Reverse prototyping makes better castings

Die castings can be in production quicker than ever before, and die designs and components evaluated before commitment is made to production tooling. Tom Shelley reports

May 2002 notebook: Reverse prototyping makes better castings

Die castings can be in production quicker than ever before, and die designs and components
evaluated before commitment is made to production tooling. Tom Shelley reports

A research project has resulted in the ability to create pressure die casting moulds in a week or two, using laminated assemblies of laser-cut steel sheet.
Not only can the moulds be made more quickly, they can also be constructed with enhanced cooling channels, permitting easier manufacture of castings with thinner sections and other potential problem features.
The project, which began in 1994, is named RADICAL: Rapid laminated die casting tooling for the automotive industry. The research has been conducted by DeMontfort and Warwick universities, and research fellow Andy Norwood revealed recently some early results of the work at a seminar at Warwick, organised by the Foresight Vehicle Design and Manufacturing Processes Thematic Group.
The research takes an idea, originally conceived in the 1970s by Professor Takeo Nagakawa, for making laminated deep-drawing dies. The idea was to come up with a faster method of making die casting moulds that would be easier to handle.
The designs are produced with the aid of standard rapid prototyping software, but working in reverse to produce slice by slice moulds instead of slice by slice products. Experiments have been conducted with unbonded assemblies of laminations and assemblies bonded together by nickel brazing in a vacuum furnace.
Die casting LM24 aluminium alloy, in an unbonded mould made of 1mm thick sheets of H13 tool steel, encountered no problems using gravity methods in the production of an initial 150 castings. This has been followed by gravity casting tests with a mould made of 66 laminations of 1mm H13 brazed together at 1,020°C. Nuneaton-based KTN has so far successfully used the mould to produce more than 2,000 gravity castings of a simple cone component.
The next phase was the manufacture of a TRW brake master cylinder. The mould was prepared by taking DXF CAD files of the product, subtracting it from a block, and applying Delcam's latest slicing software. The TRW die has produced 100 components to date. Comparative trials are now being made with bonded and unbonded moulds, sliced vertically and horizontally, with and without application of additional pressure.
One of the advantages of the approach, already established in the making of plastic injection moulds for the Contura Process, is that it is easier to incorporate cooling channels. Instead of being drilled out as round holes, the channels can be cut out of the sheets, with upper and lower surfaces parallel with sections of casting to be cooled. It is, therefore, possible to cool evenly thin sections of casting, leading to reduced cooling times. In trials, solidification times have been reduced from 3 to 0.7s, increasing production by 11%. Tools can be made very large, without incurring a heavy weight or cost penalty, and would appear to be particularly suited to the manufacture of electronic heat sinks.
(More information at www.foresightvehicle.org.uk/
programme/design01.asp) The Contura Process is available commercially in the UK through Bayer.

Producing a mould from laminations allows it to be made much more quickly, directly from CAD data, using standard rapid prototyping software and a laser cutter

The mould can be made lighter in weight and cooling can be greatly enhanced

The manufacture of shapes with thin sections and other features is less of a problem

Author
Tom Shelley

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