EME 50 Final Flashcards
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Thermosets: require "Reaction Injection Molding"
- Constituents of thermoset are introduced into a mixing chamber just before injection.
- Used for automobile bumpers, steering wheels, sports equipment, etc
- Machining of molds for large parts may cost ~$100,000 (eg autobumper or refridgerator door). Hence, injection molding is only suitbale for large volume production runs, to amortize tooling costs.
- Derived from traditional glass blowing process.
- Used for thin-walled plastic containers (eg soda bottle).
- A heated tue of hot plastic called a "parison": is extruded into a mold cavity.
- Pressurized air is forced in the parison to cause it to expand and assume shape of the mold cavity. Control of thickness is an important factor. Typically a high volume process.
- Molten platic is forced through a die orifice to produce parts of uniform cross sectional area (eg pipes,door/window moldings, insulating coating on electrical wire).
- Typically a continuous process with an extruded product cut to desired lengths on exit.
- Used with thermoplastics and eleastomers, but not with thermosets.
- Extruders are typically1-6 inches in diameter with length /diameter ratio about 10-30 and can be fitted with a varitey of dies.
- Extruder screw operates at ~60 rpm (riction in screw mechanism may generate sufficient heat to melt plastic).
- Deformation of a heated plastic sheet into a desired shape against a mold.
- COnformation of sheet against the mold may be achieved by vacuum section, pressure, or mechanically using positive and negative mold halves.
- Used extensively for packaging (eg "blister packs").
- Large parts can also be made- bath tubs, shower stalls, etc.
- Used only for thermoplastics (ABS, PVS, polystyrene, polyethylene, etc).
- Gravity casting is simpler and cheaper than injection molding, but molten plastic must be sufficiently hot and fluid to ensure complete fill of mold.
- Used for polystyrene, acrylics, vinyls (PVC), polyamides (nylon).
- Compositives are typically composed of 2 heterogeneous materials in typically distincty phases whose combination provides superior properties (stiffness, strength, weight, ratio, etc.) than individual materials.
- Typical combination involves embedding a reinforcing agent (secondary phase) in a continuous matrix (primary phase).The reinforcing agent may be in the form of particles, flakes, fibers, wire, or sheets.
- Depending on how the reinforcing agent is embedded in the matrix, the composite may have anistropic (direction-dependent) strength properties.
- The two phases remain distinct, but must have strong adhesion at their interfaces.
- Reinforcing agent may have a random or geometrically organized arrangement.
- An “interphase” agent may also be used, to promote bonding of primary and secondary phases.
- Typically, the matrix provides comprehensive strength, and transfers tensile stresses to reinforcing agent.
- Composites manufacturing can be labor intensive costly, and slow compared to metals.o Often used only for critical applications (e.g. aero industry
- Family of technologies from mid 80s that produce physical prototypes from CAD models, by curing, depositing, sintering, or bonding materials in Parallel Layers. Also called "solid free-form fabrication" (SFF) or "layered manufacturing" (LM). Uses STL file.
- Software slices CAD model into many horizontal path planning
- Support structures (removed after completion of part) required for parts with overhanging features Part bulk orientation is important to avoid “staircasing effect” or smooth part surfaces.
- Rapidly and automatically creates physical models of complex parts to check shape, fit, function, and avoid costly errors in design and tooling.
- Provides “tactile” feedback on part geometry
- Creates patterns for other manufacturing process (lost wax process)
- Can be used to fabricate “smart” structures with embedded sensors
- Software slices CAD Model into many horizontal layers for path planning.
- Support structures (removed after cmpletion of process) may be required for parts with overhanging structures.
- Part build orientation is important to avoid "staircasing effect" on smooth part structures.
What do Rapid Prototyping and STL files allow us to do?
- Rapidly create physical 3D models of complex parts, to check shape, fit, function, & avoid costly errors.Provides tactile feedback on part design.
- Commonly used to fabricate patterns for other manufacturing processes (eg casting patterns).
- Can be used to fabricate "smart" structures with embedded sensors.
- Other applications: architectural design, biomedical engineering, art and sculpture, dentistry.
- 3D systems, the original Rapic Prototyping Process.
- Uses a liquid photo polymer that cures (hardents) on exposure to UV light.
- A UV laser writes on the surface of a photopolymer reservoir to create the current part layer.
- Platform supporting part is then lowered to allow new layer to be cured.
- After removing support structures, completed part is flooded with UV light in an oven to achieve final hardening.
- Fabrication time: hours →days
- System cost: $100,000-$500,000
- Part sizes: up to 20 x 20 x 24 inches
- Material cost: $300/gallon
Open Die Forging
- AKA "upsetting or upset forging"
- reduces height and increases diameter of workpiece
- no precise control of shape
Impression (closed) die forging
- Employs die cavities that match desired part shape
- Friction of flush forced between die halves helps hold workpiec ein place and promotes full metal flow into entire die cavity.
- A succession of dies may be required to achieve complex part shapes, with several hammer blows for each die set
- Gives near net shape, but subsequent machining may be required for precision surfaces.
- Workpiece is completely enclosed between die halves
- Requires mor stringent process controls.
