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How much lifetime can you expect from your blast hose? There are two factors that are the primary determinants. The first is the durability of the hose. Make sure that you are using a hose designed for blasting. One that is the appropriate thickness (1/4" minimum - 3/8" or 1/2" even better) and preferably reinforced.
The second factor is how the hose is used during blasting. As the abrasive media flows through the hose, it will wear the hose from the inside out. If the hose is kept relatively straight, you can expect even wear and a long lifetime. If the hose is bent or looped, uneven wear will occur and bubbles or blow-outs can occur.
This picture shows a used blast hose from the outside. Notice the reinforcement and layered aspect of this properly designed hose.
While this hose might appear to be in good condition after use, take a look at the cross section of the interior. This blast hose was bent as it came off the blast pot. Notice the uneven wear around the inside - even wearing through the reinforcing cord.
Thank you to MasterBlaster for his post on contractortalk.com for sharing these pictures and insights.
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| Aluminum Oxide vs Glass Bead Blasting |
Different blast media will create different finishes on different types of metal. The aluminum part pictured was etched and corroded and need to be cleaned to a uniform surface finish. The section on the left was blasted with 120 Mesh Aluminum Oxide Grit to create a relatively smooth, matte, uniform finish. The section on the right was blasted with Medium Glass Bead to result in a bright, satin-like finish. A simple change of media can result in a dramatic change in finish.
Welding steel often creates heat discoloration and welding burns. Prior to finishing many parts with processes such as powder coating, painting, clear coats or simple final assembly, the removal of the discoloration and preparation of the surface is necessary.  Elements were welded onto this steel tube prior to powder coating. In addition to the discoloration (blue streak seen on the left side), this part started to rust in storage. The powder coating process specified that the part be clean and smooth. The right side of the part was blasted with Medium Glass Bead to remove the oxidation, discoloration and provide a smooth, lightly peened surface finish.
 Machining Marks on Aluminum
The manufacture of machined aluminum parts creates machining marks or etching on the metal (see the picture above). Abrasive blasting the surface with Glass Bead will produce a uniform surface finish. The Glass Bead is typically blasted at 60-80 psi at a perpendicular (90 degree) angle to the surface. This produces a peened effect for a unique result. Grit type blasting media such as Aluminum Oxide will also remove machine marks but will leave the aluminum or other types of metals with an etched, rough surface finish. The picture shows the same part after blasting with Fine (#13) Glass Bead to produce a very smooth, matte result.
Fine Glass Bead Blasted
10 quick abrasive blasting tips: - Nozzle size determines the air requirement (CFM), not the pot size.
- Pressure systems are more efficient than siphon or gravity fed blasters.
- More air is always better.
- DRY AIR: Use a moisture separator on the blaster and cooler on the compressor.
- Ceramic nozzles are inexpensive but carbide nozzles last much longer.
- It's rare to blast above 110 PSI - typical is 80-90 PSI.
- Ear plugs are helpful, especially if you are near the compressor.
- Avoid using a longer blast hose than is necessary. A longer air hose is preferred.
- Proper protection is important, especially an air supply helmet in a closed or contained area.
- DO NOT blast with silica sand. There are plenty of 'healthier' alternatives that might even work better.
- Bonus Tip: Expect a loss of 5 PSI per 50 feet of air and blast hose.
Removing paint from brick can often be a process that requires a difficult combination of aggressiveness and delicateness. Brick (and concrete would apply as well) is a porous surface that does an excellent job of holding onto paint and coatings. The paint can be trapped in all the nooks and crannies of the surface making it very difficult to remove all the paint. As brick can be a decorative surface, it is important to not use a process that is too aggressive as it can strip away the brick material. This leads to an uneven and damaged surface that is unappealing. Walnut shell grit offers the perfect balance of aggressiveness and safety on the brick surface. Using a finer grade of walnut shell grit will strip the paint from all the 'hidden' areas on the brick surface but will not blast away the brick itself. In addition, because walnut shell is an organic material, the process is environmentally friendly and the clean up is easy.
