Chapter 11: Metal Casting
Nonferrous Casting Alloys: Copper Alloys
- Includes bronze, brass, and aluminum bronze - Properties: ~ Corrosion resistance ~ Attractive appearance ~ Good bearing qualities - Limitation: high cost of copper - Applications: pipe fittings, marine propeller blades, pump components, ornamental jewelry
Plaster Mold Casting Disadvantages
- Mold must be baked to remove moisture ~ Moisture can cause problems in casting - Mold strength is lost if over- baked - Plaster molds cannot stand high temperatures ~ Limited to lower melting point alloys
Shell Molding: Disadvantages
- More expensive metal pattern - Difficult to justify for small quantities
Metals for Casting
- Most commercial castings are made of alloys rather than pure metals ~ Alloys are generally easier to cast, and properties of productare better - Casting alloys can be classified as: ~ Ferrous ~ Nonferrous
Ferrous Casting Alloys: Cast Iron
- Most important of all casting alloys - Tonnage of cast iron castings is several times that of all other metals combined - Several types: (1) gray cast iron, (2) nodular iron, (3) white cast iron, (4) malleable iron, and (5) alloy cast irons - Typical pouring temperatures ∼ 1400°C (2500°F), depending on composition
Overflow Modification/Results
- Porosity did relocate ~ Currently running trial ~ No quality improvement
Vacuum Molding
Uses sand mold held together by vacuum pressure rather than by a chemical binder - The term "vacuum" refers to mold making rather than casting operation itself - Developed in Japan around 1970
(MC 11.16) Which one of the following casting metals is most important commercially? a) aluminum and its alloys b) bronze c) cast iron d) cast steel e) zinc alloys
c
Steps in Investment Casting/Lost Wax Casting
(1) Wax patterns are produced (2) Several patterns are attached to a sprue to form a pattern tree (3) Pattern tree is coated with a thin layer of refractory material (4) Full mold is formed by covering the coated tree with sufficient refractory material to make it rigid (5) Mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity (6) Mold is preheated to a high temperature, the molten metal is poured, and it solidifies (7) Mold is broken away from the finished casting and the parts are separated from the sprue
Overview of Sand Casting
- Most widely used casting process, accounting for a significant majority of total tonnage cast - Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium - Castings range in size from small to very large - Production quantities from one to millions
Investment Casting: Advantages
- Parts of great complexity and intricacy can be cast - Close dimensional control and good surface finish - Wax can usually be recovered for reuse - This is a net shape process ~Additional machining is not normally required
Expanded Polystyrene Process: Advantages
- Pattern need not be removed from the mold - Simplifies and speeds mold‐making , because two mold halves are not required as in a conventional green‐sand mold
Making the Sand Mold
- The *cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern - The mold must also contain gating and riser system - If casting is to have internal surfaces, a *core must be included in mold - A new sand mold must be made for each part produced
Ferrous Casting Alloys: Steel
- The mechanical properties of steel make it an attractive engineering material - The capability to create complex geometries makes casting an attractive shaping process - Difficulties when casting steel: ~ Pouring temperature is high∼1650°C(3000°F) ~At such temperatures, steel readily oxidizes, so molten metal must be isolated from air ~ Molten steel has relatively poor fluidity
(MC 11.15) 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 e) pipe formation
c
(MC 11.3) Silica sand has which of the following compositions? a)(AL_2)(O_3) b)SiO c)Si(O_2) d)SiS(O_4)
c
Shell Molding Steps
(1) A metal pattern is heated and placed over a box containing sand mixed with thermosetting resin (2) Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell (3) Box is repositioned so loose uncured particles drop away (4) Sand shell is heated in oven for several minutes to complete curing. (5) shell mold is stripped from pattern. (6) Two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished. (7) Finished casting with sprue removed
Steps in Permanent Mold Casting
(1) Mold is preheated and coated for lubrication and heat dissipation (2) Cores (if any are used) are inserted and mold is closed (3) Molten metal is poured into the mold, where it solidifies
Steps in Expanded Polystyrene Process
(1) Polystyrene foam pattern is coated with refractory compound (2) Foam pattern is placed in mold box, and sand is compacted around the pattern (3) Molten metal is poured into the portion of the pattern that forms the pouring cup and sprue - As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus filling the mold cavity
