Polymer Matrix Composites (introduction) According to the Classification of composites:Polymer Matrix Composite (PMC) is the material consisting of a polymer (resin) matrix combined with afibrous reinforcing dispersed phase. Polymer Matrix Composites are very popular due to their low cost and simple fabrication methods. Use of non-reinforced polymers as structure materials is limited by low level of their mechanical properties: tensile strength of one of the strongest polymers - epoxy resin is 20000 psi (140 MPa). In addition to relatively low strength, polymer materials possess low impact resistance.
• Reinforcement of polymers by strong fibrous network permits fabrication of Polymer Matrix Composites (PMC) characterized by the following properties:• High tensile strength;• High stiffness;• High Fracture Toughness;• Good abrasion resistance;• Good puncture resistance;• Good corrosion resistance;• Low cost.
The main disadvantages of Polymer Matrix Composites (PMC) are:• Low thermal resistance;• High coefficient of thermal expansion.• Two types of polymers are used as matrix materials for fabrication composites: Thermosets (epoxies, phenolics) and Thermoplastics ( Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE), polypropylene, nylon, acrylics).
According to the reinforcement material the following groups of Polymer Matrix Composites (PMC) are used:• Fiberglasses – Glass Fiber Reinforced Polymers;• Carbon Fiber Reinforced Polymer Composites• Kevlar (aramid) fiber reinforced polymers.
• Reinforcing fibers may be arranged in different forms:• Unidirectional fibers;• Rovings;• Veil mat: thin pile of randomly orientated and looped continuous fibers;• Chopped strands: thin pile of randomly orientated and looped short (3-4 inches) fibers;• Woven fabric.
Properties of Polymer Matrix Composites are determined by:• Properties of the fibers;• Orientation of the fibers;• Concentration of the fibers;• Properties of the matrix.• Properties of Polymer Matrix Composites may be estimated by the Rule of Mixtures.Polymer Matrix Composites (PMC) are used for manufacturing: secondary load-bearing aerospace structures, boat bodies, canoes, kayaks, automotive parts, radio controlled vehicles, sport goods (golf clubs, skis, tennis racquets, fishing rods), bullet-proof vests and other armor parts, brake and clutch linings.
Compression MoldingCompression Molding is a Closed Mold process in which a molding charge is squeezed into a preheated mold taking a shape of the mold cavity and performing curing (cross-linking) due to heat and pressure applied to the material.• The method uses a split mold mounted in a hydraulic pressCompression Molding process involves the following steps:• A pre-weighed amount of a polymer (commonly thermosetting resin) mixed with chopped reinforcing fibers, hardening agent, anti-adhesive agent and pigment (charge) is placed into the lower half of the mold.• The charge may be in form of powders, pellets, putty-like masses or pre-formed sheets.• The charge is usually preheated prior to placement into the mold. Preheated polymer becomes softer resulting in shortening the molding cycle time.• The upper half of the mold moves downwards, pressing on the charge and forcing it to fill the mold cavity.• The mold, equipped with a heating system, provides curing (cross-linking) of the polymer matrix (if thermosetting resin is processed).• The mold is opened and the part is removed from it by means of the ejector pin.• Compression Molding cycle time is about 1-6 min, which is longer than Injection Molding cycle.• The method is suitable for mass production of flat or moderately curved parts.
Injection Molding• Injection Molding is a Closed Mold process in which molten polymer (commonly thermoplastic) mixed with very short reinforcing fibers (10-40%) is forced under high pressure into a mold cavity through an opening (sprue).• Polymer-fiber mixture in form of pellets is fed into an Injection Molding machine through a hopper. The material is then conveyed forward by a feeding screw and forced into a split mold, filling its cavity through a feeding system with sprue gate and runners.• Screw of injection molding machine is called reciprocating screw since it not only rotates but also moves forward and backward according to the steps of the molding cycle.• It acts as a ram in the filling step when the molten polymer-fibers mixture is injected into the mold and then it retracts backward in the molding step.• Heating elements, placed over the barrel, soften and melt the polymer.• The mold is equipped with a cooling system providing controlled cooling and solidification of the material.• The polymer is held in the mold until solidification and then the mold opens and the part is removed from the mold by ejector pins.• Injection Molding is used mainly for thermoplastic matrices, but thermosetting matrices are also may be extruded. In this case curing (cross-linking) occurs during heating and melting of the material in the heated barrel.• A principal scheme of an Injection Molding Machine is shown in the picture.• Injection Molding is highly productive method providing high accuracy and control of shape of the manufactured parts. The method is profitable in mass production of large number of identical parts.• One of the disadvantages of the method is limited length of fibers decreasing their reinforcing effect.
