Anchor patterns are extremely difficult to approximate due to the many variables at play. Use this information with Caution! This is only an approximation.
The type of metal use in this example was hot rolled steel with tightly adhering mil scale. The type and hardness of your substrate, how thick the scale is, the degree of cleaning you are attempting to achieve, experience and skill of the person performing the sandblasting, etc will all factor into your end result.
In this example, the pressure blasting was done using a 90-100 psi nozzle pressure. The depth of the anchor profiles listed below are approximations only, and are on average.
1 Mil Profile:
1.5 Mil Profile:
2 Mil Profile:
2.5 Mil Profile:
3 Mil Profile
What Is Friablity? Short Answer: How easily/quicky a media shatters on impact and breaks down. Think of Friability as durability. This is different from Hardness Rating. Sandblasting medias may have the same Hardness rating, but completely different Friability Ratings. One will break down fast, the other will remain intact and can be recycled multiple times.
Brown Fused Aluminum Oxides have a LOW friability, so it will break down slowly, and coarser sizes can be recycled multiple times.
White Fused Aluminum Oxides have a HIGH friabilty, but the SAME HARDNESS rating of Brown Fused Aluminum Oxides. Even though it is just as hard as Brown Fused AO, it will shatter on impact and break down quickly, without doing as much damage/impact to the substrate. This is one reason why it is often used for delicate glass etching, and why it makes a nice final polish for rock tumbling. Its high hardness rating however, makes it appropriate for anti skid flooring applications, traction, etc.
Sinterblast, or Sintered Aluminum Oxide has a MEDIUM friability. Think of it as the middle ground between Brown Fused and White Fused. It can be recycled in coarse sizes, but will not have the long life (durability) of a Brown Fused Aluminum Oxide. It is often preferred by Powdercoating companies due to its lower cost. If you are just taking off mil scale prior to coating, this is a great media to try out. It is a lower purity aluminum oxide, and cheaper in cost to manufacture due to the sintering process being less expensive than the fusing process. THE MEDIUM FRIABILITY OF THIS TYPE OF ALUMINUM OXIDE IS WHY YOU BLAST AT A 60 PSI, AND NOT THE USUAL 80-90 PSI OF BROWN FUSED ALUMINUM OXIDE. ANYTHING HIGHER AND YOU WILL NOT ACHIEVE GOOD RESULTS BECAUSE THE MEDIA WILL SHATTER UPON ITSELF IN THE AIR STREAM BEFORE GETTING TO YOUR SUBSTRATE.
When choosing an abrasive to use, make sure you take into consideration its friability, not just hardness rating. Our product descriptions of our sandblasting abrasives include both hardness ratings and friability ratings.
What is anchor pattern? It is a term used to describe the roughness (etch, or profile) that is created on a surface when sandblasted. The Anchor Pattern is measured in 1/1000th of an inch, which is called a MIL. Paint coatings are also measured in MILS as well, making it a term that is easy to remember.
Coatings require specific mils of anchor pattern in order to properly adhere to the surface it is applied to. Each coating will have different requirements. Therefore, you cannot simply sandblast a surface clean and then apply paint to it without technical information on what mils profile that particular coating requires (among other factors). If you simply guess at what profile you need, you risk premature coatings failure.
If you have no access at all to the coating manufacture’s technical data and need to make an educated guess, you will want to consider the following rule of thumb:
The mils of anchor pattern you create on the surface to be coated, should be 25 to 30% of the DRY film thickness of the total coatings system BUT NEVER greater than the dry film thickness of the primer coat unless additional coats are to be applied immediately.
Also key to understand is that the wet mils of a coating will be different than the dry mils of a coating. Many coatings shrink tremendously, depending on the percentage of solids contained within them. Make sure that the anchor pattern you create when sandblasting is much shallower than the total coatings when dry.
Other factors that impact the development of anchor pattern include the type of steel you are blasting (including hardness and chemical composition), how the steel was formed, prior use of the steel, the type of abrasive you use to create the anchor pattern (including size, shape, hardness, and velocity), as well as what type of blast nozzle you choose to use and how you use it (such as angle and distance to the work surface).
The chart below is a crude approximation of abrasive size to anchor pattern, and should thus be used only as a starting point only for both centrifugal wheel and pressure blasting.
