Metal Casting Processes - Manufacturing Engineering - Mechanical Engineering

Casting is one of the oldest manufacturing processes and remains the first step in producing many engineering components. In a casting process, molten metal is poured into a mould cavity that has the shape of the desired part, allowed to solidify and then the solidified casting is removed by breaking the mould as required. A pattern is a replica of the part to be cast and is used to prepare the mould cavity. Patterns are commonly made of wood, metal or other suitable materials. A mould is an assembly of two or more boxes (flasks) or bonded refractory particles (sand) which contains the cavity into which molten metal is poured.

Advantages of Casting

• Molten metal can flow into fine sections of the mould cavity, so intricate and complex shapes can be produced in a single operation.
• Practically any metal and many alloys can be cast.
• Tools and equipment required for many casting processes (for example, sand casting) are relatively simple and inexpensive compared with other shaping methods.
• Because cooling can be relatively uniform, castings can display uniform mechanical properties in sections having similar cooling rates.

Limitations of Casting

• For basic processes such as conventional sand casting, dimensional accuracy and surface finish are generally poorer than for wrought or machined parts.
• Casting defects (shrinkage, porosity, inclusions, misruns, cold shuts etc.) are possible and must be controlled.
• Sand casting and some other moulding processes are labour intensive and require careful process control.

Sequence of Steps in a Casting Process

1. Design and preparation of the pattern.
2. Preparation of the mould (moulding sand or other refractory mould).
3. Design and provision of the gating system (sprue, runner, ingate, pouring basin).
4. Design of risers (feeders) and chills if required.
5. Melting of the metal to be cast.
6. Pouring of molten metal into the mould.
7. Solidification and cooling of the casting.
8. Fettling (removal of gates, riser, flash) and cleaning.
9. Finishing operations (machining, heat treatment, inspection).
10. Testing and quality checks (dimensional checks, NDT where required).

Pattern Allowances

A pattern is always made different from the final casting size to compensate for changes that occur during moulding, pouring and cooling. These differences are collectively called pattern allowances.

Shrinkage Allowance

Shrinkage allowance compensates for dimensional change when the casting cools from pouring temperature to room temperature. Components of shrinkage are:

• Liquid shrinkage - contraction of the liquid metal on cooling before solidification.
• Solidification shrinkage - contraction during solidification (primary dendritic shrinkage and shrinkage porosity formation).
• Solid shrinkage (thermal contraction) - contraction of the solid casting as it cools to ambient temperature.

Cooling curve for a pure metal during casting

For linear thermal contraction (solid shrinkage), the change in length δL can be estimated as an approximation by:

δL = L α ΔT = L α (T_F - T_o)

Where:

• δL = change in dimension
• L = initial (reference) dimension of the component
• α = coefficient of linear thermal expansion of the metal
• T_F = freezing (solidus) or moulding reference temperature
• T_o = reference room temperature

Machining Allowance

Extra material is left on the casting surface to allow for machining and finishing. Typical machining allowances for sand castings are in the range 1.5 mm to 3 mm, depending on casting size, process and required tolerances.

Draft or Taper Allowance

A draft (taper) on vertical faces of the pattern facilitates easy withdrawal of the pattern from the sand mould without damaging the mould cavity. A typical draft angle is between 0.5° and 2°, depending on the pattern material and surface finish.

Shake Allowance

During pattern withdrawal and moulding, slight motion (shaking) can increase the effective size of the mould cavity. Shake allowance is provided on the pattern to compensate for this increase so that the final casting comes to correct dimensions.

Distortion Allowance

Some castings distort during cooling due to non-uniform contraction. To obtain a true final shape, the pattern may be distorted in the opposite sense by an amount equal to the expected distortion; this correction is called distortion allowance.

