Manufacturing Methods for Composites

Manufacturing Methods for Composites
Professor Joe Greene CSU, CHICO Copyright Joseph Greene 2001

Copyright Joseph Greene 2001
Objectives Identify the major manufacturing methods for composites Discuss the advantages and disadvantages of each Contrast and compare methods for thermoplastic and thermoset composites Identify areas of work for improvements in manufacturing methods Copyright Joseph Greene 2001

Overview Manual lay-up Wet and Prepreg Vacuum Bagging Autoclave curing Filament winding Pultrusion Matched-Die Molding SMC and BMC Preform molding SRIM and RTM Spray-up Thermoplastic Composite molding Copyright Joseph Greene 2001

Manual lay-up Manual lay-up Simplest technique and is called lay-up molding, wet lay-up, laminating Fabric or mat is saturated with a liquid resin and the lay-up is made by building layer upon layer to obtain the desired thickness Prepreg Wet the reinforcement with resin and partially cure the resin Careful control is taken to insure desired rein/fiber ratio Both methods have the layers placed onto a shaped surface (mold) by hand. Pressure in normally applied by hand rolling, wiping with a squeegee, or using vacuum bagging to remove trapped air and provide better uniformity. Part is cured at room temperature or at elevated temperature with heat gun or in an autoclave or oven Copyright Joseph Greene 2001

Manual lay-up Wet lay-up Oldest method, though rarely used today because of difficulty in handling wet sheet of reinforcement and high labor costs. Mold release is added to prevent resin from sticking to the mold Wax, PVOH, silicone, fluorocarbon, or plastic film Gel coat is added to the mold so that the fibers can be hidden on painted part. Gel coat is a layer of catalyzed resin that is applied to the release coated mold and allowed to cure before composite lay-up Gel coats improve flexibility, blister resistance, strain resistance, weatherability, and toughness. Surface veil glass is added to the composite lay-up on the top and bottom layers to provide better surface finish and hide the glass fiber read through. Vacuum bagging is used to remove the voids in the part. Parts are removed manually from the mold with the help of flat plastic, wooden, or metal wedges Copyright Joseph Greene 2001

Manual lay-up Molds for wet-layup Molds can be made of any materials though usually composite materials (epoxy or polyester with glass fiber)or aluminum or steel. The choice depends upon the maximum number of parts desired. The parts can be made from one side of the mold, either the outside of the part (female mold) or the inside of the part (male mold). The side of the part that you want the best finish decides whether you use the male or female maold. For bathtubs you want the best surface finish on the inside of the part so you use a male mold. For Boats, you want the best surface finish on the outside of the part so you use a female mold. Curing of wet lay-up Typically done at room temperature with the use of a promoter to speed up the reaction. Never mix the promoter with the catalyst together at the same time Mix the catalyst in the resin, then the promoter in the resin. Copyright Joseph Greene 2001

Manual lay-up Prepreg method Prepreg method is an extension of the wet lay-up method. Pre-wetting outside of the mold and then laying-up the composite. Fibers are arranged in a unidirectional tape or a woven fabric and are impregnated with initiated resin (partially cured), rolled for shipment Prepreg method is more precise than wet lay-up method Requires vacuum bagging and autoclaving for heat and pressure. Dimensions Prepreg is supplied in rolls of widths from 3” to 72 “ (usually 12” to 24”) Cut to fit mold and layed-up layer by layer until desired thickness is achieved. Prepregs have limited shelf life due to partially cured resin. Usually several days to weeks at room temperature. For unidirectionl prepreg, the strength in the cross-fiber direction is essentially the strength of the resin alone. To achieve strength in all directions, the prepreg layers are oriented in different directions, e.g., 0° (fiber direction), 90°, +45°,-45°,-45°,+45°,90°,0° Fiber ply direction lay-up should be symmetrical (same number top to bottom) Copyright Joseph Greene 2001

Manual lay-up Advatages of Wet Lay-up Tooling can consist of any material that will hold its shape under minimal pressure. Tooling can be changed easily for engineering changes. Investment in pressure devices such as press, autoclave, or vacuum pump is not required. The pump can improve the quality of parts. Curing ovens are not needed Disadvantages of Wet Lay-up Only addition-type cross-linking resin can be used, because condensation polymerization can cause bubbles and voids in part, which requires pressure. Product is nonuniform and voids are common Mechanical properties are low in comparison to other methods Tight weave fabrics are difficult to saturate with high viscosity resins Resin rich areas are common causing fracture points and shrinkage. Only one finished surface Copyright Joseph Greene 2001

