Sand casting is the most frequently specified casting process for several reasons. Tooling is generally the lowest cost amongst the casting processes, and virtually any metal can be cast. The process itself has remained unchanged for hundreds of years, where the technology has improved is in making the expendable sand molds more efficiently. This has enabled sand casting to remain competitive with more modern casting methods.
Impressions are made in two sand mold halves using a pattern of the part to be produced. Gates and runners are also added to the mold as a path for the molten metal. Voids in the parts are formed through the use of cores which are laid in the mold. The top (cope) half is then joined to the bottom (drag) half of the mold. Metal is then poured into the mold and allowed to solidify. The sand mold is then broken apart, castings are removed to the secondary areas of the foundry where gates and runners are removed. In most modern foundries the sand is recycled through the foundry where it is processed in order to assure that it has the properties necessary for molding.
Tooling selection is dependent upon the size of the part , the volume of parts to be produced, and the type of sand molding equipment to be employed at the foundry. Very low volume requirements can be produced using loose or split patterns however most modern foundries employ match plates where both the cope and drag impressions are mounted on either side of a board or aluminum plate. Often, multiple impressions are employed on a single plate. Most higher speed molding equipment requires the use of aluminum match plates. Cores are produced using core boxes (molds) the costs of which vary with the type of core making process being used.
Aluminum-319 aluminum, and 356 heat treatable aluminum are the most commonly used. 535 aluminum “Almag” is used in applications requiring a high degree of corrosion resistance or enhanced elongation properties are required.
Other commonly poured sand cast materials include…
- Brass and Bronze
- Grey and Ductile Iron
- Carbon and Alloy Steels
- Stainless and other high nickel alloys
Surface Finish – 300-600 micro inch is common. Sand casting is generally not considered a fine finish process.
Tolerance – We feel a good rule of thumb is +/- .030 for the first 6 inches, adding +/- .003 for every additional inch. Add an additional +/- .020 for cored hole diameters and features that cross the parting line.
Wall thicknesses – .125” minimum, .250” is desirable. It is important that wall thicknesses be kept as close as possible throughout the part as thicker sections solidify more slowly. Thick/thin junctions are where most process related defects will occur.
Draft – 2%
Sand cast pattern equipment is generally the property of the customer. Therefore it is important that it is insured by the customer. Patterns are generally transferable between foundries although there can be some adaptation charges depending upon what the receiving foundry must do to use the equipment. Having the following information about your patterns on file is recommended.
- Casting weight
- Pattern type-Loose, split, cope and drag plates, match plate
- Number of impressions on pattern
- Molding equipment pattern was built for- Squeezer, Rotolift, Sinto, Hunter
- Core Boxes-# of impressions and type-Shell, green sand, no bake, ect.
How we can help
Should a hole be cored or machined as a secondary operation? Will this part have enough volume to perhaps justify permanent mold tooling? What features can be cast into this part in order to reduce machining? Over the years we have tooled literally hundreds of sand castings from the simple to the extremely complex. Often we can propose alternate tooling scenarios where you can see trade offs between tooling costs and piece price. Request a quote and put our experience to work for you today.