What Are Sandblasting and Shot Peening Pretreatment for Powder Coating?

Proper surface preparation is the backbone of any successful powder coating process. Without it, even the best coatings can fail prematurely. That’s why sandblasting and shot peening are so crucial—they clean and roughen the surface, making sure the coating sticks and lasts. So what are sandblasting and shot peening?

Sandblasting or shot peening involves using an injection-type spray gun that utilizes compressed air to direct sand, steel shots, or other abrasive materials through a nozzle onto the surface of the object being coated. The intense impact of the sand or steel shots removes rust spots, oxidation layers, and reveals the true metallic luster of the object, while simultaneously creating a rough surface that significantly improves the coating’s adhesion.

But what exactly do these processes involve, and why are they so effective? Let’s dive into the details to understand how they contribute to a superior powder coating finish.

Advantages of Sandblasting or Shot Peening

1. Wide Applicability: It can be used not only on steel surfaces but also widely applied to non-ferrous metals and alloys such as aluminum, copper, and zinc.
2. Enhanced Adhesion: After sandblasting or shot peening, the surface area of the object increases, and the roughened texture significantly improves the adhesion of the powder coating, extending the coating’s lifespan, making it more effective than other methods.
3. Thorough Rust and Oxidation Removal: This process effectively removes rust spots and oxidation layers. Additionally, if the object’s surface is not very smooth, sandblasting or shot peening can noticeably improve its flatness.

The Process of Sandblasting or Shot Peening

1. High-Temperature Degreasing
   Although sandblasting or shot peening can remove some of the surface oil from the object, it cannot remove oil residues in crevices and gaps. High-temperature treatment is a simple and feasible method for removing oil from these areas. The treatment temperature ranges from 250-300°C, with a duration of 2-4 hours, though heavily soiled items may require up to 8 hours.
2. Sandblasting or Shot Peening
   In powder coating, sandblasting or shot peening is generally performed using the dry method. Dry sandblasting or shot peening equipment typically includes a spray gun, blasting chamber, air compressor, oil-water separator, and exhaust fan. The power source for dry sandblasting (or shot peening) is compressed air, typically supplied by a 40kW, 0.9MPa air compressor. The heated air generated by the compressor is cooled and sent to an air storage tank. Once the pressure reaches a certain level, the safety valve automatically opens, and the air is sent to the oil-water separator for purification before reaching the spray gun. The oil-water separator removes oil and moisture from the air, ensuring that relatively clean air is used as the power source for sandblasting (or shot peening). The compressed air pressure at the spray gun usually does not exceed 0.6MPa. The blasting chamber is relatively simple in structure, resembling a sealed box. The sand feed pipe connects to a sand hopper at the bottom of the chamber, and a foot switch controls the compressed air. Since operators work outside the chamber, they are less exposed to silica dust or iron sand dust hazards.The pressure of the compressed air during sandblasting depends on the size of the nozzle; larger nozzles require higher air pressure. The abrasive media used in sandblasting or shot peening can include quartz sand, emery, bauxite, yellow sand, river sand, glass beads, iron sand, and steel shots, with particle sizes typically ranging from 1-1.5mm. The type of abrasive media should be chosen based on the hardness of the surface being treated—harder surfaces require materials like quartz sand or emery, while softer surfaces may be treated with yellow sand or river sand.
3. Removing Residual Sand from the Object
   After sandblasting or shot peening, the surface of the object often adheres to sand particles or dust, which must be cleaned off to prevent compromising the appearance and adhesion of the coating. Typically, compressed air or a handheld dust blower is used to remove residual sand and dust, ensuring the quality of the coated product.
4. Keeping the Object Dry
   The object should be kept dry after sandblasting and stored in a clean, dry place. Powder coating should be applied as soon as possible to avoid oxidation and rusting of the blasted surface. Ideally, powder coating should be applied within 24 hours of sandblasting.

Precautions During Sandblasting and Shot Peening

• Compressed air must be cooled and purified to avoid contaminating the object being coated.
• The blasting chamber must be sealed, and dust collection equipment should be installed in the workshop to prevent abrasive media from escaping, which could harm human health or cause environmental pollution.
• The spray gun nozzles are prone to wear, so to maintain stable pressure during sandblasting, the nozzles should be replaced regularly.
• After sandblasting or shot peening, the treated object must remain clean, and should not be touched with bare hands to avoid contamination from sweat or oil, which could ultimately affect the coating quality.

Conclusion

In summary, sandblasting and shot peening are indispensable steps for ensuring the quality of powder coating. They effectively clean the surface, enhance adhesion, and lay a solid foundation for the coating’s durability.