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 includes agricultural materials such as ground nut shells or starch grit, mineral substances like aluminum 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.
Density or particle mass is a major factor in the energy that media can deliver to the surface on impact. Heavy particles pack more wallop 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 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.
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.
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.
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.
Some rules-of-thumb for media selection:
- Choose the least aggressive media that will do the work. This will result in less wear and lower equipment maintenance expense.
- Use the smallest media particle size that will do the work. More impacts per second will yield a faster process.
- 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, 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.