Lets Talk Abrasive
The information that follows will be helpful when thinking about using any blast machine in the "automotive field" ... Industrial applications almost always have specifications for processing that clearly indicate the abrasive type, blasting pressure and equipment to be used. They are lucky because they have the information about soda, plastic, glass beads, aluminum oxide, steel shot and grit on hand.
If you're new to the blasting process you should know the basic difference between a cutting and non-cutting abrasive and applications for the most popular types.
Cutting Abrasives, aluminum oxide ... silicon carbide ... garnet ... slag and simple sand, are the ones most recognized in the industry today ... Lets start at the bottom and work our way to the top ...
Sand is not used with stationary blasting cabinets because sand is soft and has a very poor recycle rate. Soft abrasive does not recycle and explodes on impact with any normal blasting operation. This produces an unnecessary amount of abrasive dust that will overload any type of machine dust collector. Using sand in any blasting cabinet will result in the machine looking like yesterday's dusty dirt road. In addition sand may contain free silica known to cause Silicoses.
Next up in the list of cutting abrasives is slag ... Slag is used for many things but the normal application for slag is one-to-three times usage for non-recycle. Slag is commonly used to clean the hulls of ships or building structures. Slag is another cutting abrasive that has a very poor recycle rate and is not recommended for use in stationary blasting cabinets.
Garnet, often talked about and seldom used, is a cutting abrasive used when clean-inexpensive iron free abrasive is needed. Seldom used in cabinets and often used in blast rooms because it has a recycle rate of about 3-6 times. Often in a blast room the contamination factor is great enough to purge the system after 3-6 uses ... Used inside a blast machine for applications requiring low pressure processing, 60 psi and less, and often in the food industry. Garnet will reach its maximum impact velocity due to its lower hardness at 60-psi siphon and 30-psi direct pressure when striking normal steel items. Operators usually increase the blasting pressure above maximum impact velocity in order to generate increased frictional heat that permits faster cleaning rates. This will exceed garnets impact velocity and result in unnecessary dust loads on any style dust collector.
Silicon Carbide is a cutting abrasive that measures 9.5 hardness on the Moe's scale of mineral hardness with diamonds equaling 10. Extremely sharp but due to its extreme hardness, very prone to breaking. Looses its cutting speed because it continually breaks into smaller pieces resulting in less particle mass. I always like to compare silicon carbide to a heat-treated part that is too hard. Often the result is a broken part ... . Silicon Carbide is used to surface etch very hard materials but at a cost of nearly double per pound over the other abrasives. This abrasive is also on the very opposite end of the cutting abrasive list ...
Aluminum Oxide is the most used cutting abrasive measuring 9.0 on the Moe's hardness scale. This produces an abrasive that does its work and bounces off the part. This type of abrasive will slow down by dulling similar to any quality sandpaper product. Aluminum Oxide is most commonly used in the brown configuration that may contains some free iron ... Aluminum oxide is also available in white with the same 9.0 hardness but no free iron. White is commonly used in the medical industry.
The above information covers the general cutting abrasive market but some others do exist. Cutting abrasives clean by stripping the surface, removing some of the part and also leaving the surface open for the best possible coating adhesions. Often cutting abrasive is necessary for removing unwanted contamination that might be present on parts.
All cutting abrasives generate higher frictional heat helping to reduce cleaning times. They do have some side affects, hard on parts ... cabinets ... nozzles ... windows and visibility, but great for reducing processing time. Cutting abrasives are often used when compressed air is inadequate. This will shorten the processing time and decrease the total volume of compressed air required to process a part. Also used as a percentage mixed with other non-cutting types to decrease cleaning rates.
Non-Cutting Abrasives, soda, walnut shells, plastic, and glass beads are all types that are easily recognized in today's market. This time we will work from the top down.
Soda is new on the market today. It is used one time for cleaning without surface texture and can also be used to clean oily parts because of this application. Because it is used only once, some oil can be present when cleaning parts. The soda is collected and held in waste containers after use to prevent dust collector overload. Small amounts of soda are discharged from the gun unlike a normal "Sandblast" cabinet.
Soda is used almost exclusively in direct pressure systems because of the concentrated nozzle pattern but it is also used in the less expensive siphon systems. Some soda is completely water-soluble making it great for graffiti removal and other sodas include starch and or plastic material.
Walnut shells have been used for years for its ability to remove coatings with little or no surface etching on the part. In addition, walnut shells have some absorption capability making it good for a specific process. Carbon removal from large diesel engines with the ability to combust makes this product a good choice. Walnut shells because of the organic nature of the product also have some disadvantages when being used in a recycle system. Mold, rot and mildew ... shells can also transfer minor amounts of shell oil to the part that can become a problem when painting fiberglass and plastic.
Walnut shells are well known as being used in blasting cabinets because of the unique nature of the product and the number of years the product has been available.
Plastic abrasive has almost replaced walnut shells completely because of its ability to remove coating without any metallurgy change and or contamination. Often used in the aircraft industry for part cleaning to prevent work-harding during the cleaning process.
Plastic cleans with a larger particle and less frictional heat and this can be an advantage with some coatings. Leaving the coating brittle can make the cleaning process faster. Plastic is used to remove paint and coatings and yes even powder coatings and Teflon. Because the particle size is larger than normal abrasives it is commonly used in direct pressure type machines equipped with a pressure pot assembly ... for every rule there is an exception ... when using larger siphon guns the plastic abrasive performs very well. The siphon gun normally used with plastic would be 50-100 cfm air volume usage.
The most commonly used non-cutting abrasive is a glass bead. Available in a range of sizes from the very large bead used in the painted lines of the highways to the .001, 25-micron size. With a size range covering 14-600 mesh you might think some sizes are better than others. The answer is yes ... large beads are slow and small beads are slow. This produces the classroom "Bell Curve" we saw and knew in school.
At the top of the bell-curve is mil-spec. Number 8 glass beads. This product is 70-100 mesh or about 0.0075 in particle size ... very good particle density with enough particle mass to clean almost any part. Another good thing to remember is the size of the bead as it relates to impact velocity of the bead. The larger the bead the lower it's maximum impact velocity. A number 8-glass bead in a siphon system will normally bounce off a cold rolled steel part at 80 psi with no damage. This would be 40 psi in any direct pressure system.
Glass beads spin polish the part sealing the surface and actually making the part appear larger. Because they make a small dent in the surface and because the bead has no real sharp edges the surface of the part around the hole is raised without part removal.
Products are always rumored to start somewhere and one very good rumor is the hydraulic industry. Produce an abrasive that can clean the surface of hydraulic rams, not reduce the size and make a better oil-retaining surface and you get glass beads. You have the ram appearing larger not smaller after processing with miniature pockets that retain that important lubricant oil and a sealed surface.
In short it might be mentioned that the majority of the abrasive used inside abrasive blasting cabinets would be glass beads and about 80% of the glass beads used would be # 8, 70-100 mesh.
Glass beads are easy on equipment and clean and seal the part surface. Beads create a very good surface for paint and do very little harm to the size of the part. Often used to de-burr, clean, look for surface cracks in parts and many other applications.
Strongly recommended for the abrasive of first choice for any automotive application when the part is important.