CHAPTER - 1 INTRODUCTION

Transcription

1 CHAPTER - 1 INTRODUCTION

2 Polymer Matrix Composites Composite materials are formed by combining two or more materials that have different properties. The constituent materials work together to give the composite unique properties. The constituents of a composite are generally arranged so that one or more discontinuous phases are embedded in a continuous phase. The discontinuous phase is termed as the reinforcement and the continuous phase is the matrix. The reinforcements can be fibres, particulates, or whiskers, and the matrix materials can be metals, plastics, or ceramics. The reinforcing fibre or fabric provides strength and stiffness to the composite, whereas the matrix gives rigidity and environmental resistance. Polymer Matrix Composite (PMC) uses a polymer as the matrix and a fibre as the reinforcement. The resin matrix spreads the load applied to the composite between each of the individual fibres and also protects the fibres from damage caused by abrasion and impact. Polymer matrix composites offer many attractive characteristics that cannot be attainable with other materials. High strengths and stiffness, ease of moulding complex shapes, high environmental resistance all coupled with low densities, make the resultant polymer matrix composite superior to metals for many applications [Peters (1998)].. Light weight coupled with higher strength and tailor-made properties make them suitable for high performance applications such as aircrafts, satellites and submarines. Recently, particular attention has been focused on using polymer matrix composites in automotive, transport, consumer, infrastructure and sporting goods because of their advantages such as integrated component production with lower assembly costs. However, the installation of polymer matrix composites to automotive, transport, infrastructure, consumer and sport industries has been slow due to the lack of understanding of production 1

3 process, the lack of validated experimental and raw material characteristics data, absence of material design facilities, lack of clear guidelines and limited hands-on experience. Thus, the successful and cost effective fabrication of a composite component requires a proper product design and manufacturing facilities [Mazumdar (2002)]. Depending upon the type of polymer matrix used, PMC s are classified into thermoplastic PMC and thermoset PMC. Thermoplastic PMC currently represent a relatively small part of the PMC industry. Thermoplastic PMC are prepared using heat and pressure and there is no chemical reaction process occurs during processing. The thermoplastic matrix is supplied in solid form and the thermoplastic PMC is formed by inserting the reinforcement material into the molten thermoplastic matrix. The production of thermoplastic PMC s are difficult because of higher melt viscosity of the thermoplastic matrix. However, thermoplastic PMC s are prepared with filler, powder and short fibre reinforcements using injection and extrusion mouldings. Recently, long fibre thermoplastic composites manufacturing methods have been developed with the (i) injection and extrusion of relatively long fibre filled pellets of thermoplastics, (ii) reinforced reaction injection process, (iii) coating of thermoplastics on long fibres coupled with thermoforming, and (iv) Pultrusion of long fibres into the thermoplastic matrix [Mallick (2008)]. The thermoset PMC uses a thermoset resin as the polymer matrix. Thermoset resins can be cross-linked and converted to hard solid using a curing agent or an application of heat. This cross-linking operation is called curing. The thermoset PMC is formed by impregnating the resin onto a reinforcing material, followed by a curing step to produce the finished part. Thermoset resins are initially available in liquid state and hence it is easy to introduce the inserts like fillers and reinforcement fibres. 2

4 Some of the thermosets commonly used as polymer matrices are polyesters, epoxies, phenolics, polyurethanes, polyimides etc. Unsaturated polyester resins are the most widely used resin systems in many applications because of their versatility in curing, low cost and better resistance to many chemicals and water. Most unsaturated polyester resins are viscous, pale coloured liquids consisting of a solution of polyester in a monomer which is usually styrene. The addition of styrene makes the resin easier to handle by reducing its viscosity. The styrene also performs the vital function of enabling the resin to cure from a liquid to a solid by cross-linking. Moreover, the polyester resins can be moulded without the use of higher pressures, hence called contact or low pressure resins. Reinforcement fibres are fine diameter one-dimensional elements with a fairly large aspect ratio. The most common reinforcements are glass, carbon, aramid and boron fibres. More than 90 % of composites made uses glass fibre because of its all round properties and relatively lower cost. The glass fibre is commercially available in the forms namely yarns, rovings, chopped strand mats, chopped strands and woven rovings. Each of these forms has its own special application. Yarns and rovings are continuous fibres used in composite production processes such as filament winding and pultrusion. Chopped strands are used for making injection and compression moulding compounds. Chopped strand mats and woven rovings are three dimensional preforms used for making laminates for a variety of applications [Peters (1998), Mallick (2008), Leonard Hollaway (1994)]. 1.2 Processing Methods The main purpose of all polymer composite processing methods is to bring the resin and reinforcement fibre together in the required shape of the product targeting minimum void with maximum resin-fibre wet-out. Hence, the objective of any composite processing method is to accomplish a maximum wet-out, satisfying the part performance requirements with the 3