Types of Patterns

• Solid or single-piece pattern: simplest, made as one piece without detachable parts; suitable for simple shapes.
• Split pattern (two-piece pattern): made in two halves for more intricate castings; eases pattern withdrawal.
• Match-plate pattern: two pattern halves are mounted on opposite faces of a single plate (match plate) to improve production rate and dimensional accuracy.
• Cope and drag pattern: pattern halves are mounted on separate match plates or flasks (cope and drag) which are assembled to form the complete mould.
• Gated pattern: pattern(s) with attached gates and runners to produce moulds having gating channels for molten metal flow.
• Loose-piece pattern: pattern has detachable parts (loose pieces) which are removed independently to form undercuts and projections in the mould.
• Sweep pattern: a movable template (sweep) is rotated or swept about an axis to form large surfaces of revolution in the mould; useful for large, axisymmetric castings.
• Follow-board pattern: used when portions of the mould are weak and require support during ramming; follow boards support the sand close to the pattern surface.
• Skeleton pattern: minimal framework pattern used for very large castings produced in small numbers where full wooden patterns are uneconomical.

Effect of Moisture Content and Properties of Moulding Sand

Moisture content of the moulding sand strongly affects its compactibility, green strength and permeability. Proper control of sand moisture is essential to avoid defects such as gas porosity and mould collapse.

Important Properties of Moulding Sand

• Permeability: ability of the moulding sand to allow gases and air to pass through and escape from the mould cavity. Good permeability prevents gas-related defects.
• Permeability number (P_n) can be measured: P_n = V H / (P A T) where V is volume of air collected (cm³), H is specimen height (cm), P is applied air pressure (gm/cm²), A is cross-sectional area (cm²) and T is time (minutes). For some standard test arrangements P_n is expressed by empirical relations such as P_n = 50.127 / T (T in minutes) depending on the equipment calibration.
• Strength: ability of the compacted sand to resist crushing; measured on universal sand strength testing machines.
• Green strength: strength of the moist (green) sand after compaction; required to hold mould cavity during handling and pouring.
• Dry strength: strength of sand after drying; relevant when mould is dried or baked.
• Hot strength: strength of sand when exposed to molten metal temperatures; important to prevent mould collapse during pouring.
• Refractoriness: ability of the sand to withstand high temperatures without fusion or loss of strength.
• Mould hardness: related inversely to permeability; very hard moulds may have lower permeability.
• Cohesiveness: ability of sand grains to bond with each other (binders and clay content contribute to this).
• Adhesiveness: tendency of sand to stick to other materials, including the pattern or cores.
• Collapsibility: ability of the sand to collapse or give way during solidification of the casting; desirable to reduce internal stresses and avoid cracking of the casting.
• Flowability: ability of sand to flow and fill thin sections around the pattern during moulding; affects surface detail reproduction.

Cores, Core Prints, Chaplets and Chills

Core and Core Print

A core is a sand insert used to produce internal cavities, holes or undercuts in a casting. A core print is the recess provided in the mould to locate, position and support the core during moulding and pouring.

Buoyancy force on Core

The net buoyant force acting on a core immersed in molten metal is given by:

F = V g (ρ_m - ρ_c)

Where:

• F = net upward buoyant force
• V = volume of displaced liquid (projected volume of the core immersed)
• g = acceleration due to gravity
• ρ_m = density of molten metal
• ρ_c = average density of the core assembly

Chaplets

Chaplets are small metallic supports placed inside the mould to support cores and maintain their position against buoyant forces during pouring. Chaplets are usually of the same or compatible metal as the casting and are melted or fused into the casting during solidification.

Chills

Chills are metallic or insulating inserts placed in or on the mould surface to promote rapid local cooling of the casting. Chills are used to control the direction of solidification, reduce shrinkage defects and produce a desired grain structure or mechanical property in local regions.

Pads or Padding

Padding (pads) are additional blocking pieces or supports provided at corners and thin sections of the mould to prevent erosion or breakage of the sand during ramming and while pouring the molten metal.

Gating System

The gating system provides a controlled path for molten metal from the pouring basin to the mould cavity. A well-designed gating system reduces turbulence, prevents air entrainment and controls the metal flow rate.

• Pouring basin
• Sprue
• Runner
• Ingate (gate)

Gating Ratio

Gating ratio refers to the relative cross-sectional areas of the sprue, runner and ingate. It is often expressed as:

A_S : A_R : A_G

Where A_S is sprue area, A_R is runner area and A_G is ingate area. Proper gating ratio ensures desired flow velocity, reduced turbulence and minimised defects.