Manual lay-up Advatages of Pre-preg Method Resin/initiator (hardener) ratio is more accurately controlled. Resin distribution is more controlled resulting in more uniform part. Fewer health and safety concerns due to lack of liquid resin Automation can improve output. Higher fiber content, better part definition, and better consolidation Disadvantages of Pre-preg Method Methods are slow and labor intensive compared to automated methods High reject rate because of faulty bagging procedures. Difficult to bag complex shapes Curing equipment (autoclaves) are expensive Inside surfaces are not satisfactory as in matched-die molding. Copyright Joseph Greene 2001

Manufacture of Lay-up Materials
Manufacture of Prepreg Materials Solvent impregnation: Fiber bobbins are placed on a pay-off device such as creel, Fibers are collimated into a ribbon Ribbon is passed through a resin/solvent bath. Resin/fiber is squeezed through nip rollers for proper ratio. Resin/fibers are heated to remove solvent and cure resin. Pre-preg is wound onto drum and stored at 0°F. Hot melt impregnation: Resin is calendered (rubber mixing rollers) or cast into a film. Collimated fibers are sent to hot rollers with hot resin applied to them. Tape and resin are heated and pressed in rollers. Roll is cooled and stored. Sizes Prepreg are available in styles of fabric up to 72” (1.83 m) wide Sizes range from 12 to 48” are most common Sizes from 3, 6, or 12” are used in automated lamination Narrow tapes have tighter tolerances on width, longer defect free lengths, and lower and more repeatable tack. Copyright Joseph Greene 2001

Manufacture of Prepreg Materials (ref: Copyright Joseph Greene 2001

Manufacture of Prepreg Materials (ref: Companies (ref: » Aeroform Ltd.; Automated Dynamics » Brenner International ; Composite Matrix Corporation » Diaphorm; Eurocarbon BV » Gerber Technology Inc.; Magnum Venus Products - M.V.P. » Mikrosam; Owens Corning » Sutherland & Assoc. of Los Angeles Copyright Joseph Greene 2001

Manufacture of Automated Tape Lamination
Ref: New machines and concepts can speed-up the lay-up process. An overhead gantry moves a tape application head across the mold, and up inclined faces to apply a prepreg tape, 3" width is typical. Cutting and trimming is done automatically. NC programs direct the tape layup, often in geodesic paths. Advantages Time and labor savings More uniform parts due to more consistent lay-up pressures. Lower stresses in fibers during lay-down. Disadvantages Cost to program the machine plus cost of the machine Presence of consistent gap between adjacent paths; Restricted to simple parts. Long production time for small parts. Copyright Joseph Greene 2001

Cutting of Uncured Composite Materials Manual cutting of uncured composite parts has been a major method for many years. Utility knives, carbide disc cutters (pizza cutters), shears (powered or manual) Template is used to guide the cut on a soft urethane, rubber, plywood, or aluminum sheet that is placed on a hard cutting table. Multiple sheets can be cut at the same time to reduce costs and increasing consistency. Automated cutting can be used for high volume parts and more relatively simple (planar) parts. Ultrasonics, water jets (care is required not to wet the materials), die cutting, laser cutting Punch Presses Copyright Joseph Greene 2001

Vacuum Bagging Application of a vacuum to the resin helps eliminate residual materials/gas trapped in the uncured resin, e.g., air pockets, solvents, and low molecular weight resin components Copyright Joseph Greene 2001

Vacuum Bagging Procedure Coat the mold with mold release agent to allow the part to easily separate later. Remove prepreg materials from the freezer. Allow the materials to warm to room temperature to reduce condensation which can contaminate the polymer materials. Build up the prepreg layers of the part. Inserts, ribs, can be placed in mold. Put a layer of release film on the part. This allows resin to flow out under vacuum, and leaves a good bonding surface for subsequent composite layers. Add the bleeder layer. This layer will soak up excess resin. It is typically a mat of cotton, polyester felt, or fiberglass (with teflon coat), etc. Add a layer of barrier to prevent resin movement to the vacuum valve, but allow air movement. A resin trap should be used in the vacuum system if this step is omitted. Add a layer of breather material. This will act as a buffer between the wrinkles in the bag, and the part surface. It also allows better distribution of the vacuum. Apply a sealant around the edges of the part. This can be a foam tape. Put the vacuum bag over the part, and seal at the edges. A typical material is nylon. The vacuum is then applied, and possibly a curing oven is used to accelerate curing. Copyright Joseph Greene 2001