 Question: I am looking at using media blasting to identify/mark items made from stainless steel sheet using stencils to allow shielding of areas I don't want to blast. The stainless steel (used in a food factory) is highly polished. I was hoping media blasting would give a different enough surface finish to make the marking easily visible but what media would be best to achieve this? Also, would a direct pressure or suction system media blasting cabinet be best? The factory air system produces up to 90 psi and supplied more than enough air to run lots of machinery. Response: You can use either Aluminum Oxide or Glass Bead to create the marking you described. Aluminum Oxide will produce an etched finish with finer grit sizes producing a smoother, lighter etch. Glass Bead will produce a smooth, peened finish with finer grades producing a more uniform, matte finish while coarser grades will leave a brighter finish. Either a suction system or direct pressure blast system could be used. The pressure system will typically shorten the cycle time, produce a more uniform surface finish and allow for more media re-use since you can blast at lower pressures.
Steel Shot and Steel Grit are some of the most effective media due to their extreme re-usability and lack of dust generation during blasting. These two types of media are used most in wheel blasting equipment. In this process, the media is literally thrown against the parts and then recycled again and again. Unlike air blast (i.e., sandblasting) systems, wheel blasting does not require an air compressor and can be used with any size of Steel Shot or Grit. Air blasting with Steel Shot and Steel Grit requires a little more consideration. If a cabinet system is being used, the steel media will be re-used many times but the type of blasting (suction or pressure), hose size and durability and nozzle type and size will determine what grades or sizes of steel media that can be used. If a portable air blasting system is used, the size of the fittings, hoses and nozzles are also important in identifying which grades can be used. Re-usability can be a challenge as there typically is no system to automatically collect and recycle the media. The most important consideration concerns the operator and the weight of the hose while blasting. Steel Shot and Steel Grit can weigh more than three times as much as other blast media. The weight of the blast hose filled with media can fatigue the operator very quickly. Finally, pushing heavy weights (a 50 foot hose filled with steel media) will require lots of air pressure and air volume.
The hardness of the metal (or plastic) to be finished is a critical factor in determining the ultimate surface finish in a tumbling or blasting process. Harder metals (steel, titanium, etc.) will typically require more aggressive media to remove burrs off an edge or machine marks on a surface. These types of metals are also much easier to polish and have a greater opportunity to finish close to a near mirror finish. Softer metals (aluminum, copper, etc.) can be finished with the same aggressive media but the surface finish will be rougher and the edges will be more rounded. It is also much more difficult to get a high polish on soft metals without some amount of hand polishing. While it is desirable from a operational standpoint to use one media for every part or process, efficiency, productivity and quality can be improved with changes of media. Sometimes just a minor change in procedure may be all it takes.
The choice of abrasive blasting cabinet needs to made based on the type of blasting being done including the media type and frequency of blasting. The primary benefit of a blasting cabinet versus a portable system is the containment of the media allowing for re-cylcing. Many re-usable blast media are abrasive and demand a durable cabinet construction and heavy duty fittings and nozzles. A cabinet constructed of continuously welded 14 gauge or thicker steel will provide long-term wear with most blast media. Adding optional rubber mats (if available) to the inside of the cabinet will further increase the lifetime of the cabinet. Accessories such as heavyweight gloves and long-lasting tungsten carbide nozzles are also worth the additional expense. A nice addition is an underlayment on the inside of the window to prevent etching of the glass and keep operating costs to a minimum. The selection of a siphon/suction or pressure cabinet will be described in another post. This decision will impact overall operational efficiency and performance. The blasting cabinets offered by Trinity Tool Company (Trinco) are excellent examples of the proper construction for long term durability and high value.
The Steel Structures Painting Council ( SSPC) has designated fifteen different surface preparation standards including simple solvent cleaning (SP-1) to Commercial Blast Cleaning (SP-6) to Industrial Blast Cleaning (SP-14). Some of these standards are simply visual while others designate specific profiles. These standards are applicable to a large variety surfaces. If a specification has not been designated for a surface to be cleaned or blasted, using an SP standard will provide consistent results from part to part and job to job.