Casting Yield

The casting yield is the proportion of the mass of the actual casting to the total mass of metal poured into the mould:

Casting yield = (m / M) × 100

Where m is mass of the finished casting and M is mass of metal poured (including gates, risers and scrap).

Choke Area

The choke area is the minimum cross-sectional area in the gating system (it may occur at the sprue base, runner or in-gate). The choke controls the flow rate through the system.

Types of Gating Systems

Non-pressurised gating system: the choke area is at the bottom of the sprue base; runner and ingate areas are larger than the sprue area so the system does not build a back pressure and pressure decreases downstream.

Pressurised gating system: the smallest area is the in-gate area so that back pressure exists throughout the gating system and the metal in the runner may be under pressure when the mould cavity fills.

Top Gating

In top gating the molten metal flows from the sprue directly into the mould cavity. The gating channels are arranged so that metal enters the cavity from the top; the sprue is commonly vertical. Top gating is simpler but may cause greater turbulence and oxidation unless carefully designed.

Pouring or filling time for top gating depends on the gating geometry and head available. Sufficient filling time should be provided to ensure smooth, laminar fill where required.

Bottom Gating (Bottom Fill)

Bottom gating introduces metal at the bottom of the mould cavity (or runner) allowing fill from bottom to top, which reduces turbulence, splashing and oxidation. This method is often preferred for sound castings where surface finish and minimised gas entrapment are important.

Where h_t = total head height and h_m = height of the mould, the filling times may differ for top and bottom gating arrangements.

Special case:
If height of the mould (h_m) is equal to total head (h_t), then the bottom filling time may be approximately twice the top filling time under certain simple assumptions:

(t_f)_Bottom = 2 × (t_f)_Top

Solidification Time and Chvorinov's Rule

Solidification time is the time required for the entire casting to solidify after pouring. It depends on casting size, shape and the mould material. A simple and widely used empirical relation for solidification time is Chvorinov's rule.

Chvorinov's rule:

t_s = k × (V / SA)²

Where:

• t_s = total solidification time
• k = mould constant (depends on metal and mould properties)
• V = volume of the casting
• SA = surface area of the casting

The term modulus (M) used in riser and solidification analysis is defined as:

M = V / SA

The larger the modulus, the longer the solidification time. The same relation applies to risers; risers must have a larger modulus than the casting they feed to ensure the riser solidifies after the casting.

Methods of Riser Design

Risers (feeders) supply liquid metal to compensate shrinkage during solidification. Proper riser design ensures directional solidification and avoids shrinkage defects.

Caine's Method

Caine's method (also spelled Caine) is an empirical method used to estimate riser dimensions for simple casting shapes. It uses empirical charts and relations based on casting modulus, riser shape and material properties to determine riser volume and dimensions.

Shape Factor Method

The shape factor method estimates riser size based on a shape factor (S.F.) which accounts for the geometry of the casting section to be fed. One expression used for shape factor in thin rectangular sections is:

S.F. = (L + w) / t

Where L, w and t are characteristic dimensions of the section (length, width and thickness) depending on the chosen geometry. Shape factor correlates geometry to cooling behaviour and aids selection of suitable riser size and placement.

Finishing, Testing and Quality Control

After fettling (removal of gates, risers and parting burrs) and cleaning, castings commonly undergo finishing operations such as mechanical machining, grinding, heat treatment and surface treatment. Quality control includes dimensional inspection, hardness testing, microstructure checks and nondestructive testing (radiography, ultrasonic, dye-penetrant) where required.

Applications and Selection Considerations

Casting processes are chosen for applications where complex shapes, internal cavities, large sizes or special metallurgical requirements are needed. Process selection (sand casting, investment casting, die casting, centrifugal casting, etc.) depends on production quantity, required tolerances, surface finish, material, and economics.

Key terms: pattern, mould, core, gating system, riser (feeder), chill, chaplet, permeability, green strength, refractoriness, modulus, Chvorinov's rule, casting yield, gating ratio.