Reading Assignment
- 13.1 Introduction to Multiple Use Molld Casting
- 13.2 Permanent Mold Casting
- 12.3 Die Casting
- 12.4 Squeeze Casting & Semisolid Casting
- 13.5 Centrifugal Casting
- 13.6 Continuous Casting
- 13.7 Melting
- 13.8 Pouring Practice
Outline
Introduction
In expendable mold casting, a separate mold is produced for each casting
Low production rate for expendable mold casting
If multiple-use molds are used, productivity can increase
Most multiple-use molds are made from metal, so most molds are limited to low melting temperature metals and alloys
Permanent-Mold Casting
Also known as gravity die casting
Mold can be made from a variety of different materials
Gray cast iron, alloy cast iron, steel, bronze, or graphite
See www.youtube.com/watch?v=xL8ZCp93ulU
Most molds are made in segments with hinges to allow rapid and accurate closing
Molds are preheated to improve properties
Liquid metal flows through the mold cavity by gravity flow
Process can be repeated immediately because the mold is still warm from the previous casting
Most frequently cast metals
Aluminum, magnesium, zinc, lead, copper, and their alloys
If steel or iron is to be used, a graphite mold must be used
Advantages of Permanent-Mold Casting
Near- net shapes
Little finish machining
Reusable molds
Good surface finish
Consistent dimensions
Directional solidification
Disadvantages of Permanent Mold Casting
Limited to lower melting temperature alloys
High mold costs
Mold life is dependent on the following
Alloys being cast
Mold material
Pouring temperature
Mold temperature
Mold configuration
High production runs can validate high mold costs
Molds are not permeable
Limited mold complexity
Low Pressure Permanent-Mold Casting
Tilt-pour permanent-mold casting
Mold is rotated to force flow into the cavity
Low pressure permanent-mold casting
Mold is upside down and connected to a crucible that contains the molten metal
Pressure difference induces upward flow
Metals are exceptionally clean because it is fed directly into the mold
Little or no turbulence during flow
Typical metals cast using low pressure process
Aluminum, magnesium, and copper
Low-Pressure and Vacuum Permanent-Mold Casting
Vacuum Permanent-Mold Casting
Atmospheric pressure in the chamber forces the metal upward after the vacuum is drawn
Thin-walled castings can be made
Excellent surface quality
Cleaner metals than low pressure
Lower dissolved gas content
Better mechanical properties than low pressure casting
Die Casting
Molten metal is forced into the mold under high pressure
Held under high pressure during solidification
Castings can have fine sections and complex details
Long mold life
Typical metals cast
Zinc, copper, magnesium, aluminum, and their alloys
Advantages of Die Casting
High production rates
Good strength
Intricate shapes
Dimensional precision
Excellent surface qualities
Small-medium sized castings
Die Modifications and Die Life
Die complexity can be improved through the use of
Water cooled passages
Retractable cores
Moving pins to eject castings
Die life
Limited by erosion and usage temperature
Surface cracking
Heat checking
Thermal fatigue
Die-Casting Dies
Basic Types of Die-Casting
See North American Die Casting Association
Hot chamber castings
Fast cycling times
No handling or transfer of molten metal
Used with zinc, tin, and lead-based alloys
Heated-manifold direct injection die casting
Molten zinc is forced though a heated manifold
Next through heated mini-nozzles directly into the die cavity
Eliminates the need for sprues, gates and runners
Cold-chamber machines
Used for materials not suitable for hot chamber machines
Typical materials
Aluminum, magnesium, copper, and high-aluminum zinc
Longer operating cycle than hot-chamber
High productivity
Summary of Die Casting
Dies fill so fast with metal that there is little time for the air in the runner and die to escape
Molds offer no permeability
Air can become trapped and cause defects
Risers are not used because of the high pressures used
Sand cores can not be used due to high pressures
Cast-in inserts can be used
High production rates
Little post casting finishing necessary
Die Casting
Squeeze Casting and Semisolid Casting
Advantages
High production
Thin-walled parts
Good surface finish
Dimensional precision
Good mechanical properties
Squeeze Casting
Large gate areas and slow metal velocities to avoid turbulence
Solidification occurs under high pressure
Intricate shapes with good mechanical properties
Reduced gas and shrinkage porosity
Rheocasting and Thixocasting
Rheocasting
Molten metal is cooled to semisolid
Metal is stirred to break up dendrites
Thixocasting
No handling of molten metal
Metal is stirred as in rheocasting and produced into blocks or bars
Metal is then reheated to semisolid and can be handled as a solid but processed as a liquid
Injection system used is similar to the one used in plastic injection molding
Die Cast Materials
Centrifugal Casting
Inertial forces due to spinning distribute the molten metal into the mold cavity
True centrifugal casting
Dry-sand, graphite or metal mold can be rotated horizontally or vertically
Exterior profile of final product is normally round
Gun barrels, pipes, tubes
Interior of the casting is round or cylindrical
If the mold is rotated vertically, the inner surfaces will be parabolic
Specialized equipment
Expensive for large castings
Long service life
No sprues, gates, or risers
Semicentrifugal casting
Several molds may be stacked on top of one another
Share a common basin and sprue
Used for gear blanks, pulley sheaves, wheels, impellers, etc.