Expanded Polystyrene Process: Disadvantages
- A new pattern is needed for every casting - Economic justification of the process is highly dependent on cost of producing patterns
Product Design Considerations: Geometric Simplicity
- Although casting can be used to produce complex part geometries, simplifying the part design usually improves castability - Avoiding unnecessary complexities: ~ Simplifies mold‐making ~ Reduces the need for cores ~ Improves the strength of the casting
6) Heat Treatment
- Castings are often heat treated to enhance properties - Reasons for heat treating a casting: ~ For subsequent processing operations such as machining ~ To bring out the desired properties for the application of the part in service
Semi-Solid Metal Casting: Advantages
- Complex part geometries - Thin part walls possible - Close tolerances - Zero or low porosity, resulting in high strength of the casting
Product Design Considerations: Dimensional Tolerances and Surface Finish
- Dimensional accuracy and finish vary significantly, depending on process ~ Poor dimensional accuracies and finish for sand casting ~ Good dimensional accuracies and finish for die casting and investment casting
Applications and Metals for Permanent Mold Casting
- Due to high mold cost, process is best suited to high volume production and can be automated accordingly - Typical parts: automotive pistons, pump bodies, and certain castings for aircraft and missiles - Metals commonly cast: aluminum, magnesium, copper‐base alloys, and cast iron ~ Unsuited to steels because of very high pouring temperatures
Buoyancy in a Sand Casting Operation
- During pouring, buoyancy of the molten metal tends to displace the core, which can cause casting to be defective - Force tending to lift core = weight of displaced liquid minus the weight of core itself, (Fb =Wm ‐Wc), where Fb = buoyancy force; Wm = weight of molten metal displaced; and Wc = weight of core
Vacuum Mold Casting Advantages
- Easy recovery of the sand, since no binders - Sand does not require mechanical reconditioning done when binders are used - Since no water is mixed with sand, moisture‐related defects are avoided
Die Casting: Advantages
- Economical for large production quantities - Good accuracy and surface finish - Thin sections possible - Rapid cooling means small grain size and good strength in casting
Furnaces for Casting Processes
- Furnaces most commonly used in foundries: ~ Cupolas ~ Direct fuel‐fired furnaces ~ Crucible furnaces ~ Electric‐arc furnaces ~ Induction furnaces
Nonferrous Casting Alloys: Aluminum
- Generally considered to be very castable - Low pouring temperatures due to low melting temperature ~Pure Aluminum Tm = 660°C (1220°F) - Properties: ~ Lightweight ~ Range of strength properties by heat treatment ~Easy to machine
Permanent Mold Casting: Limitations
- Generally limited to metals of lower melting point - Simpler part geometries compared to sand casting because of need to open the mold - High cost of mold
Die Casting: Limitations
- Generally limited to metals with low metal points - Part geometry must allow removal from die
Die Polish Analysis
- Goals: ~ Increase Quality, ~ Cost Savings - Hypothesis: ~ Lengthening the polish time will increase quality - Procedure: ~ Compare Leak Rate vs. Duration of Polish
Plaster Mold Casting Advantages
- Good accuracy and surface finish - Capability to make thin cross sections
Permanent Mold Casting: Advantages
- Good dimensional control and surface finish - Rapid solidification caused by metal mold results in a finer grain structure, so castings are stronger
Investment Casting: Disadvantages
- Many processing steps are required - Relatively expensive process
Foundry Sand: Binders Used w Foundry Sand
- Sand is held together by a mixture of water and bonding clay ~ Typical mix: 90% sand, 3% water, and 7% clay - Other bonding agents also used in sand molds: ~Organic resins (e g , phenolic resins) ~ Inorganic binders (e g , sodium silicate and phosphate) - Additives are sometimes combined with the mixture to increase strength and/ or permeability
Product Design Considerations: Corners on the casting
- 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
Vacuum Mold Casting Disadvantages
- Slow process - Not readily adaptable to mechanization
Shell Molding: Advantages
- Smoother cavity surface permits easier flow of molten metal and better surface finish - Good dimensional accuracy - Mold collapsibility minimizes cracks in casting - Can be mechanized for mass production
Desirable Mold Properties
