metalcasting 101 M Exploring Sand Molding Cast Metals Institute, Schaumburg, Illinois I
n the metalcasting process, molten metal is poured into a mold and solidifi es into a component. Sand is the most widely used material for mold
making. To create the mold, sand grains are packed around a metal pat- tern that is roughly the shape of the desired metal part. Sand is available to the metalcasting facility in various sizes, shapes and chemical composi- tions, and metalcasting sand systems must conform to a standard of sizes and shapes in order to maximize mold properties and casting quality. Silica sand is the most common
type used in metalcasting due to its availability and price, but specialty sands such as zircon, chromite and olivine also are used. T ese sands provide better mold properties, which lead to higher quality castings. To produce a mold, metalcasters
can use one of two basic methods of making sand grains stick together. T e less expensive of the two, “green sand,”
Silica sand is used in both green sand sys- tems and nobake systems (shown).
mixes clay and water with the sand. Other constituents are added in small quantities to boost performance and minimize defects.
Sand grains also can be chemi-
cally bound by resins that harden in the presence of a catalyst. Chemi- cally bound molds are stronger and typically produce castings with more precise dimensions than green sand molds. Most cores (which create hollow spaces in castings) and large mold assemblies are made using chemically bound sand. Sand is defi ned as decomposed
or crushed rock ranging from 0.002 to 0.083-in. in diameter. Sand grains can be round, angular or sub-angular in shape. Silica sand from lakes and dunes tends to be angular to sub-an- gular, while pure silica sand is typi- cally round. Round sand grains have minimal surface area and need less binder to coat the surface than angular grains, but they expand more when a metalcasting mold is fi lled with liquid metal, which can lead to casting defects. Strength and permeability are other competing factors that must be measured in the molding process.
To keep a green sand facility running effectively, the metalcaster must maintain precise control over the size and shape of the sand used. 48 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2011
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60