Abrasive Blasting 101
Updated January 2022: Do you often wonder why your blast cabinet has abrasive flow problems after roughly 30 minutes of operation? It’s a common issue that we field questions about, and we’ve compiled a list of the five most common culprits. Read below for our media blasting troubleshooting for our sand blasting tips.
Often particles removed from the part being processed get into the abrasive and sieve down to the abrasive flow valve, blocking abrasive flow to the gun. Just imagine turning over an hourglass that has marbles in it along with sand, you can see how quickly things would get stopped up.
Typically it’s things like decals, gaskets, rust chunks, and carbon pieces that cause issues. Most machines have a scalper screen that captures these items, but if your machine doesn’t have a scalper screen, drain and sift the abrasive to remove nested trash you can manually replace abrasive media after sifting it through a window screen or pasta strainer. Both common household items will separate out any items that may restrict abrasive flow. If you find you are constantly back pressuring the nozzle, holding something over the nozzle to reverse the abrasive flow, you most likely have too much trash in the abrasive.
Low abrasive volume.
As you use your blasting cabinet, abrasive lands in low airflow places and is no longer part of the recycling process. If the abrasive valve is starving after a short time using the machine, try adding more abrasive to your system. It should be visually apparent if this solution eliminates your problem.
Worn out abrasive.
As you use any abrasive it wears out, gets physically smaller, and takes longer to clean. Different abrasive types have different recycle rates, and typically using a larger abrasive offers more recycles. As abrasive gets smaller, its angle of repose gets greater and greater. When it’s worn out, and no longer part of the recycle process, it requires an abrasive change out. Drain the abrasive, throw it away, and replace it with a new, larger abrasive. Just like changing the oil in your car, the abrasive in your blast cabinet has a finite lifespan.
ProTip: Using any abrasive in excess of its maximum impact velocity will drop its recycle rate to one or two times. This requires you to switch to new abrasives far more frequently and becomes very costly.
Drain the Air Filter/Water Trap.
If you don’t drain the air filter/water tap when it is full the water can easily pass into the cabinet. This problem is easy to control, just visually check and remove the water when visible to eliminate the problem. If you are collecting water in the air filter water trap it is indicating the last condition, the next section is for you!
Keep in mind Air Filter/Water Traps only removes moisture already in a liquid state in the compressed air line. This moisture is then collected in an air filter water trap on the machine itself. Air filters do not remove humidity in the air.
Unfortunately, hot air isn’t something you can easily diagnose on sight. This problem commonly arises when the air compressor is close to the blasting cabinet and just large enough, or maybe a bit too small, to operate the machine without allowing the compressor to start, stop and cool during the blasting operation.
If this sounds like your setup, most likely hot air traveling to the blasting cabinet is creating moisture inside the cabinet and preventing abrasive flow after about 30 minutes of blasting. That time varies based on your hose size, tank size, gun size, and compressor pump speed. Possibly the compressor tank size is small making this condition happen, but what is most likely happening is the compressor tank starts at room temperature then begins to warm as hot compressed air enters the tank.
This is how you get water in the tank: the tank acts as the first air cooler. Compressed air is created, and the hot air warms the tank above room temperature. While some of the hot air’s moisture is captured in the tank, some is not. Less and less moisture is captured in the tank as the tank warms. Next the warm tank passes warm air to the supply hose, and the hose starts to warm. This supply line often creates the water collected in the Air Filter/Water Trap on the machine. Finally, the warm air passes through the air filter water trap with some water vapor. This hot, wet, compressed air becomes condensation when it enters the cold steel cabinet – which is cooler than room temperature because the dust collector blower is removing air from the cabinet and creating a “wind chill cooling effect” inside the cabinet.
Feel the air hose at the tank and assess if it’s warm as it passes into the cabinet. Remember if it feels warm to you, it’s more than 98.6 degrees as it enters a much cooler steel cabinet! The difference in temperature between the cabinet and the compressed air can vary as much as 30 to 40. Even a higher difference is possible if the machine is located in a very cold shop during the winter!
You need an air cooler or air dryer to drop the compressed air temperature well below room temperature, which will remove the water vapor before it enters the tank. There are many types of air coolers, and dryers are sized so that compressed air spends long enough inside the cooler to drop the air temperature and remove moisture. Some types absorb the moisture, others types drop the temperature below ambient but they all dry the air.
At Media Blast, we recommend ambient dryers; they operate at the machine air inlet and only cool what the machine uses. Remember some facilities are cooling massive amounts of compressed air if they are cooling the entire compressed air volume. Ambient dryers look similar to an air filter water trap but they chill the air inside the dryer and self-dump the moisture out, cooling the air before it enters the cabinet. You should purchase a dryer for the volume of air your abrasive blasting gun uses.
ProTip: You can easily test your compressed air for moisture and also see how an ambient dryer works at the same time. Use an older type blow-off gun, with a small high velocity tip, held about 1/4″ from the cabinet steel after you have run for 30 minutes. Hold it perpendicular and start the blow-off gun holding it in place for 10 seconds. If you see small beads of moisture being created against the steel cabinet, your air is wet. This is often also how an ambient dryer works by super chilling compressed air inside the dryer.
If you’ve got a compressed air flow problem and you don’t think it’s due to one of the five issues above, call our team of experts and we’ll help with your blast cabinet troubleshooting. Media Blast & Abrasive has been manufacturing sandblasting cabinets since the 1970s. We make more than 180 standard models and sizes of abrasive blast cabinets and have a vast knowledge and understanding of most blast cabinet related topics.