- Strength ‐ to maintain shape and resist erosion - Permeability ‐ to allow hot air and gases to pass through voids in sand - Thermal stability ‐ to resist cracking on contact with molten metal - Collapsibility ‐ ability to give way and allow casting to shrink without cracking the casting - Reusability ‐ can sand from broken mold be reused to make other molds? Typically yes
Product Design Considerations: Machining Allowances
-Almost all sand castings must be machined to achieve the required dimensions and part features - Additional material, called the *machining allowance, is left on the casting in those surfaces where machining is necessary - Typical machining allowances for sand castings are around 1.5 and 3 mm (1/16 and 1/4 in)
Additional Steps After Solidification
1) Trimming 2) Removing the core 3) Surface cleaning 4) Inspection 5) Repair, if required 6) Heat treatment
Steps in Sand Casting
1. Pour the molten metal into sand mold 2. Allow time for metal to solidify 3. Break up the mold to remove casting 4. Clean and inspect casting - Separate gating and riser system 5. Heat treatment of casting is sometimes required to improve metallurgical properties (can increase yield strength)
Example 2: True centrifugal casting is performed horizontally to make copper tube sections. The tubes have a length = 1.0 m, diameter = 0.25 m, and wall thickness = 15 mm. (a) If the rotational speed of the pipe = 700 rev/min, determine the G-factor on the molten metal. (b) Is the rotational speed sufficient to avoid "rain?" (c) What volume of molten metal must be poured into the mold to make the casting if solidification shrinkage and solid thermal contraction for copper are = 4.5% and = 7.5%, respectively (both are volumetric contractions).
?
General Defects- Misrun
A casting that has solidified before completely filling mold cavity
Centrifugal Casting
A family of casting processes in which the mold 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
Investment Casting (a.k.a. Lost Wax Process)
A pattern made of wax is coated with a refractory material to make the mold, after which wax is melted away prior to pouring molten metal - "Investment" comes from a less familiar definition of "invest" - "to cover completely," which refers to coating of refractory material around wax pattern - It is a precision casting process ~ Capable of producing castings of high accuracy and intricate detail
Die Casting
A permanent mold casting process in which molten metal is injected into mold cavity under high pressure - Pressure is maintained during solidification, then mold is opened and part is removed - Molds in this casting operation are called *dies; hence the name die casting - Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes
Sand Casting Defects - Mold Shift
A step in the cast product at parting line caused by sidewise relative displacement of cope and drag
Sand Casting Defects - Sand Blow
Balloon‐shaped gas cavity caused by release of mold gases during pouring
Shell Molding
Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder
Semicentrifugal Casting
Centrifugal force is used to produce solid castings rather than tubular parts - Molds use risers at center to supply feed metal - Density of metal in final casting is greater in outer sections than at center of rotation - Often used on parts in which center of casting is machined away, thus eliminating the portion where quality is lowest ~ Examples: wheels and pulleys
4) Electric‐Arc Furnaces
Charge is melted by heat generated from an electric arc - High power consumption ~ But electric- arc furnaces can be designed for high melting capacity - Used primarily for melting steel
Squeeze Casting
Combination of casting and forging in which a molten metal is poured into a preheated lower die, and the upper die is closed to create the mold cavity after solidification begins - Differs from usual closed-mold casting processes in which die halves are closed before introduction of the molten metal - Compared to conventional forging, pressures are less and finer surface details can be achieved
General Defects- Shrinkage
Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze
Draft
Design change to eliminate need for using a core: (a) original design, and (b) redesign
Die Casting Machines
Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity - Two main types: 1. Hot‐chamber machine 2. Cold‐chamber machine
Semi-Solid Metal Casting
Family of net-shape and near net-shape processes performed on metal alloys at temperatures between liquidus and solidus - Thus, the alloy is a mixture of solid and molten metals during casting (mushy state) ~ To flow properly, the mixture must consist of solid metal globules in a liquid ~ Achieved by stirring the mixture to prevent dendrite formation
Buoyancy Example 1 An aluminum-copper alloy (92% Al, 8% Cu) casting is made in a sand mold using a sand core that weighs 15 kg. Determine the buoyancy force in Newtons tending to lift the core during pouring.