Autoclave An oven that allows for high pressures to be used. Composites cure under heat and pressure provides a superior part because the voids are reduced due to the pressure. Process The part is placed in the pressure vessel, and heated, pressure is applied simultaneously. Vacuum bagging can be used in an autoclave. Thermoset composites are crosslinked. Thermoplastics are melted. Advantages The pressure helps bond composite layers, and remove more voids in the matrix. Very large parts can be made with high fiber loadings. Properties are improved. Many different parts can be cured at the same time. Disadvantages Autoclaves are expensive Copyright Joseph Greene 2001

Filament Winding Ref: Filament Winding Process Fig 5-1 For Round or Cylindrical parts A tape of resin impregnated fibers is wrapped over a rotating mandrel to form a part. These windings can be helical or hooped. There are also processes that use dry fibres with resin application later, or prepregs are used. Parts vary in size from 1" to 20‘ Winding direction Hoop/helical layers Layers of different material High strengths are possible due to winding designs in various direction Winding speeds are typically 100 m/min and Typical winding tensions are 0.1 to 0.5 kg. Copyright Joseph Greene 2001

Filament Winding Ref: Demolding To remove the mandrel, the ends of the parts are cut off when appropriate, or a collapsible mandrel (e.g., low melt temperature alloys ) is used. Curing in done in an Autoclave for thermoset resins (polyester, epoxy, phenolic, imide, silicone) and some thermoplastics (PEEK) Fibers are E-glass, S-glass, carbon fiber, and aramids (toughness and lightweight) Inflatable mandrels can also be used to produce parts that are designed for high pressure applications, or parts that need a liner, and they can be easily removed. Advantages Good for wide variety of part sizes Parts can be made with strength in several different directions Very low scrap rate Non-cyclindrical parts can be formed after winding Flexible mandrels can be left in as tank liners Reinforcement panels, and fittings can be inserted during winding Due to high hoop stress, parts with high pressure ratings can be made Disadvantages Viscosity and pot life of resin must be carefully chosen NC programming can be difficult Some shapes can't be made with filament winding Factors such as filament tension must be controlled Copyright Joseph Greene 2001

Laminating EHA - Coating machines are used for the manufacture: paper and plastic coating manufacture of flexible abrasives slitting and cross-cutting of webs application of glue laminating manufacture of PVC floors special scopes of application in the field of coating technology Copyright Joseph Greene 2001

Pultrusion Ref: Manufacturing Fibers are brought together over rollers, dipped in resin and drawn through a heated die. A continuous cross section composite part emerges on the other side. Design (Fig 5-4) · Hollow parts can be made using a mandrel that extends out the exit side of the die. · Variable cross section parts are possible using dies with sliding parts. · Two main types of dies are used, fixed and floating. Fixed dies can generate large forces to wet fiber. Floating dies require an external power source to create the hydraulic forces in the resin. Multiple dies are used when curing is to be done by the heated dies. · Very low scrap. Up to 95% utilization of materials (75% for lay-up). · Rollers are used to ensure proper resin impregnation of the fiber. · Material forms can also be used at the inlet to the die when materials such as mats, weaves, or stitched material is used. · For curing, tunnel ovens can be used. After the part is formed and gelled in the die, it emerges, enters a tunnel oven where curing is completed. · Another method is the process runs intermittently with sections emerging from the die, and the pull is stopped, split dies are brought up to the sections to cure it, they then retract, and the pull continues. (Typical lengths for curing are 6" to 24") Copyright Joseph Greene 2001

Pultrusion Ref: Materials Most fibers are used (carbon, glass, aramids) and Resins must be fast curing because of process speeds. (polyester and epoxy) Processing speeds are 0.6 to 1 m/min; thickness are 1 to 76 mm; diameters are 25mm to 5m double clamps, or belts/chains can be used to pull the part through. The best designs allow for continuous operation for production. diamond or carbide saws are used to cut sections of the final part. The saw is designed to track the part as it moves. these parts have good axial properties. Advantages good material usage compared to lay-up high throughput and higher resin contents are possible Disadvantages part cross section should be uniform. Fiber and resin might accumulate at the die opening, leading to increased friction causing jamming, and breakage. when excess resin is used, part strength will decrease void can result if the die does not conform well to the fibers being pulled quick curing systems decrease strength Copyright Joseph Greene 2001

