Abrasive blasting will produce an effect that may combine both a cleaning and finishing action. The finishing effect may vary by controlling such factors as hardness of the abrasive, abrasive particle size, velocity of abrasive stream, angle of abrasive gun, distance from the work, method of application and work flow.
As it is applied to preparation of surfaces prior to finishing, abrasive blasting is generally used to replace sanding, wire brushing and pickling. Ordinarily, no other cleaning is necessary because the blasted surface is chemically and mechanically cleaned.
Abrasive blasting can save from 25% to 75% of the time normally required by hand cleaning. Blasting is considered economical. The abrasives are relatively inexpensive and reusable. The general economical advantages of abrasive blasting lie in the reduction of man-hours required to clean and finish parts and needing only minimally trained personnel, yet still having high productivity per hour per dollar of equipment.
Abrasive blasting can make a good finish better and cleaner. It also produces a better tooth for bonding. It is estimated that the surface area of metal increases as much as ten times as a result of the abrasive impact action. This increases the surface to which paint, coating or plating can adhere.
Direct pressure machines require less pressure. Whereas a siphon machine is normally operated at 60-90 PSI, the direct pressure machines can function at 15-80 PSI. Operating at lower pressure reduces the work hardening of the surface and reduces warping of thin parts.
Most people sandblast at an air pressure that is too high. When you blast at pressures above 90 PSI, there is an excessive breakdown of the media and very little improvement of the cutting rate.
The Sandblast Gun
Abrasive blasting is supposed to be a scrubbing action, not a peening process. Therefore, the gun should always be aimed at a 60° to 45° angle to the surface being cleaned. When the gun is aimed at 90°, peening occurs and, due to the abrasive particles colliding with the abrasive bouncing off the surface, a very high rate of media wear occurs.
The gun in a siphon machine should be kept at least six inches from the surface being blasted. This allows the spray to spread out and cover a larger area. Blasting a larger circle allows for better overlap of the pattern and yields a more even and appealing finish. The direct pressure units can effectively operate at distances of one foot or more.
The Gun Nozzle
Nozzles made of tungsten carbide are the best choice. Settling for a less expensive, lower quality nozzle ultimately increases operational costs. If your compressor cannot keep up with the blaster, chose a smaller nozzle for the gun. If you have plenty of pressure at the gauge, but don’t seem to feel it at the gun, look for an obstruction in the abrasive pickup line or something stuck in the nozzle.
In a siphon machine, remember to change the air jet (behind the nozzle) every few nozzle changes. A worn air jet will deflect the flow in the gun and cause the abrasive to wear a hole in the side of the gun. If you have enough pressure at the gun, but very poor flow of abrasive, your nozzle is worn, there is a hole in the siphon tube pick-up hose, or the abrasive is so fine that it won’t flow down to the pickup area.
Replace the siphon hose on a regular basis. When the walls get too thin the hose will collapse and obstruct the flow.
Many types of finish may be obtained by the selection of abrasive and by the adjustment of air pressure in the blasting unit. The more commonly used abrasives are
- aluminum oxide,
- white aluminum oxide,
- urea and other plastic abrasives,
- corn cob grit,
- walnut shell grit,
- glass beads,
- crushed glass grit,
- silicon carbide,
- steel grit and
- steel shot.
For the most efficient performance, when the abrasive in the machine has broken down too much, the entire load should be replaced. Adding new material to the old load greatly reduces the performance of the abrasive and increases the amount of dust.
If you are getting a sporadic flow of abrasive, it is being caused by fine material not flowing down to the pick-up area or too much pressure. Banging on the side of the cabinet hopper can test this. If the flow is good after this, your material is too fine or may be moist.
- Glass beads can be used to texturize, descale, or remove light burrs and die-cast flash leaving a smooth bright satin finish. Used at 40 to 80 PSI.
- Abrasive grits can be used for more aggressive work leaving a dull satin finish and are useful for creating a good surface for bonding. Use up to 120 PSI.
- Walnut shell grit can be used for deflashing thermoset plastics without destroying the original polish. Use 30 to 80 PSI.
Blasting machines occasionally cause shocks from static electricity. If the operator stands on a mat grounded to the machine and the gun is grounded to the cabinet, this will be eliminated. The cabinet can also be grounded to any conduit for insurance.
Try not to hold a part up to the window. This will cause frosting of the window and make it difficult to see inside.