Air blasting with either Steel Shot or Steel Grit provides a couple of unique challenges. Because of the weight of the media (2-3 times 'standard' abrasive blast media), it is important to check with the manufacturer to make sure that your blasting system is designed to handle this media. Many pressure systems are OK but siphon or suction systems will probably have some issues. Clean and dry (and I mean dry) air is critical for air blasting with steel shot or grit. If moisture gets into the pressure pot, the steel can rust over time reducing the effectiveness and requiring more frequent replacement of the media. If the steel media is left in the pot overnight with moisture you will probably need to purchase a new blasting pot.
 Here is a great picture of a do-it-yourself log home blasting project. The blaster being used is a relatively small KF100DM Portable Pressure Blaster with a 3/16" nozzle. Fine Corn Cob Grit is the media being blasted. The objective was to take off the old stain and sealant so that the logs could be re-stained and sealed. I believe the job was completed in one weekend.
Question: I would like to know if there is a standard for matching Nozzle opening size to media size. I am just starting out and I am not sure what size media I can use with my sand blaster. The literature on the blaster doesn't say. Can you help? Response: Generally, the nozzle size you use should be at least 3-4 times the diameter of the grit size. {If the grit is still getting stuck in the nozzle, try reducing the blasting pressure (psi) and making sure there is no moisture in the system.}
Question: I want to clean/polish an aluminum motorcycle engine. Would walnut shell be the best media for the job? I want a clean, lightly polished look typical of stock bikes. Response: Walnut Shell Grit will clean the surfaces relatively well but will not have an impact on the surface finish. A Medium-Fine or Fine Glass Bead will clean the metal and leave a satin-like finish on the surface.
Aluminum Oxide (a.k.a. aluminium oxide, alumina, aloxite, corundum) is generally sourced from bauxite ore. While bauxite is the primary ore for aluminum metal, Aluminum Oxide is extracted using the Bayer Process to remove unwanted impurities. Aluminum Oxide is a very hard, versatile blasting media that can be used in a variety of applications for removing oxidation and scale off metals to etching stone and glass surfaces. The pure (>99%) White Aluminum Oxide is used when surface non-contamination is critical like when blast cleaning medical parts. The hardness and large grit sizes available also make Aluminum Oxide an excellent choice for non-skid flooring. Aluminum Oxide is commonly used as an abrasive for sandpaper. I'll bet you didn't know that precious stones like rubies and sapphires are simply Aluminum Oxide (corundum) crystals contaminated with other metals like chromium, titanium and iron.
Just one more quick comment on CFM, nozzle size and blasting... I discussed a little about the differences and relationship between CFM and PSI in a previous post. PSI is basically the amount of force you blast with. CFM is the volume of air being used during blasting. It is sometimes suprising to realize how much air (CFM) is required during blasting. At a given PSI setting, the amount of air needed is directly related to the nozzle size being used. The larger the nozzle, the more air that is required. This also means that if you want to use a large nozzle, you will need a large compressor. For example, at 80 psi, a 1/8" nozzle will require 20 CFM. At the same pressure a 1/4" nozzle will require 85 CFM and a 1/2" nozzle will demand 340 CFM. A basic chart that shows the air requirements for different nozzles at different pressures is on the PPB Series page. Rule of thumb: double the nozzle size, quadruple the CFM required.
All solid materials can be measured for hardness on the Mohs scale. The scale was created in 1812 by Freidrich Mohs and is based how easily one substance can scratch another. Diamond is the hardest rated at a 10 while talc is a very soft 1. Suprisingly, Corn Cob Grit and iron have the same hardness at about 4.5. Even more amazing is that Corn Cob Grit is harder than brass (3-4), copper (2.5-3) and aluminum (2-3). The really cool thing is that blasting with Corn Cob Grit won't frost or pit glass and can be used to absorb grease, oils and water from metal surfaces. Corn Cob Grit is also great for blasting on wood surfaces (more to come on that). I wouldn't recommend using iron grit near glass or on wood!
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