Fb=111.3 N
Sand Casting Defects - Pinholes
Formation of many small gas cavities at or slightly below surface of casting
Core
Full‐scale model of interior surfaces of part - Inserted into mold cavity prior to pouring - The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces - May require supports to hold it in position in the mold cavity during pouring, called *chaplets
The Pattern
Full‐sized model of part, slightly enlarged to account for shrinkage and machining allowances in the casting -Pattern Materials: ~ Wood- common material because it is easy to work , but it warps ~ Metal- more expensive to fabricate, but lasts longer ~ Plastic- compromise between wood and metal - Types of patterns used in sand casting: (a) solid pattern, (b) split pattern, (c) match‐plate pattern, (d) cope and drag pattern (Picture in notes)
1) Hot-Chamber Die Casting
Metal is melted in a container, and a piston injects liquid metal under high pressure into the die - High production rates (500 parts per hour not uncommon) - Applications limited to low melting‐point metals that do not chemically attack plunger and other mechanical components - Casting metals: zinc, tin, lead, and magnesium
3) Crucible furnaces
Metal is melted without direct contact with burning fuel mixture -Sometimes called *indirect fuel‐fired furnaces - Container (crucible) is made of refractory material or high‐temperature steel alloy - Used for nonferrous metals such as bronze, brass, and alloys of zinc and aluminum - Three types used in foundries: (a) Lift‐out crucible, (b) stationary pot - molten metal must be ladled, and (c) tilting-pot furnace
General Defects- Cold Shots
Metal splatters during pouring and solid globules form and become entrapped in casting
Centrifuge Casting
Mold is designed with part cavities located away from axis of rotation, so molten metal poured into mold is distributed to these cavities by centrifugal force - Used for smaller parts - Radial symmetry of part is not required as in other centrifugal casting methods
Two Categories of Casting Processes: 2) Permanent mold casting
Mold is made of metal and can be used to make many castings - Advantage: higher production rates - Disadvantage: geometries limited by need to open mold
Two Categories of Casting Processes: 1) Expendable mold casting
Mold is sacrificed to remove part - Advantage: more complex shapes possible - Disadvantage: production rates often limited by the time to make mold rather than casting itself
True centrifugal casting
Molten metal is poured into rotating mold to produce a tubular part - In some operations, mold rotation commences after pouring rather than before - Parts: pipes, tubes, bushings, and rings - Outside shape of casting can be round, octagonal, hexagonal, etc , but inside shape is (theoretically) perfectly round, due to radially symmetric forces
2) Cold‐Chamber Die Casting Machine
Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity - High production but not usually as fast as hot‐chamber machines because of pouring step - Casting metals: aluminum, brass, and magnesium alloys - Advantages of hot‐chamber process favor its use on low melting‐point alloys (zinc, tin, lead)
Sand Casting Production Sequence
Production sequence in sand casting, including pattern‐making and mold‐making (picture in notes)
3) Surface Cleaning 4) Inspection
Removal of sand from casting surface and otherwise enhancing appearance of surface - Cleaning methods: tumbling, air‐blasting with coarse sand grit or metal shot, wire brushing, buffing, and chemical pickling - Surface cleaning is most important for sand casting ~ In many permanent mold processes, this step can be avoided - Defects are possible in casting, and inspection is needed to detect their presence
1) Trimming
Removal of sprues, runners, risers, parting‐line flash, fins, chaplets, and any other excess metal from the cast part - For brittle casting alloys and when cross sections are relatively small, appendages can be broken off - Otherwise, hammering, shearing, hack‐sawing, band‐sawing, abrasive wheel cutting, or various torch cutting methods are used
