Investment casting of metal parts

Highlights

Investment casting requires a pattern that is sacrificed for each metal casting. Conventionally this is made from wax, but some RP materials are also suitable, such as polystyrene from laser sintering or Quickcast patterns from Stereolithography.

Where size or quantities warrant tooling for waxes, this can be aluminium, model board or silicone giving a variety of benefits, costs and leadtimes.

As a generality the shelling (investment) process takes longer than generating the equivalent sand mould, however the surface finish and accuracy are better with a potential decrease in machining time. Thus the process needs careful selection to meet the project requirements.

Process Features

Accuracy

Linear tolerances of ± 0.25mm for dimensions of less than 25mm and an extra allowance of ± 0.12mm/25mm after that. Tighter tolerances can be obtained in consultation.

Minimum Feature Size

With investment casting there are two issues, firstly the wax must ‘form’ the feature and secondly the chosen material must fill the cavity. Both are a function of how well the materials flow before they chill. Thus this is not just about features but also minimum wall thicknesses, areas and material selection. As a general guide wall sections below 0.7mm should not be considered.

Layer Thickness

Not applicable.

Materials

Aluminium, Magnesium and Zinc are currently available.

Part Sizes

Parts up to 2000 x 1500 x 500 mm can be cast.

Machining Allowances

Machining allowances are required on all faces that are to be post machined. These should be incorporated into the casting model.

Draft

Draft is not required for this process.

Post Processing

After casting any remnants of the shell will be blasted off, feeds and risers removed and basic inspection undertaken. Depending on application, further options including heat treatment and machining are available.

Description

The first phase of investment casting is the creation of the sacrificial pattern. Options for this include.

• generation of aluminium or modelboard (CNC) tooling and wax injection
• generation of Stereolithography tooling and wax injection
• generation of silicone tooling and wax injection
• production of Quickcast style Stereolithography patterns
• production of polystyrene (or Castform) patterns using Laser sintering

The patterns are then mounted on a 'tree' before being shelled, dried, and cast in the desired material.

The use of RP patterns to avoid the need for wax tooling allows smaller quantities to be run economically. This route also facilitates the casting of complex geometries in relatively short timescales.

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