EG182 — Definitions & Diagrams

Definitions with Diagrams

Every key term defined from first principles, paired with annotated diagrams showing exactly what the words mean visually.

Topic 1 — General Manufacturing

Production quantity vs product variety

The fundamental trade-off in manufacturing: as production volume increases, product variety must decrease. Each region of this chart has a matching type of manufacturing system.

PRODUCTION QUANTITY (parts / year) → ← PRODUCT VARIETY 1 10² 10⁴ 10⁶ 10⁷+ JOB SHOP Qty: 1–100 Unique / custom parts e.g. prototypes, custom tools, aircraft frames General-purpose machines Highly skilled operators BATCH Qty: 100–10,000 e.g. machine tools, furniture, equipment Semi-dedicated tooling Changeover between runs MASS PRODUCTION Qty: 10,000–10⁷ Cars, appliances, electronics Dedicated automated lines CONTINUOUS wire, strip, pipe Left → Right: Automation ↑ | Unit cost ↓ | Flexibility ↓ | Skilled labour ↓
Production quantity vs product variety — the fundamental manufacturing system classification
Job shop
Low-quantity (1–100), high-variety production. General-purpose machines. Each job is unique. Highest unit cost, maximum flexibility. Skilled workers essential.
Batch production
Medium-quantity (100–10,000) production runs. Semi-dedicated tooling. Line reset between different product batches. Intermediate cost and flexibility.
Mass production
High-quantity (10,000–10⁷) of identical parts. Dedicated automated equipment. Lowest unit cost but zero flexibility. Product changes require total line redesign.
Continuous process
Single product produced without interruption (e.g. wire drawing, strip rolling). Extreme volume. Usually a single continuous operation, not discrete parts.
Manufacturing lead time
Time from order placed to finished part delivered. Job shop = long (custom setup). Mass production = short (line always running). Key competitive metric.
Hard automation
Fixed automated tooling designed for one specific product. Very high throughput, zero flexibility. Example: transfer line for engine blocks.

Topic 2 — Metal Casting

Sand casting mould anatomy

The gating system delivers molten metal from outside the mould to the cavity, while the riser provides a reservoir to compensate for shrinkage. Every component has a specific function.

COPE (top half) DRAG (bottom half) PARTING LINE CAVITY (part shape) POURING BASIN SPRUE (tapered vertical) RUNNER (horizontal channel) GATE (narrow entry) RISER solidifies LAST CORE (creates hollow features) FLASK (rigid box containing the mould) Metal flows: pouring basin → sprue → runner → gate → cavity
Sand casting mould anatomy — cross-section view
Cope
Top half of the sand mould.
Drag
Bottom half of the sand mould. Sits on moulding board during preparation.
Flask
Rigid box (wood or metal) containing and supporting both mould halves.
Parting line
Interface where cope and drag meet. Leaves a visible seam on the finished casting.
Pouring basin
Funnel at top of mould. Receives molten metal smoothly, reducing turbulence and splash.
Sprue
Tapered vertical channel. Metal flows down. Tapered to prevent air ingestion as metal accelerates.
Runner
Horizontal channel at base of sprue. Distributes molten metal laterally to the gates.
Gate
Narrow entry point into mould cavity. Controls flow rate and is easy to snap off after solidification.
Cavity
The hollow space within the mould that defines the final shape of the casting.
Core
Sand insert placed in cavity to create hollow internal features. Also allows for shrinkage.
Chaplet
Metal support holding the core in correct position during pouring. Must have higher melting point than the cast metal.
Riser
Reservoir of molten metal that solidifies LAST. Feeds liquid metal into cavity to compensate solidification shrinkage.

Solidification grain zones

When metal solidifies in a mould, three distinct grain structure zones form from outside to centre, each with different mechanical properties.

