pump is used to maintainlowest possible vacuum throughout the process.Resin Transfer MediaTop Release FabricFigure 2. Vacuum BaggingBag leaks are the most common problems observed during the fabrication process. Thismay be due to damage of the nylon film before cure. Nylon film is hygroscopic and subjected tomoisture changes due to changes in the moisture level in the surrounding environment. Dry andbrittle film can cause cracking when it is handled too much. There is also a possibility of leaks atthe nylon material and sealant interface. Once the leaks have been removed and the vacuum bagcompletely sealed, the vacuum pump is kept running for at least for 12 hrs to achieve a goodvacuum in the bag.Pleating is an important step involved in the fabrication of the panel. The pleats help to“Proceedings of the 2003 American Society for Engineering Education Annual conference & ExpositionCopyright 2003, American Society for Engineering Education”Page 8.785.8avoid air pockets in the panel. The pleats go along the edges of the fabric lay-up. The pleats help
the mold to direct the air, if entering the mold through any of the leaks, to go through them andthen subsequently through the vacuum line. At the end of the first laboratory session, thecomposite panel is vacuum bagged and lowest possible vacuum is maintained in the bag.LABORATORY SESSION 2:In the second laboratory session, students perform resin preparation and degassing, resinimpregnation operations. Details of these operations are provided below:Resin preparation and degassingThe resin is mixed with catalyst, promoter and gel time retarder in the precalculatedpercentage suggested by the resin manufacturer. The following are the proportions of resin andthe ingredients [5]: Resin – Derakane Momentum 411-350 Epoxy Vinyl Ester Cobalt Napthenate-6% - 0.02 to 0.13% by weight CoNap6% is promoter in chemical reaction 2,4-Pentanedione – 0.015 to 0.0175% by weight2, 4-P is gel time extender (retarder) Methylethylketone Peroxide – 1.0 to 1.5% by weightMEKP is catalyst in chemical processCatalyst is the substance that promotes or controls curing of the resin without beingconsumed in the reaction. Promoter is chemical additive that accelerates the curing process.Curing of theroset resin is exothermic reaction. It dissipates the heat to the surroundings. Geltime retarder is chemical additive, which absorbs free radicals once exothermic reaction hasstarted. It retards curing process, so that molding will be complete before resin gels.Before adding the resin to the mold, it has to be free of all the air pockets that may causevoids if they enter the mold. For this purpose, the resin is kept in a cylinder that maintains avacuum of approximately 5 torr. This enables the suction of all the gases that have been trappedinto the resin.Copyright 2003, American Society for Engineering Education”Page 8.785.9“Proceedings of the 2003 American Society for Engineering Education Annual conference & Exposition
Resin impregnationOnce the resin is ready it is injected into the mold at a very slow rate. The flow of resin iscontrolled with the help of peristaltic pump in such a way that it is allowed to flow in thedistribution medium for some distance and then the resin inlet is shut off to enable the resin gothrough the thickness. This cycle is repeated until the whole panel is soaked in resin. Figure 3shows the resin impregnation setup. It took approximately 50 minutes of flow time for completesoaking of the panel. The panel was kept in the mold for 24 hours at the room temperature forcuring which is called green cure.Temperature andHumidity GageSealant TapeResin FlaskVacuumDistribution LineVacuum BagResinDistribution LineFigure 3. Resin ImpregnationLABORATORY SESSION 3:Before students come for the third laboratory session, at the end of second laboratorysession (after green cure), the panel is removed from the mold and kept in the oven at 1800 F for8 hours for post cure.In the third laboratory session, student use density method to calculate the fiber volumefraction of the cured panel [1]. Details of the density method are given below.The density of composite panel, post cured resin and carbon fibers were found by usingthe techniques explained by ASTM D792-86. The Figure 4 shows the experimental set-up. The“Proceedings of the 2003 American Society for Engineering Education Annual conference & ExpositionCopyright 2003, American Society for Engineering Education”Page 8.785.10expression for fiber volume fraction based on the density of the composite is:
