1. Types of Abrasive Materials for Sand Blasting Machines

The sandblasting process for rust removal can utilize metal shot, metal cuttings, garnet, quartz sand, glass, slag, plastic, and other materials as abrasives. For specific applications, the type, hardness, density, size, and shape of the abrasive are the primary factors determining its performance. The classification of abrasives is shown in Figure 5-26.

Quartz sand is a commonly used non-metallic abrasive with sharp edges. When it is喷射 onto the surface of workpieces, it exhibits strong scraping action, delivering excellent rust removal and leaving a smooth, bright finish with reduced roughness. It is widely used in on-site construction, but it poses significant environmental pollution and poses considerable health hazards to workers.

(2) Copper sand is a byproduct from the smelting process, known for its low cost and high consumption rate. It is highly suitable for open-pit sand processing. To achieve optimal treatment results, copper sand with particle sizes ranging from 0.6 to 1.8mm is generally preferred.

(3) Metal abrasive is relatively inexpensive and contains less sand. It is widely used in sandblasting during the preprocessing of steel plates.

The steel shot cutting has minimal impact, thereby extending the lifespan of the equipment, although it results in a finer sanding roughness.

Steel shot cutting has high efficiency, low strength, minimal rebound, moderate roughness, but high cost.

Wire cutting has a significant cutting effect, but the surface roughness is too high.

2. Selection of Sandblasting Abrasive Materials

The optimal particle size for abrasives is below 1/3 of the nozzle diameter. The smaller the abrasive particle size, the finer the surface roughness achieved, and the wider the area it can act upon. Larger particle sizes have a greater impact force on the surface, resulting in weaker cleaning action, a rougher finish, and fewer impacts per unit time on the workpiece surface. Higher hardness of the abrasive leads to faster cleaning but has a shorter lifespan and greater consumption. After selecting an abrasive, its overall cleaning effectiveness is related to the impact force. Therefore, the ideal particle size should be a combination of large, medium, and small sizes with moderate hardness (approximately HRG 40-60). Larger particles are primarily used for breaking through harder oxide scales, while smaller particles are used for cleaning rust layers. The relationship between abrasive hardness and cleaning effectiveness is shown in Figure 5-27.

3. The Relationship Between Sandblasting Abrasive Material and Surface Roughness

Surface roughness refers to the microscopic geometric feature composed of smaller spacings and troughs on the machined surface. Roughness is categorized into roughness and average roughness (also known as average centerline roughness). Roughness is the vertical distance between any crest and the adjacent trough that is parallel to a straight line, while average roughness is the average vertical distance from each point on the surface to the centerline.

The surface roughness of workpieces is related to process parameters such as the particle size, shape, material, spray angle, distance, and action time of the abrasive. Among these, the particle size of the abrasive has a significant impact on the roughness (Table 5-12).

4. Sandblasting machine abrasive position

(1) The typical particle size distribution for the grit blast treatment is as follows: 0.4-0.8mm for finer surface roughness; 0.4-1.2mm for coarser surface roughness; 0.2-1.9mm for severely pitted old steel plates, requiring a high degree of roughness; and 1.2-1.9mm for new steel plates without pitting, also requiring a high degree of roughness.

For metal abrasives, the test can be conducted according to ISO 11126, whereas for non-metallic abrasives, the test should be carried out based on ISO 11127.

(2) After testing the pH value of 100g of abrasive powder, take 50g and mix it with 200mL of distilled water. Ensure thorough mixing. The pH value of the mixture, measured with an electronic pH meter with an accuracy of ±0.01, must not be below 6.2.

(3) Electrical Conductivity of Water-Soluble Salts

① Conductivity testing equipment: Conductivity meter, measuring range of 0-10000 s/cm; two glass test tubes, 400mL and 100mL; one measuring cup, 250mL; a stirring rod; distilled water; plastic examination gloves; plastic sample bags; a stopwatch; Quantab sodium chloride test strips.

② The test procedure requires the use of plastic disinfectant gloves to prevent contamination from sweat throughout the entire sampling, calibration, and testing process. Approximately 5g of sample is needed for each test. To ensure representative sampling, one can collect samples during the sand pouring or from the abrasive pile or bag. Before each test, the conductivity meter must be calibrated using calibration fluid. Pour the calibration fluid into a clean 100mL beaker, wash the probe inside. Discard the used calibration fluid and rinse with fresh calibration fluid once more. Then, add another 50mL of calibration fluid to the beaker, submerge the probe, turn on the conductivity meter, and adjust the knob to 1.0 ohm·cm. Before the formal test, rinse the beakers, measuring cups, funnels, stirrers, and probes. Take 200mL of the abrasive sample and pour it into a 400mL wide-mouth beaker, stirring for 1 minute until the water and abrasive are evenly mixed. Then, stir for an additional 8 minutes, let the water settle, and stir for another 1 minute. Ensure the total stirring time is about 10 minutes to allow the salt in the abrasive to fully dissolve in the water. Then, filter the solution soaked in the abrasive through filter paper into a 100mL beaker, discard the first 10mL of filtrate, and use the middle 50mL for testing. Submerge the probe in the solution, turn on the conductivity meter, and measure the conductivity. To ensure the accuracy of the measurement data, it is recommended to conduct several tests.