MOULD WALL CHILL ZONE COLUMNAR ZONE EQUIAXED ZONE (isotropic — best properties) Fine, random Long, directional Equal size, all directions Heat flow →
Solidification grain zones in a casting — cross-section view showing chill, columnar, and equiaxed zones
Chill zone
Outermost layer at mould wall. Very fine, randomly oriented grains from extremely rapid cooling. Thin.
Columnar zone
Long grains growing inward against heat flow direction. Anisotropic — strong along grain direction, weak perpendicular.
Equiaxed zone
Centre grains — equal size in all directions. Isotropic. Best for general mechanical applications.
Isotropic
Properties same in all directions — equiaxed grains. Desirable for most engineering components.
Anisotropic
Properties differ with direction — columnar grains. Undesirable in castings, deliberate in composites.
Thermal arrest
Flat section on pure metal cooling curve. Temperature stays constant while latent heat of fusion is released.
Mushy zone
Semi-solid state between liquidus (TL) and solidus (TS). Part liquid, part solid. Prone to segregation.
Segregation
Non-uniform composition in alloy casting. Different elements freeze at different temperatures, causing concentration variations.

Cooling curves — pure metal vs alloy

A pure metal solidifies at a single temperature (flat thermal arrest). An alloy solidifies over a range — between its liquidus and solidus temperatures — creating a mushy semi-solid zone.

PURE METAL TEMPERATURE TIME → Tm THERMAL ARREST Latent heat released at Tm LIQUID SOLID ALLOY TEMPERATURE TIME → TL liquidus TS solidus MUSHY ZONE liquid + solid coexist prone to segregation LIQUID SOLID Pure metal: single Tm, flat arrest | Alloy: range TL→TS, sloping mushy zone
Cooling curves for pure metal (thermal arrest) vs alloy (mushy zone between liquidus and solidus)
Thermal arrest
Flat horizontal section on a pure metal cooling curve. Temperature stays constant while latent heat of fusion is released during solidification at Tm.
Liquidus (TL)
Temperature at which an alloy begins to solidify on cooling (first solid appears). Above TL the alloy is fully liquid.
Solidus (TS)
Temperature at which an alloy is fully solidified on cooling (last liquid disappears). Below TS the alloy is fully solid.
Mushy zone
Semi-solid state between TL and TS where liquid and solid coexist. Prone to segregation. Alloys with wide TL–TS ranges are harder to cast cleanly.
Chvorinov's rule
Solidification time ∝ (V/SA)². Shapes with high V/SA ratio (sphere) solidify slowest. Risers must have higher V/SA than the casting to solidify last.

Die casting — high-pressure process

Metal is injected at high pressure directly into a permanent steel die. Much faster and more accurate than sand casting, but only viable for non-ferrous metals (aluminium, zinc, magnesium).

FIXED DIE HALF MOVING DIE HALF CAVITY PLUNGER → SHOT SLEEVE (molten metal) EJECTOR PINS (push casting out) CLAMP FORCE holds die shut against injection pressure Injection pressure: 7–700 MPa | Fast fill: 10–100 ms | Permanent die: up to 1,000,000 shots
High-pressure die casting — plunger injects melt into clamped permanent steel die at high speed
Hot-chamber die casting
Shot sleeve is submerged in molten metal (gooseneck design). Faster cycle, but only for low-melting-point alloys (zinc, tin, lead). Injection system can't survive in high-temperature Al.
Cold-chamber die casting
Metal ladled into separate shot sleeve each cycle — not submerged. Slower but handles higher-melting alloys (aluminium, magnesium, copper). Standard for Al components.
Ejector pins
Pins on moving die half that push the solidified casting out of the cavity when the die opens. Leave small circular witness marks on casting surface.
Porosity in die casting
Major defect. Air trapped by fast fill, or dissolved gas released on solidification. Can be reduced by vacuum die casting or slow injection (squeeze casting). Makes parts unsuitable for welding.
Investment casting
Wax pattern coated in ceramic slurry, dried (shell), wax melted out, metal poured in. Excellent dimensional accuracy and surface finish. Complex shapes, thin walls. Used for turbine blades.

Topic 3 — Plastics Shaping

Extruder anatomy

The extruder is the workhorse of plastics processing. Understanding each component and its function is essential — and the die is not part of the extruder.

