Ch 8 Metal Casting Processes
What are hot spots?
Thick sections cool slower than other sections causing abnormal shrinkage. Defects such as voids, cracks and porosity are created.
Which of the following metals would typically be used in die casting (three best answers): (a) aluminum, (b) cast iron, (c) steel, (d) tin, (e) tungsten, and (f) zinc?
(a) aluminum (d) tin (f) zinc
In plaster mold casting, the mold is made of which one of the following materials: (a) Al2O3, (b) CaSO4-H2O, (c) SiC, or (d) SiO2?
(b) CaSO4-H2O
Shell molding is best described by which one of the following: (a) casting operation in which the molten metal has been poured out after a thin shell has been solidified in the mold, (b) casting process in which the mold is a thin shell of sand bonded by a thermosetting resin, (c) sand-casting operation in which the pattern is a shell rather than a solid form, or (d) casting operation used to make artificial sea shells?
(b) casting process in which the mold is a thin shell of sand bonded by a thermosetting resin
Which one of the following casting metals is most important commercially: (a) aluminum and its alloys, (b) bronze, (c) cast iron, (d) cast steel, or (e) zinc alloys?
(c) cast iron
A misrun is which one of the following defects in casting: (a) globules of metal becoming entrapped in the casting, (b) metal is not properly poured into the downsprue, (c) metal solidifies before filling the cavity, (d) microporosity, and (e) "pipe" formation?
(c) metal solidifies before filling the cavity
Investment casting is also known by which one of the following names: (a) fast-payback molding, (b) full-mold process, (c) lost-foam process, (d) lost-pattern process, or (e) lost-wax process?
(e) lost-wax process
What are the furnaces for casting processes?
- Cupolas - Direct fuel-fired furnaces - Crucible furnaces - Electric-arc furnaces - Induction furnaces
Properties of malleable cast iron?
- It is obtained by heat treatment of white cast iron. - This structure promoted toughness and ductility. - It is used in making small tools, pipe fittings, automobile parts etc.
There are various types of patterns used in sand casting. What is the difference between a split pattern and a match-plate pattern?
- Split pattern: consists of two pieces, dividing the part along a plane where the two halves align. - Match-plate pattern: The two halves are to the opposite sides of a wood or metal plate.
The properties of cast iron are affected by what factors?
- chemical composition of the iron - rate of cooling ot the casting in the mold - type of graphite formed (if any)
VACUUM MOLDING, advantages
- easy recovery of the sand, since binders not used - sand does not require mechanical reconditioning normally done when binders are used - since no water is mixed with sand, moisture-related defects are absent
DIE CASTING, advantages
- economical for large production quantities - good dimensional accuracy and surface finish - thin sections are possible - rapid cooling provides small grain size and good strength to casting
Advantages DIE CASTING
- excellent surface finish - excellent dimensional accuracy - high production rate - complex shapes can be cast - little or no finishing cost
PERMANENT MOLD CASTING, advantages
- good dimensional control and surface finish - more rapid solidification caused by the cold metal cold results in a finer grain structure, so stronger castings are produced - the cold can be reused many times
Advantages PERMANENT MOLD CASTING
- good surface finish - good dimensional accuracy - high production rate
Name different types of cast iron
- gray cast iron - white cast iron - malleable cast iron - ductile or nodular cast iron - compacted graphite iron
Limitations INVESTMENT CASTING
- limitations on part size - expensive pattern and mold - high labour cost
What are the advantages of cast iron?
- low tooling and production cost - good machinability - ability to cast into complex shapes - excellent wear resistance and high hardness
PLASTER MOLD CASTING, disadvantages
- moisture in plaster mold causes problems - plaster holds cannot stand high temperatures
SHELL MOLDING, disadvantages
- more expensive metal pattern - difficult to justify for small quantities
INVESTMENT CASTING, advantages
- parts of great complexity can be cast - close dimensional control and good surface finish - wax can usually be recovered for reuse - net shape process - formation of hollow interiors in cylinders without cores - less material required for gate
Properties of white cast iron?
- produced by rapid cooling - carbon remains chemically combined in the form of iron carbide (cementite) - it is hard, brittle and posses excellent wear resistance
What does avoiding unnecessary design complexities lead to?
