Surface Roughness in Abrasive Blasting: Grit Size vs Profile
The critical relationship between abrasive grit size, air pressure, and surface profile depth. Essential for coatings, bonding, and corrosion protection.
1. Why Surface Profile Matters in Blasting
Unlike machining, where roughness is often minimized, abrasive blasting is usually performed to increase the surface area. This creates a "mechanical anchor" for coatings like paint, powder coat, or thermal spray. If the profile is too shallow, the coating will peel; if it is too deep, the peaks may poke through the coating and cause premature rusting.
2. Parameters: Roughness (Ra) vs Profile Depth (Rz)
In the blasting industry, the term "Surface Profile" is more common than "Roughness." Standard parameters include:
- Rz (Maximum Profile Depth): The distance from the highest peak to the lowest valley. This is the parameter most often specified on coating data sheets.
- Ra (Average Roughness): Sometimes used for general QC but less common than Rz in this field.
- Peak Count (Pc): The number of peaks per linear distance. A high peak count provides more "teeth" for the coating to grip.
3. Grit Size vs Achievable Profile
The choice of abrasive grit is the primary driver of profile depth. Below is a typical reference chart:
| Grit Type | Grit Size (Mesh) | Profile Depth Rz (µm) | Typical Application |
|---|---|---|---|
| Garnet | 30/60 | 50 - 75 | Heavy coating removal |
| Garnet | 80 | 35 - 50 | New steel preparation |
| Aluminum Oxide | 24 | 75 - 100 | Thermal spray prep |
| Aluminum Oxide | 60 | 40 - 60 | Standard powder coating |
| Glass Bead | Fine | 10 - 25 | Satin finish, no removal |
| Steel Shot | S-230 | 40 - 60 | Shot peening, cleaning |
4. Other Factors Influencing Profile
- Blast Pressure (PSI): Higher pressure increases the velocity of the grit, leading to deeper craters and a higher Rz.
- Blast Angle: A 90-degree angle (perpendicular) creates the deepest profile, while a shallow angle (30-45 degrees) creates more "tearing" and a different texture.
- Standoff Distance: Moving the nozzle closer increases the concentration of energy, but moving it too far can reduce the profile depth.
- Material Hardness: Blasting soft aluminum with the same grit and pressure as hard steel will yield a much rougher surface on the aluminum.
5. Measuring Blasted Surfaces
Measuring blasted surfaces is challenging because of the random texture.Replica Tape (Testex Tape) is the industry standard for field measurement. A piece of tape is rubbed onto the surface to create a physical "reverse copy" of the profile, which is then measured with a micrometer.