Matched Die Molding Matched die molding involves molding of composites with two sided molds. Materials The molds are typically steel molds and Production rates Used for high volume production (up to 1 million parts per year) Processes SMC molding- compression molding with polyester and glass. BMC molding- similar to SMC but with a lot of filler. GMT molding- compression molding of thermoplastic PP and glass. Preform molding- spray process of polyester with chopped glass. SRIM molding- molding of liquid polyurethane resin and glass preform. RTM molding- molding of polyester resin and glass preform. Copyright Joseph Greene 2001

BMC Materials BMC- Bulk Molding Compound Materials BMC is a combination of chopped glass strands with resin as a bulk prepreg. Unlike SMC, it is not necessary to have recourse to a maturation step, and consequently, BMC prepreg formulations contain higher filler contents. The reinforcements are essentially chopped glass strands of 6 or 12mm. The reinforcement content is generally between 10 and 20%. The filler is usually calcium carbonate with consequent economic benefit. The resins are polyester, vinyl ester, epoxy, urea, melamine, phenolic, or polyurethane Processing BMC is suitable for either compression or injection molding. Design Injection molding of BMC can produce complex components such as electrical equipment, car components (headlamps), housings for electrical appliances and tools in large industrial volumes. Copyright Joseph Greene 2001

Sheet Molding Compound (SMC)
SMC is the paste that is compression molded 33% polyester resin and styrene, which polymerizes and crosslinks 33% glass fibers (1” fibers) 33% Calcium Carbonate Copyright Joseph Greene 2001

Materials Resins: polyester; Fibers: 0.5” to 1” E-glass; Filler: Calcium Carbonate Body panel Class ‘A’ material- Phase epsilon from Ashland Company has less glass Structural material- Has more glass fibers (up to 50%) SMC Lite- Lower density SMC that has glass bubbles in it to lower density to 1.2 g/cc Processing SMC is produced in paste form and made into a roll which is kept for 1 month to allow for viscosity and molecular weight increases. SMC is cut off the roll and cut into charges (2” x 8” strips typically) SMC strips are placed in a hot mold at strategic locations (charge pattern) and then compression molded at 400F for 1 minute. SMC molded parts typically have blisters and areas of defects at the edge of the parts that need to be sanded Design SMC Alliance Any information you may desire about SMC and BMC thermoset, crosslinking composite systems will be available from the Web site of the European Alliance for SMC. Copyright Joseph Greene 2001

Design Copyright Joseph Greene 2001

Compression Molding Process
Materials Thermosets: Polyester, Vinyl ester, or Epoxy resins with glass fiber Sheet Molding Compound (SMC), Bulk Molding Compound (BMC) Thermoplastics: Polypropylene, polyester, or others with glass fibers Glass Mat Thermoplastic (GMT), thermoplastic BMC Elastomers: Thermoplastic or Thermoset rubbers Thermoplastic Olefin (TPO), Thermoplastic Elastomer (TPE), Thermoplastic Rubber (TPR) Thermoset Styrene Butidiene Rubber Thermoplastic: Heat Plastic prior to molding Thermosets: Heat Mold during molding Copyright Joseph Greene 2001

GMT Compression Molding
GMT: Glass Mat Thermoplastic Materials Products: General Electric Company- Azdel AZDEL Laminate, GMT Resin: thermoplastic polypropylene – 60% by weight Glass: continuous glass mat- 40% by weight Five layer composite of glass fiber and thermoplastic resin. Usually compression molded in a process similar to SMC, this material may also be thermoformed using several industry standard methods of production. Typically used in more structural applications where surface finish is not an issue. Typical applications include vehicle bumper beams, underbody shields, etc. Available in three types of glass fiber mat: Chopped, Random & Uni-directional. AZDEL SuperLite® A low pressure, thermoformable composite of polypropylene and long chopped fiber combined with outer layers as needed for the application (i.e.; adhesive film, barrier film, tough PP film, non-woven, reinforcing, or just the bare surface.) A sheet product that is primarily thermoformed into shape. Typically used in less demanding structural applications where a high stiffness-to-weight ratio is required. Typical applications include vehicle interior substrates (headliners, shelves, etc.) Copyright Joseph Greene 2001

