Epoxy granite

Epoxy granite is a novel material being developed as an alternative material for precision machine tool bases. Precision granite castings, also known as synthetic granite, epoxy granite or polymer concrete, are used to replace cast iron and steel for superior vibration damping, better performance, longer tool life and lower assembly cost.

Machine Tool Base

Machine tools and other high precision machines rely upon high stiffness, long term stability and excellent damping characteristics of the base material for their static and dynamic performance. The most widely used material for these structures are cast iron, welded steel fabrications and natural granite. Steel fabricated structures are seldom used where high precision is required due to the lack of long-term stability and very poor damping properties. Good quality cast iron which is stress-relieved and annealed, will give the structure dimensional stability and can be cast into complex shapes, but needs an expensive machining process to form precision surfaces after casting.

Good quality natural granite is becoming more and more difficult to find but has a higher damping capacity than cast iron. Again, as with cast iron, the machining of natural granite is labour intensive and expensive.

Process

Precision granite castings are produced by mixing granite aggregates, which are crushed, washed, dried and mixed with an epoxy resin system at ambient temperature (i.e. cold curing process). Quartz aggregate filler can also be used in the composition. Vibratory compaction during the molding process tightly packs the aggregate together.

Threaded inserts, steel plates, coolant pipes can be cast-in during the casting process. To achieve an even higher degree of versatility, linear rails, ground slide-ways and motor mounts can be replicated or grouted-in to the ultimate accuracy, therefore eliminating the need for any post-cast machining. The surface finish of a precision granite casting is as good as the mould surface and in many instances the need for painting is not necessary.

Epoxy granite material has an internal damping factor up to ten times better than cast iron, up to three timer better than natural granite and up to thirty times better than steel fabricated structure. It is more cost-effective, un-affected by coolants, has excellent long-term stability, improved thermal stability, high torsional and dynamic stiffness, excellent noise absorption and it's low exothermic curing property aids to negligible internal stresses.

Advantages

* Vibration dampening.
* Flexibility- custom linear ways, hydraulic fluid tanks, threaded inserts, cutting fluid, and conduit piping can all be integrated into the polymer base.
* Assembly time is reduced by incorporating multiple components into one casting.
* Does not require a uniform wall thickness allowing for greater design flexibility of your base.
* Chemical resistance to most common solvents, acids, alkalis, and cutting fluids.
* Does not require painting.
* Composite has a density approximately the same as aluminum (but pieces are thicker to achieve equivalent strength).
* The composite polymer concrete casting process uses much less energy than metallic castings. Polymer cast resins use very little energy to produce and the casting process is done at room temperature.

Avoid

* Thin sections (<1").
* High tensile loading.
* Shock loading.
* Tapped holes too close to the edge.

History

Although new to many machine tool builders, especially those in the United States, the composite base has been available in Europe for several years. Fritz Studer AG of Switzerland first came up with its Granitan S-100 technology in the early 1970s. The Granitan base is a mixture of reactable epoxy-resin binder developed by Ciba-Geigy Ltd., Switzerland, and granite or gravel. Granitan produced with sand and gravel was developed specifically for the requirements of machine structure manufacturing. Studer has also been selling licenses for its technology. So far, between 20 and 30 companies worldwide have purchased such licenses, including six U.S machine tool builders.

Other synthetic granite products:

*Motema-AC -Helgard Koblischk GmbH of Germany
*Quazite - Plastibeton Inc., Montreal
*Zanite - ITW Polymer Castings
*Harcrete - Hardinge Brothers, Inc. (Granitan licence)
*Celith - Microplan Group, France
*Epucret - RAMPF Group, Germany

Further reading

Terry Capuano "Polymer Castings take on metals", Machine Design 2004