Abrasive Blasting 101
When you think about using a media blasting cabinet, i.e. sandblasting cabinet, with enclosed containment and recycling abrasive, one of the first things you will find are different types of abrasive deliveries to the blast gun. This post will help to uncover which delivery – wet or siphon or direct pressure – is best for your application.
There are three types of abrasive deliveries for cabinets. Abrasive delivery is how the abrasive get to the blast nozzle and exits into the cabinet.
Wet cabinets use an injector suction gun to deliver the slurry consisting of water and suspended abrasive to the part surface. The abrasive suspension can be created using different methods but know abrasive being suspended can be as high as 23% and this high percentage can help to decrease cleaning speed in the slower processing wet cabinets. An injector gun has two hoses attached to the gun, one for compressed air supply and one for abrasive delivery to the blast gun. Slurry supplied to the blast gun is accelerated by compressed air inside the gun exiting the nozzle at higher velocity to clean or texture the part surface.
Normally, you know you need a wet cabinet, but if you’re not sure this applies to you, here’s a few quick reasons you’d want to use wet blasting:
Wet blasting is much slower than dry blasting because no frictional heat is present. In addition wet blasting cabinets normally use finer abrasives, which clean much slower than larger abrasives. Media Blast offers wet blasting models with abrasive sizes up to 80 mesh at high slurry percentage unlike the other just now entering the wet blast machine market.
Think about using the above reasons as an advantage and you will understand why companies that refurbish painted automotive wheels use wet blasting cabinets to etch and clean the original coating using finer abrasive for smother coated finish while making sure a clean bonding surface is created. With this chemically clean surface you eliminate “fish-eye” when the wheel is coated again. Wet blasting is a good choice for remanufactured wheels but so are other applications like critical gold plating, medical implants and other parts damage by frictional heat.
Real World Example: Years ago phone cases were blasted wet to remove all the dirt and pen marks in the cases. After cleaning the cases were heated again and allowed to re-skin creating an almost new case appearance.
Siphon blasting cabinets are easy to identify because they use the same “two hose injector gun” that wet blasting cabinets use. One hose for compressed air and one hose for suction abrasive delivery to the gun where the gun design accelerates the abrasive at high speed to the part surface.
Dry siphon cabinets are about 80% of the machine cabinet market today and they can cost a few hundred dollars to well over $20,000. Most are manually operated using a single hand held directional blast gun, but automated siphon cabinets also exist and tend to be on the much more costly side. These cabinets, while consisting of a large market share, do have limitations created by heavy steel shot and grit, larger mesh sizes and deep pocket or blind hole cleaning.
The main advantage of siphon abrasive delivery is that the operator does not need to stop to refill the pressure pot and siphon has a wider blast pattern capable of cleaning most parts quickly, always remember the deciding factor about cleaning speed is the cfm used by the gun and guns vary in size.
Pressure cabinets are more expensive, harder to operate, and clean more slowly than siphon when compressed air usage, abrasive velocity and abrasive pounds per minute delivery are equal. Dry pressure cabinets can operate with the same abrasives used inside siphon cabinets, but they can also use other materials, such as steel shot, and allow through-hole blasting.
Myths about pressure abrasive delivery machines:
Most pressure machines using a pressure pot to pneumatically accelerate the abrasive can easily exceed maximum abrasive impact velocity of any abrasive type being used. This more than likely creates a higher abrasive use rate for pressure due to operator error about maximum abrasive impact velocity. Pressure cabinets generate more frictional heat than siphon, allowing some cleaning applications to require pressure models. If you want to shot peen to the highest peening intensity you must use pressure.
Pro Tip: Pressure cabinets do some things siphon cabinets cannot – through-hole cleaning, operating using certain large or heavy abrasives, etc.
Depending on your application, you’ll want a wet, siphon, or pressure abrasive delivery system. What helps decide is what is being blasted and why. Many ill-informed buyers believe that pressure is faster, and this might be true with heavy coatings, but remember pressure is also more expensive. Working with a knowledgeable manufacturer can help you spend smart and buy the right machine for your needs the first time!
Pro Tip: The CFM used by any gun or nozzle decides speed of cleaning more than the abrasive and blasting pressure… Not the abrasive delivery system.