Metal Casting Processes: Objectives and Intro

Objectives of Metal Casting Processes

• Describes the casting process and its characteristics
• Classifies different types of casting processes
• Describes a mold and its main components
• Explains the effects of the solidification process

Introduction to Metal Casting

• Casting is a process where molten metal flows, by gravity or other force, into a mold
• The metal then solidifies in the shape of the mold cavity
• The end product is called a casting
• Important considerations include the flow of molten metal, solidification and cooling, mold material, geometry, and production time

Advantages of Metal Casting

• Complex geometries and both internal and external shapes can be created
• Some processes produce final pieces without further processing
• It covers any component size dimension
• Batch production is possible

Disadvantages of Metal Casting

• Limitations exist on the mechanical properties of the metal or alloy
• Metals with high melting points cannot be used with certain types of molds
• Poor dimensional accuracy and surface finish can occur
• Safety hazards arise due to hot molten metals

Mould Features

• The most important aspect to carry out a casting process:
  • The mold can be generated with different materials.
  • It reproduces the external shape of the piece.
  • Auxiliary elements are used for the interior geometries of the piece.
  • The mold must be oversized due to the later contraction of the metal as metals shrink during solidification.
  • It must allow the removal of the piece without deteriorating it.
• Parts of a mold:
  • Pouring cup
  • Cast metal in cavity
  • Riser
  • Core
  • Downsprue
  • Runner
  • Flask
  • Mold
  • Cope
  • Parting line
  • Drag

Aspects to design its element

• The goal is to completely fill the cavity.
• Avoid solidification during pouring.
• Avoid defects of the pieces.
• Take into account superheat temperature, filling time, and coil speed

Types of Casting Processes

• Includes Expendable Mold Casting and Permanent Mold Casting

Expendable Mold Casting

• Mould Materials: Sand, plaster or similar.
• One single use of the mould for each piece.
• The mould must be destroyed to remove casting
• Complex shapes are possible
• Production rates are limited by the time to make the mould

Permanent Mold Casting

• Mould materials: metals or ceramic
• Moulds are reused more than once
• Permanent mold for many castings
• Moulds have two separable parts to allow the removal of the piece
• Higher production rates can happen
• Geometries are limited by the need to open the mould

Solidification

• After molten metal is poured into the mould, a sequence of events happens during solidification and cooling of the metal to ambient temperature including:
  • Liquid shrinkage: Occurs during cooling up to the liquidous temperature. It is about 0.5% of the room temperature volume.
  • Solidification shrinkage: Occurs during cooling from liquidous-to-solidus temperatures.
  • Solid Shrinkage: Occurs during cooling from solidus temperature to room temperature.

Pure Element Solidification

• Pure metal solidifies at a constant temperature equal to its freezing point (same as melting point)

Alloy Solidification

• Most alloys freeze over a temperature range rather than at a single temperature

Sand Casting

• Placing a pattern (having the shape of the desired casting) in sand to make an imprint
• Incorporating a gating system
• Removing the pattern and filling the mold cavity with molten metal
• Allowing the metal to cool until it solidifies
• Breaking away the sand mold
• Removing the casting
• There are different types of sand moulds:
  • Green-sand moulds: the sand in the mold is moist while the metal is poured in; this is the least expensive method.
  • Skin-dried method: the old surfaces are dried by storing the mould in air or drying it with torches.
  • Cold-box moulds: organic and inorganic binders are blended for greater strength.
  • No-bake moulds: synthetic liquid resin is mixed with the sand and the mixture hardens at room temperature without added heat.
  • Flask: supports the mould
  • Cope: on top
  • Drag: on the bottom
  • Paring line: in between
  • Pouring cup: receives the metal
  • Sprue: molten metal flows downward
  • Runner system: carries the metal
  • Gates: inlets into the mould cavity
  • Risers: Supply additional molten metal as it shrinks
  • Cores: inserts made from sand placed in the mould to form hollow regions
  • Vents: to carry away gases in order that the molten metal does not come into contact with the sand.
• The patterns used to mould the sand may be made of wood, plastic, or metal
  • One-piece: used for simpler shapes & low-quality production
  • Split: 2-piece patterns that form an area for the cavity
  • Match-plate: constructed by securing each half of one or more split patterns to the opposite side of a single plate
• Advantages
  • Low capital investment, viable for short production runs
  • Sand cores allow complex shapes to be cast
  • Large components can be produced
  • Suitable for small batch production
• Disadvantages
  • High unit cost due to labor and time intensity
  • Sand leaves a bad surface finish
  • Cannot make thin sections

