Sand casting: Cope and Drag, Mold Cavity, Pattern, Pouring Cup, Sprue, Runner, Gate, Risers, Vents, Cores and Important considerations for casting

Sand casting: Cope and Drag, Mold Cavity, Pattern, Pouring Cup, Sprue, Runner, Gate, Risers, Vents, Cores and Important considerations for casting

Work flow in typical sand-casting foundries

Sand casting uses natural or synthetic sand (lake sand) which is mostly a refractory material called silica (SiO2). The sand grains must be small enough so that it can be packed densely; however, the grains must be large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized molds use green sand (mixture of sand, clay and some water). Sand can be re-used, and excess metal poured is cutoff and re-used also.

Schematic showing steps of the sand casting process
Schematic showing steps of the sand casting process

Typical sand molds have the following parts:

• The mold is made of two parts, the top half is called the cope, and bottom part is the drag.

• The liquid flows into the gap between the two parts, called the mold cavity. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is (almost) identical to the shape of the part we need to make.

• A funnel shaped cavity; the top of the funnel is the pouring cup; the pipe-shaped neck of the funnel is the sprue – the liquid metal is poured into the pouring cup, and flows down the sprue.

• The runners are the horizontal hollow channels that connect the bottom of the sprue to the mould cavity. The region where any runner joins with the cavity is called the gate.

• Some extra cavities are made connecting to the top surface of the mold. Excess metal poured into the mould flows into these cavities, called risers. They act as reservoirs; as the metal solidifies inside the cavity, it shrinks, and the extra metal from the risers flows back down to avoid holes in the cast part.

Vents are narrow holes connecting the cavity to the atmosphere to allow gasses and the air in the cavity to escape.

Cores: Many cast parts have interior holes (hollow parts), or other cavities in their shape that are not directly accessible from either piece of the mold. Such interior surfaces are generated by inserts called cores. 

Cores are made by baking sand with some binder so that they can retain their shape when handled. The mold is assembled by placing the core into the cavity of the drag, and then placing the cope on top, and locking the mold. After the casting is done, the sand is shaken off, and the core is pulled away and usually broken off.

Important considerations for casting

(a) How do we make the pattern?

Usually craftsmen will carve the part shape by hand and machines to the exact size.

(b) Why is the pattern not exactly identical to the part shape?

– you only need to make the outer surfaces with the pattern; the inner surfaces are made by the core

– you need to allow for the shrinkage of the casting after the metal solidifies

(c) If you intersect the plane formed by the mating surfaces of the drag and cope with the cast part, you will get a cross-section of the part. The outer part of the outline of this cross section is called the parting line. The design of the mold is done by first determining the parting line (why ?)

(d) In order to avoid damaging the surface of the mould when removing the pattern and the wood-pieces for the vents, pouring cup and sprue, risers etc., it is important to incline the vertical surfaces of the part geometry. This (slight) inclination is called a taper. If you know that your part will be made by casting, you should taper the surfaces in the original part design.

Taper in design
Taper in design

(e) The core is held in position by supporting geometry called core prints (see figure below). If the design is such that there is insufficient support to hold the core in position, then metal supports called chaplets are used. The chaplets will be embedded inside the final part.

Design components of a mold showing chaplets
Design components of a mold showing chaplets

(f) After the casting is obtained, it must be cleaned using air-jet or sand blasting

(g) Finally, the extra metal near the gate, risers and vents must be cut off, and critical surfaces are machined to achieve proper surface finish and tolerance.

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