A Beginner’s Guide to Abrasive Media Blasting for Powder Coating and Industrial Painting
The key to getting a good powder coated or painted finish is preparing the surface of the parts you are coating. If the part is not thoroughly cleaned and ready to accept powder or paint, the finished product may be compromised. There is nothing more frustrating than applying a top-notch coating only to get an unacceptable finish because the surface of the part wasn’t adequately cleaned. Depending on the nature of the material you are working with, chemical pretreatment and blasting with abrasive media are two popular choices for prepping parts.
Blasting does a great job of eliminating surface defects and trash like rust, mill scale, laser scale, welding splatter, and other surface contaminants. It also provides a textured surface for the powder or paint to grab on to while it starts to cure. If you are worried about powder coating adhesion, blasting is one of the best ways to prepare the metal.
Where to start?
Well, setting up a blasting process requires the purchase of a handful of key components. Let’s review what you’ll need and consider the critical information about each piece of equipment:
Air Compressor, Filtration, and Air Lines
This is probably the most important set of components to research thoroughly. Without the correct air volume and pressure, your blast pot will not function correctly. It’s also important that the compressed air delivered to your blasting equipment is dry, cool, and free of oil and contaminants. Moist blasting media will not blast correctly, and if the media is wet enough, it may not feed at all. If you live in an area with a hot, humid climate, a refrigerated air dryer after the compressor may be needed. In any case, you need to make sure that you have a good in-line filter system that will trap airborne contaminants in your air supply, as well as a desiccant type dryer with a drain if you don’t add a more expensive refrigerated dryer instead.
The compressed air lines you run are also critical. We’ve frequently seen coaters that have large compressors get sub-standard results from their blasting equipment. It’s almost always because they’ve choked down the air supply from their compressors by running small 1/4” to 1/2” air lines over long distances to save money on piping costs. Connect the blasting pot to your compressed air supply with piping that is at least as big as the fitting that is used by the blasting pot manufacturer at the point where the pot connects to your air lines. DO NOT run small piping and then adapt it up at the connection point of the pot. Smaller pots usually require 1” pipe while larger pots can need up to 2”. Don’t skimp on your air lines.
The distance from the compressor to your blasting equipment also matters. Even with smaller pots, once the compressor is more than about 200’ away, it may be necessary to upsize the piping to 1 ½” or 2” in order to get proper volume and pressure at the blast pot.
IMPORTANT: Read the set-up guide for your blasting equipment very carefully to avoid making costly modifications to your air piping after you’ve installed your equipment.
Another issue can occur when you run multiple machines off the same compressor. You can have trouble blasting because the airflow requirements of the blasting pot aren’t being met when the other equipment is in operation. Having a dedicated compressor just for your blasting operation is a good idea, especially if you are blasting on a daily basis.
Typically, most blasting pots require 80 PSI or more to function correctly. If you are using smaller media with a 3/16” nozzle orifice, then you’ll need a compressor that can consistently generate about 40 CFM. If you run a larger nozzle orifice, you will need a larger compressor. A 1/4” orifice needs about 70 CFM and a 5/16” orifice requires about 115 CFM.
When possible, we suggest using a rotary screw type compressor to power your blasting operation. Although more expensive than piston type compressors, they are better suited to an environment where they will see extended use at high CFM flow rates. Try to use a slightly larger compressor than what is detailed in the instructions for your equipment. We recommend that the compressor you use is sized so it can operate at 70-80% of its rated maximum output. This will extend its service life and assure that you aren’t at risk of “running the system dry” on days where you are blasting for long periods.
A blasting booth, also known as a blast booth or blast room, contains the dust and surface contamination removed by the blasting process and its overhead lights and dust collection system help the operator see his work. If you’ve ever watched someone try to blast without a booth, you know it’s a messy process and the equipment operator often seems to be struggling to see what he is doing.
Just like it’s important to have a booth to enclose the process and keep from contaminating the shop environment, it’s critical to have a dust collection system so the operator isn’t surrounded by a cloud of airborne dust while he works. Good visibility is critical for good results. Most blast rooms feature an exhaust with either cyclonic dust removal or an array of cartridge type filters that remove airborne dust particles. It’s important to understand that these filtration systems do not remove spent media or large pieces of debris from the booth. We’ll discuss reclamation systems that remove media and debris later in this article.
Most blast booths have plastic or rubberized shields to protect the walls from the constant abrasion of the blasting. Without these shields, blasting can eat away at the walls of booth in a matter of time. The rubberized shields also help keep the spent media from bouncing back onto the operator. This is especially valuable when using steel shot.
Blasting pots, also known as blasting pressure vessels, are usually measured in terms of how many cubic feet or pounds of blasting media they can hold. Common blast pot sizes range from 50-pound to 650-pound capacity. If you have a small shop that only does a couple small parts or a set of wheels per day, then the 50-pound pot might be fine. However, if you don’t want to be constantly refilling the pot, a 250-pound pot is usually the minimum for a full-service powder coating shop and a 450-pound or larger pot is common at larger job shops or in a production environment.
Using the smallest common nozzle orifice (3/16”), a pot can typically spray about 215-220 pounds of media in an hour if the operator is blasting non-stop for the entire hour without releasing the controls. The amount of media being used goes way up with a larger nozzle orifice. The difference in cost between a small pot and a large pot is minimal in comparison with the amount of time you burn up during the workday reloading a blast pot.