- simplifies mold-making - reduces the need for cores - improves the strength of the casting
VACUUM MOLDING, disadvantages
- slow process - not readily adaptable to mechanisation
SHELL MOLDING, advantages
- smoother cavity surface permits easier flow of molten metal and better surface finish - good dimensional accuracy - cold collapsibility minimises cracks in casting - can be mechanised for mass production
On what factors depends the selection of the most appropriate furnace?
- the casting alloy - it´s melting and pouring temp - capacity requirements of the furnace - costs of investment, operation and maintenance - environmental pollution considerations
Why should the pattern be oversized?
- to account for shrinkage in cooling and solidification - to provide enough metal for the subsequent machining operation
What are the considerations when selection of casting process?
- type of metal - number of castings required - shape and size of castings - dimensional accuracy required - casting finish required - economics
Characteristics of zinc alloys?
1. HIGHLY CASTABLE, commonly used in die casting 2. LOW MELTING POINT, melting point of zinc 419 C 3. GOOD FLUIDITY, for ease of casting
Characteristics of copper alloys?
1. High cost 2. corrosion resistance 3. attractive appearance 4. good bearing qualities
Name the two main types of die casting machines
1. Hot-chamber machine 2. Cold-chamber machine
What are the steps in sand casting?
1. Prepare mold for pour 2. Pour molten metal into sand mold 3. Allow metal to solidify 4. Break up the cold to remove casting 5. Clean and inspect casting 6. Heat treatment of casting is sometimes required to improve metallurgical properties
What are the benefits of continuous casting?
1. The castings material will possess uniform properties 2. The castings solidify 10 times the rate that a casting solidifies during ingot production 3. With less loss of material, cost reduction, higher productivity rate, and superior quality of casting, continuous casting manufacture is often the choice over ingot production
Define centrifugal casting
A group of casting processes in which the old is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavity The group includes - True centrifugal casting - Semicentrifugal casting - Centrifuge casting
Which of the following qualifies as a precision-casting process (two correct answers): (a) ingot casting, (b) investment casting, (c) plaster- mold casting, (d) sand casting, and (e) shell molding?
(b) investment casting (c) plaster- mold casting
INVESTMENT CASTING, disadvantages
- many processing steps are required - relatively expensive process - requires skilled labor - contamination of internal surface of casting with non-metallic inclusions - inaccurate internal diameter
What are the material properties which affect castability?
- melting temp - fluidity - coefficient of expansion - chemical affinity Not only material property, it is affected by casting temp, mold type, and mold temp.
What properties determine the quality of a sand mold for sand casting?
The usual properties are (1) strength - ability to maintain shape in the face of the flowing metal, (2) permeability - ability of the mold to allow hot air and gases to escape from the cavity, (3) thermal stability - ability to resist cracking and buckling when in contact with the molten metal, (4) collapsibility - ability of the mold to give way during shrinkage
Which one of the following casting processes is the most widely used: (a) centrifugal casting, (b) die casting, (c) investment casting, (d) sand casting, or (e) shell casting?
(d) sand casting
What are the common defects in sand casting?
- Mold shift, cope and drag are not placed exactly - Penetration, when fluidity is high it may penetrate inte the sand mold and mix - Pin holes, formation of small gas cavities at or slightly below surface of casting - Sand blow, balloon-shaped gas cavity caused by release of mold gases during pouring
Difficulties when casting steel?
- Pouring temperature for steel is higher than for most other casting metals, 1650 C - At such temperatures, steel readily oxidizes, so molten metal must be isolated from air - Molten steel has relatively poor fluidity
EXPANDED POLYSTYRENE PROCESS, disadvantages
- a new pattern is needed for every casting - economical justification of the process is highly dependent on cost of producing patterns
Advantages INVESTMENT CASTING
- almost any metal can be cast - good surface finish - good dimensional accuracy - fairly high production rate - intricate shapes can be cast - low finishing cost
Advantages SAND CASTING
- almost any metal can be cast - no limit on size and shape - low equipment cost - economical for low-volume production
Limitations SAND CASTING
- coarse finish - dimensional accuracy not good - finishing required - low production rate
Limitations PERMANENT MOULD CASTING
- high mould cost - suitable for high volume production - suitable for casting of simple shapes - suitable for low-melting-point metals
Define castability
It reflects the easy with which a metal can be poured into a mold to obtain a casting without defects. Depends on part design and material properties.
The properties of grey iron are affected by what factors?
The amount of graphite present as well as the shape, size and distribution of the graphite flakes.