HOPPER pellets feed in BARREL (heated) SCREW (rotates — conveys, melts, pressurises) SCREEN PACK DIE NOT part of extruder! extrudate continuous output polymer flow direction → heating elements
Single-screw extruder cross-section — showing all components and polymer flow direction
Hopper
Feed funnel at inlet of extruder. Holds plastic pellets and feeds them continuously into the barrel.
Barrel
Cylindrical heated tube containing the screw. Heats and pressurises polymer toward the die.
Screw
Rotates inside barrel — simultaneously conveys solid pellets forward, melts them by friction and conduction, and builds pressure for injection into the die.
Screen pack
Wire mesh filter positioned before the die. Removes contaminants, builds back-pressure, and straightens the rotating melt flow.
Die (extrusion)
Separate, replaceable shaping tool — NOT part of the extruder. Each cross-section profile needs its own die. Defines what shape the extrudate takes.
⚠️ Exam trap: commonly said to be part of the extruder — it is not.
Mandrel
Pin held in the centre of the die by spider legs. Creates the hollow centre in hollow profile extrusions such as pipes and tubes.
Die swell
Extrudate swells wider than the die opening as it exits. Polymer chains relax from their stretched, aligned state back to their natural coiled state.
Qx = Qd − Qb
Net output flow = drag flow (forward, from screw) minus back-pressure flow (reverse, from die resistance). The die resists flow, pushing some polymer back.

Polymer melt behaviour

Shear Thinning VISCOSITY SHEAR RATE → Water (Newtonian) Polymer melt (shear thinning) viscosity drops as speed↑ Die Swell die opening extrudate (swells wider) die size extrudate wider than die Push faster → flows easier Chains relax → extrudate swells
Left: shear thinning behaviour vs Newtonian fluid. Right: die swell from viscoelastic memory
Viscosity
Resistance to flow. Higher molecular weight → higher viscosity. Higher temperature → lower viscosity. The measure of how "thick" the melt is.
Shear thinning
Viscosity decreases as shear rate increases. Polymer melts flow more easily the faster you push them. Non-Newtonian, pseudoplastic behaviour.
Like ketchup — shake hard and it suddenly flows.
Pseudoplastic
Technical term for shear-thinning behaviour. Viscosity depends on shear rate. Opposite of Newtonian (constant viscosity).
Viscoelasticity
Combined viscous (flow) and elastic (memory) behaviour in polymer melts. Chains are stretched during flow and want to return to coiled state.
Die swell
Extrudate swells wider than the die opening on exit. Viscoelastic chains relax from stretched, aligned state back to natural coiled state.
Frost line
In blown film extrusion: the height at which the rising bubble solidifies. Higher frost line = more stretching = thinner film. Controls final film thickness.

Injection moulding — machine anatomy

The injection moulding machine has two distinct units: the injection unit (melts and injects plastic) and the clamping unit (holds the mould shut against injection pressure).

INJECTION UNIT HOPPER pellets BARREL + RECIPROCATING SCREW heating bands NOZZLE CLAMPING UNIT STAT. PLATEN CAVITY MOVING PLATEN TIE BAR CLAMP FORCE sprue+ runner+gate Mould closes → screw injects → pack + hold → cool → mould opens → eject → repeat (cycle ~10–60 s) ⚠️ Clamp force must exceed injection pressure × projected area — or mould flashes open
Injection moulding machine cross-section — injection unit (left) and clamping unit with mould (right)
Reciprocating screw
Rotates to melt and meter polymer forward, then moves axially (forward) to inject the shot. Acts as both conveyor and piston. One mechanism, two jobs.
Clamp force
Force holding mould shut during injection. Must be greater than injection pressure × cavity projected area or the mould flashes open at the parting line. Rated in tonnes (100–5000 t).
Sprue, runner, gate
Delivery system: sprue (entry from nozzle) → runner (channels to cavity) → gate (narrow entry into cavity). Gate freezes first to seal the cavity. All are waste material.
Packing stage
Extra pressure applied after cavity is filled. Compensates for solidification shrinkage by forcing in more material. Prevents sink marks and voids.
Sink marks
Depressions on surface of injection moulded part. Caused by insufficient packing pressure over thick sections where internal shrinkage pulls surface inward.
Weld lines
Lines on moulded part where two melt fronts meet and bond. Weak because polymer chains don't fully interdiffuse. Appear around holes and at flow convergences.

Extrusion blow moulding — making hollow parts

A hollow tube of molten plastic (the parison) is extruded, captured in a mould, then inflated with air to press it against the mould walls. Used for bottles, tanks, and hollow containers.

