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11 C - METAL CASTING , Casting quality


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Common casting defects
Some defects are common to all casting processes, and those are: [a] Misruns, [b] Cold shuts, [c] Cold shots, [d] Shrinkage cavity, [e] Microporocity, [f] Hot tearing. >>> MyCoC SeeM Huge - MISRUNS, COLD SHUTS, COLD SHOTS, SHRINKAGE CAVITIES, MICROPOROCITY, HOT TEARING.
Misruns
CASTING SOLIDIFIES BEFORE COMPLETELY FILLING THE MOLD CAVITIES, usually because: [1] Insufficient fluidity; [2] Low pouring temperature; [3] Slow pouring; [4] Thin mold cavity cross-section. >>> COLD, SLOW VISCOUS METAL STICKS IN THIN CROSS-SECTIONS.
Cold shuts
PORTIONS OF MATERIAL FAIL TO FUSE due to the same reasons as with misruns: cold material, slow pouring temperature, lack of fluidity, thin cross-sections.
Cold shots
SOLID GLOBULES OF METAL TRAPPED IN CASTING, globules formed by splattering during pouring - eliminate by improving pouring procedures and gating design. >>> POURING GAIT REMOVES SOLID GLOBULES.
Shrinkage cavity
LACK OF LIQUID METAL PRODUCES A CAVITY, a cavity or a depression in the part resulting from restriction of molten metal available due to a solidification shrinkage, a problem easily solved by proper riser design. >>> ROUND RISER SUPPLIES MOLTEN STEEL
Microporosity
A NETWORK OF SMALL VOIDS THROUGHOUT THE CASTING, a defect resulting from localized solidification shrinkage in the dendritic structure, a defect usually occurring in alloys, since they tend to freeze a bit unevenly. >>> DENDRITIC ALLOY SHRINKS TO SMALL BUBBLES.
Hot tearing
CRACKING DUE TO INABILITY TO CONTRACT, since if a metal can't contract freely in the mold, it would crack and tear, a defect removed in sand molds by the mold's collapsibility, and in permanent molds by removing the part from the mold after solidification. >>> COLLAPSIBLE MOLD PREVENTS HOT CRACKS
Sand casting defects
Some defects occur only in sand castings, sometimes occurring in other expendable mold castings as well, and the defects are: [a] Sand blow; [b] Pinholes; [c] Sand wash; [d] Scabs; [e] Penetration; [f] Mold shift; [g] Core shift; [h] Mold crack >>> BeeP WaSP CuM: BLOW, PINHOLES, WASH, SCABS, PENETRATION, CORE_SHIFT, MOLD_CRACK
Sand blow
MOLD HAS A GAS BUBBLE SHAPED DENT, a defect occurring due to the inability of the gazes to escape - increase permeability, venting, and decrease sand moisture content to prevent gas bubble shaped cavities. >>> SUFFOCATING SAND BLOWS GREAT BUBBLES.
Pinholes
GAS BUBBLE FORMED HOLES UNDER THE SURFACE. A multitude of small, gas formed cavities below the casting's surface, a gas release related problem. >>> SMALL GAS BUBBLES TRAPPED UNDER THE SURFACE.
Sand wash
HOT METAL WASHES AWAY PARTS OF CAST, producing a casting surface irregularity caused by sand erosion during the pouring - redesign gating system, remove turbulence, pour slower >>> TURBULENT METAL STEALS SAND WALLS
Scabs
FLAKING MOLD SCARS PART SURFACE. Irregularities on surface casting, defects formed by mold flaking off some bits during solidification, and embedding them in the casting's surface >>> FLAKING MOLD SCARS PART SURFACE.
Penetration
PERMEABLE WALLS CREATE FUNNY SHAPES ON THE CAST, since the fluid metal penetrates deep into sandy walls, taking on sand grain and peculiar patterns after solidification, a problem solved by densely packing the sand. >>> DENSE WALLS PREVENT NASTY STALACTITES.