Centrifuging
Uses centrifugal acceleration to force metal into mold cavities that are offset from the axis of rotation
Centrifuging
Continuous Casting
Used for the solidification of basic shapes for feedstock
Can be used to produce long lengths of complex cross sections
Selection of melting method is based on several factors
Temperature needed to melt and superheat the metal
Alloy being melted
Desired melting rate and quantity
Desired quality of metal
Availability and cost of fuels
Variety of metals or alloys to be melted
Batch or continuous
Required level of emission control
Capital and operating costs
Cupolas
Cupola- refractory-lined, vertical steel shell
Alternating layers of carbon, iron, limeston, and alloy additions
Melted under forced air
Simple and economical
Melting rate can be increased by using hot-blast cupolas, oxygen-enriched blasts, or plasma torches
Types of Furnaces
Indirect Fuel-Fired Furnace
Crucibles or holding pots are heated externally which in turn heats the metal
Low capital and operating costs
Direct Fuel-Fired Furnace
Similar to small open-hearth furnaces
Flame passes directly over metal
Arc Furnaces
Preferred method for most factories
Rapid melting rates
Ability to hold molten metal for any period of time
Greater ease of incorporating pollution control equipment
Induction Furnaces
Rapid melting rates
Two basic types of induction furnaces
High-frequency (coreless)
Contains a crucible surrounded by a water-cooled coil of copper tubing
High-frequency electrical current induces an alternating magnetic field
The magnetic field, in turn, induces a current in metal being melted
Low-frequency (channel-type)
Small channel is surrounded by the primary coil and a secondary coil is formed by a loop or channel of molten metal
Induction Furnaces
Ladles are used to transfer the metal from the melting furnace to the mold
Concerns during pouring
Maintain proper metal temperature
Ensure that only high-quality metal is transferred
Pouring may be automated in high-volume, mass-production foundries
Automatic Pouring
Cleaning, Finishing, and Heat Treating of Castings
Post-casting operations
Removing cores
Removing gates and risers
Removing fins, flash, and rough surface spots
Cleaning the surface
Repairing any defects
Cleaning and finishing may be expensive, so processes should be selected that minimize necessary operations
Cleaning and Finishing
Sand cores may be removed by mechanical shaking or chemically dissolved
Flash may be removed by being tumbled in barrels containing abrasive materials
Manual finishing
Pneumatic chisels, grinders, blast hoses
Porosity at surfaces may be filled with resins (impregnation)
Pores may also be filled with lower-melting point metals (infiltration)
Heat Treatment and Inspection of Casting
Heat treatments alter properties while maintaining shape
Full anneals reduce hardness and brittleness of rapidly cooled castings
Reduce internal stresses
Nonferrous castings may be heat treated to provide chemical homogenization or stress relief
Prepares materials for further finishing operations
Automation in Foundries
Most manufacturing operations may be performed by robots
Dry mold, coat cores, vent molds, clean or lubricate dies
Plasma torches
Grinding and blasting
Investment casting
Lost foam process
Casting can be dangerous for workers; by automating these processes, safety is increased
Process Selection
Each casting process has advantages and disadvantages
Typical requirements
Size, complexity, dimensional precision, surface finish, quantity, rate of production
Costs for materials (dies, equipment, and metal)
