Materials

The acrylic material is polymethylmethacrylate (PMMA) this is a widely used thermoplastic. PMMA is common in everyday use in many forms such as sheets (glazing and signs) and mouldings (light fittings and automotive parts) etc. The process we carry out is unusual in that we polymerise a mass of polymer/monomer mixture in bulk and subsequently cut and polish the resulting casting. This method of production is ideal for the embedment of specimens.

The embedment is achieved by pouring the slurry of monomer/polymer into a mould and allowing it to gel. When the gel is sufficiently hardened the articles for embedment are positioned and fresh slurry poured on top. This operation is repeated for multi-layer assemblies. The casting is then placed in a pressure vessel which is pressurised to 10 Bar (150psi). This is then heated and after a few hours the mass polymerises. The polymerisation reaction is strongly exothermic (gives out heat) and the temperature of the casting rises rapidly.

The temperature in the centre of a thick section may reach up to 150 deg.C.

When the reaction is complete the castings cool slowly to ambient temperature. They are removed from their moulds and are subsequently machined and polished to final size and shape. During polymerisation the monomer/polymer matrix shrinks by approximately 10% when the molecular chains form. This shrinkage is not necessarily even, consequently each casting has to be made significantly oversize to allow for subsequent machining to the final size.

The finished parts are polished manually to a very fine standard. The material itself is of high optical clarity and perfectly clear. It is very durable and will not deteriorate significantly with the passage of time. In use cleaning materials containing abrasive compounds should be avoided and the PMMA should not be exposed to solvents.