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10 - FUNDAMENTALS OF METAL CASTING


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Solidification processing
Processing in which the material solidifies from a liquid to a solid state >>> [1] Casting of metals, [2] Glassworking, [3] Processing of polymers.
Casting
A PROCESS OF POURING MOLTEN MATERIAL INTO A FORM - engine blocks, parts of heavy machinery, complex shapes, a form that can produce a product close to a net shape, shape in which no more processing is required, yet casting can also produce imperfection, innacurate forms, porosity, and the material also shrinks, thus the form needs to be a bit bigger than the final form. >>> MOLTEN BRONZE SHINES BRIGHTER THAN THE EVENING SUN
Net shape
Final shape of the part - no processing is needed: the part has designed geometry and dimensions.
Mold
A PART WITH THE CAVITY, WHICH IS BIGGER THAN THE ORIGINAL PART, FOR MOLTEN MATERIAL TO FILL. Molds have to be a bit bigger than the original part, since the material contracts when it cools. Molds can be either [a] open or [b] closed, and those two categories can be either [1] expendable molds - sand casting molds for example, or [2] permanent - heat resistant mold out of refractory materials, the permanent molds are limited by the need to open and close the mold, in order to remove the part. >>> CLOSED, EXPENDABLE MOLD MAKES WORSE SPEAR TIPS THAN A PERMANENT, CERAMIC ONE.
Sand casting mold
A MOLD MADE OF SAND WHICH HAS THE FEATURES COMMON TO MOST MOLDS - it's devided into a cope and a drag that sit within a two-part flask, devided by the parting line, housing the pattern for the part, a pattern with the core to make holes, a part that red hot, boiling metal will fill when poured through the poring cup, rushing through the downsprue, filling the riser on it's way to the pattern through the runner while evacuating the gases through the vent holes.
Cope and drag
Upper and lower parts of the mold.
Flask
A two part box housing the mold's cope and drag.
Parting line
Line that separates the two parts of the mold.
Pattern
The enlarged part model, half of the pattern is in the cope and another half is in the drag.
Core
Form inside the mold cavity in order to define internal geometry such as holes.
Gaiting system
A channel or a system of channels by which metal flows into the cavity.
Pouring cup
The part in which the molten metal is poured.
Downsprue
Part that channels liquid material into the runner.
Runner
Channel that leads molten material into the main cavity.
Riser
Holds liquid metal to compensate for shrinkage during solidification. MUST FREEZE AFTER PART
Vent holes
A SYSTEM TO EVACUATE GASES when liquid material is poured - drilled holes in permanent molds, sand porosity in sand molds
Heating the metal
In order to heat the metal to the pouring temperature you need to: [1] Invest enough energy to bring it to a melting point. [2] Heat it up until it melts - heat of fusion. [3] Invest even more energy to bring it up to a desired pouring temperature. [4] Consider heat loss to environment, the fact that most metals are alloys with liquidus and solidus rather than one melting point, specific heat changes with temperature, lack of data for a specific alloy. >>> HMCP - HEAT, MELT, COOK, POOR
Pouring the metal
The metal must be poured at a temperature higher than it's freezing point in order not to freeze prematurely (difference between pouring temperature and freezing temperature sometimes referred to as "superheat"), at a correct rate - too slow and some metal will freeze; too fast and turbulence will form, and with as little turbulence as possible, since turbulence in a heavy, chemically reactant metal will erode the mold and promote oxide formations - make curved corners to remove turbulence. >>> HOT METAL SLOWLY POURS IN A LAMINAR STREAM
Analysis of pouring
Using the bernouly equation, while neglecting lossed due to friction, the velocity at the base of the sprue is the square root of earth's gravity and twice of sprue's hight: {{v=sqrt(2*g*h)}}. The volumetric flow rate is velocity multiplied by the cross-section area: {{Q=v*A}}, and the time to fill a mold is the volume divided by the flow rate {{T=V/Q}} >>> VFT: VELOCITY, FLOW-RATE, TIME
Fluidity
CAPABILITY OF THE METAL TO FLOW AND FILL THE MOLD BEFORE FREEZING. Affected by a great multitude of factors: (1) pouring temperature above the melting point, (2) metal alloy composition - better fluidity in pure or eutectic metals, (3) viscosity of the liquid metal, and (4) heat transfer to the surroundings. Can be tested in a spiral mold test for fluidity >>> EUTECTIC FLUIDITY FILLS HOT SAND CAVERNS
Effects of high pouring temperature
The metal will be more fluid, but it would also stay fluid for longer, causing some problems: oxide formation, gas porosity, penetration of metal between sand grains - that will produce a part with embedded sand grains and a poor surface finish. >>> HIGH TEMPERATURE MAKES POROUS AND SANDY SURFACE FINISH.