5 desired rate of production. The measure of resin impregnation is governed by the processing parameters such as applied pressure, cure temperature of the manufacturing method employed. Irrespective of the selected manufacturing technique, factors such as raw material characteristics including reinforcement permeability,fibre volume fraction, resin curing kinetics, viscosity, and product dimension and complexity affects the outcome of the finished part. These factors may get affected with changes made in the processing parameters and hence, the dependency of these factors with the process parameters should be revealed for the successful production of high quality products [Mazumdar (2002)]. The mechanical strength and the stiffness of a composite component come primarily from the reinforcement fibres, making a higher volume fraction desirable. However, increase in fibre volume fraction, decreases the degree of resin impregnation due to reduced porosity as a cause of reduced space between the fibre bundles. Consequently, this poor resin distribution can result in entrapment of air and formation of voids, affecting the final product quality. The proper selection of processing parameters of manufacturing method helps to maximize the fibre volume content. The viscosity changes and the cure kinetics during fiber wet-out are the main resin characteristics to be considered during production process. Lower the viscosity, easier the resin flow and the saturation of reinforcement fiber. Cure kinetics increases the viscosity with increase in degree of cross-linking. Moreover, cure kinetics directly affects the process efficiency, as the time for complete cure governs the production rate. Several resins have been developed specifically for each manufacturing process in accomplishing processing traits with desirable physical properties. Heat is often used to speed up the curing process [Advani et al (2003)]. 4

6 The product geometry can often dictate the selection of production process, by its size and complexity. As the geometry becomes more complicated, it becomes more difficult to force the resin and saturate the whole product domains. The resin flow can be hindered in the presence of with ribs and design features of varying thickness. The presence of uneven surfaces and hollow structures has the impact to select one process over another. Various composite production techniques such moulding, winding and other continuous automated production methods such as pultrusion are currently in use. However, the choice of the composite production process for a particular application is governed by a trade-off between lower manufacturing cost, high performance part, production rate, size, shape and ease in making complex geometries. In the winding method such as filament winding method, the resin impregnated fibres are wound on a mandrel surface in a precise geometric pattern. This is accomplished by rotating the mandrel while a delivery head precisely positions fibres on the mandrel surface. By winding continuous strands of fibre in very precise patterns, structures can be built. Filament winding machines operate on the principles of controlling machine motion through various axes of motion. The most basic motions are the spindle or mandrel rotational axis, the horizontal carriage motion axis and the cross or radial carriage motion axis. Additional axes may be added, typically a rotating eye axis or a yaw motion axis, and when the pattern calls for more precise fiber placement further additional axes may be added. Axes oriented products such as cylinders, pipes, pressure vessels, casings, nose cones etc. can be manufactured through filament winding process. Pultrusion is a manufacturing process for producing continuous lengths of reinforced plastic structural shapes with constant cross-sections. The process involves pulling the resin impregnated fibres through a heated steel forming die using a continuous pulling device. The 5

7 reinforcement materials are in continuous forms such as fibre glass roving. The resin gelation and curing is initiated by the heat from the die and a rigid structure is formed that corresponds to the shape of the die [Mazumdar (2002)]. 1.3 Moulding Techniques Composite manufacturing process through moulding techniques uses a cavity that has the shape of the product. Moulding of a composite product is accomplished by using either an open mould or a closed mould. In the open mould process, only one half of the mould is used for the development of the product. Only the surface that is in contact with the mould will be smooth and the other surface will be rough. The thickness is also not precisely controlled. All the open mould processes use wet resin lay-up. In the closed mould process, both halves of the mould are used and the product is made within the cavity of mould. The product will have good finish on both surfaces. The thickness also can be correctly controlled. The moulding processes namely hand lay-up method and resin transfer moulding method are described in the following sections [Peters (1998), Mallick (2008), Mazumdar (2002)] Hand Lay Up Process Figure 1.1 Schematic of Hand Lay Up Process. 6

8 A schematic of the hand lay-up (HLU) process is shown in Figure 1.1. Production of a composite component is done by manual laying up of reinforcement layers and liquid resin layers in sequence. A roller is used for the compaction and the homogeneous fibre wetting. Then the component is cured under room temperature and it is removed after solidification. Hand lay-up process allows the manufacture of product wide range of applications and geometries with low initial investment. Despite these advantages, there are several disadvantages which includes low reinforcement volume fraction, non uniform quality leading to uneven thickness and non uniform distribution of reinforcement material and matrix. Being an open mould process, it emits a large volume of styrene which makes the process non-environmental friendly. Furthermore, post processing fabrication is more often required which compromises the reliability of the product. Longer production time, lower production rate and high involvement of skilled labour make the process unsuitable for large scale and complex geometries production forcing the manufacturers to explore the options of closed mould alternatives such as Liquid Composite Moulding (LCM) techniques[peters (1998), Mallick (2008), Mazumdar (2002)] Resin Transfer Moulding Process Figure 1.2 Major Stages of RTM Process Resin Transfer Moulding (RTM) consists of a mould cavity that is in the shape of the part to be manufactured. The fibre preform is placed in the cavity. The mould is closed and clamped. 7