Which of the following casting processes are expendable mold operations (four correct answers): (a) centrifugal casting, (b) die casting, (c) investment casting, (d) low pressure casting, (e) sand casting, (f) shell molding, (g) slush casting, and (h) vacuum molding?
(c) investment casting (e) sand casting (f) shell molding (h) vacuum molding
What considerations exist when designing corners?
- Sharp corners and angles should be avoided, since they are sources of stress concentrations and may cause hot tearing and cracks. - Generous fillets should be designed on inside corners and sharp edges should be blended.
What are the common defects in all castings?
- Shrinkage cavity, depression in surface or internal voids - Cold shot, metal splatters during pouring and solid globules form and become entrapped in casing - Cold shut, two portions of metal flow together but there is a lack of fusion due to premature freezing - Misrun, castings are solidify before completely filling the mold cavity
DIE CASTING, disadvantages
- generally limited to metals with low melting points - part geometry must allow removal from die cavity
What are desirable characteristics for grey cast iron?
- it is among the cheapest of ferrous materials available - it is one of the most easily cast of all metals in the foundry - it has excellent machining qualities
Limitations DIE CASTING
- limitations on part size - high cost of die - generally limited to casting of nonferrous metals
PERMANENT MOLD CASTING, limitations
- limited to metals of lower melting point - simple part geometries compared to sand casting because of the need to open the cold - high cost of mold
CENTRIFUGAL CASTING, disadvantages
- more segregation of alloy component during pouring under the faces of rotation - contamination of internal surface of castings with non-metallic inclusions - inaccurate internal diameter
What are the characteristics of cast iron?
- most important of all casting alloys - typical pouting temps around 1400 C, depending on composition - all cast irons, in general, are brittle materials - they are easy to form shapes, by casting into molds - they are cheaper than forming steel shapes
EXPANDED POLYSTYRENE PROCESS, advantages
- pattern need not be removed from the mold - simplifies and expedites cold-making, since two cold halves are not required as in conventional green-sand mold
Considerations when designing a riser
- riser can not solidify before casting - riser must be places so the liquid metal can be delivered to locations where it is most needed - riser volume must be large enough to provide a sufficient amount of liquid metal to compensate for shrinkage - junctions between casting and feeder should not develop a hot spot where shrinkage porosity can occur
What are the sand mold classifications?
1. GREEN-SAND MOLD - most common and least expensive - sand, clay and water - sand is moist when metal is poured into cold 2. DRY-SAND MOLD - mold is baked to improve strength - organic binders are mixed with the sand 3. SKIN-DRIED MOLD - drying the surface of the green-sand cold
Characteristics of aluminium?
1. Generally very cartable 2. Low pouring temp, due to low melting temp, 660 C 3. Light weight 4. Corrosion resistance 5. Easy to machine 6. Range of strength properties by heat treatment
Different carbon content?
LOW CARBON STEELS (C<0,2%) soft & and very ductile"commercial steels", used for construction MEDIUM CARBON STEELS (C<0,5%) used in machine parts HIGH CARBON STEELS (C>0,5%) very hard, used in drills, saw blades etc
Mechanical properties of grey cast iron?
Large graphite flakes produce good dampening capacity, dimensional stability, resistance to thermal shock and ease of machining. Small graphite flakes results in higher tensile strength, high modulus of elasticity, resistance to crack and smooth machined surfaces.
For which one of the following reasons is a green mold named: (a) green is the color of the mold, (b) moisture is contained in the mold, (c) mold is cured, or (d) mold is dry?
(b) moisture is contained in the mold
Which of the following casting processes are permanent mold operations (two correct answers): (a) centrifugal casting, (b) die casting, (c) expanded polystyrene process, (d) sand casting, (e) shell molding, (f) slush casting, and (g) vacuum molding.
(a) centrifugal casting (e) slush casting
Given that Wm = weight of the molten metal displaced by a core and Wc = weight of the core, the buoyancy force is which one of the following: (a) downward force = Wm + Wc, (b) downward force = Wm - Wc, (c) upward force = Wm + Wc, or (d) upward force = Wm - Wc?
(d) upward force = Wm - Wc
What are the factors affecting the properties of steel?