Shell Molding

• Casting process where the mold is a thin shell made of sand held together by a thermosetting resin binder
• Smoother surface in order that it permits easier flow of molten metal during pouring
• Better surface finish on the casting for a good dimensional accuracy of 0.25mm tolerances.
• Expensive metal pattern
• Steps
  • A match-plate or cope-and-drag metal pattern is heated
  • A box containing sand mixed with thermosetting resin is put in place
  • The box is inverted to where the sand and resin fall onto the hot pattern which causes a layer in order that the mixture partially cures on the surface to form a hard shell.
  • The box is repositioned in order that loose uncured particles drop away
  • The sand shell is heated for several minutes to complete the curing
  • The shell mold is stripped from the pattern
  • Half the shell molds are assembled; supported by sand or metal shot in a box and pouring is accomplished
  • The finished casting with sprue is removed
  • Smoother cavity surface permits better flow of molten metal
  • Good dimensional accuracy
  • machining is recommended
• Disadvantages include its expensive metal pattern, and because of this, it is not recommended for small quantities

Investment Casting

• Wax pattern is produced
• Several patterns are attached to a sprue to form a pattern tree
• The pattern tree is coated with a thin layer of refractory material
• The full mould is formed by coating with slurry
• The full mould is formed by coating with Stucco
• The mould is held and inverted position and heated to melt the wax
• The wax patter is melt-out the cavity. The mould is preheated to a high temperature that ensures all contaminants are eliminated from the mould; it also permits the liquid metal flow more easily into the detailed cavity
• The molten metal is poured and it solidifies
• The mould is broken away from the finished casting, and parts are separated from the sprue

Plaster-mould Casting

• A precision casting due to high dimensional accuracy and good surface finish The mould is made of plaster of Paris (gypsum or calcium sulphate) with the addition of silica powder to improve strength
• The components are mixed with water and the resulting slurry is poured over the pattern
• After it sets, it is removed and the mould is dried at a temperature range (120-260°C)
• The mould halves are assembled to the form the mould cavity and preheated to about 120°C.
• The molten metal is then poured into the mould.

Ceramic-mould Casting

• Is similar to the plaster-mold process but uses refractory mold materials suitable for high temperature exposure.

Permanent-mould Casting

• Hard mould casting where the part is made from materials that have high resistance such as cast iron, steel, bronze or refractory metal alloys.
• Types:
  • Basic permanent mould casting
  • Die casting
  • Centrifugal casting

Basic Permanent-Mould Casting

• Uses a metal mould constructed of two sections designed for easy, precise opening and closing
• Molds used for casting low 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 high pouring temperatures.
• Advantages & Disadvantages:
  • Good dimensional control and surface finish -Stronger castings are produced due to rapid solidification
  • Higher cost of mold
  • Limited to metals of lower melting temperature
  • Simple part geometries compared to sand casting due to the need to open the mould.

Die Casting

• Is a process of permanently moulding a casting process in which molten metal is injected into mould cavity under high pressure.
• Moulds in this casting operation are called dies
• Uses a die to force metal into die cavity, what makes this different from other permanent mould processes

Centrifugal Casting

• The mold rotates so that centrifugal force distributes molten metal to outer regions of die cavity.
• Types include:
  • True centrifugal casting => Cast iron pipes, tubes, bushings, rings.
  • Semicentrifugal casting => Pulleys and wheels
  • Centrifuge casting => Parts with non-radial symmetry.

Casting Defects

• Cold Shot: metal splatters during pouring which forms solid globules
• Misrun: solidified before filling mold cavity
• Cold Shut: Lack of fusion due to premature freezing
• Shrinkage Cavity: depression or internal void caused by solidification
• Sand Blow: Balloon-shaped gas cavity
• Pin Holes
• Mold Shift