Friday, 14 June 2013

The Journey to Find Cost Efficient Aluminium Rapid Cast Parts. Part One - The Dilemma

We are an engineering design consultancy based in Kingston. Engineering is our work and hobby. So to put it mildly we enjoy what we do and we are always on the lookout for new processes and ideas to help us along our way. In-house we have a 3D printer which is a fantastic asset for prototyping initial ideas that can be shown to clients in a physical form and then tweaked as required.

Often we require rapid cast metal parts. In the past we have designed in CAD and then sent the 3D files to a company that has a special wax printer that can print out a wax master allowing the part to be investment cast. This process , using the lost wax technique, is one of the oldest casting methods known.

Typically parts takes a few weeks to manufacture, the process is quite involved as the wax printing takes quite some time to create resulting in an expensive process.

Below illustrates the different stages required to achieve the aluminium parts using the example of a seat pad.

The seat pad was modelled from plaster to achieve the optimum shape required. Using a 3D scanner the pad was scanned and the data was transferred into Solidworks.
CAD rendering of the seat pad including an inscription of a finger print.

To show the stresses of the design finite element analysis was used to test the CAD model.

The CAD was sent off to create a wax replica using a 3D wax printer (similar to a prototyping machine).

This is then attached to a central wax stick to form the assembly and mounted onto a pouring cup. In this case the wax replica was also finished so as to produce a higher quality result.

A shell is built by immersing the assembly in a liquid ceramic slurry and then into a bed of fine sand forming several layers.

The end results of the investment cast process. The ceramic is then dried in the oven.

The ceramic is dried while the de-waxing occurs as the wax is melted out in an autoclave. This creates a negative impression of the assembly. The shell mould is then fired in a high temperature oven.

The final part (sorry no photos) involves casting as the shell is filled with molten aluminium. When this has cool  and solidified the ceramic shell is broken off and then cut away from the central assembly.

The end result is the aluminium part identical to the original wax replica.

We were really pleased with the quality of the parts, the tolerances were good and the surface finish was impressive, just what we were looking for. However, they took around 2 to 3 weeks to be completed and the process was also extremely expensive. 

This got us all thinking to explore some different routes for a low cost alternative shorter process.
The following blogs over the course of the next few months will be our ideas and thoughts on what went well as well as wrong and most importantly of all if we managed to create a more cost effective part to the same quality.

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