Solidification of metals
Results in a different structure depending on what is solidifying in the mold - pure or eutectic metals have fine grains at the skin, the part where the part touches the mold's walls, and long, course, columnar grains inside (dendrites). An alloy will form the fine grained skin, a fine grained core, and long, course, columnar dendritic grains between them, due to segregation between components. >>> PURE OR EUTECTIC FREEZE AT FREEZING POINT, ALLOYS FREEZE GRADUALLY
Solidification time
Can be divided into; [1] Local solidification time - the time from beginning to the end of freezing, a the time it takes for the metal to to release its latent heat of fusion into the mold, and [2] Total solidification time - the time between pouring and complete solidification which can be calculated using the following formula: $$T_{ts} = C_m \left( \frac{V}{A} \right) ^{N}$$, with the $C_m$ - the mold constant, $V$ - the volume of the casiting, $A$ - surface area --- thus the higher the volume to surface area ratio is, the longer it takes for the mold to solidify, a fact that used in riser design >>> ROUND SPHERE COOLS LONGER THAN SQUARE CUBE
Shrinkage
Occurs in three stages: [1] Shrinking of the liquid due to cooling, about 0.5%, [2] Contraction during phase change from liquid to solid - usually forms a shrinkage cavity in the solid, [3] Contraction of the solid due to cooling a contraction that is planned ahead for by using linear shrinkage tables - tables that indicate how much a linear dimension would shrink during casting. >>> LIQUID, PHASE CHANGE, SOLID - THEY ALL SHRINK
Pattern shrinkage allowance
Amount by which the mold must be made larger relative to the final casting size.
Directional solidification
Solidification starts away from the riser and progresses towards it, thus minimizing chance of cavities inside part due to solidification. Achieved by: [1] Sections with low V/A ratio away from the riser. [2] Internal chills - embedded inserts of metal, same metal that is cast to promote solidification. [3] External chills - removal of heat from sections that are difficult to supply with molten metal. [4] Sufficiently thick connector between the riser and the part. >>> V/A, CHILLS, THICK RUNNER
Riser design
Riser must remain molten after the casting solidifies, a feat achieved by designing a riser with a high volume to surface area ration, a V/A ratio - use Cvorinov's rule. A riser can be a top or a side riser - attached to a side or the top of the casting, or a open or blind riser - open to the air or totally enclosed withing the mold >>> ROUND RISER STORES MELTED METAL
Hot spots
Places in the mold that inhibit uniform distribution of metal after solidifying, such as a middl of a T or Y junction, thus they are usually redesigned or assisted by chills. >>> CHILLING METAL CHILLS HOT HOLES
Weakness planes
When metal castings solidify, columnar grain structures tend to develop, in the material, pointing towards the center. Due to this nature, sharp corners in the casting may develop a plane of weakness. By rounding the edges of sharp corners this can be prevented. >>> ROUND EDGES REMOVE WEAK PLANES
Chaplet
A support for a core, a support made of a metal with a higher melting temperature than the cast metal, in order not to melt but to become embedded in the final part >>> SUPPORTING CHAPLET HOLDS FAT CORE