9 The resin mixed with the curing agents is then injected into the preform through one or more gates from a pressurized container. Once the cavity is full, resin injection is discontinued. Finally, the resin is allowed to cure either at ambient or at elevated temperatures [Kevin Potter (1997)] Resin Transfer Moulding Vs. Hand Lay Up RTM offers numerous advantages over HLU method. It offers higher reinforcement volume percentage, better distribution of resin and reinforcement, good double sided surface finish and uniform thickness contributing to good quality product. Cycle time in RTM process is much lower compared other moulding processes such as hand lay-up process making the process suitable for large production volume. Also, RTM being a closed mould process, styrene emission becomes less and wastage of material can be kept at minimum. Selective reinforcement and accurate fibre management, incorporation of inserts, low tooling costs, ability to produce net shape complex structures and hollow shapes, all together makes RTM process, a competitive composite manufacturing process when compared to HLU method [Kevin Potter (1997)]. 1.4 Scope and Objectives Most of the industrial composite parts that are large and complex geometry are manufactured by hand layup method. RTM is a better substitute to it, but is not used readily due to the very high cost of developing a proper mould design and determining most effective process parameters to produce the component with minimum mould filling time without the formation of dry spot. A proper mould design requires not only the geometry of the mould but also the effective injection strategy, pressure and temperature distributions. In addition, the difficulty in the mould design and also higher fabrication cost with increase in size and complexity of the component and chances of formation of dry spot is extremely high if the proper injection strategy is not used [Kevin Potter (1997), Advani et al (2003)]. 8

10 Currently, RTM process is developed by trial and error methods, leading to high development cost and sub-optimum solution. Process simulation can be effectively used to determine the effective injection strategy including the position of injection gates and vents, and the most effective values of parameters for minimum mould filling time without formation of dry spot. The effective injection strategy, and the resulting pressure and temperature distribution can be utilized to design and fabricate the mould [Kevin Potter (1997)]. A proper scaling down methodology based on process simulation can be used to manufacture a scaled down prototype for product testing and validating the process simulation of the full scale product. Manufacturing a scaled down prototype to validate the mould design and injection strategy will reduce the development cost and will enable the industry to use RTM technology instead of HLU for large and complex composite structures. While researchers have worked on different aspects on RTM, no effort has gone into develop RTM procedure with a scaled down component and use the data for scaling up to production level. The objectives of the current research are 1. To develop resin transfer moulding technology for large and complex composite structures by process simulation. Resin transfer moulding technology includes the effective injection strategy to determine the number and location of injection ports and vents, proper mould design depending on the pressure and temperature distribution and suitable process parameters such as injection and vent pressure, temperature of the mould. 2. To develop a scaling down methodology for manufacturing a scaled down prototype in which the mould filling time and mould filling pattern will be same as in the full scale product. The RTM technology can be validated by manufacturing the scaled 9

11 down prototype using the developed scaling down methodology in a cost effective way. 3. To validate the developed RTM technology for full scale composite structure using experiments on scaled down prototype. 1.5 Work Plan To meet the above objectives, first a component of industrial relevance which is large and complex in geometry and currently being manufactured using hand lay-up method was selected. Proper reinforcement materials and proper polymer matrix resin for manufacturing the component was identified. Since the manufacturing method and simulation of the manufacturing process is dependent on the properties of the reinforcement and polymer matrix resin, the important and relevant properties of resin and reinforcement matrix were identified and characterized. Viscosity, curing kinetics and rheokinetics (inter dependence of kinetic behaviour and viscosity) of resin were identified as the key characterizing parameters affecting the RTM process. Permeability of reinforcement mat, wet ability of the mat and the porosity of the mat were the key parameters for the reinforcement which need to be characterized and the characterized data need to be used for the simulation of the realistic RTM process. The characterized resin and reinforcement data were used to simulate the flow behaviour of the resin through the reinforcement structure. The flow simulation allows determining number and the location of injection ports and vents, injection pressure, pressure and temperature distribution inside the mould, and mould filling time. Validation of the model using a full scale prototype manufacturing involves a larger mould which is costly. To avoid this, a scaled down strategy were developed keeping the same flow pattern in the full scale and scaled down simulation. The simulated RTM process was validated manufacturing a scaled mould and performing the flow tracking experiments using the scaled down mould. 10

12 The schematic of this research road map is shown in Figure 1.3. Details of the activities outlined in this road map are described in the subsequent Chapters. Component Selection for a Particular Application Physical Modeling using Computer Aided Design (CAD) Resin Characterization Cure Kinetics Model Viscosity Model Gel Time Compare and Ensure Mould Fill before Resin Gelation Analysis & Feed Back Scale Down Model Mesh Full Scale Model Mesh with Visual Mesh Software Development of RTM Technology for Full Scale Model based on Simulation Isothermal Mould Filling Air Entrapment Curing Development of Scaled Down Strategy using Isothermal Mould Filling Simulations Fibre Characterization Fiber Permeability Porosity Injection Strategy Mould Fill Time Pressure Distribution Temperature Distribution Dry Spots Cure Time Heat Evolution Behavior Compare and Ensure Same Mould Fill Time, Mould Fill Pattern, Pressure Distribution Pattern Analysis & Feed Back Validation Scaled Down Mould Design & Mould Fabrication Experimental Isothermal Filling Validation of Isothermal Mould Filling Simulations on Scaled Down Model with Experimental Data Figure 1.3 Road Map of Proposed Research 11