Shaping Processes for Polymer Matrix Composites (PMC) General
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ME477 Fall 2004 General Classification Hand lay-up Automated Shaping Processes for Open Mold Tape laying Compression Closed Mold Polymer Matrix Composites Continuous- Molding Fibers Pultrusion Resin Transfer Molding (PMC) FRP Filament Composites Winding Tube rolling Others 1. Starting Materials for PMC Metals Shaping Spray-up Process Open Mold Compression 2. Open Mold Processes Molding Ceramics Polymer Transfer 3. Closed Mold Processes Closed Mold Molding Short- Injection 4. Filament Winding Fibers Molding Glass Rubber Centrifugal 5. Pultrusion Processes Casting Others Continuous 6. Other PMC Shaping Proceess Laminating 1 2 Polymer and Reinforcement Combining • In a PMC, polymer and reinforcing phase (fibers, • Prepreg Prepregs particles and flakes) are processed separately – Polymers – Thermoplastics, Thermosets on most molding compounds & Elastomers with carbon black – Reinforcing fibers Resin Bath • Roving(Untwisted strands)-> Woven roving • Sheet Molding Compounds • Yarn (Twisted strands)-> Cloth (SMC) – polymer sheet • Mat – a felt made of randomly oriented short fibers cuts to • Thick Molding Compounds + + shape called preforms which are impregnate with resin. (TMC) – thicker polymer + + • They are subsequently combined into sheet composites. • Bulk Molding Compounds (BMC) – polymer billets + + – Combining Matrix and Reinforcement in a intermediated form called prepregs or sheet-, thick- SMC, TMC & BMC or bulk-molding compounds before shaping. 3 4 Open Mold Process Hand Lay-up & Spray-up • Mold – Negative or positive mold • Lay-up – wet lay-up or Prepregs – Hand lay-up – high labor cost but strong – Spray lay-up - randomly oriented short fibers, not as strong • Boat hulls, bathtubs, automobile body parts, furniture, large structural panels, containers, Movie and stage props – Automated Tape-laying - dispensing a prepreg tape onto a mold following a programmed path • Curing for thermosetting resins (Crosslinking) – Room temp, Oven, Microwave, Autoclave – Autoclave - an enclosed chamber equipped to apply heat (1) mold is treated with mold release agent; and/or pressure at controlled levels (2) thin gel coat (resin, colored) is applied, to be the outside surface of molding; (3) layers of resin and fiber, the fiber in the form of mat or cloth; each layer is rolled to impregnate the fiber with resin and remove air; (4) part is cured; 5 (5) fully hardened part is removed from mold 6 Kwon 1 ME477 Fall 2004 Automated tape-laying machine Closed Mold Processes • Match Die (negative and positive) Molding – Compression molding – Transfer molding – Injection molding • More Tooling cost due to the more complex equipment • Advantages: – good finish on all part surfaces – higher production rates – closer control over tolerances, and (courtesy Cincinnati Milacron) – more complex three-dimensional shapes 7 8 Filament Winding & Pultrusion Pultrusion • Common resins: unsaturated polyesters, epoxies, and silicones, all thermosetting polymers • Reinforcing phase: E-glass is most widely, in proportions from 30% to 70% • Products: solid rods, tubing, long flat sheets, structural sections (such as channels, angled and flanged beams), tool handles for high voltage work, and third rail covers for subways. 9 10 Other PMC Shaping Cutting methods • Centrifugal casting • Uncured (prepregs, preforms, SMCs, and etc.): • Tube rolling – Cut to size for lay-up, molding, etc. • Continuous laminating – Typical cutting tools: knives, scissors, power shears, and – Gathering either impregnated or woven steel-rule blanking dies fabric with resin – Nontraditional methods (laser beam cutting and water jet cutting) – Compacting with roller and curing • Cured FRPs are hard, tough, abrasive, and difficult-to-cut • Many of the traditional thermoplastic shaping processes – To trim excess material, cut holes and outlines, etc. are applicable to FRPs (with short – For glass FRPs, cemented carbide cutting tools and high speed fibers) steel saw blades – Blow molding – For other advanced composites, diamond cutting tools – Thermoforming – Water jet cutting reduces dust and noise problems – Extrusion • Cutting of FRPs 11 12 Kwon 2 ME477 Fall 2004 Introduction • Feasible when 1. the melting point of a metal is too high such as W, Ta, Mo POWDER METALLURGY 2. the reaction occurs when melting such as Zr and for superhard tool materials (P/M) • Powder Metallurgy (PM) (around 1800s) – Pressing – Powders are compressed into the desired shape 1. Characterization of Powder in a press-type machine using punch-and-die tooling designed specifically for the part. 2. Production of Metallic Powder – Sintering - Heating at a temperature well below melting. 