1 — EXTRUDE PARISON 2 — MOULD CLOSES 3 — BLOW + EJECT die PARISON (hollow tube) mould open Mould pinches & seals bottom of parison AIR → BOTTLE pressed to mould wall Air inflates; cool; open mould; eject
Extrusion blow moulding — parison extruded, captured in mould, inflated to bottle shape, cooled and ejected
Parison
Hollow tube of molten plastic extruded downward from a circular die with a mandrel. The raw form before blowing. Wall thickness is controlled by the die gap.
Extrusion blow moulding
Continuous extrusion of parison, mould closes and pinches bottom, air inflates parison against mould walls. Best for large hollow shapes (tanks, drums, bottles).
Injection blow moulding
Preform is injection moulded first (precise neck/thread), then transferred to blow station and inflated. Better neck accuracy, no pinch weld. Used for pharmaceutical bottles.
Pinch weld
Weld line at the bottom of an extrusion blow moulded part where the mould pinches and seals the parison. Weak line — a potential failure point.

Topic 5 — Powder Metallurgy

PM process sequence

POWDER PRODUCTION Gas/water atomise Chemical/electrolysis BLENDING / MIXING Blending = same comp Mixing = diff comp COMPACTION F = Ap × Pc punch ↓ GREEN ↑ punch SINTERING 70-90% of Tm Diffusion bonds necks form → pores shrink SECONDARY OPERATIONS Sizing / Coining Infiltration Impregnation HIP / Heat Treat Key concept: metal is NEVER fully melted Sintering uses atomic diffusion at 70-90% Tm to bond particles without liquefaction This is why PM can process tungsten (Tm = 3422°C) that cannot be conventionally cast
Complete PM process sequence — from raw powder to finished part
Green compact
Part after compaction — particles mechanically interlocked. Holds shape. Weak, porous, handleable. NOT yet sintered.
Like pressed chalk — holds shape but crumbles easily.
Sintering
Heating compacted powder to 70-90% of melting point (in Kelvin). Atomic diffusion creates bonds across particle boundaries. No melting occurs. 1-5% shrinkage results.
Neck formation
Stage 1 of sintering. Atoms diffuse across particle contact boundaries, forming small bonding bridges called necks. Fastest densification stage.
Pore shrinkage
Stage 2 of sintering. Necks grow larger, pores between particles reduce in size and become more rounded. Continued densification.
Grain growth
Stage 3 of sintering. Little further densification. Grain boundaries migrate and grains enlarge. Excessive grain growth reduces mechanical properties.
Blending vs Mixing
Blending = same composition + different particle sizes → improves packing. Mixing = different compositions → creates alloy precursors. Fe + graphite = MIXING.
⚠️ Exam trap: commonly confused.
Infiltration
Lower-melting metal (bronze) placed on sintered part, heated to melt, wicks into pores by capillary action. Increases density and thermal conductivity.
Impregnation
Oil forced into pores of sintered part. Creates self-lubricating bearings — rotation generates heat, oil seeps out and lubricates automatically. No maintenance needed.

Topic 6 — Metal Forming

Blanking vs Punching

These two operations both use a punch and die to cut sheet metal — but the desired product is different in each case.

BLANKING cut piece = desired part PUNCH ↓ Sheet with hole = SCRAP BLANK = PART ✓ e.g. coins, washers, discs PUNCHING cut piece = scrap (slug) PUNCH ↓ ← hole created Sheet with hole = PART ✓ slug = SCRAP e.g. bracket with holes
Blanking vs Punching — the product distinction. Same tooling setup, different desired output.
Blanking
The cut piece IS the desired part. Surrounding stock is scrap. Used for coins, washers, discs, gaskets.
Memory: the "Blank" is what you keep.
Punching
The cut piece (slug) is the scrap. The surrounding sheet with the hole is the desired part. Used to create holes in brackets, panels, housings.
Springback
After bending, the elastic portion of the deformation recovers on releasing the force — the bend angle opens up. Fixed by overbending past the desired angle.
Deep drawing
Punch draws a flat blank into a die cavity, forming a hollow cup. Used for drinks cans, car body panels, kitchen sinks, ammunition shells.
Blankholder
Clamps the outer edges of the blank during drawing. Essential to prevent the flange from buckling under compressive stress (wrinkling defect).
Thread rolling
Cold rolling threads onto fasteners instead of cutting them. Stronger threads — grain flow preserved along thread profile, surface work-hardened. Better fatigue resistance.