- Typically used only for simple part shapes and soft metals (aluminum, magnesium, etc)
- Mismatch of workpiece and die cavity results in incomplete forging (die under-filled) or excessive forces (die over-filled).
- Coin manufacture is a typical example.
- Reduce part thickness (or change cross sectional shape) by feeding between counterrotating tools called rolls on a machine called a Rolling Mill.
- Rotating rolls draw workpiece into gap between them and compress it with a force F
- Rolling Mills are typically massive machines with large power consumption.
- Friction between rolls and workpiece is essential for drawing material through.
- Usually a hot-working process, but some parts can be cold-rolled.
Uses cylindrical rolls to reduce thickness of workpiece
- Empolys contoured rolls to produce various cross-section shapes (eg an L beam)
- A succession of progressively shaped rolls (a "roll pass") is required.
- Produces helical screw threads of given pitch on cylindrical stock piece (bolt, screw).
- Uses flat dies with slanted V grooves, that reciprocate relative to each other.
- Usually Cold Worked.
- Deform a thick, small diamter ring into a thinner, large diameter ring.
- Used to make railways XXXXXXXX for ball bearings.
- Shaping a bar to a desired cross sction shape by forcing it through a shaped die opening.
- Exit aperture of die defines final sectional shape.
- Process developed ~1800 in England during Industrial Revolution to make lead pipes.
- May be performed with hot or cold metal billet.
- Limited to parts of uniform cross section.
- But high tolerances are possible.
Direct Extrusion (Forward Extrusion)
- involves forcing metal billet through die using a ram, which must overcome friction of billet aginst container walls, as well as mechanical deformation forces.
- Hollow or semi hollow extrusions are possible using a shaped ram/man drill.
Indirect Extrusion (backward or reverse extrusion)
- Die is attached to a hollow ram and material is forced through it in direction oppostite to RAM motion.
- Avoids friction of billet motion against container walls so ram force is lower.
- However, hollow ram is less rigid than solid ram used in direct extrusion.
- Similar to extrusion, but workpiece is shaped by pulling (rather than pushing) it through a die.
- Commonly used to manufacture wire of various stock diameters.
- Final shape may be achieved by drawing through many dies of successively smaller diameters.
- Area reduction determined by tensile strength of metal.
- Dies are lubricated with die or soap to reduce friction
- Sut (ultimate tensile strength) depends on the amount of cold working.
Sheet Metal Working
- Performed cold on thin workpieces by compressing them between punch (positive) and die (negative) tool pairs.
- Typical Operations include bending, deep drawing, (forming a concave shape in a workpiece held by a blank holder) and cutting by shear action (not really a deformation process).
- In deep drawing, punch and die must have suitable fillet (corner) radii and clearance to ensure metal is deformed rather than cut. Friction between workpiece, die, blankholder and punch plays imporatant role in proper metal flow.
- Holding force of blankholder against workpiece must be carefully calibrated.
- substances made of long molecules with repeating units, usually coarbon based. may be synthetic (plastics) or natural (rubbers).
- 3 Basic Types: thermoplastics, thermoset polymer, electromers.
- Solid at room temperature.
- Melted at relatively low temperatures and reformed into diff shapes repeatedly.
Thermoset Polymer (crosslinked/irreversible)
Undergoes chemical reaction on initial heating and forming that prevents further melting and reshaping.
- Highly eleastic
- Can exhibit extreme recoverable elastic strains under relatively low stresses.
Engineering Pros of Plastics
- easily formed, cheap, lightweight
- easily painted/coated, plated
- corrosion resistant, thermally and electrically insulaitng
- can be used in composities.
Engineering Cons of Plastics
- Low strength, stiffness, melting point.
- Relative to metals, creep under steady stress
- may degrade under exposure to sunlight or atmosphere.
- plastics manufacturing developed in 1st half of 20th c.
- Computer automation was introduced in 70s, 80s.
- Basic processes include injection molding, blow molding, extrusion, casting, thermoforming.
- process derived from die casting of metals.
- Heated plastic forced into mold under pressure by reciprocating punger or screw (pressure is essential to overcome viscosity of molten pastic and allow "net shape" production of complex parts with diverse shapes and short ~10-30 sec cycle times.
- mostly used with thermoplastics, but elastomers and thermosets are also moldable with suitable modifications.
Metal Forming (Mechanical Deformation)
- Involves large compressive stresses (exceeding yield strength of the metal) to change shape of workpiece through plastic deformation (non-recoverable)
- Large forces exerted on workpiece by press or hammer through tools called “dies”.
- Desired final part shape may be achieved through a sequence of dies and loading cycles.
- Part assumes shape of final die.
- Metals used in forming processes should have low yield strength (high malleability) and high ductility (accommodates large deformation without fracture).