AZDEL GMT Material Applications
AZDEL is a global leader in the manufacture and supply of high-performance thermoplastic composite materials serving a wide variety of markets and industries including automotive, large truck, materials handling, HVAC, and building & construction. AZDEL is a joint venture company between GE Advanced Materials (PP plastic) and PPG, Inc.(glass-fiber technology) Products Bumper beams, seat backs, instrument panels, interior parts, roof racks, A/C housings Copyright Joseph Greene 2001

Processing of Composites
Open Mold processes Lay-up on one-sided molds Resin is added to fiber preform Pultrusion Fiber are drawn through bath before being pulled through a profile shape. Filament winding Fibers are dipped in resin before winding to shape Spray-up of glass and resin Resin is sprayed like paint as the fibers are chopped onto a screen with vacuum to form the part. Fiber-resin combination is heated for 5 to 10 minutes to cure and crosslink the resin. Part is removed from screen and then is trimmed of the flash. Copyright Joseph Greene 2001

Preform Molding: RTM and SRIM
Closed mold processing Place fiber preform inside two sided molds and inject resin onto fiber preform, heat to cure resin, demold, and trim part of flash Materials Resin: polyester, epoxy, polyurethane liquid thermoset resins Reinforcements: Continuous: glass, carbon fiber, kevlar; Discontinuous: chopped glass Processing RTM: Resin Transfer Molding VARTM: Vacuum assisted RTM RIP: Resin infusion process SCRIMP: Seeman composites resin infusion process SRIM: Structural Resin Injection Molding Copyright Joseph Greene 2001

Resin Transfer Molding
In the RTM process, dry (i.e.,non-impregnated ) reinforcement is pre-shaped and oriented into skeleton of the actual part known as the preform which is inserted into a matched die mold. The heated mold is closed and the liquid resin is injected The part is cured in mold. The mold is opened and part is removed from mold. Copyright Joseph Greene 2001

Polyurethane Processing
RIM mixing RIM flow into mold Foam machine Polyurethane processed by Casting, painting, foaming Iso and polyol are mixed under low pressure and injected into open mold. Reaction Injection Molding (RIM) Iso and polyol are mixed under high pressure and flow into closed mold Copyright Joseph Greene 2001

Processing of Composites
Open Mold processes Vacuum bag, pressure bag, SCRIMP VARTM, Vacuum Assisted Resin Transfer Molding autoclave: Apply Vacuum Pressure and Heat in an oven which can be 5 feet to 300 feet long Copyright Joseph Greene 2001

Structural RIM Fiber preform is placed into mold. Polyol and Isocyanate liquids are injected into a closed mold and reacted to form a urethane. Copyright Joseph Greene 2001

Injection Molding Glass Reinforced Composites
Plastic pellets with glass fibers are melted in screw, injected into a cold mold, and then ejected. Glass filled resin pellets Copyright Joseph Greene 2001

Thermoset Reacting Polymers
Process Window Temperature and pressure must be set to produce chemical reaction without excess flash (too low a viscosity), short shot (too high a viscosity), degradation (too much heat) Copyright Joseph Greene 2001

Tooling Molds are cheaper than equivalent steel stamping dies Materials Soft- polyester resin with glass mat (like the skateboard) Low cost- typically $20,000 and 2 weeks for a bumper beam mold. Low production runs of 20 to 200 parts depending upon the quality. Kirksite: Zn (94%) and Al (6%) cast alloy; machinable and weldable Low cost- typically $40,000 and 6 weeks for a bumper beam mold. Medium production runs of 500 to 2,000 parts depending upon the quality. Aluminum Moderate cost- typically $80,000 and 6 weeks for a bumper beam mold. Medium production runs of 5,000 to 20,000 parts depending upon the quality. Steel (typically P-20) Moderate cost- typically $250,000 and 12 weeks for a bumper beam mold. Medium production runs of 1 million parts depending upon the quality. Comparison to Steel stamping tooling of $1.5 Million and 24 weeks for bumper beams. Copyright Joseph Greene 2001

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