If your air-blast system is being used for technical surface preparation, and you are trying to consistently produce a specific profile within a narrow range of roughness values, then you need to control the sizing of the media particles that are actually doing the work.
The same is true for peening systems, where over-sized or under-sized shot would make it impossible to induce a predictable amount of compressive stress in the target surfaces. How do you control shot or grit size in an air blasting machine that contains and recirculates media?
Most industrial blast cabinets are supplied with a cyclone separator/media reclaimer that serves the purpose of removing dust and fines from the media. The cone-shaped cyclone separates particles by their mass, with the effective-size media spiraling down the interior of the cone into the media delivery system and the lightweight material in the extraction zone of the cyclone being suspended in the flow of air to the dust collector.
No matter how carefully you adjust it, the cyclone by itself is not capable of separating-out many of the fractured media particles that are somewhat heavier than fines, but lighter than full size media particles. We generally advise that, in a blast system with a properly adjusted cyclone, from 5% to 10% of the contents of the dust drum should be good media.
Media Classification
The addition of a vibrating screen classifier to the media reclaim system provides a more precise means of separating fractional media particles by sieving all the media each time it cycles through the machine and returning only the correct-sized particles to the media delivery system. Under-sized, fractured material is diverted to a separate drum for re-use elsewhere or for disposal.
By maintaining the working media mix in a more uniform condition and preventing the build-up of under-sized particles in the mix, classifier media reclamation makes it possible to gain the tightest control over the surface roughness being produced. That is why a classifier is specified for most grit-blast systems being used for technical surface preparation.
Guyson works with many companies doing prep work for what are regarded as critical coating and bonding applications. From time to time, we have encountered a different approach to media size control that might be mentioned in this discussion.
One-Time Use of Media
When you have no intention of reclaiming and recirculating the grit used in one particular surface preparation process, there is a good chance that you already have a buyer for it or another use for it in your own operations.
In this case, we can build you a system that removes the dust and fines, then deposits the grit directly in a roll-away barrel. We can make it even more convenient by adding a level sensor to signal when the media drum is full!
