Casting Processes and Gating Design

Casting shapes metal by pouring it molten into a mould. GATE asks most often about filling time of the mould and solidification time (riser design).

Sand casting basics

• Pattern: a replica of the part, made larger to allow for shrinkage. Allowances: shrinkage, machining, draft (taper for removal), distortion and shake.
• Core: a sand shape that forms internal holes; supported by core prints and chaplets.
• Moulding sand: silica + clay binder + water. Key properties: permeability, strength, refractoriness, collapsibility.

Gating system

Sprue, runner and ingates carry the metal into the cavity.

Velocity at the sprue bottom (Bernoulli): V = √(2gh).

Filling time (cross-section A_g at the gate, mould volume V):

• Top gating: t = V/(A_g √(2gH)) with H = height of the sprue.
• Bottom gating: the head falls as the mould fills: t = (A_m/A_g)(2/√(2g))(√H − √(H − h_m)), where A_m is the mould area and h_m the mould height.

Aspiration: a straight sprue would make the stream pull away from the walls and suck in air. The sprue is tapered so that A₁/A₂ = √(h₂/h₁).

Solidification and risers

Chvorinov's rule: solidification time

t_s = B (V/A)ⁿ, n ≈ 2

(V/A is the casting's modulus.) A riser must freeze after the casting, so its modulus must be larger. For a given volume, a sphere has the smallest surface area and freezes slowest; a cylindrical riser with H = D is commonly used.

Special casting processes

Defects

Blowholes and gas porosity (gas), shrinkage cavities (poor feeding), cold shuts and misruns (metal too cold), hot tears (restrained contraction), and sand inclusions.

Worked example

A cube of side 10 cm and a cylinder riser with D = H must freeze together. The cube's modulus is 10/6 = 1.667 cm. A cylinder with D = H has V/A = D/6, so for the same modulus, D = 10 cm; a riser is made larger, for example 20% more modulus.

Practice