Rolling — reducing thickness by plastic deformation

Workpiece is drawn between two rotating rolls. The reduction in thickness (draft) causes elongation in the rolling direction. Work hardening occurs in cold rolling; recrystallisation anneal may be used between passes.

axle ↻ rotation ↺ rotation INCOMING thickness h₀ h₀ feed → OUTGOING h₁ < h₀ h₁ DRAFT = h₀ − h₁ Elongation = h₀/h₁ | Width increase (spread) is small compared to length increase
Rolling process — draft reduces thickness h₀→h₁; material elongates in rolling direction

Deep drawing — forming hollow cups

A flat blank is drawn into a die cavity by a punch. The flanged material is compelled to flow radially inward and axially downward, forming a seamless cup or can shape.

START OF STROKE BLANK HOLDER BLANK HOLDER PUNCH ↓ BLANK (flat sheet) DIE DIE Die opening die radius MID-STROKE PUNCH flange flows inward → CUP forming Blankholder prevents wrinkling of flange under hoop compression
Deep drawing — punch draws flat blank into die cavity; blankholder suppresses flange wrinkling; cup forms seamlessly
Draft (rolling)
Thickness reduction per pass: d = h₀ − h₁. Limited by friction at the roll surface. Larger draft = more work hardening per pass. Multiple passes used for large total reductions.
Work hardening
Dislocation density increases during cold plastic deformation, dislocations impede each other → harder and stronger but less ductile. Quantified by Hollomon equation: σ = Kε^n.
Hollomon equation
σ = K·ε^n where K = strength coefficient, n = strain hardening exponent (0–1). Higher n = greater work hardening rate. Al: n≈0.2, Cu: n≈0.54, austenitic SS: n≈0.52.
Wrinkling (deep drawing)
Compressive hoop stress in the flange during drawing causes buckling waves. Prevented by blankholder applying controlled pressure to flatten flange against die face.
Tearing (deep drawing)
Cup wall fails in tension near the punch radius — the thinned section can't carry the drawing force. Prevented by good die radius, lubrication, and limiting draw ratio (D/d < 2.1 typical).
Recrystallisation
New, strain-free grains nucleate and replace work-hardened grains on annealing above Tr (≈0.4 Tm). Restores ductility. Allows further cold work without fracture (process annealing).

Topics 7–9 — Machining, Cutting Tools & Grinding

Turning vs Milling — the critical distinction

TURNING WORKPIECE ROTATES single-point tool (moves linearly) feed ↓ Machine: LATHE Produces cylindrical shapes MILLING TOOL ROTATES WORKPIECE moves → feed direction Machine: MILLING MACHINE Flat surfaces, slots, profiles
Turning (workpiece rotates, tool fixed) vs Milling (tool rotates, workpiece moves)

Tool wear — crater and flank

RAKE FACE (top) FLANK FACE (side) cutting edge CRATER WEAR chip sliding over rake face FLANK WEAR rubbing against machined surface Replace at 0.5mm chip flows here WORKPIECE ↑ newly machined surface
Single-point cutting tool showing crater wear (rake face) and flank wear (flank face) locations

Grinding wheel structure

arbor hole BOND holds grains together ABRASIVE GRAINS Al₂O₃, SiC, CBN, diamond Do the actual cutting POROSITY Chip clearance + coolant access to cutting zone BOND MATERIAL Holds grains — releases dull ones to expose fresh cutting ROTATION
Grinding wheel cross-section showing the three components — grains, bond, and porosity
Abrasive grains
Hard cutting particles embedded in the wheel. Must be harder than the workpiece. Al₂O₃ (general), SiC (hard materials), CBN (steels), diamond (non-ferrous only).
Bond material
Holds abrasive grains together. Must be strong enough to hold grains during cutting but allow worn, dull grains to be released so fresh ones are exposed.
Porosity
Spaces between abrasive grains. Essential for: chip escape during grinding, coolant access to the cutting zone, preventing wheel clogging.
Friability
The ability of abrasive grains to FRACTURE and expose new sharp cutting edges. Causes self-sharpening behaviour. Too little → glazing. Too much → rapid wear.
⚠️ Exam trap: friability ALLOWS fracture, it does NOT prevent it.
Glazing
Grinding wheel surface becomes smooth and shiny. Dull grains remain because friability is too low. Wheel stops cutting, generates excessive heat, poor surface finish.
Wheel grade
How strongly the bond holds the grains. Hard grade = grains stay longer. Soft grade = grains released easily. Rule: HARD material → SOFT grade wheel.
Wheel structure
Spacing between abrasive grains. Dense = more cutting edges, better finish, clogs faster. Open = more chip clearance, higher MRR, less clogging.