3. Conventional Processing and Sintering • Advantage – Near-net shape, No waste, controlled porosity, 4. Alternative Processing Dimension control better than casting 5. Materials and Products for PM • Disadvantage 6. Design Consideration – High cost of tooling and powder, Powder Harder to handle Geometric & Size limitation, Density variation 13 14 1. Characterization of Powders Particle Shapes and sizes • Particle Shape • P/M materials – Alloys of iron, steel, and aluminum, copper, nickel, and refractory metals such as molybdenum and tungsten and metallic carbides • Geometric feature • Particle Size (PS) Mesh count refers to the number of openings per – Particle shape linear inch of screen. Thus, higher mesh count – Particle size means smaller particle size 1 – Particle distribution PS = − t – They affects surface area, packing density, porosity, interparticle MC w friction (Flow Characteristics), Green Strength where PS = particlesize • Other Factors MC = mesh count – (Chemistry and Surface Film) tw = wire thickness πD3 A 6 • Surface Area (SA) For spherical powder, A = πD2 ; V = ; = 6 V D How do SA and PS affect the final product? 15 16 2. Production of Metallic Powder Shape(Morphology) Powders • Atomization – The molten metal is injected in a stream of Gas, Water or Centrifugal forces (e.g. rotating disk) • Chemical Reduction – Liberation of metals from oxides. (Iron, Tungsten and Copper) • Precipitation of metallic elements from the salts dissolved in water • Electrolytic: Anode made of desired metal is dissolved into the solution. Cathode collects the deposit. (Beryllium, Copper, Iron, Silver, Tantalum and Titanium 17 18 Kwon 3 ME477 Fall 2004 Gas and Water Atomization Friction Collection Chamber • Interparticle Friction & Flow Characteristics Nozzle – Friction between particles affects ability of a powder to flow Gas readily and pack tightly Metal Siphon Powder – A common test of interparticle friction is the angle of repose, which is the angle formed by a pile of powders as they are Molten poured from a narrow funnel Metal – Smaller particle sizes generally Molten • greater friction and steeper angles Funnel Metal – Spherical shapes Water Water • the lowest interpartical friction – As shape deviates from Metal spherical, friction between Pile of powders powder particles tends to increase Collection Chamber Angle of Repose 19 20 3. Conventional Pressing and Packing Factor (PF) Sintering • True density • Blending and Mixing – density of the true volume of the sintered material – Blending – intermingling of powders • Bulk density – Mixing – combining powders of different chemistries – density of the powders in the loose state after – Additives – Lubricants, binders and deflocculants pouring • Pressing (Compaction) – green compact, low – bulk density ? true density density and strength • Typical PF for loose powders: 0.5 and 0.7 • Sintering – increases strength and density – powders of various sizes vs. uniform size – Reduction of surface energy – Compaction Pressure increases packing – Necking, reduction of pore and grain growth • Porosity (P) - Ratio of the volume of the empty spaces in the powder to the bulk • Secondary Operation – Densification & sizing, volume. (P+PF=1) Impregnation & Infiltration, Heat Treating & 21 Finishing 22 Sintering on a microscopic Blending, compacting & sintering (Pressing) scale Sintering at between 70% and 90% of the metal's melting point (absolute scale) Die (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and Loose powder a green compact A Fully sintered part (4) grain boundaries develop between particles in place of the necked regions 23 24 Kwon 4 ME477 Fall 2004 Pressure Distribution in Powder Model – Density Gradient Compaction po ⎛ πD2 ⎞ ⎛ πD2 ⎞ ⎜ ⎟ ⎜ ⎟ Under the same axial stress ⎜ ⎟ px − ⎜ ⎟()()()px + dpx − πD µσ r dx = 0 ⎝ 4 ⎠ ⎝ 4 ⎠ p Ddpx + 4µσ r dx = 0 Assuming σ r = kpx (interparticlefriction) dp 4µkdx dx x = − L Ho=4.113mm px D R=11.125mm p x For px = po at x = 0 −4µkx / D px = p0e σr µσr D p +dp x x Max. Density Difference =0.46 x Max. Density Difference =0.30 Ho=15.746mm 25 R=6.55mm 26 Densification and Sizing Impregnation & Infiltration • Impregnation - when oil or other fluid is • Secondary operations to increase density, permeated into the pores of a sintered part. improve accuracy, or accomplish additional – oil-impregnated bearings, gears, and similar shaping components – Repressing - pressing the sintered part in a closed die – An alternative application is when parts are to increase density and improve properties impregnated with polymer resins to create a – Sizing - pressing a sintered part to improve pressure tight part dimensional accuracy • Infiltration - An operation in which the pores – Coining - pressworking operation on a sintered part to of the PM part are filled with a molten