We should begin a discussion of blast media with some observations about the term 'sand blasting'. Natural silica sand or beach sand may be cheap, but this granular mineral material and the vitreous slag by-products that are marketed as substitutes for sand in outdoor blasting are generally not suitable for indoor cabinet-type blast systems that recirculate media. All such materials have an extremely high rate of fracture, leading to problems that will shortly be explained.

More importantly, you must understand that the use of silica sand for blasting may result in the release into the air of free silica that can cause the respiratory ailment called silicosis.

If you need the surface finish generated by such mineral grit, you can choose from many other media that will do the work and that do not bring with them the liabilities and health hazards associated with sand blasting.

Recently there have also been some environmental issues with cheap imported glass bead blast media, where the original source of the recycled glass has been manufactured including some 'heavy metals' banned in the EU and these continue to transfer through the process line.

So it pays to investigate the source of your media or buy from reputable media specialists.

* So, what can be used to make blast media?

- The simple answer is anything! If a product can be produced in a size small enough to allow it to be propelled through a blasting system, then it qualifies for the job.

However, how it performs and what effects it creates are down to many different factors.

In summary, some of these are, as follows.

* Mass.

* Shape.

* Size.

* Density.

* Hardness.

* Chemical composition.

* Speed at impact.

* Direction of impact.

* Substrate being blasted.

The most important properties that should be considered in the selection of blast media for a particular application are the material or chemical composition, hardness, density, particle shape, screen or particle size and impact resistance.

Both the technical performance of the media and the cost of the process are at stake in the choice of blasting materials.

Commonly available blast media include agricultural materials such as ground nut shells or starch grit, mineral substances like aluminium oxide or silicon carbide, ceramic shot and grit, glass in the form of beads or granular crushed glass; various plastics formed into beads or ground up into angular particles and metals such as steel shot and iron grit.

Today, all or most of these media are engineered materials, formulated or processed to emphasize useful characteristics for impact treatment.

It should be noted that some of the media in many of these categories of materials are primarily marketed for outdoor or single-pass blasting operations versus use in longer-cycle cabinet blast media delivery systems.

* Typically, what products are used for blast media and where would they be used?

- They can be subdivided in the following general categories.

* Aluminium oxide - different qualities of this exceptionally hard material can be used for many different cleaning, etching and surface preparation applications.

* Glass - mainly used in bead form for general cleaning and peening of metallic components.

* Metals - steel, iron, aluminium and brass are formed into pellets or cut wire for cleaning, derusting / descaling and peening applications.

* Plastics - both thermoset and thermoplastic products can be used for a wide range of cleaning, deburring / deflashing and paint stripping applications.

* Ceramics - useful for finishing and peening applications where long bead life is important.

* Natural products - corncob and granulated shells and kernels can be used for sensitive cleaning applications.

Density or particle mass is a major factor in the energy that media can deliver to the surface on impact.

Heavy particles pack more punch than light ones and may be capable of greater surface modification, however, increased particle velocity can compensate for lower density up to the point where an excessive fracture rate becomes noticeable.

* Media shape - media particle shapes fall broadly into the categories of shot and grit.

Spherical particles distribute their impact over a larger area, moderating the impact and potentially creating a round-bottomed dimple in the surface.

Sometimes called a peened finish, the effect of shot blast treatment is likely to be a semi-reflective sheen appearance.

With angular grit media, the impact may be concentrated on a point of the particle or a sharp edge, generating an etched, matte finish that is characteristically bright, but non-reflective.

In terms of surface modification capabilities, comparing peened versus etched surfaces created by different-shaped particles, the contrast is not so much in the final texture or depth of impression, but in the nature of the indentations in the surface and its reflectivity.

* Media size - particle size has an important effect on the number of impacts per second of blasting, so it is advisable to use media of the smallest screen size that will do the work, in order to reduce process time to a minimum.

Larger particles may be capable of creating bigger indentations and more texture in the surface, whereas smaller ones produce dimples or angular dents of lesser diameter.

In some instances, the choice of media screen size is dictated by surface features such as holes or narrow places where shot or grit may penetrate less effectively, become entrapped or lodge in recesses of the component.

* Media hardness - the hardness of media is a critical factor in almost every case.

Usually reckoned by the Rockwell Scale or the mineral order of hardness (MOH Scale), it often expresses the 'aggressiveness' of the material and its ultimate potential for surface modification.

A particle of softer material, even when it is of greater size and density and propelled at a higher velocity, will be unlikely to alter the finish of a harder substrate.

Hardness may determine whether a blast particle will deliver its energy with effect or absorb some of the impact energy by deformation or fracture.

One of the best pieces of advice to guide in the selection of media is to know the hardness of your substrate.

* Media breakdown - finally, one of the keys to economical blasting is to consider the impact strength or fracture resistance of the material.

This factor manifests itself in the attrition rate of media and your consumables cost, but it can also be an issue in terms of the generation of dust from the breakdown of media and the volume of waste material for disposal.

Blast pressure or particle velocity plays a major part in the equation, but the fracture resistance of media under the conditions of use has a direct effect on the technical quality of surface preparation and the consistency of your surface finish.

As previously mentioned, most of the materials used for outdoor blasting are highly friable and do not survive their first impact with the substrate.

Obviously, all of the properties of media that we have outlined must be taken together to determine the usefulness of any media for a given blasting application.

Component material and the type of blast equipment to be used dictate certain media choices.

Media selection is usually an educated process of elimination that should primarily be based on your technical surface finish requirements.

Cost considerations for a particular choice, both in terms of media consumption and wear-and-tear on equipment, can be estimated to fill in the economic dimensions in your evaluation of alternatives.

* How do we select the right blast media?

- Firstly, decide what the application fully requires.

Are you removing deposits from the surface?

Must the surface remain dimensionally stable?

Is a fine etch or roughening effect required?

Once the final effect is defined, then selection is made much easier.

The type of blast equipment available or being chosen will also have a bearing upon the type of media being selected.

Compressed air systems, both suction and pressure fed can generally use most types of media whereas turbine wheel systems have a much smaller range available.

However, the different characteristics of each of these types of blast system mean that the same blast media may react in a totally different way in each instance.

Dependent upon this effect, the appropriate blast media will be either harder or softer than the substrate of the object being blasted.

* Some 'rules-of-thumb' for blast media selection are the following.

1 - Choose the least aggressive media that will do the work.

This will result in less wear and lower equipment maintenance expense.

2 - Use the smallest media particle size that will do the work.

More impacts per second will yield a faster process.

3 - Find the lowest blast pressure that will do the work.

This offers the benefits of energy savings in reduced compressed air requirements, as well as less wear and lower maintenance costs.

The best advice about media selection is to avoid making assumptions about what worked for somebody else.

Take advantage of laboratory testing services offered by some equipment manufacturers - prospective user of Guyson manual or automated blast systems are encouraged to submit sample components for free feasibility testing, insist that a potential supplier demonstrate acceptable results on your own components and expect satisfactory answers to your questions about the testing and the recommendations.

* Blast media guide available from Guyson - leading surface finishing specialist Guyson International has produced an informative and unbiased blast media selection guide.

This handy four page PDF leaflet with accompanying diagrams and pictures is ideal material for anyone who specifies blast media or will make useful reading for those new to blasting finishing.