Mold shift
DISPLACEMENT OF THE COPE RELATIVE TO DRAG, a defect producing a step on a parting line
Core shift
LIQUID, HEAVY METAL DISPLACES LIGHT, SAND CORE, probably due to a lack of proper anchoring and not enough chaplets holding it in place.
Mold crack
CRACKED MOLD GROWS STRANGE FINS, the cracks letting the molten, hot, fluid metal to flow in, creating strange protrusions upon solidification.
Inspection methods
VaDiM PaRMa: VISUAL, DIMENSIONAL, METTALURGICAL,,,, PRESSURE, RADIOGRAPHIC, MECHANICAL >>> Foundry inspection methods: [1] Visual inspection; [2] Dimensional measurements - check casting is of required size; [3] Metallurgical, chemical, physical and other tests: [a] Pressure testing to locate leaks, [b] Radiographic testing to detect surface or internal defects, [c] Mechanical testing to determine tensile strength and hardness. >>> VaDiM PaRMa: VISUAL, DIMENSIONAL, METTALURGICAL,,,, PRESSURE, RADIOGRAPHIC, MECHANICAL
Ferrous casting alloys
CAST IRON AND STEELS USED FOR CASTING - [1] Cast iron - the most used alloy in casting. [2] Steels - a strong metal with poor fluidity and propensity to oxidize, thus limiting its use for casting when thin sections are needed.
Nonferous casting alloys
AMoC NeT ZiT - ALUMINUM, MAGNESIUM, COPPER, NICKEL, TITANIUM, ZINC, TIN. [1] Aluminum - very castable alloys with good fluidity and low melting points. [2] Magnesium - the lightest of all castable metals and has good corrosion resistance. [3] Copper alloys - bronze, brass, aluminum bronze: expensive yet corrosion resistant alloys with good bearing qualities. [4] Tin based alloys - easy to cast due to low melting temperature and are corrosion resistant. [5] Zinc alloys - common in die casting due to a low melting point and good fluidity, having a major weakness of creep strength. [6] Nickel alloys - good hot strength and corrosion resistance thus are not easy to cast. Used for jet engine parts and rocket components. [7] Titanium alloys - Strong, corrosion resistant, yet hard to cast due to low fluidity and high melting point. >>> AMoC NeT ZiT - ALUMINUM, MAGNESIUM, COPPER, NICKEL, TITANIUM, ZINC, TIN.
Design considerations
SIMPLE, THICK CORNERS DRAFT TOLERANT MACHINED SURFACES >>> Before casting one must consider the following design considerations, in order to produce a good part: [1] Geometric simplicity; [2] Corners; [3] Section thickness; [4] Drafts; [5] Use of cores; [6] Dimensional tolerances; [7] Surface finish; [8] Machining allowances. >>> SCaT DoCTa SaM: SIMPLICITY, CORNERS, THICKNESS,,, DRAFTS, CORES, TOLERANCES,,, SURFACES, MACHINING
Geometric simplicity
The simpler the part is, the simpler it is to cast.
Corners
Round corners remove weakness planes
Section thickness
Uniformly thick sections eliminate hot spots, preventing shrinkage cavities.
Draft
A WEDGE SHAPED PART IS POSSIBLE TO REMOVE FROM THE MOLD - draft or taper the part, using 1[deg] for sand, 2[deg] - 3[deg] for permanent molds
Use of cores
Minimize core use as possible by making better design parts.
Dimensional tolerances
CONSIDER THE TOLERANCE A CASTING PROCESS IMBUES UPON YOUR CREATION - die casting will have small tolerances, sand casting will have bigger tolerances, due to a rougher finish and a lesser degree of dimensional control.
Casting surface finish
ROUGH FOR SAND CASTING, GENTLE FOR DIE AND CERAMICS CASTING. Consider the rough N9 to N10 surface finish of the sand mold, while dreaming of the most exquisite, gentle, smooth N3 to N4 surfaces of plaster, ceramic, investment casting and die casting processes. >>> N10 SAND ::: N4 CERAMICS AND DIES
Machining allowances
LEAVE SOME MATERIAL TO MACHINE AFTERWARDS, bringing the part up to the desired surface finish and dimensional accuracy.