Blast Media Reclamation
Although not exactly a “best practices” approach, some smaller shops sweep up the spent media and put it through one or more screens to remove large particles. The material that falls through the screen is then reloaded by hand into the blast pot. This only works with media that can be reused.
If your shop does a good bit of blasting on a daily basis, you may want to consider a reclaim unit that will help you quickly reuse spent blast media. The most basic of these include a pick-up bin that is about the size of a mailbox. Spent media is swept into the bin and a powerful vacuum draws it into the reclaim unit where it is spun until debris and broken-down grit is separated into a trash barrel and the material that can be reused is then manually or automatically loaded into the blast pot. Larger, more sophisticated systems may include a grated floor or troughs in the floor that either rely on a vacuum system or a powered auger and bucket elevator to return spent media to the blast pot.
Blast Nozzle Selection
As stated before, the smallest blast nozzle orifice is usually 3/16”. We recommend trying to set up your blast process to use the smallest, least aggressive media possible. This will leave a smaller blast profile that has to be covered and “evened out” by powder or paint. Your blast media data sheet will indicate a mil profile that is left after blasting. The larger the mil profile, the more powder or paint it will take to achieve a smooth surface.
However, if you have a lot of deep rust or mill scale, you should try larger nozzles and suitable media to save time when blasting. Just remember to check the mil profile rating on the media and that will tell you the basic powder mil amount needed to cover the profile. For example, if you have a 1.5 mil profile, you will need at least 3.5 mils of cured powder to adequately cover the steel and fill in all the recesses caused by the blasting.
Remember, if you use larger nozzles, your air consumption will go up—way up. Make sure you have enough compressor capacity to accommodate the larger nozzles, or you’ll be frustrated by the results.
Blast Media Selection
Most people think of blasting as sand blasting. Sand is not recommended anymore due to the health hazards of silicosis. Sand breaks down and forms dust clouds that remain suspended in the air. Even in the most controlled environment there is a chance that these extremely fine particles can be inhaled and cause problems. Therefore, these other medias are now commonly recommended and are silicone free.
- Aluminum Oxide – This popular abrasive has been in use for decades. Countless grit sizes available. Reusable and has a great shelf life. Aggressive cutting and hard enough to texture stainless and titanium. Requires care when working with softer metals.
- Coal Slag – Many different sizes of media are available. Good cutting power for rust or old paint. Relatively inexpensive but not reusable.
- Eco-Friendly/Organic – Walnut shells and corn cob fragments are among these types of media. Bio-degradable and similar to glass in maintaining metal details on softer metals.
- Garnet – Different grits available. Cleaner and can be more affordable than coal slag. Very consistent mil profile. Reusable and gives predictable results.
- Glass or Glass Beads – Mostly for fine blasting where metal details need to be maintained. Also good for softer metals. Beads are reusable and crushed glass is disposable.
- Steel Shot – Probably the world’s most popular reusable blasting media. Does not degrade with multiple uses like other media (most reusable media types can’t be used more than 3-6 times). Can be contaminated by oils, so not the best choice for oily parts. A little too harsh for fine finishes and soft metals. Requires robust reclamation equipment to effectively reuse.
These are the main types of blasting media we see in use by small powder shops and big manufacturers alike. There are others, like grit made from recycled plastic for applications where little surface texturing is desired, or silicon carbide for when a heavily etched surface is desired. If possible, start a relationship with a local blast media supplier who can help you find the right media for your blasting operation.
Personal Protective Equipment (PPE)
If your shops blast 6-8 hours a day, OSHA says you should supply your operator with a full suit and gloves, along with an enclosed helmet that supplies air for him to breath. The air supply needs to have a carbon dioxide monitor and an inline cooling unit. The helmet/respirator needs to be supplied with Grade “D” compressed air. Each operator needs their own suit and helmet. Make sure if you use the air from the same compressor as the blasting pot, that the plumbing does not rob air from the operator or the pot. Get some extra face shields, fittings, gloves, and a spare cooling unit to avoid downtime during breakdowns.
What Can You Blast?
Blasting pots and blast media are most commonly used for preparing regular mild carbon steel. If you are blasting aluminum, lower pressures (40-60 PSI) and smaller, less aggressive media are recommended so you do not damage the surface of the parts. Large, aggressive types of grit can tear into the metal and cause finishing inconsistencies like dull spots, rough textures, and irregular gassing out of the metal. Steel shot is also not a great choice for aluminum or stainless steel since the shot can leave behind carbon steel residue that may interfere with the properties of those parts.
Setting up the Blast Pot
Fill the pot with blast media but do not exceed the bottom of the sealing plunger.
Adjusting the blast pot is usually a two-person operation. One operator will handle the nozzle while the other slowly increases the media feed at the bottom of the pot while the nozzle is triggered. You want to barely be able to see the media coming out of the nozzle. Too much media will clog the hose, slow down the media velocity, waste the media, and yield unacceptable results.
Watch your air pressure when triggering the nozzle. If your air pressure drops below about 20 PSI while the nozzle is triggered, then you have an air volume problem. Look for airline chokepoints and/or stop using other machinery that requires air if they are connected to the compressor you use for blasting.
Need Help? We’re Here for You.
Hopefully, this guide will help you plan the set-up of your blasting operation. As mentioned before, carefully review the literature for the specific blasting pot and nozzle orifice you purchase and be sure to get a big enough compressor to operate your system properly.
In addition to manufacturing professional powder coating equipment, Reliant also provides a wide range of services to help your existing finishing operation run smoothly–from on-site troubleshooting to training, equipment refurbishment, and more. Call us today at (877) 418-5550.