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Foundry Presentation

An introduction to various casting processes.
by

Tony Gude

on 16 July 2013

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Transcript of Foundry Presentation

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Expendable mold processes - mold is sacrificed to remove part
Advantage: more complex shapes possible
Disadvantage: production rates often limited by time to make mold rather than casting itself
Permanent mold processes - 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Advantages of permanent mold casting:
Good dimensional control and surface finish
More rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are stronger
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
Advantages and Limitations
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
The Basic Permanent Mold Process
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Economic disadvantage of expendable mold casting: a new mold is required for every casting
In permanent mold casting, the mold is reused many times
The processes include:
Basic permanent mold casting
Die casting
Centrifugal casting
Permanent Mold Casting Processes
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Advantages of investment casting:
Parts of great complexity and intricacy can be cast
Close dimensional control and good surface finish
Wax can usually be recovered for reuse
Additional machining is not normally required; this is a net shape process
Disadvantages
Many processing steps are required
Relatively expensive process
Advantages and Disadvantages
Investment Casting
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
A pattern made of wax is coated with a refractory material to make 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
Investment Casting (Lost Wax Process)
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Advantages of shell molding:
Smoother cavity surface permits easier flow of molten metal and better surface finish
Good dimensional accuracy - machining often not required
Mold collapsibility minimizes cracks in casting
Can be mechanized for mass production
Disadvantages:
More expensive metal pattern
Difficult to justify for small quantities
Advantages and Disadvantages
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Shell Molding
Investment Casting (Lost Wax)
Vacuum Molding
LFC (Lost Foam)
Plaster Mold and Ceramic Mold Casting
Here is a good reference web site:
http://www.custompartnet.com/wu/SandCasting
Other Expendable Mold Processes
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Types of Sand Mold
Binders
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Silica (SiO2) or silica mixed with other minerals
Good refractory properties ‑ capacity to endure high temperatures
Small grain size yields better surface finish on the 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 tends to reduce permeability
Foundry Sands
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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?
Desirable Mold Properties
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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.
Applications of Permanent Mold Casting
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e











Figure 11.10 Steps in permanent mold casting: (2) cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies.
Permanent Mold Casting
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e










Figure 11.10 Steps in permanent mold casting: (1) mold is preheated and coated.
Permanent Mold Casting
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Figure 11-9 A one-piece compressor stator with 108 separate airfoils made by investment casting (photo courtesy of Howmet Corp.).
Investment Casting
(3) the pattern tree is coated with a thin layer of refractory material,
(4) the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid.
(4) sand shell is heated in oven for several minutes to complete curing;
(5) shell mold is stripped from the pattern;
(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 that loose uncured particles drop away;
Shell Molding
Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder
Types of Patterns
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Pour the molten metal into sand mold
Allow time for metal to solidify
Break up the mold to remove casting
Clean and inspect casting
Separate gating and riser system
Heat treatment of casting is sometimes required to improve metallurgical properties
Steps in Sand Casting
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
Making the Sand Mold
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Sand Casting
Other Expendable Mold Casting Processes
Permanent Mold Casting Processes
Foundry Practice
Casting Quality
Metals for Casting
Product Design Considerations
METAL CASTING PROCESSES
Figure 11.4 (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting with internal cavity.
Full‑scale model of interior surfaces of part.
It is inserted into the 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
Core
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
A full‑sized model of the 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 make, but lasts much longer
Plastic - compromise between wood and metal
The Pattern
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
Overview of Sand Casting
(6) two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished;
(7) the finished casting with sprue removed.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
Heat Treatment
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
Surface Cleaning
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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 possible alloying elements, is loaded through a charging door located less than halfway up height of cupola

Cupolas
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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)
Product Design Considerations
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Dimensional Tolerances and Surface Finish:
Significant differences in dimensional accuracies and finishes can be achieved in castings, depending on process:
Poor dimensional accuracies and finish for sand casting
Good dimensional accuracies and finish for die casting and investment casting
Product Design Considerations
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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 if solid cores are used
Product Design Considerations
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
Product Design Considerations
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
Product Design Considerations
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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

Nonferrous Casting Alloys: Copper Alloys
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Generally considered to be very castable
Pouring temperatures low due to low melting temperature of aluminum
Tm = 660°C (1220°F)
Properties:
Light weight
Range of strength properties by heat treatment
Easy to machine
Nonferrous Casting Alloys: Aluminum
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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 of steel is higher than for most other casting metals  1650C (3000F)
At such temperatures, steel readily oxidizes, so molten metal must be isolated from air
Molten steel has relatively poor fluidity
Ferrous Casting Alloys: Steel
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Most important of all casting alloys
Tonnage of cast iron castings is several times that of all other metals combined
Several types:





Typical pouring temperatures  1400C (2500F), depending on composition

Ferrous Casting Alloys: Cast Iron
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Most commercial castings are made of alloys rather than pure metals
Alloys are generally easier to cast, and properties of product are better
Casting alloys can be classified as:
Ferrous
Nonferrous
Metals for Casting
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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

Foundry Inspection Methods
Sand Casting Defects: Pin Holes
Figure 11.23 Common defects in sand castings: (b) pin holes
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Formation of many small gas cavities at or slightly below surface of casting
General Defects: Cold Shot
Figure 11.22 Some common defects in castings: (c) cold shot
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Metal splatters during pouring and solid globules form and become entrapped in casting
General Defects: Cold Shut
Figure 11.22 Some common defects in castings: (b) cold shut
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Two portions of metal flow together but there is a lack of fusion due to premature freezing
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
Casting Quality
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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 casting, either manually or mechanically.
In rare cases, cores are removed by chemically dissolving bonding agent
Solid cores must be hammered or pressed out.

Removing the Core
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
Trimming
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Trimming
Removing the core
Surface cleaning
Inspection
Repair, if required
Heat treatment

Additional Steps After Solidification
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Moving molten metal from melting furnace to mold is sometimes done using crucibles
More often, transfer is accomplished by ladles






Figure 11.21 Two common types of ladles: (a) crane ladle, and (b) two man ladle.
Ladles
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
Electric-Arc Furnaces
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Figure 11.19 Three types of crucible furnaces: (a) lift-out crucible, (b) stationary pot, from which molten metal must be ladled, and (c) tilting-pot furnace.
Crucible Furnaces
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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 type, (b) stationary, (c) tilting

Crucible Furnaces
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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

Direct Fuel‑Fired Furnaces
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Furnaces most commonly used in foundries:
Cupolas
Direct fuel-fired furnaces
Crucible furnaces
Electric-arc furnaces
Induction furnaces
Furnaces for Casting Processes
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Minor changes in part design can reduce need for coring






Figure 11.25 Design change to eliminate the need for using a core: (a) original design, and (b) redesign.
Draft
Sand Casting Defects: Sand Blow
Figure 11.23 Common defects in sand castings: (a) sand blow
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Balloon‑shaped gas cavity caused by release of mold gases during pouring
General Defects: Shrinkage Cavity
Figure 11.22 Some common defects in castings: (d) shrinkage cavity
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze
General Defects: Misrun
Figure 11.22 Some common defects in castings: (a) misrun
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
A casting that has solidified before completely filling mold cavity
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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 in liquid metal
Since metal does not contact heating elements, environment can be closely controlled to produce molten metals of high quality and purity
Melting steel, cast iron, and aluminum alloys are common applications in foundry work
Induction Furnaces
Sand Casting Defects: Mold Shift
Figure 11.23 Common defects in sand castings: (f) mold shift
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
A step in cast product at parting line caused by sidewise relative displacement of cope and drag
Sand Casting Defects: Penetration
Figure 11.23 Common defects in sand castings: (e) penetration
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
When fluidity of liquid metal is high, it may penetrate into sand mold or core, causing casting surface to consist of a mixture of sand grains and metal
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
And 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
Sand Waste
Sand Waste
(cc) image by nuonsolarteam on Flickr
Sand Flow
Metals Flow
Molding
Pouring
Cooling
Shakeout
Trim
Product Finishing
Product & waste
Mixing
Mold Formation
Sand Cooling
Sand Processing
Losses
Recycling
Recycling
SandWaste
Steps in shell‑molding:
(1) a match‑plate or cope ‑and‑ drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin.










(7) the mold is broken away from the finished casting and the parts are separated from the sprue
(5) the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity,
(6) the mold is preheated to a high temperature, the molten metal is poured, and it solidifies
Steps in investment casting:
(1) wax patterns are produced,
(2) several patterns are attached to a sprue to form a pattern tree
Types of patterns used in sand casting:
Solid Pattern
Split Pattern
Matchplate
Cope and Drag
(1) gray cast iron,
(2) nodular iron,
(3) white cast iron,
(4) malleable iron, and
(5) alloy cast irons
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