CompositesPartAAppliedScienceandManufacturing03(586)

Resin infusion between double fl exible tooling prototype development J R Thagard O I Okoli Z Liang H P Wang C Zhang Department of Industrial Engineering Florida A and M University Florida State University College of Engineering 2525 Pottsdamer Street Tallahassee FL 32310 6046 USA Received 1 September 2002 revised 10 April 2003 accepted 21 May 2003 Abstract The Resin Infusion between Double Flexible Tooling RIDFT technique is a novel two stage process which incorporates resin infusion and wetting with vacuum forming The fl ow front of the infused resin is two dimensional and avoids fl ow complexities prevalent in the three dimensional fl ow seen in other liquid composite molding techniques It employs a one sided mold which provides obvious cost benefi ts when compared with resin transfer molding On going prototype development of the RIDFT process has yielded positive results Composite laminates with good surface quality micro structural characteristics and mechanical properties have been repeatedly produced with cost savings of 24 when compared with SCRIMP This paper describes the RIDFT process outlining its merits and presenting its challenges whilst identifying potential benefi ts to industry Current work being undertaken include the refi ning of production parameters the construction of a larger prototype to examine the full extent of its suitability for the manufacture of large composite components and the incorporation of the UV curing technique to reduce the cycle time in the manufacture of large structures Keywords E Forming Infusion 1 Introduction The transport sector continues to provide signifi cant growth opportunities for polymer composites with advan tages of weight savings corrosion resistance and functional integration 1 However the available production processes have limited the utilization of composite materials in the mass production sector Many of the current processes do not readily lend themselves to mass production due to long cycle times and high emissions of harmful volatile organic compounds VOCs Nevertheless liquid composite mold ing LCM techniques are technologically promising Examples include resin transfer molding RTM fl exible resin transfer molding FRTM and resin infusion by fl exible tooling RIFT These closed molding techniques also have the advantage of reducing emissions of VOCs by 90 2 Cost is a primary consideration in the development of composite production processes The marine industry manufacturers have stuck to the validated and cheaper open molding technique of hand lay up despite the US Environmental Protection Agency EPA regulations The development of the Resin Infusion between Double Flexible Tooling RIDFT technique further advances resin infusion technology reducing the higher costs of closed mold methods 2 Liquid composite molding processes 2 1 Resin transfer molding RTM Traditionally RTM has been used as the choice for the manufacturing of composite parts RTM offers many advantages over other processes for the manufacturing of fi ber reinforced thermosetting polymer composites These advantages include improved component thickness toler ances better surface fi nish and reduced emissions of volatiles One of the critical issues for the success of RTM processes is the proper understanding and prediction of resin fl ow during mold fi lling Considerable work has been done in this area and some models and simulation tools are available 3 7 Nonetheless the analysis of resin fl ow for parts with complex geometry and permeability variations still present diffi culties However preform preparation and Composites Part A 34 2003 803 811 Corresponding author Tel 1 850410 6352 fax 1 850 410 6342 E mail address okoli wombat eng fsu edu O I Okoli tooling costs can be prohibitively large for parts of more than a few meters in dimension particularly for one off or small production runs when compared with the hand lay up process 8 Fig 1 shows a schematic of the RTM process Further development of LCM processes have been targeted at reducing complexity and associated costs Some of these will be discussed in the following sections 2 2 Resin infusion under fl exible tooling RIFT Resin Infusion under Flexible Tooling RIFT is a relatively new process introduced in the 1980s A version of RIFT dates back to the 1950s when it was used in the production of boat hulls Fig 2 shows the RIFT process developed by Ciba and Geigy 9 A fl exible female splash tool was the basis behind this process During the 1980s the use of a rubber bag as the fl exible tool was investigated and several patents were fi led 8 The process was rediscovered during the 1990s and has found application particularly in the marine and automotive industries A version of RIFT is used to strengthen offshore structures with carbon fi ber 8 In the RIFT process fi bers are fi rst placed onto a female mold that is typically coated with a release agent Next a fl exible tooling layer is placed over the fi ber and sealed around the edges vacuum tight The fi ber is then vacuum infused between the mold and fl exible tooling layer thereby forming the shape of the part RIFT retains many of the environmental advantages of RTM but at a much lower tooling cost since half of the conventional rigid closed mold is replaced by a bag Adapting existing contact molds for the RIFT process may be feasible This becomes very important in mass production as there isa potential for millions of dollars to be saved from reduced tooling and manufac turing costs RIFT has some disadvantages over the RTM process RIFT offers limited direct control over the thickness or fi ber content of the fi nal composite laminate in the RIFT process These parameters depend on the compressibility and relaxation of the reinforcement under pressure and interactions with bagging fi lm breather and other ancillary materials 8 Compression studies of dry fi ber assemblies have been subject to much research 8 10 15 Pearce and