Metal Casting Process
Sand Casting Other Expendable Mold Casting Processes Permanent Mold Casting Processes Foundry Practice Casting Quality Metals for Casting Product Design Considerations
Other Expendable Mold Processes
Shell Molding Vacuum Molding Expanded Polystyrene Process Investment Casting Plaster Mold and Ceramic Mold Casting
Foundry Sand: Desirable Properties
Silica (SiO2) or silica mixed with other minerals - Good refractory properties ‐ for high temperatures - Small grain size for better surface finish on cast part - Large grain size is more permeable, allowing gases to escape during pouring - Irregular grain shapes strengthen molds due to interlocking, compared to round grains ~ Disadvantage: interlocking reduces permeability
Ceramic Mold Casting
Similar to plaster mold casting except that mold is made of refractory ceramic material that can withstand higher temperatures than plaster - Can be used to cast steels, cast irons, and other high‐temperature alloys - Applications similar to those of plaster mold casting except for the metals cast - Advantages (good accuracy and finish) also similar
Plaster Mold Casting
Similar to sand casting except mold is made of plaster of Paris (gypsum ‐ CaSO4‐2H2O) - In mold-making, plaster and water mixture is poured over plastic or metal pattern and allowed to set ~Wood patterns not generally used due to extended contact with water - Plaster mixture readily flows around pattern, capturing its fine details and good surface finish
2) Direct Fuel‐Fired Furnaces
Small open‐hearth in which charge is heated by natural gas fuel burners located on side of furnace - Furnace roof assists heating action by reflecting flame down against charge - At bottom of hearth is a tap hole to release molten metal - Generally used for nonferrous metals such as copper‐base alloys and aluminum
Hot-Chamber Die Casting Steps
Step 1- with die closed and plunger withdrawn, molten metal flows into the chamber Step 2- plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification. Step 3- plunger is withdrawn, die is opened, and casting is ejected
Cold‐Chamber Die Casting Machine Steps
Step 1- with die closed and ram withdrawn, molten metal is poured into the chamber Step 2- ram forces metal to flow into die, maintaining pressure during cooling and solidification Step 3- ram is withdrawn, die is opened, and part is ejected
Ladles
Two common types of ladles to transfer molten metals to molds: (a) crane ladle, and (b) two‐man ladle3
General Defects- Cold Shut
Two portions of metal flow together but there is a lack of fusion due to premature freezing
The Basic Permanent Mold Process
Uses a metal mold constructed of two sections designed for easy, precise opening and closing - Molds used for casting lower melting point alloys are commonly made of steel or cast iron - Molds used for casting steel must be made of refractory material, due to the very high pouring temperatures
Expanded Polystyrene Process/Lost Foam Molding
Uses a mold of sand packed around a polystyrene foam pattern which vaporizes when molten metal is poured into mold - Other names: lost‐foam process, lost pattern process, *evaporative‐foam process, and full‐mold process - Polystyrene foam pattern includes sprue, risers, gating system, and internal cores (if needed) - Mold does not have to be opened into cope and drag sections
1) Cupolas Furnace
Vertical cylindrical furnace equipped with tapping spout near base - Used only for cast irons ~ Although other furnaces are also used, the largest tonnage of cast iron is melted in cupolas - The "charge," consisting of iron, coke, flux, and any alloying elements, is loaded through a charging door located less than halfway up height of cupola
(MC 11.2) In sand casting, the volumetric size of the pattern is a) Bigger than b) Same size as c) Smaller than than the cast part?
