Sand casting

Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold), and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2300-2700 kg. Minimum part weight ranges from 0.075-0.1 kg. The sand is bonded together using clays (as in green sand) or chemical binders, or polymerized oils(such as motor oil. Sand in most operations can be recycled many times and requires little additional input.

Preparation of the sand mold is fast and requires a pattern which can "stamp" out the casting template. Typically, sand casting is used for processing low-temperature metals, such as iron, copper, aluminum, magnesium, and nickel alloys. Sand casting can also be used for high temperature metals where other means would be unpractical. It is said to be the oldest and best understood of all techniques. Consequently, automation may easily be adapted to the production process, somewhat less easily to the design and preparation of forms. These forms must satisfy exacting standards as they are the heart of the sand casting process - creating the most obvious necessity for human control.

[edit] Plaster casting (of metals)
Plaster casting is similar to sand molding except that plaster is substituted for sand. Plaster compound is actually composed of 70-80% gypsum and 20-30% strengthener and water. Generally, the form takes less than a week to prepare, after which a production rate of 1-10 units/hr-mold is achieved with items as massive as 45 kg and as small as 30 g with very high surface resolution and fine tolerances.

Once used and cracked away, normal plaster cannot easily be recast. Plaster casting is normally used for nonferrous metals such as aluminium-, zinc-, or copper-based alloys. It cannot be used to cast ferrous material because sulfur in gypsum slowly reacts with iron. Prior to mold preparation the pattern is sprayed with a thin film of parting compound to prevent the mold from sticking to the pattern. The unit is shaken so plaster fills the small cavities around the pattern. The form is removed after the plaster sets.

Plaster casting represents a step up in sophistication and requires skill. The automatic functions easily are handed over to robots, yet the higher-precision pattern designs required demand even higher levels of direct human assistance.


Casting of plaster, concrete, or plastic resin
Main article: Resin casting
Plaster itself may be cast, as can other chemical setting materials such as concrete or plastic resin - either using single use waste molds as noted above or multiple use piece molds, or molds made of small ridged pieces or of flexible material such as latex rubber (which is in turn supported by an exterior mold). When casting plaster or concrete the finished product is, unlike marble, relatively unattractive, lacking in transparency, and so is usually painted, often in ways that give the appearance of metal or stone. Alternatively, the first layers cast may contain colored sand so as to give an appearance of stone. By casting concrete, rather than plaster, it is possible to create sculptures, fountains, or seating for outdoor use. A simulation of high quality marble may be made using certain chemically set plastic resins (for example epoxy or polyester) with powdered stone added for coloration, often with multiple colors worked in. The later is a common means of making attractive washstands, washstand tops and shower stalls, with the skilled working of multiple colors resulting in simulated staining patterns as is often found in natural marble or travertine.


Shell moulding
Shell molding is also similar to sand molding except that a mixture of sand and 3-6% resin holds the grains together. Set-up and production of shell mold patterns takes weeks, after which an output of 5-50 pieces/hr-mold is attainable. Aluminium and magnesium products average about 13.5 kg as a normal limit, but it is possible to cast items in the 45-90 kg range. Shell mold walling varies from 3-10 mm thick, depending on the forming time of the resin.

There are a dozen different stages in shell mold processing that include:

initially preparing a metal-matched plate
mixing resin and sand
heating pattern, usually to between 505-550 K
inverting the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill)
curing shell and baking it
removing investment
inserting cores
repeating for other half
assembling mold
pouring mold
removing casting
cleaning and trimming.
The sand-resin mix can be recycled by burning off the resin at high temperatures.