Abrasive Blasting Media - Steel

This final installment of the Abrasive Blasting Media series will discuss the merits of using both Steel Shot and Steel Grit for blasting. Both of these media types are produced from scrap steel which is melted, modified and then solidified into a generally round shape (Shot) or angular particles (Grit).

Steel Shot produces a very smooth finish on the surface of metals. The rounded shape does not strip a surface very effectively but rather peens the surface to produce a clean, smooth and (sometimes) polished finish. The smaller the shot the better the polish but the less aggressive the peening operation. The peening action of the shot is also used to impart compressive strength or hardening to metals.

Steel Grit is used when aggressive stripping action is required. Steel is softer than Aluminum Oxide so will not fracture as easily but the higher density produces a forceful blasting process. Smaller sizes and softer formulations of grit are used when lighter cutting is required while larger sizes and harder steel formulations will maintain the particle angularity and a high strip rate.

Since both Steel Shot and Steel Grit have a high bulk density of about 250 pounds per cubic foot, special or modified equipment is necessary to blast these media using air pressure alone. Steel is often used in wheel blast operations where the media is propelled by a wheel using controlled force.

Abrasive Blasting Media - Crushed Glass Grit

Traditional sand blasting and the need for healthy and environmentally sensitive operations meet up perfectly when using Crushed Glass Grit. Glass has the same hardness and other blasting properties as silica sand but none of the inhalation dangers from free silica. Crushed Glass Grit is 100% recycled glass diverting this material away from landfills.

Crushed Glass Grit is available in a wide variety grit sizes making it suitable for many different applications. Coarse sizes can be used for heavy duty stripping of paint, rust and other coatings off structural steel, concrete, brick and other structures. Fine sizes can be used (if careful) on thin metal surfaces such as automobile door panels without warping and even on wood surfaces without etching.

With recent performance and environmental approvals from the military and government agencies, Crushed Glass Grit is increasingly being used as a replacement for coal slag and other 'dirty' media. As Glass contains no heavy metals or free silica this media is showing increasing versatility as an economical, consumable blasting media.

Stainless Steel Deburring and Polishing

Question: I modify stainless steel flatware and dinner service pieces for other uses. In the process of modifying I use heat to bend, cutting wheels, grinding wheels, drilling and some sawing. The pieces are blued from the heat and have grinding marks, cutting marks and holes with burrs. I am presently de-burring and removing grinding and cutting marks by hand. After cleaning up the pieces I polish them to a high shine by hand. I have recently obtained a vibrating tumbler. What suggested media can be used to do the clean up and the polishing? How long would it take to tumble in these processes?

Response: The surface roughness of your parts will determine what media to use. Typically the first step would use a preformed ceramic aggressive cut media (the larger the better) for material removal. The second step would be a general purpose ceramic. Both operations should be done with a nonferrous deburring liquid compound diluted to a proper user strength. The operation should be wet with minimal water or suds. Cycle time are typically 1-4 hours for these deburring steps.

For a bright finish a precision ceramic polishing or steel media can be used. A compound with good lubricity is recommended for this third step. Processing time may be 4 hours with steel media and up to 8 hrs with precision ceramic. For a higher polish and smoother finish, the parts can be tumbled in a pretreated walnut shell grit for 8 to 24 hours for a jewelry quality finish.

Abrasive Blasting Media - Dry Ice

Dry ice (frozen carbon dioxide) blasting is similar in many respects to other types of blasting. The dry ice pellets are blasted at a surface to remove surface contamination, coatings, etc. This method is ideal for use with electronics, food processing equipment and areas where moisture and/or grit contamination can not be tolerated.

The Dry ice blasting process is not completely understood. The dry ice sublimates immediately after blasting. The process seems to involve 'attacking' the surface contamination with a combination of kinetic energy transfer and microscopic thermal variations at the blasting point. Because of the non-abrasiveness of carbon dioxide very sensitive surfaces can be blasted with little or no affect on the substrate. Due to the sublimation of the media the only residue to clean up is the material blasted off the surface.

Because specialty equipment is required to keep the dry ice cold and the obvious non-reusability of the media, dry ice blasting is an expensive and specialty blasting process. Care should also be taken to make sure that proper air flow is maintained in any enclosed environments as carbon dioxide can build up over time and replace oxygen in the area.

Decorative Black Carbon Removal

After molding these decorative aluminum plates are treated with a black carbon/oxide that then needs to be removed to create a striking, shadow effect.

Before tumbling

After tumbling

While it might seem that an abrasive media with lots of corners (like a triangle or angle cut cylinder) would be best to remove the carbon, this part was vibratory tumbled with Precision Ceramic Spheres and Kramco 1030 to remove the black coating from the high spots. This combination of round media and mild acid cleaner significantly enhanced the 3-dimensional effect desired by the customer.

Abrasive Blasting Media - Soda

Soda blasting is specialty blasting process that is ideal for applications where damage to the surface is unacceptable. This process was originally developed for blasting the Statue of Liberty. Soda (or baking soda or sodium bicarbonate) blasting is a versatile process used to remove paint, oils/grease, rust, paint and other surface contaminants. Soda will not damage glass or create heat during the blasting process and cause warping of thin metals. If desired and local regulations allow, soda can be washed away after blasting due to it's water solubility.

Since soda is hydroscopic (a 'love' for water) it will have a tendency to clump and flow improperly. To minimize this issue, special equipment is required that is dedicated for soda blasting. Also, specialty soda formulas for blasting (not the standard stuff in your refrigerator) have been developed to improve the media flow properties.

Unlike other dry abrasive blast media, if the surface being blasted is sensitive to chemicals or the surface will be coated after the blasting a rinse with water or a water/vinegar solution will be required to neutralize the soda.

Abrasive Blasting Media - Plastic

Plastic Abrasive Blast Media (PABM) are considered specialty media that are typically used in very specific applications. There are three primary types of PABM that are differentiated by their hardness; although, all of these media are considered softer and less aggressive than more popular blast media. PABM are manufactured to meet the military specification MIL-P-85891-A which details the different types.

Urea (Type II) is the most popular plastic abrasive media. Urea is typically used for auto paint stripping. Urea is a soft media {in between Melamine (Hardest) and Acrylic (Softest)}. Urea media can be used for mold and tool die cleaning/stripping.

Melamine (Type III) is typically used when a technician is not as concerned about the surface finish of the substrate relative to blasting cycle time. Melamine offers the quickest strip rate of all three PABM. Melamine media also breaks down the quickest of the three.

Acrylic (Type V) is the least used of all three medias. Acrylic is a truly specialized media and is used for applications like fiberglass, boats, helicopters and aircraft. Acrylic media will last the longest of all three medias.

Fluted Wood Columns

Question: I am needing to strip 80 years worth of paint from some fluted wood columns. The columns are in good condition and worth saving if I can remove the paint effectively. What media and grading do you recommend? How much media do I need?

Response: Corn cob grit will be the best option to protect the wood from being etched or 'raising' the grain. If the paint is too difficult to strip using this media then a Fine Walnut Shell Grit will be a better option taking care to protect the wood surface. Depending on how difficult it is to strip the paint I would conservatively suggest 2-3 pounds per square foot will be required.

Slightly edited from the Shop Talk Forum.