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09/12/2009
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Sand that has been bonded with a special adhesive binder is being used to produce low cost plastic moulds and other items. Tom Shelley reports
 Strong fabrications of sand and other materials can be made by forcing a modified cyanoacrylate based adhesive through the interstices between solid particles. The basic idea started off being applied to shell mouldings but by improving the adhesives properties, it is now possible to make items that are sufficiently strong and detailed for use as vacuum and, more recently, plastic injection mouldings. Because the material has greater abrasion resistance than most plastics, the technique is being considered for making other products as well. Inventor Dave Darby has been developing the technology in conjunction with Permabond.
Darby says the project started off when he was trying to find a simple way of taking an impression of a tyre tread in order to make a cast that would be used to make a tyre mould. This led to the development of what he terms his 'D' process, which involves pouring sand and binder over a rapid prototype produced by stereolithography. This is then cured in an oven at 130ºC and soaked in cyanoacrylate under vacuum.
This produces moulds hard enough to be used for plastic injection and vacuum mouldings. The latest development involves sucking the adhesive binder through sand to form a tool with small holes in it. It is this process that has been used to make the thin walled samples shown in the illustrations. Key to the process is the formulation by Permabond of an adhesive binder that Darby says is 'mainly cyanoacrylate'. The binder has a low enough viscosity to allow it to easily penetrate the interstices between the grains of sand. Curing time is delayed so the binder has enough time to be sucked completely through the tool, leaving a honeycomb structure that is totally permeable. The green object is a replica of a tool that is normally used to cut moulding sand away from the mass of material in a hopper, in order that it can pour into a metal casting mould. Wall thickness in some parts is around 1mm. It is a zircon sand that has a tensile strength of 700kg/cm2 (69MPa).
The material has been measured by a student undertaking a PhD study into the processes used to make it. Its strength is substantially greater than most plastics, HDPE for example has a tensile strength of 37MPa. But, steel is still significantly stronger. It is harder and has greater abrasion resistant then conventional plastics, but this is still not quite as great as most straight ceramics. Its particle size is sufficiently fine to allow the reproduction of fine detail when used to make mould tools. It has been used experimentally by The Welding Institute to make boxes that retain metal during friction welding processes. However, the main use has been to produce sanding blocks.
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Author
Tom Shelley
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