Transfer MoldingTransfer Molding (Resin Transfer Molding) is a Closed Mold process in which a pre-weighed amount of a polymer is preheated in a separate chamber (transfer pot) and then forced into a preheated mold filled with a reinforcing fibers, taking a shape of the mold cavity, impregnating the fibers and performing curing due to heat and pressure applied to the material.• The method is used primarily for molding Polymer Composites with thermosetting resin matrices, but some thermoplastic parts may also be produced by Transfer Molding.• The picture below illustrates the Transfer Molding Process.• The method uses a split mold and a third plate equipped with a plunger mounted in a hydraulic press.• The method combines features of both Compression Molding - hydraulic pressing, the same molding materials (thermosets) and Injection Molding – ram (plunger), filling the mold through a sprue.Transfer Molding process involves the following steps:• The mold cavity is filled with preformed reinforcing fibers.• A pre-weighed amount of a polymer mixed with additives and fillers (charge) is placed into the transfer pot.• The charge may be in form of powders, pellets, putty-like masses or pre-formed blanks.• The charge is heated in the pot where the polymer softens.• The plunger, mounted on the top plate, moves downwards, pressing on the polymer charge and forcing it to fill the mold cavity through the sprue and impregnate the fibers.• The mold, equipped with a heating system, provides curing (cross-linking) of the polymer (if thermoset is processed).• The mold is opened and the part is removed from it by means of the ejector pin.• If thermosetting resin is molded, the mold may be open in hot state – cured thermosets maintain their shape and dimensions even in hot state.• If thermoplastic is molded, the mold and the molded part are cooled down before opening.• The scrap left on the pot bottom (cull), in the sprue and in the channels is removed. Scrap of thermosetting polymers is not recyclable.• Transfer Molding cycle time is shorter than Compression Molding cycle but longer than Injection Molding cycle.• The method is capable to produce very large parts (car body shell), more complicated than Compression Molding, but not as complicated as Injection Molding.
Pultrusion• It is an automated, highly productive process of fabrication of Polymer Matrix Composites in form of continuous long products of constant cross-section.• A scheme of the process is presented on the picture:• Pultrusion process involves the following operations:• Reinforcing fibers are pulled from the creels. Fiber (roving) creels may be followed by rolled mat or fabric creels. Pulling action is controlled by the pulling system.• Guide plates collect the fibers into a bundle and direct it to the resin bath.• Fibers enter the resin bath where they are wetted and impregnated with liquid resin. Liquid resin contains thermosetting polymer, pigment, fillers, catalyst and other additives.• The wet fibers exit the bath and enter preformer where the excessive resin is squeezed out from fibers and the material is shaped.• The preformed fibers pass through the heated die where the final cross-section dimensions are determined and the resin curing occurs.• The cured product is cut on the desired length by the cut-off saw.• Pultrusion process is characterized by the following features:• High productivity.• The process parameters are easily controllable.• Low manual labor component.• Precise cross-section dimensions of the products.• Good surface quality of the products.• Homogeneous distribution and high concentration of the reinforcing fibers in the material is achieved (up to 80% of roving reinforcement, up to 50% of mixed mat + roving reinforcement).• Pultrusion is used for fabrication of Fiberglasses, Carbon Fiber Reinforced Polymer Composites and Kevlar (aramid) fiber reinforced polymers.
Open mold fabrication of Polymer Matrix Composites• Open Molding (Contact Molding) is the simplest method of fabrication of Polymer Matrix Composites.• Open Molding is usually used for manufacturing large individual parts (swimming pools, boat bodies).• Open Molding method is mostly used for fabrication Glass fiber reinforcing Polymers (Fiberglasses) with polyester (sometimes epoxy or vinylester) matrix. The methods of open mold fabrication:• Hand Lay-up• Spray-up method• Tape Lay-up• Filament Winding• Autoclave Curing
Hand Lay-up• The most popular type of Open Molding is Hand Lay-up process. The Hand Lay-up is a manual, slow, labor consuming method, which involves the following operations:• The mold is coated by a release anti-adhesive agent, preventing sticking the molded part to the mold surface.• The prime surface layer of the part is formed by applying gel coating.• A layer of fine fiber reinforcing tissue is applied.• Layers of the liquid matrix resin and reinforcing fibers in form of woven fabric, rovings or chopped strands are applied. The resin mixture may be applied by either brush or roll.• The part is cured (usually at room temperature).• The part is removed from the mold surface.• The disadvantages of the Hand Lay-up method are: low concentration of reinforcing phase (up to 30%) and low densification of the composites (entrapped air bubbles).
Spray-up• In Spray-up process liquid resin matrix and chopped reinforcing fibers are sprayed by two separate sprays onto the mold surface.• The fibers are chopped into fibers of 1-2” (25-50 mm) length and then sprayed by an air jet simultaneously with a resin spray at a predetermined ratio between the reinforcing and matrix phase.• The Spray-up method permits rapid formation of uniform composite coating, however the mechanical properties of the material are moderate since the method is unable to use continuous reinforcing fibers.
• Tape Lay-up• In this method layers of prepreg (reinforcing phase impregnated by liquid resin) tape are applied on the mold surface by a tape application robot.
Filament Winding• Filament Winding method involves a continuous filament of reinforcing material wound onto a rotating mandrel in layers at different layers. If a liquid thermosetting resin is applied on the filament prior to winding the, process is called Wet Filament Winding. If the resin is sprayed onto the mandrel with wound filament, the process is called Dry Filament Winding.• Besides conventional curing of molded parts at room temperature, Autoclave Curing may be used.