Recycling abrasive to be used again and again is unique to abrasive blast cabinets. Recycling isn’t possible if you’re performing portable blasting without a containment cabinet. So, how many times can abrasive be recycled? This is a very hard question to answer as an absolute because the number of recycles depends on many variables factors which can extend or shorten any abrasive recycle rate we’ve published at the end of this article. The more you know about your application, the more accurately you can determine how long to use any abrasive.
What variables affect recycling abrasive?
- Hardness of the abrasive measured on the Mohs Scale or Mineral Scale for Hardness (this is the standard measure for most abrasives)
- Hardness of the part being blasted, many recycles rates are based on a Cold Roll Steel Part hardness meaning that aluminum will have a higher abrasive recycle rate and heat treated steel a shorter recycle rate. Part hardness can be related to how hard or easy a part can be dented or scratched.
- The type of abrasive delivery used, direct pressure using a pressure pot or siphon delivery using a two hose injector gun, both produce very different abrasives speeds at the same blasting pressure and affect how many times the abrasive can be reused.
- Amount of CFM and blasting pressure used with any delivery type, cubic feet of air per minute used with the delivery type and blasting pressure will determine frictional heat that is constantly changing.
- Blasting pressure used is very different between direct pressure and siphon delivery. At 80 psi using siphon delivery, beads will bounce off the part. Using direct pressure delivery at 80 psi will turn the beads into dust by exceeding maximum impact velocity. Many people don’t understand pressure delivery doubles the abrasive velocity at any specific pressure.
- Starting size of the abrasive used, measured in mesh or grit size, larger abrasives recycle more times and can be used longer as they wear to a finer size.
- Efficiency of the abrasive separator reclaimer used to remove the dust and spent abrasive and return usable back to the stored abrasive volume.
- Distance of the blasting nozzle to the part, which affects the maximum impact velocity and the frictional heat created.
- The volume of abrasive being delivered in pounds per minute also effects frictional heat created, more heat can shorten the number of times abrasive can be used… this is apparent with walnut shells or plastic abrasives.
- Volume of abrasive inside the machine does not affect the actual recycles but does affect the time before you reach maximum recycles. Having a smaller gas tank does not affect mileage but does affect how many times you need to stop for gas.
As you can see from the above variables simply saying glass beads recycle “X” times can be a very iffy answer at best. Know that most recycles you see in print are not considering any of the above variable items, doing so would turn a chart into a ten page article.
Rules for Recycling Abrasive
Rule #1: Never use soft abrasive inside any blasting cabinet built to recycle abrasives. Soft abrasives reach maximum impact velocity at a low blasting pressure, explode, and turn into dust upon impact with almost any part material. Soft abrasives, such as Slag, Sand and Garnett, quickly overload and plug the dust collector regardless of the size of the dust collector. Even small, low cost siphon blasting cabinets will delivery five pounds of abrasive per minute resulting in 300 pounds of dust being created per hour. Additionally, the dust may weigh 100 pounds per cubic foot, requiring lots of dust storage to accommodate 300 pounds of dust.
Rule #2: Know the abrasive hardness before you purchase. Normally the harder the abrasive the longer it lasts, but if the abrasive is too hard it must be used against softer parts or it will also quickly shatter into small particles. Silicon Carbide is the hardest abrasive commonly used, but it wears out quickly when used against hard titanium or any super metals. Silicon Carbide, with its sharp cutting shape, is really great when blasting glass or hard against soft.
Rule #3: Never blast above the abrasive’s maximum impact velocity, which is normally measured in blasting pressure or psi. Doing this will allow maximum use or maximum recycles, thus lowering the cost of operation.
Rule #4: Understand that harder parts wear out abrasive faster than softer parts. There is nothing wrong with the abrasive if it wears out faster when blasting harder parts.
Rule #5: You can use hours of time blasted to estimate how many recycles have occurred, you need to know the volume inside the machine in pounds and the pounds per minute abrasive flow from the nozzle to calculate how many times per hour the abrasive is being recycled.
Rule #6: If you are using a blasting cabinet without a separator reclaimer you will quickly contaminate the recycled abrasive with the dust and spent particles. When this happens, the good abrasive becomes over-contaminated with the dust and spent abrasive to the point that everything has to be discarded, good and bad. Combining a cabinet without an inefficient separator reclaimer and cleaning a dirty part can really shorten the recycle rate of any abrasive to a maximum of two or three uses.
Rule #7: Know that all abrasives rank on a sizing scale normally measured in mesh or grit size. The cleaning speed relative to abrasive size resembles a bell curve, large and small abrasives are slow, and medium size abrasives are the fastest. When using blasting cabinets with 0 to 100 cfm of compressed air being used the fastest abrasive cleaning size is normally between 60 and 100 mesh in abrasive particle size. Starting slightly larger will produce more recycles and as the larger then wears down it then gets faster.
Rule #8: Using any abrasive past its recycle rate will slow down part processing just like using dull sandpaper slows down sanding time. It’s good to think of abrasive as similar to oil in a car, because all types of abrasive must be changed and replaced eventually, don’t mistakenly assume that just because it’s still there it’s still good.
You can see the logic of the above rules with respect to changing any published abrasive recycle rate you might find.
One exception to any rule always exists, and here’s ours: glass beads may have the same hardness as garnet stone but they also have a very strong, round shape. They can take a higher impact pressure for the part hardness because of their shape. Larger beads break at a lower pressure than smaller beads, and 70 to 100 mesh beads are normally the fastest cleaning bead size.
The chart below is a good guide for estimating how many times you can recycle a specific type of abrasive but you do need to know nozzle flow in pounds per minute and the volume inside to know how many time you have recycled your abrasive. We will try and list all the abrasives you can use inside any blasting cabinet and also list ones you should avoid, showing you how quickly they turn into dust.
The recycle rates are calculated on the 60 to 100 mesh abrasive size that is normally the fastest cleaning. Starting with larger abrasive would naturally produce higher recycle rates, starting smaller will result in shorter recycle rates. Keep in mind that the rule of thumb is direct pressure blasting reaches maximum abrasive impact velocity at half the blasting pressure of siphon.
Real World Example:
You purchased a siphon delivery blasting cabinet operating on 16 to 18 cfm of compressed air at 80 to 90 psi blasting pressure. The manual advises you to use 50-pound glass bead volume inside the cabinet, using less simply means replacing the glass beads sooner.
The siphon gun uses 18 cfm at 80 psi and delivers 5 pounds of abrasive per minute. 60 minutes of blasting will deliver approximately 300 pounds of abrasive against the part. If you turn up the blasting pressure higher than maximum impact velocity you will get about 3 to 4 recycles period!
300 pounds divided by the 50 pounds inside the cabinet can equal about 6 recycles per hour. By dividing the average recycles for glass beads (30 times) by recycles per hour (in this case, 6) gives you 5 hours of blasting time before the glass beads need to be replaced. History tells us that most people use this kind of abrasive for eight hours or more or twice as long as it should be used.
Five (5) hours for a machine delivering 5 pounds per minute with a 50 pound charge is not an 8 hour day but you do extend blasting time when you are loading and unloading, flipping and inspecting the part inside the cabinet. Charging the same machine with only 25 pounds means would need to replace in 2-1/2 hours. Not replacing the beads after meeting maximum recycles is the same as using worn out sandpaper, you simply slow down the cleaning pressure and waste time you could use being more productive!