- Carbon content - Heat treatment and shaping method - Presence of harmful elements - Presence of alloying materials
CENTRIFUGAL CASTING, advantages
- formation of hollow interiors in cylinders without cores - less material required for gate - fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity - slag free
PLASTER MOLD CASTING, advantages
- good dimensional accuracy and surface finish - capability to make thin cross-sections in casting
What are the following elements effect in cast iron? 1. Aluminum 2. Carbon 3. Manganese 4. Phosphorus 5. Silicon 6. Sulfur
1. Deoxidizes molten cast iron 2. Depending on form, affects machinability, ductility, shrinkage 3. Below 0,5 % reduces hardness, above 0,5 % increases hardness 4. Increases fluidity and lowers melting temp 5. Below 3,25 % softens iron and increases ductility, above 3,25 % hardens iron, above 13 % increases acid and corrosion resistance 6. Increases hardness
Which of the following are advantages of die casting over sand casting (four best answers): (a) better surface finish, (b) closer tolerances, (c) higher melting temperature metals, (d) higher production rates, (e) larger parts can be cast, and (f) mold can be reused?
(a) better surface finish (b) closer tolerances (d) higher production rates (f) mold can be reused
Which die-casting machines usually have a higher production rate, cold- chamber or hot-chamber, and why?
Hot-chamber machines are faster because cold-chamber die casting machines require molten metal to be ladled into the chamber from an external source. Ladling takes more time than injecting the molten metal into the die as in the hot-chamber operation.
What are the Machining allowance?
It is the additional material that must be left on the casting in those surfaces where machining is necessary.
In sand casting, the volumetric size of the pattern is (a) bigger than, (b) same size as, or (c) smaller than the cast part?
(a) bigger than
Silica sand has which one of the following compositions: (a) Al2O3, (b) SiO, (c) SiO2, or (d) SiSO4?
(c) SiO2
Advantages CENTRIFUGAL CASTING
- high production rate - good dimensional accuracy and surface finish
Cupolas are furnaces used to melt which of the following metals (one best answer): (a) aluminum, (b) cast iron, (c) steel, or (d) zinc?
(b) cast iron
Name the two basic categories of casting processes
Expendable mold process and Permanent mold process
Limitations CENTRIFUGAL CASTING
- expensive set-up - good for production of cylindrical parts only
What is a cupola?
A cupola is a vertical cylindrical furnace equipped with a tapping spout near its base. Cupolas are used for melting cast irons.
What are the most common metals used in die casting?
Common die-casting metals include zinc, tin, lead, aluminum, brass, and magnesium.
What are the microstructure of compacted graphite cast iron?
Compacted graphite cast iron consists of a microstructure similar to that of gray iron, except that the graphite cells are coarser and more rounded.
What are the properties of compacted graphite cast iron?
Compacted graphite cast iron have properties between those of grey cast iron and nodular cast iron.
What is flash in die casting?
Flash is a thin portion of metal at the exterior of a casting that results from molten metal being squeezed into the spaces between the die halves of the mold at the parting line, or into the clearances around the cores and ejector pins.
What are some of the general defects encountered in casting processes? Name and briefly describe three.
General defects include: (1) misruns, in which the casting solidifies before filling the mold cavity (2) cold shuts, in which two portions of metal flow together but there is lack of fusion at the joint (3) cold shots, where solid globules of cast metal become entrapped in the casting (4) shrinkage cavity, which is a depression on the casting surface or an internal void in the casting caused by solidification shrinkage (5) microporosity, which is a network of small voids throughout the casting caused by localized solidification shrinkage (6) hot tearing, which is a crack in the casting caused by a mold that does not yield to the metal during the early stages of solidification shrinkage.
What are the properties of Ductile/Nudular Cast iron?
It has good ductility, high strength, toughness, wear resistance and excellent casting properties. The carbon is in the form of spherical rather than flakes, thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name.
What is a chaplet?
Metal supports that hold the position of the core.
What are some of the operations required in sand casting after the casting is removed from the mold?
The operations include (1) trimming, in which the sprues, runners, risers, and flash are removed, (2) core removal, (3) surface cleaning, (4) inspection, (5) repair if needed, (6) heat treatment, and (7) machining.
What is the difference between vacuum permanent-mold casting and vacuum molding?
Vacuum permanent-mold casting is a form of low-pressure casting in which a vacuum is used to draw molten metal into the cavity. Vacuum molding is sand casting in which the sand mold is held together by vacuum pressure rather than by a chemical binder.