Chip types — identification and causes

The type of chip produced reveals cutting conditions and material properties. Each type has a distinct cause and implication for surface finish and tool life.

CONTINUOUS Ductile material Low feed, sharp tool Good finish Can be hazardous (long birds-nest) SEGMENTED Alternating hard/soft zones; titanium alloys hardened steels Periodic tool forces → vibration (also called serrated) DISCONTINUOUS Brittle material cast iron, ceramics Easy to handle Rough finish BUILT-UP EDGE BUE Soft ductile material welded to tool tip BUE breaks off → rough finish, tool wear BUE fix: increase speed (outrun adhesion), use cutting fluid, coat tool (TiN reduces friction)
Four chip types — continuous (ductile, good conditions), segmented (Ti/hard steels), discontinuous (brittle), BUE (soft workpiece, low speed)

Cutting parameters — Vc, f, d on a turning operation

Three independent parameters control material removal rate, surface finish, and tool life. They must be balanced — MRR = Vc × f × d.

Vc CUTTING SPEED Surface tangential velocity (m/min) Vc = π·D·N / 1000 TOOL d d — DEPTH OF CUT Radial engagement into workpiece (mm) ↑d → more tool force f → f — FEED RATE Axial advance per revolution (mm/rev) ↑f → rougher surface finish MRR = Vc × f × d (mm³/min)
Three independent cutting parameters on a turning operation — cutting speed Vc, feed f, depth of cut d
Cutting speed Vc
Tangential velocity of workpiece surface relative to tool tip. Vc = π·D·N/1000 (m/min). Biggest effect on tool life — doubling Vc dramatically reduces tool life (Taylor equation).
Feed rate f
Axial advance per revolution (mm/rev) in turning. Controls surface roughness: Ra ∝ f²/r where r = nose radius. Increasing f improves MRR but worsens finish.
Depth of cut d
Radial depth the tool engages the workpiece. Controls the radial cutting force. Large d with small f is better than small d with large f for productivity.
Taylor's equation
V·T^n = C. T = tool life (min), V = cutting speed, n = Taylor exponent (0.1–0.5), C = speed giving T = 1 min. Higher V → exponentially shorter tool life.
MRR (material removal rate)
MRR = Vc × f × d (mm³/min). Proportional to all three parameters equally. Increasing any one gives linearly more MRR but different effects on finish and tool life.
Built-up edge (BUE)
Work material welds to tool tip at low-to-medium cutting speeds. Acts as a false cutting edge. Breaks off periodically, tearing workpiece surface. Cured by higher speed, cutting fluid, or coated tools.

Topic 4 — Rubber & PMC

Rubber processing sequence

COMPOUNDING Add all additives: carbon black, clay, plasticisers, ZnO MIXING ⚠️ TWO STAGES Stage 1: everything Stage 2: add sulphur SHAPING Extrusion Calendering Moulding VULCANISING Sulphur cross-links chains 15-20 min (modern) Goodyear 1839 / 5 hrs ⚠️ SULPHUR ADDED LAST — early addition causes premature vulcanisation in the mixer This is the "rubber processor's nightmare" — a ruined batch stuck in the machine
Rubber processing sequence — compounding → mixing (two stages!) → shaping → vulcanising
Elastomer
Polymer with very large reversible elastic deformation. Three requirements: long flexible chains, light cross-links, glass transition temperature below room temperature.
Entropy-driven elasticity
Stretched polymer chains spontaneously recoil to their natural disordered state when released. Driven by thermodynamic tendency toward higher entropy (disorder).
Vulcanisation
Sulphur atoms form chemical bridges between polymer chains during heating. Creates permanent cross-links. Discovered by Charles Goodyear in 1839. Modern: 15-20 minutes.
Carbon black
Primary reinforcing filler. Increases tensile strength dramatically and provides UV protection. That's why most rubber is black.
⚠️ Exam trap: carbon black DOES provide UV protection.
China clay
Non-reinforcing filler (hydrous aluminium silicate). Cheaper bulk filler used when the product cannot be black in colour.
Cross-link density
Number of sulphur bridges between chains. Low density → soft rubber (elastic bands, tyres). High density → ebonite (hard rubber, bowling balls).