Cold Working (room temperature)
- Good accuracy, surface finish, repeatability
- Strain hardening increases part strength
- Controllable directional properties of part
- Disadvantage: requires very high forces, only limited geometry changes possible
Hot Working (~50-70% of Tm, Kelvin scale melting temperature)
- Above “re-crystallization” temperature – internal structure of metal can re-organize
- Lower forces than cold-working and large deformations are possible
- May be feasible for metals that are brittle at room temperature
- Isotropic strength properties
- No strength improvement by strain hardening
- Poorer accuracy, surface finish, repeatability
Warm-working: intermediate process ~30% of Tm
- Bulk deformation process for parts with low surface area and volume ratio (A/V) –forging, rolling, extrusion/drawing
- Sheet-metal forming (also called presswork or stamping) for parts with high A/V ratio
- die defining desired part shape. Usually a hot-working process but some parts are also cold-forged.
- Dates to ~5000 B.C.( used to make coins, weapons, etc.).
- Essentially an extension of blacksmith practice of hammering workpiece on anvil.
- Forged parts may require finishing operations - machining or heat treatment.
- Deformation force may be applied gradually (forging press) or as an impact load (forging hammer) using open dies, impression dies, or flashless dies.
- Flashless dies prevent flow of excess metal along 'parting line' between two die halves - this is "flash" Wh
Common “simple” composites:
- Adobe bricks - mud reinforced with straw
- Steel reinforced concrete (buildings, bridges, etc.)
- Plywood – crossgrained wood sheets bonded with resin
Modern Composites: Polymer matrix composites (PMCs)
- Thermoset resin (epoxy/polyester mix) reinforced with fibers
- Thermoplastics with particle reinforcements
- Elastomers with reinforcing agent called “carbon black” (nanoparticles produced by combustion)
- Helps conduct frictional heat from tire treads
Modern Composites: Metal Matrix Composites (MMCs)
Include metal/ceramic mixtures (cemented carbides) and fiber-reinforced aluminum/magnesium
Modern Composites: Ceramic Matrix Composites (CMCs) – also called “cermet” (ceramic + metal)
- Provide extreme hardness and temperature resistance, also wear resistance
- Tungsten carbide used for cutting tools, drill bit, indentation tool, dies, et.
Reinforcement in Composites
- may be continuous (wire, woven fabric, etc.) or randomly oriented short strands or particles
- work by inhibiting propagation of crystal dislocations in matrix material.
- Reinforcing fibers must be impregnated with matrix polymer for good adhesion to matrix. “Prepeg” tape or sheets (requires refrigerated storage) may be cut, dipped in resin, and stacked to form laminated shapes. Can be done by CNC tape/wire laying machine.
- Reinforced plastic (possibly heated) can be formed by various molding processes – compression, vacuum, or open (contact) molding – e.g. “hand lay-up”Filament winding, pultrusion, pulforming ombine continuous reinforcement fiber with resin while laying on a mold or pulling through die.
Composites: Advantageous Properties
- Strength, similar to metal part, but at ~1/5 weight
- Good fatigue life and corrosion resistance
- Low thermal expansion and dimensional stability
- Parts with an isotropic strength are possible
- Good creep resistance under steady loads
- Rapid increase of acceptance in aero/auto industry
Fused Deposition Modeling (FPM): Stratasys
- Employs an extrusion mounted in an (x,y)stage above a platform raised/lowered in z-direction.
- Thermoplastic supplied from a spool in filament form is heated on passing through extruder, and solidifies on exit.
- Different material may be used for support structures, to facilitate removal.
- An “office-friendly” technology that uses no lasers or toxic chemicals.
- Fabrication times: hours
- System cost: $25,000 - $150,000
- Part sizes: 9-30in, depending on system
- Material cost: ~$100/lb
Selective Laser Sintering (SLS):
- originally DTM corporation
- Later merged with 3D systems
- Sintering = fusing powder particles by raising temperature close to melting point
- Can be used with polymer, metal, or ceramic powders.
- Powder heating is achieved by laser
- Once current layer is sintered, part platform is lowered and new powdered layer is rolled over it. Does not require support structures. Only RP method that can make metal parts, but quality not comparable toCast/forged/machined metal parts (sintered parts may be porous)
- Fabrication times: hours→days
- System cost: ≥$500,000
- Part size: 24 x 24 x 24 inches
- Material Cost: ~$5-60/ lb
Laminated object Manufacturing (LOM):
- originally Helisys
- Folded in 2000 and succeeded by cubic technologies
- Uses laser to cut coated paper or other sheet material into shape of each part layer, and bonds layers using a heated roller
- Can model large parts (e.g. engine blocks) with complex geometries.
- Models have a “sculpted wood” appearance.
- Not as accurate as other process, but low material costs
- Finished part must be manually extracted by “de-cubing” process
- Fabrication times: hours→days
- System cost: $500,000 - $1,000,000
- Part size: several feet (largest RP methods)
- Material Cost: $10-20/ lb
Solid Object Printing (SOP)
- aka 3D printing
- Z Corp
- based on a method developed @ MIT
- Uses a wide-arc inkjet to spray binding agent onto powder material (typically plaster or starch).
- Very cheap and fast but limited part strength and accuracy.
- Post-processing steps may be used to improve part strength.
- Can also be used to make multicolored parts
- Competes most directly with FDM process.
- Fabrication time: hrs
- System Cost: $20,000-$70,000
- Part Size: up to ~12 inches
- Material Cost: minor
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