Abrasive Blast Rooms

Abrasive blast rooms offer a lot of advantages when blasting large parts and when using a re-usable media such as Aluminum Oxide or Steel Shot. These rooms provide an area that can collect the media, send it to a recycling and separating system and then be re-blasted. Blast rooms can also be equipped with exhaust and dust collection systems. The major drawback is the cost typically in excess of $100,000.

An alternative to a blast room is to perform the abrasive blasting in either an outdoor area that is 'closed' in or to blast in a separate room such as a garage or shipping container. These areas can be set up to contain the media and dust but will limit the media choices to consumable, less expensive blasting media such as Crushed Glass Grit.

In order to use a re-usable media, a system such as the Blast Room Package is needed to collect, separate, clean and blast the media. Simply sweeping the used media, dust, dirt, paint chips, etc. into a feed hopper will transfer the recycled material to the abrasive separator, clean out large pieces, remove the dust and send the good media to the pressure pot for re-blasting. The Blast Room Package can simulate nearly all the benefits of a full-fledged blast room but at a fraction of the cost.

Steel Shot & Grit Hardness

As a follow-up to the SAE specifications to identify the different types of shot peening media types, there are also specifications that specify the hardness of the Steel Shot or Steel Grit. Hardness is reported in the Rockwell 'C' range and is stated as Rc or HRC with higher numbers indicating a harder grade. SAE specifies that "ninety percent of random hardness checks performed on a representative sample shall fall within the ranges" and that the standard deviation allowed is +/- 3 HRC.

The typical ranges are as follows:
40 to 51 HRC (KGP)
47 to 56 HRC (KGB)
54 to 61 HRC (KGL)
61+ HRC (KGH)

Generally, the softer grades will round over more quickly making it better for removal of oxides and cleaning. Harder grades will maintain the angular shape (with Steel Grit) and are best for stripping and etching applications.

Recyclable Abrasive Blast Media

Many types of abrasive blast media can be re-used multiple times. The rate at which the media fractures during blasting will determine the recyclability. Media such as Steel Shot have an extremely high re-use rate while more consumable media such as Crushed Glass Grit can only be blasted once or twice. This chart details the recyclability of many common abrasive blast media.

Some factors to consider when selecting a media (as it relates to re-usability):

• Blasting system: Blasters that automatically collect the media (i.e., cabinets, blast rooms) make it much easier to use recyclable media.
• Cost: Typically, re-usable media are more expensive to purchase; however, the re-usability can make these media very economical.
• Surface finish: The hardness and aggressiveness of the media is not directly related to re-usability. Consider the surface finish required in combination with the recyclability.
• Cleanliness: Collected used media can also include contaminants that can reduce efficiency (i.e., dust particles), impact blasting performance (i.e., paint chips blocking the nozzle) or even damage the surface (i.e., concrete chips or small stones). Be sure to screen and clean the media as best as possible.

Shot Peening Specifications

The Society for Automotive Engineers (now SAE International) has created technical standards for all kinds of engineering and products and processes. AMS 2431 (AMS is Aerospace Material Specification but is not used only in aerospace applications) is the specification related to the general requirements for peening media. In addition to noting the applicable ASTM testing specs, there are 8 different sub-specifications (separate SAE documents) used to identify and define each of the individual media types including:

AMS2341/1 - Cast Steel Shot (45-52 HRC)
AMS2341/2 - Cast Steel Shot (55-62 HRC)
AMS2341/3 - Carbon Steel Cut Wire Shot
AMS2341/4 - Stainless Steel Cut Wire Shot
AMS2341/5 - Peening Balls
AMS2341/6 - Glass Shot (Glass Bead)
AMS2341/7 - Ceramic Shot
AMS2341/8 - Carbon Steel Cut Wire Shot, High Hardness

Each individual sub-spec details technical requirements such as chemical composition, hardness, shape, size among others and the test methods to determine if the shot being used is 'standard'. Using shot peening media that meets these specs ensures consistent and quality results from the media in whatever blasting process is being used.

Blasting with Steel Shot

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.

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 Blast Polishing?

I wish there was a way to polish metal, plastic and wood surfaces using abrasive blasting. Unfortunately, using abrasive media (no matter how soft or round) just doesn't produce a polished finish. Glass Bead will result in a uniform, satin-like finish on some metals and Steel Shot can 'almost polish' hardened steel surfaces.

So far, we have not identified a media and process to consistently create a high polish using a blasting process - especially on plastics and wood.

Have you had any success in this area?

All-Time High for Steel

In a February post I explained the origins of the steel scrap surcharge. Basically, the base price of steel shot and steel grit is fixed but a surcharge is added (and changed each month) to bring the price of steel to current market value.

Well, the demand for steel in both Asia and Eastern Europe continues to grow and much of the scrap is being sent overseas to satisfy this demand. Obviously, increasing demand leads to increased pricing and even product shortages.

The current scrap surcharge is up about 500% in the past 5 years and has nearly doubled in the past six months. In addition, the lead times are continuing to extend for certain sizes and grades. Unfortunately, it does not look like this is going to be a short term problem unless steel manufacturers can figure out a way to make a lot more steel.

We're at an all-time high for steel prices but I fear we will continue to say the same thing many times in the near future.