Summers cales 10 noted that the response of a dry preform was dynamic Time dependent compression and relaxation were observed and repeated loading and unloading of the reinforcement achieved higher compaction at a given pressure The compression of the reinforcement during RIFT is further complicated by the arrival of the fl owing resin This provides lubrication for the fi bers and may affect the deformation of the laminate under the vacuum bag Furthermore the effective compressive force acting on the reinforcement is not constant during the process Saunders et al 15 investigated the compressibility of different fabrics plain weave twill satin non crimped stitch bonded and determined that the compressibility of a fabric depended on its type Twill weave fabrics were the most diffi cult to compress in the wet and dry states Before the arrival of the resin at a given point the dry laminate is subject to atmospheric pressure As the resin fl ows past this point the pressure in the resin rises so the new compression on the reinforcement reduces The prevailing is indicative of the possibility of fl ow induced defects with an increase in complexity of part geometry A theoretical and practical understanding of these compaction mechanisms is required in order to assess whether molded laminates can be produced with a consistent reproducible and predictable fi ber content and quality Any interaction between the laminate and the ancillary materials during the process must be quantifi ed 8 Summerscales 9 showed that the RIFT process reduces worker contact with liquid resin while increasing component mechanical properties and fi ber content by reducing voidage compared to hand lay up Furthermore RIFT offers the potential for reduced tooling costs where matched tooling RTM or compression molding is currently used 9 RIFT has many advantages over the traditional RTM process These advantages include 11 Fig 1 Schematic of the RTM process Fig 2 Schematic of the Ciba Geigy RIFT method 4 J R Thagard et al Composites Part A 34 2003 803 811804 Use of existing hand lay up molds with only minor alterations Low investment in additional equipment Reduced void content as compared to 3D infusion techniques Ability to produce very large components Nevertheless part thickness consistency is a problem with RIFT 2 3 Flexible resin transfer molding FRTM A similar process to RIFT is FRTM FRTM is an innovative composite manufacturing process developed based on detailed cost analysis which is intended to be cost effective by design FRTM is a hybrid process which combines the technical characteristics and respective favorable economics of diaphragm forming and RTM Separate sheets of dry fi ber and solid resin are placed between elastomeric diaphragms and heated so that the resin liquefi es The fi ber and resin are then compacted by drawing a vacuum between the diaphragms and formed to shape by drawing the diaphragm assembly over hard tooling 16 Fig 3 shows a schematic of the FRTM process The FRTM process was optimized to produce high quality parts with low thickness variation low void content and high fi ber volume Finally the cost effectiveness of the FRTM process was verifi ed through a mini production run 16 FRTM was designed and developed to allow for parts to be made cheaper and faster than traditional methods such as RTM A need for new cost effective means of production is often a starting point for the development of a new process such as FRTM from the classical RTM process The comparative advantages and disadvantages of the vacuum forming version of FRTM and several other currently available processes such as RIFT are shown in Table 1 Conceptually FRTM is a hybrid process which combines favorable characteristics of RTM and diaphragm forming Like RTM FRTM uses the lowest cost constitutive raw materials possible dry fi ber and resin but eliminates the labor intensity typically associated with preparation of the three dimensional fi brous preform used in RTM In FRTM fabric is formed in a one step double diaphragm forming process This reduces labor intensity and decreases cycle time FRTM can also reduce the tooling costs typically associated with RTM because no heavy matched tooling is required 16 The second advantage of the FRTM process arises from the fact that the diaphragm system is by nature deformable and provides a low cost reconfi gurable tooling surface Through the use of various forming methods such as vacuum forming and matched mold stamping it is possible to reduce the tooling costs associated with dedicated matched tooling in the traditional RTM process Reduced tooling costs can come from lighter weight tooling one sided tools or through the economic advantages of a fl exible reconfi gurable forming mechanism FRTM also reduces or eliminates tool cleaning which is typically labor intensive 16 The third advantage of the FRTM process is the repeatability of the impregnation process which is quicker and more easily controlled This results from conducting the resin impregnation along the part thickness direction which is relatively shorter than the other two in plane directions Additionally by impregnating in the fl at placement of sprues and vents is independent of fi nal part geometry The traditional costly experimentation necessary to optimize processing variables and redesign tooling to achieve void free uniform wet out is eliminated and development time for new parts is greatly reduced since new learning is not required Given that the resin begins in a position very close to its fi nal location the process is inherently quicker and more controllable than the transverse impregnation method typically associated with RTM 16 Table 1 shows the disadvantages of the FRTM process Many forming processes have limitations in the geometries that can be formed Undercuts cannot be produced with the vacuum forming mechanism The control of thickness variation and achievable fi ber volumes with the FRTMFig 3 Schematic of the FRTM process 16 Table 