Vf ρc ρmρ f ρmwhere,ρ f , ρm and ρ c densities of fiber, matrix and composite.Figure 4. Density MeasurementThese fabricated panels later can be cut into the tension test coupons as suggested byASTM 3039. These tensile coupons can be tested by same group of students in the Strength ofMaterials Laboratory.DeliverablesAt the end of three laboratory sessions, following deliverables are expected from thestudents:1. Good quality composite panel free from any defects such as random air pockets at the surface,dry fibers, large variation in thickness.2. The batch of student will write laboratory report, which will have following details. Process parameters during manufacturing which are temperature of mold, temperature of“Proceedings of the 2003 American Society for Engineering Education Annual conference & ExpositionCopyright 2003, American Society for Engineering Education”Page 8.785.11resin before and after mixing, temperature after degassing, vacuum in the bag before and
during impregnation, vacuum during degassing, time for degassing, time forimpregnation, time for green cure and time for post cure. Students are also encouraged tonote unusual things occurring during composite panel manufacturing. Overall fiber volume fraction in the composite panel.This new experiment generated lot of interest in undergraduate students to perform theresearch in the general area of manufacturing of composites. This laboratory experiment has yetto be assessed. It is planned to assess this laboratory experiment upon class completion this year.Bibliography1.Daniel I. M.; Ishai Ori; 1994, “Engineering Mechanics of Composite Materials”, Oxford University Press2.Strong A. B.; 1989, “Fundamentals of Composite Manufacturing: Materials, Methods, and Applications”,Society of Manufacturing Engineers3.BGF Industries, Inc.; “http://www.bgf.com/cchart.htm4.Hexcel Corporation, Inc.; ntinuous / Productlist.htm”5.The Dow Chemical Company, Inc.; 1-350.htm”6.Kelar Ajit D. and Tate Jitendra S.; 2002, “Low Cost Manufacturing of Textile Composites Using VacuumAssisted Resin Transfer Molding”, All India Manufacturing Design and Research Conference, Ranchi,India, December 2002.BiographyDR. AJIT D.KELKAR is a Professor of Mechanical Engineering at North Carolina A&T State University. Hisresearch interests include manufacturing of composite materials, finite element modeling, fracture mechanics, hightemperature materials, ceramics and composites. Specifically, his work has included the low cost manufacturing ofcomposite materials, damage characterization of thin and thick composite laminates subjected to low velocity impactloadings, three dimensional finite element micro mechanics modeling of composites, modeling of textilecomposites, geometrically nonlinear plate and membrane problems, modeling of ceramic composites, fracturetoughness studies of high strength materials, finite element modeling of offset car crash simulations. He is themember of several professional societies including ASME, AIAA, and ASEE.Copyright 2003, American Society for Engineering Education”Page 8.785.12“Proceedings of the 2003 American Society for Engineering Education Annual conference & Exposition
JITENDRA S. TATE is a doctoral student at North Carolina A&T State University. Currently, he is researching onFederal Aviation Administration’s project “Performance Evaluation and Modeling of Braided Composites”. Hisresearch interests include low cost manufacturing of composites, fatigue behavior of composites, finite elementmodeling, CADD, mechanical event simulations, and statistical analysis. He is the student member of ASME.RONNIE BOLICK is Research Projects Manager of the Mechanical Engineering Laboratory, a Researcher and PhDcandidate at North Carolina A&T State University. His research areas have been in embedded fiber optic sensors,fatigue and durability studies for the automotive industry for replacement of mechanical fasteners, manufacturing ofcomposite materials, high temperature materials, both ceramics and composites and low cost manufacturing processdevelopment for composite laminates. He has extensive experience in testing and data acquisition ranging from:low velocity impact studies, stress and strain measurement using laser displacement sensors, strain gauges and loadcells, and component life/endurance limit prediction. He is a member of several professional societies includingASEE, ISA and SAE.Copyright 2003, American Society for Engineering Education”Page 8.785.13“Proceedings of the 2003 American Society for Engineering Education Annual conference & Exposition
fabric or a mat) into the mold and applying the resin. The wet composite is rolled by hand to evenly distribute the resin and thereby removing the air pockets. Another layer of reinforcement is laid on top, after which more
