Shot blasting and sand blasting: a double-edged sword in metal surface treatment.
Shot blasting involves projecting high-speed shot (such as steel grit, aluminum grit and more.) onto the workpiece surface using compressed air or mechanical force. The impact and friction of the shot blasting agent remove dirt, oxide layers, and old coatings, while also improving surface quality. Sand blasting involves using compressed air to propel sand or other abrasives onto the workpiece surface. The impact and friction of the sand particles remove dirt, oxide layers, and old coatings, while also creating the desired surface effect.
In the field of metal surface treatment, shot blasting and sand blasting are two common processes. Both effectively clean workpiece surfaces, remove burrs, increase surface roughness, and enhance coating adhesion. However, these two techniques differ significantly in their principles, equipment, media, and application results, requiring a professional distinction from multiple perspectives.
Process Principles and Equipment Differences
The shot blasting process uses a high-speed rotating impeller to accelerate metal shot (such as steel or stainless steel shot) to a velocity of 60-110 m/s and eject it onto the workpiece surface. The core equipment is the shot blasting machine, which accelerates the shot through centrifugal force, providing a large processing area and high efficiency. Automotive wheel hub production lines utilize suspended shot blasting machines, capable of processing over 200 pieces per hour. Sandblasting, on the other hand, uses compressed air (0.4-0.8 MPa) to propel abrasives (such as quartz sand and brown corundum) through a nozzle at speeds typically between 30-50 m/s.
Comparison of Surface Treatment Media
The metal shot used in shot blasting typically has a diameter between 0.2 and 2.0 mm and a regular shape (spherical or cut) that can be recycled 5-15 times. This medium produces a uniform “cratering effect,” and the surface roughness Ra of castings treated with S330 steel shot can reach 6.3-12.5 μm. Sandblasting, on the other hand, uses irregular abrasives, ranging from 80-mesh quartz sand to 36-mesh brown corundum, which provide a more robust cutting action but also result in faster wear. In the treatment of aluminum alloy die-castings, white corundum sandblasting can achieve a surface with an Ra of 10-20μm, but the abrasive consumption is 3-5 times that of shot blasting.
Surface Topography Analysis
Electron microscopic observation reveals that the shot-blasted surface exhibits a uniformly concave, “orange peel”-like structure with regularly distributed peaks and troughs. The arithmetic mean deviation (Rz) of the surface profile after shot blasting is 25-40μm, and a compressive stress layer of 0.1-0.3mm is formed on the surface. The sandblasted surface exhibits random cutting marks, with large Rz fluctuations (30-60μm). Significant anisotropy was observed on titanium alloy specimens, with roughness deviations of up to 15% along the blasting direction.
Functional Comparison
1. Cleaning Capability: Sandblasting has a higher removal rate for scale and weld slag, making it particularly suitable for stainless steel welds. Quartz sand blasting can achieve a cleanliness level of Sa3 for 304 stainless steel welds, while shot blasting only achieves Sa2.5. 2. Strengthening Effect: Shot blasting has a greater impact energy, increasing the surface hardness of medium-carbon steel by 15-20 HRC and extending fatigue life by 3-5 times. Gear manufacturers using 0.8mm cast steel shot have seen a 40% increase in contact fatigue strength.
3. Substrate Impact: Sandblasting has a smaller thermal impact and is suitable for thin plates (<3mm). Experiments show that a 1mm thick aluminum plate deforms by up to 0.5mm after shot blasting, while sandblasting limits this to less than 0.1mm.
In actual production, the two processes are often used in combination. Of course, when selecting the right method, it’s important to consider over 20 parameters, including workpiece material (cast iron is more suitable for shot blasting), thickness (shot blasting should be used with caution for thin parts), and subsequent processes (sandblasting is often used before electroplating). Orthogonal testing can be used to determine the optimal solution if necessary.
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