a
(MC 11.13) Which of the following are advantages of die casting over sand casting? (4) a) better surface finish b) closer tolerances c) higher melting temperature metals d) higher production rates e) larger parts can be cast f) mold can be reused
a, b, d, f
(MC 11.11) Which of the following casting processes are permanent mold operations? (3) a) centrifugal casting b) die casting c) expanded polystyrene process d) sand casting e) shell molding f) slush casting g) vacuum molding
a, b, f
(MC 11.12) Which of the following metals would typically be used in die casting? (3) a) aluminum b) cast iron c) steel d) tin e) tungsten f) zinc
a, d, f
(MC 11.14) Cupolas are furnaces are used to melt which of the following metals? a) aluminum b) cast iron c) steel d) zinc
b
(MC 11.4) For which of the following reasons is green mold named? a) green is the color of the mold b) moisture is contained in the mold c) mold is cured d) mold is dry
b
(MC 11.7) Shell molding is best described by which 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 processes 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 d) casting operation used to make artificial sea shells
b
(MC 11.9) In plaster mold casting, the mold is made of which of the following materials? a) (Al_2)(O_3) b) CaS(O_4)-(H_2)O c) SiC d) Si(O_2)
b
(MC 11.10) Which of the following qualifies as a precision-casting process? (2) a) ingot casting b) investment casting c) plaster-mold casting d) sand casting e) shell casting
b and c
(MC 11.6) Which of the following cast processes are expendable mold operations? (4) a) centrifugal casting b) die casting c) investment casting d) low pressure casting e) sand casting f) shell casting g) slush casting h) vacuum casting
c, e, f, h
(MC 11.1) Which one of the following casting processes is the most widely used? a) centrifugal casting b) die casting c) investment casting d) sand casting e) shell casting
d
(MC 11.5) Given that (W_m)=weight of the molten metal displaced by a core and (W_c)=weight of the core, the buoyancy force is which one of the following? a) downward force=(W_m)+(W_c) b) downward force=(W_m)-(W_c) c) upward force=(W_m)+(W_c) d)upward force=(W_m)-(W_c)
d
(MC 11.8) Investment casting is also known by which of the following names? a) fast-payback molding b) full-mold process c) lost-foam process d) lost-pattern process e) lost-wax process
e
(11.1)
hi
(11.2)
hi
Foundry Sand: Types of Sand Molds
- Green‐sand molds: mixture of sand, clay, and water ~ "Green" means mold contains moisture at time of pouring - Dry‐sand mold: organic binders rather than clay ~ Mold is baked to improve strength - Skin‐dried mold: drying mold cavity surface of a green‐sand mold to a depth of 10 to 25 mm, using torches or heating lamps
2) Removing the Core
- If cores have been used, they must be removed ~ Most cores are bonded, and they often fall out of casting as the binder deteriorates ~ In some cases, they are removed by shaking the casting, either manually or mechanically ~ In rare cases, cores are removed by chemically dissolving bonding agent ~ Solid cores must be hammered or pressed out
Product Design Considerations: Draft Guidelines
- In expendable mold casting, draft facilitates removal of pattern from mold ~ Draft = 1° for sand casting - In permanent mold casting, purpose is to aid in removal of the part from the mold ~Draft = 2° to 3° for permanent mold processes - Similar tapers should be allowed for solid cores
Permanent Mold Casting Processes
- In permanent mold casting, the mold is reused many times - The processes include: ~ Basic permanent mold casting ~ Die casting ~ Centrifugal casting - Economic disadvantage of expendable mold casting: ~A new mold is required for every casting
Casting Quality
- There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product - The defects can be classified as follows: ~ General defects common to all casting processes ~ Defects related to sand casting process
5) Induction Furnaces
- Uses alternating current passing through a coil to develop magnetic field in metal ~ Induced current causes rapid heating and melting ~ Electromagnetic force field also causes mixing action - Since metal does not contact heating elements, environment can be closely controlled to produce molten metals of high quality and purity - Common alloys: steel, cast iron, and aluminum
Molds for Die Casting
- Usually made of tool steel, mold steel, or maraging steel - Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron - Ejector pins required to remove part from die when it opens - Lubricants must be sprayed onto cavity surfaces to prevent sticking
Nonferrous Casting Alloys: Zinc Alloys
- Very castable, commonly used in die casting - Low pouring temperatures due to low melting temperature ~ Pure zinc Tm = 419°C (786°F) - Good fluidity for ease of casting - Properties: ~ Low creep strength, so castings cannot be subjected to prolonged high stresses
Foundry Inspection Methods
- Visual inspection to detect obvious defects such as misruns, cold shuts, and severe surface flaws - Dimensional measurements to insure that tolerances have been met - Metallurgical, chemical, physical, and other tests concerned with quality of cast metal