1 Process comparison chart 16 ProcessAdvantagesDisadvantages Hand lay up Can produce complex shapes Expensive raw material Well understoodLabor intensive Not cost effective at high volumes RTMUse of low cost raw material Labor intensive perform preparation Produce complex highly integrated parts High tooling cost3D fl ow diffi cult to control FormingLabor cost is reduced Expensive raw material One step bulk deformation Complexity limited to formable shapes FRTM and RIFT Uses low cost raw materials Complexity limited to formable shapes Less labor content bulk deformation Thickness variation potential 2D impregnation easier to control Limits in achievable fi ber content J R Thagard et al Composites Part A 34 2003 803 811805 process is potentially limited Control of thickness variation is optimized using close loop process control and through judicious selection of resins whose properties were best suited for the unique requirements of the FRTM process Fiber volume is closely related to the compaction pressure applied to the fabric during cure therefore varies depending on the forming method employed 16 2 4 Vacuum bag molding VBM and Seaman composites resin infusion molding process SCRIMP The VBM technique is a closed mold technique and a cost effective alternative to the open mold processes SCRIMP is a popular version of the VBM In this process a network which consists of grooves or channels is used to distribute the resin and reduce the fl ow resistance and fi lling time The resin fi lls the grooves or channels fi rst by vacuum pressure and then the resin infuses into the fi ber perform In VBM a one sided rigid mold and a bag are used to form a mold cavity 17 The VBM process can be divided into fi ve steps First in pre molding the mold surface is cleaned and then a mold release agent and a gel coat are sprayed on the surface Next during reinforcement loading dry fi ber mats are mounted into the mold and covered by a fl exible bag The cavity is sealed by vacuum tapes or other techniques and channel networks or grooves form In the third step the cavity of the mold is vacuumed and resin infuses into the fi ber mats by the vacuum force After the cavity is fi lled with resin resin begins curing and solidifying into the composite part called the resin curing step Finally the cured composite is taken out of the mold and the next cycle begins 17 2 5 Resin infusion between double fl exible tooling RIDFT RIDFT intends to solve problems associated with other LCM techniques These problems include achievable fi ber volume part thickness consistency manufacturing cycle time and process complexity Although not all problems have been currently addressed it was the intent of this research to use RIDFT to overcome the shortcomings and limitations of other LCM techniques Fig 4 shows a schematic of the RIDFT process Unlike the FRTM process the RIDFT process does not use dry solid sheets of resin but currently uses a low viscosity room temperature curing thermoset The room temperature thermosets can vary hardener content to allow for partial curing within 10 minutes of completed infusion which allows for the partially cured part to be removed Furthermore the low viscosity resin may provide better lubrication for reinforcing fi bers thus enhancing process formability An advantage of the RIDFT process is that the fl ow of resin is two dimensional eliminating the complexity of the three dimensional fl ow front experienced with RTM 18 Other advantages of RIDFT include lower tooling costs when compared with RTM reduced production times the incorporation of UV curing techniques and the potential for attaining higher fi ber contents The inherent limitations restrict the part geometries to formable shapes An advantage of RIDFT over RIFT is in the use of a second fl exible tooling that reduces cleanup and manufac turing preparatory work With RIDFT resin does not contact the mold surface and eliminates the need to prepare the mold before each cycle In addition to reducing a manufacturing step this does not lend itself to tool wear experienced from continuous use as seen in the RTM process For the RIDFT process porous aluminum mold technol ogy can be utilized International Mold Steel 19 constructed the porous mold seen in Fig 5 for use with RIDFT The Swiss manufacturer Portec introduced a unique patented material with a trade name METAPOR 19 This commercially available product consists of aluminum granules encased by epoxy resin and compressed under high pressure The combination of materials and the manufacturing process results in a cast block having the appearance and feel of solid metal while being completely micro porous and permeable to air 19 Fig 5 International mold steel porous mold METAPOR Fig 4 Schematic of the RIDFT process J R Thagard et al Composites Part A 34 2003 803 811806 The METAPOR technology allows the RIDFT process to overcome potential problems The vacuum driven forming step in the RIDFT process is the key to forming part shapes Micro pores in the mold allows for vacuum to be pulled from all areas within the mold which allows part intricacy to be increased as problems associated with air pockets are no longer an issue Fig 6 shows that when using a non porous mold the vacuum cannot form the fl exible layer into the V shaped groove Once the vacuum is evacuated from between the non porous mold and the silicone sheet the forming can no longer occur However with the porous mold surface the air is evacuated from all areas on the mold surface and allows for the silicone sheet to form into the V shaped groove Due to low forming pressures and the lack of contact between the resin and the mold surface RIDFT mold cost is signifi cantly less than with other liquid molding processes 3 Modeling of RIDFT forming Understanding the forming mechanics and the prediction of the formability of desired geometries within the RIDFT process necessitates the creation of a simulation model The RIDFT process is dynamic and simulation software