Vulcanisation — molecular cross-linking

Before vulcanisation, polymer chains slide freely past each other. Sulphur bridges lock them together, preventing permanent deformation while allowing elastic stretching.

UNVULCANISED Chains slide freely Chains slide → permanent deformation VULCANISED Sulphur bridges lock chains S₂ S₂ S₂ S₂ Stretches elastically, snaps back
Vulcanisation — sulphur (S₂) bridges between polymer chains prevent permanent sliding, enabling elastic recovery

PMC fibre architectures

The arrangement of reinforcing fibres in a polymer matrix composite determines its strength, stiffness, and directionality.

UNIDIRECTIONAL (UD) Max strength IN fibre direction only e.g. CFRP wing spar WOVEN FABRIC Balanced 0°/90° strength Good drapability e.g. GFRP panels RANDOM CHOPPED Isotropic, lowest cost Lower strength than UD e.g. SMC panels, SFRP
PMC fibre architectures — unidirectional (max directional strength), woven (balanced), random chopped (isotropic, lowest cost)
Fibre volume fraction (Vf)
Proportion of composite that is fibre by volume. Higher Vf → higher stiffness and strength. CFRP aerospace: Vf ≈ 60%. Bulk moulding compound: Vf ≈ 20%.
Rule of mixtures
Along fibre direction: E_c = Vf·Ef + Vm·Em. Composite modulus is the volume-weighted average. Only valid in fibre direction for UD composites.
Filament winding
Continuous fibre tow wound onto rotating mandrel at precise angles. Produces cylinders and pressure vessels. High Vf, excellent strength-to-weight. Rocket motor cases, pipes.
Delamination
Separation between plies in a laminated composite. The dominant failure mode. Driven by interlaminar shear or peel stress. Invisible to naked eye — requires ultrasonic inspection.

Topic 10 — Heat Treatment, AM & SSM

Heat treatment — quenching media and cooling rate

TEMPERATURE TIME → Austenitising temperature (heated) Brine (FASTEST) → Martensite Fresh water Still oil Air (SLOWEST) → Pearlite Faster quench → harder product (martensite) | Slower cool → softer product (pearlite)
Quenching media cooling curves — brine (fastest) through to air (slowest). Rate determines final microstructure.
Annealing
Heat below melting point, soak, then SLOW cool (in furnace). Softens metal, improves ductility, relieves residual stresses. Reverses effects of cold working.
Hardening
Heat steel above critical temperature (~723°C) to form austenite, then rapidly quench. Produces martensite — very hard but brittle. Always followed by tempering.
Tempering
Heat previously hardened steel to intermediate temperature. REDUCES brittleness while retaining most hardness. Does NOT increase hardness.
⚠️ Exam trap: tempering reduces brittleness — it does NOT increase hardness.
Martensite
Forms by rapid quenching of austenite. Carbon has no time to diffuse — trapped in distorted crystal structure. Extremely hard and brittle. Highest hardness of any steel microstructure.
Pearlite
Forms by slow cooling of austenite. Layered structure of ferrite and cementite. Much SOFTER than martensite. Better ductility.
⚠️ Exam trap: martensite is harder than pearlite — not the other way round.
Precipitation hardening
Solution treatment (high temp) → quench → precipitation treatment (lower temp). Fine precipitate particles form that block dislocation movement → hardening. NOT available for all alloys.
Rheo-route (SSM)
Semi-solid processing from LIQUID. Cooled + sheared simultaneously into mushy zone. ONE step — no intermediate solidification. Lower cost, less control.
Thixo-route (SSM)
Semi-solid processing from SOLID billet. Pre-processed billet reheated into mushy zone. TWO steps — includes intermediate solidification. Higher control, higher cost.

