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塑料罩注塑模具設計【說明書+PROE】,說明書+PROE,塑料罩注塑模具設計【說明書+PROE】,塑料,注塑,模具設計,說明書,仿單,proe Resin infusion between double flexible 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 flow front of the infused resin is two-dimensional and avoids flow complexities prevalent in the three- dimensional flow seen in other liquid composite molding techniques. It employs a one-sided mold, which provides obvious cost benefits 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 benefits to industry. Current work being undertaken include the refining 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 significant 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), flexible resin transfer molding (FRTM), and resin infusion by flexible 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 fiber-reinforced thermosetting polymer composites. These advantages include improved component thickness toler- ances, better surface finish, and reduced emissions of volatiles. One of the critical issues for the success of RTM processes is the proper understanding and prediction of resin flow during mold filling. Considerable work has been done in this area and some models and simulation tools are available 37. Nonetheless, the analysis of resin flow for parts with complex geometry and permeability variations still present difficulties. However, preform preparation and Composites: Part A 34 (2003) 803811 * Corresponding author. Tel.: 1-850410-6352; fax: 1-850-410-6342. E-mail address: okoliwombat.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 flexible 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 flexible female splash tool was the basis behind this process. During the 1980s, the use of a rubber bag as the flexible tool was investigated and several patents were filed 8.Theprocesswas 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 fiber 8. In the RIFT process, fibers are first placed onto a female mold that is typically coated with a release agent. Next, a flexible tooling layer is placed over the fiber and sealed around the edges vacuum tight. The fiber is then vacuum infused between the mold and flexible 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 is a 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 fiber content of the final composite laminate in the RIFT process. These parameters depend on the compressibility and relaxation of the reinforcement under pressure, and interactions with bagging film breather and other ancillary materials 8. Compression studies of dry fiber assemblies have been subject to much research 8,1015. 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 flowing resin. This provides lubrication for the fibers 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 difficult 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 flows 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 flow-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 fiber content and quality. Any interaction between the laminate and the ancillary materials during the process must be quantified 8. Summerscales 9 showed that the RIFT process reduces worker contact with liquid resin while increasing component mechanical properties and fiber 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 CibaGeigy RIFT method 4. J.R. Thagard et al. / Composites: Part A 34 (2003) 803811804 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 fiber and solid resin are placed between elastomeric diaphragms and heated so that the resin liquefies. The fiber 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 fiber volume. Finally, the cost effectiveness of the FRTM process was verified 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 fiber and resin), but eliminates the labor intensity typically associated with preparation of the three-dimensional fibrous 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 reconfigurable 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 flexible, reconfigurable 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 flat, placement of sprues and vents is independent of final 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 final 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 fiber volumes with the FRTMFig. 3. Schematic of the FRTM process 16. Table 1 Process comparison chart 16 Process Advantages Disadvantages Hand lay-up Can produce complex shapes Expensive raw material Well understood Labor intensive Not cost effective at high volumes RTM Use of low cost raw material Labor intensive perform preparation Produce complex /highly integrated parts High tooling cost3D flow difficult to control Forming Labor 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 fiber content J.R. Thagard et al. / Composites: Part A 34 (2003) 803811 805 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 flow resistance and filling time. The resin fills the grooves or channels first by vacuum pressure, and then the resin infuses into the fiber 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 five 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 fiber mats are mounted into the mold and covered by a flexible 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 fiber mats by the vacuum force. After the cavity is filled 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 flexible tooling (RIDFT) RIDFT intends to solve problems associated with other LCM techniques. These problems include achievable fiber 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 fibers, thus enhancing process formability. An advantage of the RIDFT process is that the flow of resin is two-dimensional eliminating the complexity of the three-dimensional flow 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 fiber contents. The inherent limitations restrict the part geometries to formable shapes. An advantage of RIDFT over RIFT is in the use of a second flexible 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) 803811806 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 flexible 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 significantly 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 must be chosen that can account for various materials used within the process, interactions of these materials and the force applied during forming. The current effort will investigate the PAM-FORM software since it is a general-purpose finite element package for the industrial virtual manufacturing of non-metallic sheet forming. 3.1. Materials property Within the PAM-FORM simulation model of the RIDFT process, the following three distinct material types were used and defined 20. Material type 121 for the flexible silicone sheets Material type 140 for the fiber reinforcement Material type 100 for tooling and vacuum chamber 3.1.1. Material type 121 for the flexible silicone sheets The material model is characterized as nonlinear thermo- visco-elastic for shell elements (GSell Model). Inputs for this material model include initial thickness, Youngs modulus, Poissons ratio, and mass density. The governing equation used within the PAM-FORM software is given in Eq. (1) 20. s k1 2 exp2vEexphE 2 E m 1 where, k scaling factor or material consistency in software model 1 2 exp2vE visco-elastic term for low E (strain) exphE 2 strain hardening for high E (strain) E m strain rate sensibility m strain rate hardening exp h strain hardening coefficient E modulus of the wetted fabrics 3.1.2. Material type 140 for the fiber reinforcement The material model is characterized as thermo-visco- elastic matrix with elastic fibers for shell elements. Inputs for this material model include the following 20: Material density Locking angle from a picture frame test Youngs Modulus in 0 and 908 * Stress vs. strain curves in 0 and 908 at different strain rates Shear modulus using picture frame test