Sand Casting of metal parts

Highlights

The attractions of sand casting include the speed of the process. Patterns may be made using machining or various RP processes depending on the life required of the tooling and the complexity sought. For simple parts it is possible to be looking at metal parts in 7 days, although these may require post machining.

Coring can be achieved with either sintered sand cores, separate core boxes or ceramic cores, allowing considerable complexity to be introduced.

Post casting the feed system will need to be removed, the part inspected and any machining undertaken. Inspection criteria may vary from simple visual inspection through to pressure test and x-ray.

Process Features

Accuracy

Tolerances vary from one foundry to another, but as a guide it is best to allow +/-0.75mm on dimensions up to 300mm, and an additional ± 0.2mm/100mm after that.

Minimum Feature Size

For casting in sand there is a minimum wall section of 2mm.

Layer Thickness

Not applicable. Even where an RP pattern has been used it is normal practice to dress out any layering on the pattern as this may cause problems when removing the pattern from the sand mould.

Materials

Plunkett Associates is currently supplying aluminium and zinc sand castings

Part Sizes

These are usually limited by dimension and material weight that can be achieved in a single pour. Current constraints are 2m x 1.5m x 0.3m and 50 kg. (note this has to include the feed system)

Machining Allowances

As most sand castings undergo secondary machining, these surfaces should have additional stock added to the model. Unless specifically agreed otherwise this is the responsibility of the customer to ensure cad files are supplied with this included.

Draft

The pattern equipment must be removed from the sand mould and hence draft is required on vertical faces. Typically this should be 1 degree on draws of less than 100mm and 1.5 degrees over this.

Post Processing

Most cast parts are heat treated and post machined as the application demands.

Description

Sand is used to make a mould for the molten metal by compacting it around a pattern that includes an associated feed system. The pattern is made either by machining or one of the RP techniques and incorporates the parting plane (pattern plate). Each plate is then used to form half of the cavity, known as the cope and drag.

Once assembled with any associated cores, the sand defines a full negative of the desired part, including any allowances for shrinkage incurred as the molten material solidifies.

After the metal has been poured and allowed to solidify, the sand is knocked away and any remnants removed by blasting. The feed is then cut away, to leave the cast geometry.

Inspection conditions vary depending upon the application and will be recommended accordingly.

For more information, please contact:  stefan@smartprototype.net