Fe-Fe₃C phase diagram (simplified)

The iron-carbon phase diagram governs which microstructure forms during heat treatment of steels. The eutectoid point (0.76%C, 723°C) is the key reference — steels are classified relative to it.

600°C 723°C 1148°C 1538°C 0% 0.76% 2.14% 4.3% 6.67% Carbon content (wt%) → Temperature (°C) → LIQUID AUSTENITE (γ) FCC, paramagnetic max 2.14%C soluble FERRITE (α) BCC, soft, ductile <0.022%C α + PEARLITE hypoeutectoid PEARLITE PEARLITE + CEMENTITE (Fe₃C) hypereutectoid steels + cast iron region A₁ = 723°C 1148°C EUTECTOID 0.76%C 723°C EUTECTIC 4.3%C 1148°C 2.14%C STEEL (<2.14%C) CAST IRON (>2.14%C)
Simplified Fe-Fe₃C phase diagram — eutectoid point (0.76%C, 723°C) governs steel microstructure; above A₁ = austenite; below A₁ = ferrite + pearlite or pearlite + cementite
Eutectoid reaction
At 0.76%C, 723°C: austenite → pearlite (ferrite + cementite) on slow cooling. A single-phase solid transforms to two-phase solid. This exact composition is called eutectoid steel.
Hypoeutectoid steel
Carbon content < 0.76%. On slow cooling below A₁: ferrite + pearlite microstructure. Soft ferrite grains at grain boundaries (proeutectoid ferrite) then pearlite fills remainder.
Hypereutectoid steel
Carbon content > 0.76%. On slow cooling: proeutectoid cementite at grain boundaries, then pearlite. Hard but brittle network of Fe₃C makes these steels less tough than hypoeutectoid.
A₁ critical temperature
723°C — the eutectoid temperature. Steels must be heated above A₁ (and then above A₃ or Acm) to form 100% austenite before hardening. Quenching from below A₁ achieves nothing.
Pearlite
Lamellar (layer) structure of alternating ferrite (α) and cementite (Fe₃C) formed by slow cooling through eutectoid. Named for pearl-like iridescence under microscope. Moderate strength.
Austenite (γ)
FCC iron that can dissolve up to 2.14%C. Required starting phase for all steel heat treatments. Forms by heating above A₁ (and A₃/Acm for hypo/hypereutectoid steels).

Additive manufacturing — FDM layer deposition

Fused Deposition Modelling (FDM) builds parts by extruding thermoplastic filament layer-by-layer. Each layer bonds to the one below by thermal fusion. The fastest, cheapest AM process.

BUILD PLATFORM (moves down each layer) Z↓ Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 (current) NOZZLE X → filament ~220°C heater block Filament (1.75mm) → heated → extruded → deposited → bonds to previous layer Layer height: 0.1–0.4 mm | Materials: PLA, ABS, PETG, nylon | Z-direction weakest (anisotropic)
FDM (Fused Deposition Modelling) — nozzle deposits molten filament; each layer fuses to the one below; build platform descends per layer
FDM (Fused Deposition Modelling)
Thermoplastic filament melted in heated nozzle and deposited layer by layer. Cheapest, fastest. Anisotropic — Z-direction (between layers) much weaker than X-Y. PLA, ABS, PETG.
SLA (Stereolithography)
UV laser cures liquid photopolymer resin layer by layer. Best surface finish and accuracy of AM processes. Parts brittle and UV-sensitive. Slow, expensive resin.
SLS (Selective Laser Sintering)
Laser sinters powder (polymer, metal, ceramic) bed layer by layer. No support structures needed — unfused powder supports the part. Produces porous parts unless infiltrated.
Support structures (AM)
Sacrificial material printed under overhangs in FDM/SLA to prevent collapse. Must be removed post-print. Soluble support materials (PVA, HIPS) dissolve in water/limonene.
STL file format
Standard input for all AM processes. 3D model surface approximated by triangular facets. Sliced into 2D layers by slicer software before printing. Finer facets = better accuracy.
Anisotropy in AM
FDM parts weakest in Z-direction (inter-layer bond is weakest). Design parts so critical load paths are in X-Y plane. SLS and SLA are more isotropic than FDM.