Blasting media
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Contents |
Introduction
Whether cleaning, peening, de-burring or polishing via some type of blasting process; you must use some type of media. The classic example of this would be to use "Silica sand" as the media to “Sand blast”.
Media Attributes
There are four main attributes that determine the performance of an abrasive. Those are Size, Bulk Density, Angularity, and Hardness. Additionaly each type of media produces a different surface profiles.
Size
Bulk Density
Bulk Density is usually measured in lbs per cubic ft. Media with a high bulk density can impart more energy upon impact at a given speed and size. If you are looking for a significant surface profile then, a media with a higher bulk density may work for you. While if you are looking for a low or no profile, then a low bulk density media would be a better choice.
Angularity
Angularity is a measurement of the shape of the media. Media with a high amount of angularity is rough and of jagged shape. The usually results in a faster cutting action. Low angularity media may be ball or similar shape. Angularity also affects the resulting surface profile.
Hardness
Hardness (usually measured in Mohs) can somewhat be equated to the cleaning rate of a media. The harder the media the faster it will clean.
Dust Level
As media is used, it creates varying levels of dust. For example Sodium Bicarbonate may create a large amount of dust as the media breaks up upon impact while Coal Slag may create very little dust. Silica Sand creates a dust that is very hazardous to your health.
Reuse
As media is used, it deteriorates. Some media such as Sodium Bicarbonate and Dry Ice are designed for one time us only. While others such as Glass Beads can be strained and reused. Eventually all media looses its effectiveness.
Surface Profile
One common measurement of all blasting media is the resulting "surface profile". The surface profile describes the resulting change to the surface caused by the blasting process with a specific media and technique. One way measure a profile is the depth (which usually ranges from 0 to 4+ mils). The other is the shape of the profile. For example a peening process may produce a smoother profile, while a course abrasive may product a very angular (rough) profile.
Media Types
| Type | Bulk Density (lbs/ft3) | Hardness (Moh) | Angularity | Surface Profile | Dust | Reuse | Note |
|---|---|---|---|---|---|---|---|
| Silica Sand | 100 | 5-6 | High | Rough | Medium | Yes | Silica Sand (plain sand) should generally not be used for media blasting. The major reason is that the resulting silica dust can cause Silicosis. Silicosis is a deadly disease and should be taken seriously. While sand (such as playground sand) may be cheap, other low or no silica media (such as Coal Slag) are available and are very cost effective. |
| Glass Bead | 100 | 6-7 | Low | Low | Medium | Yes | Glass spheres/beads can be used for cleaning, polishing or peening. This is a common media used in blast cabinets. Glass beads work well when used to clean aluminum parts. |
| Crushed Glass | 100 | 6-7 | High | ? | Medium | Yes | Crushed glass can be used as a substitute for Silica Sand |
| Metal Shot | 230-280 | 6-9 | Low | ? | None | Yes | Metal shot (Aluminum, Steel and Stainless Steel) is commonly used for peening or deburring. |
| Cut Metal Wire | 230-280 | 6-9 | Low | ? | None | Yes | |
| Metal Grit | 230-280 | 6-9 | High | High | None | Yes | |
| Coal Slag | 80-90 | 7-8 | High | Rough | Low | Yes | Sold under trade names such as "Black Beauty", "Black Diamond", "Black Magnum", this is a byproduct of coal fired boilers. It consists of an black amorphous glass like compound mostly made up of Silicon Dioxide and usually having less than 1% crystalline silica. It is a very low cost alternative to Silica Sand due to the very low silica content. |
| Silicon Carbide | 95-105 | 9-9.5 | As the name implies it is made up of either a black, blue or greenish compound that mostly consists of Silicon Carbide. | ||||
| Aluminum Oxide | 120-125 | 8-9 | High | Rough | ? | Low | Aluminum Oxide is a popular media. As the name implies it is made up of either a white or brown aluminum compound that mostly consists of Aluminum Oxide. It has a high Bulk Density, high hardness and angular shape. Because of this, the media is well suited for removal of heavy rust. However it will most likely result in a deeper surface profile. |
| Garnet | 130-145 | 7-8 | High | Rough | ? | ? | For a given quantity, Garnet is usually higher in cost than Aluminum Oxide. However, due to it's higher hardness and bulk density (which results in higher performance) it can be an econmical alternative to Aluminum Oxide. As previously noted, Garnet has a high Bulk Density, high hardness and angular shape. Because of this, the media is well suited for removal of heavy rust. However it will most likely result in a deeper surface profile. |
| Sodium Bicarbonate | 60 | 2-3 | High | None | High | No | AKA Baking Soda, Sodium Bicarbonate is a newer media that is popular due to it's ease of clean up as you can just wash it away with water. Dry Ice pellets has a low to medium Bulk Density, low Hardness and angular shape. Because of this, the media is not well suited for removal of heavily coated items or for rust removal. |
| Dry Ice | 50 | ? | High | None | High | No | Dry Ice is a newer media that is popular due to it's ease of clean up as it sublimates directly into gas. Dry Ice uses the traditional abrasive kinetic impact as well as thermal shock as part of the blasting process. Dry Ice pellets has a low to medium Bulk Density and angular shape. Because of this, the media is not well suited for removal of heavily coated items or for rust removal. |
| Walnut Shell Grit | 42-47 | 4 | High | None | ? | ? | This media is a soft grit made from Walnut Shells. Walnut shell grit has both a low Bulk Density, low Hardness and angular shape. Because of this, the media is not well suited for removal of heavily coated items or for rust removal. But it will work well when zero surface profile is required (such as with fiberglass). It also is absorptive so it works well when removing oil buildup. It works well when used to clean internal automotive parts. The idea being that if any of the soft abrasive material left is left behing it will break down with much less damage than a much harder media. |
| Corn Cob Grit | 35-45 | 2-4 | High | None | ? | ? | This media is a soft natural cellulose grit made from corn cobs. Corn cob grit has both a low Bulk Density, low Hardness and angular shape. Because of this, the media is noy well suited for removal of heavily coated items or for rust removal. But it will work well when zero surface profile is required (such as with fiberglass). It also is absortie so it works well when removing oil buildup. |
| Plastic Bead | 45-60 | 3-4 | Low | None | None | Yes | Used as a non-toxic and dust free media. It usually can be used in place of walnet shells. It works well for deflashing plastic and deburring metal. Used to strip paint or power coating from sheet metal and fiberglass. Plastic Beads has a low Bulk Density, low Hardness and low Angularity. Because of this, the media is not well suited for removal of heavily coated items or for rust removal. But it will work well when zero surface profile is required (such as with fiberglass) and when low dust is required. |
914 Relevance
Repair of various 914 items may require stripping of rust, paint and other coatings prior to repair or painting. Media blasting is a common way to accomplish this.
