English
Shot blasting (sand blasting) is a common surface treatment process prior to hot-dip galvanizing. It uses compressed air to propel shot or sand media to strike workpieces and remove contaminants such as welding slag, paint and rust. It is especially suitable for malleable iron castings and gray iron castings.
## I. Complete Shot Blasting (Sand Blasting) Treatment System
It consists of four major parts: compressed air and air distribution, shot blasting equipment, media recovery, and ventilation and dust removal.
1. Compressed air: Generated by air compressors, stabilized and purified through oil-water separators and air filters. The shot blasting pressure shall be ≤600kPa, and sand blasting pressure ranges from 200 to 300kPa. Water removers, oil removers, purifiers and air fresheners are equipped to eliminate water and oil and ensure operators breathe clean air.
2. Shot blasting main machine: Driven by compressed air, shot media is ejected through spray guns to impact workpieces. Fallen shot media is screened for recycling and reuse, while waste residues are collected.
3. Shot media recovery device: Divided into mechanical type (screw/scraper conveying, prone to wear and material jamming) and pneumatic type (no rotating parts, easy maintenance, commonly used in production). Iron shot can be recycled repeatedly.
4. Ventilation and dust removal system: Air inlets and exhaust outlets are installed in the blasting chamber to maintain negative pressure and prevent dust leakage. Multi-stage dust removal (sedimentation + cyclone + baghouse dust removal) is adopted, with silencers fitted to reduce fan noise.
## II. Classification of Shot Blasting (Sand Blasting) Equipment
1. Three categories by media conveying method
- Pressure type: High efficiency, suitable for medium and large workpieces; this type is exclusively used for cleaning inner pipe walls. It is divided into single-chamber and double-chamber types, with manual and remote control versions. The pneumatic remote control type uses compressed air to control air valves; abrasives are mixed with compressed air in the mixing chamber before ejection.
- Suction type: No pressure vessel for media storage, simple structure, suitable for small workpieces and intermittent production of large workpieces.
- Direct-flow type: Equipped with fixed spray guns, suitable for automated cleaning and rarely used in practical applications.
2. Spray guns
- Direct jet type: Composed of gun body and nozzle. Nozzles suffer wear from long-term impact and are made of highly wear-resistant materials such as tungsten carbide and ceramics. Nozzles must be replaced when wear exceeds the limit.
- Ejector type: Includes mixing chamber and compressed air nozzle; the nozzle position can be finely adjusted.
An air replenishment gap shall be reserved at elbows of sand delivery hoses to prevent sand blockage.
## III. Abrasives (Shot Media)
1. Types: Natural abrasives (silica sand, mineral sand), by-product abrasives (slag, copper slag), synthetic abrasives (alumina, iron shot/steel shot, plastics, glass beads, etc.). Iron shot/steel shot features good reusability and serves as the mainstream option in the hot-dip galvanizing industry. Silica sand generates heavy dust and breaks easily and is gradually phased out.
2. Selection criteria: Degree of workpiece rusting, plate thickness, surface roughness requirements and equipment type. It is recommended that abrasive grain size shall not exceed 1/3 of the spray gun nozzle diameter.
- Coarse grain size: Strong impact force and high capability to remove scale, yet resulting in high surface roughness.
- Fine grain size: More frequent impacts and lower surface roughness. Mixing coarse and fine particles delivers an optimal combination.
## IV. Key Factors Affecting Treatment Effect
1. Jet angle: For hard scale, an incident angle ≥70° is recommended (for hammering and peeling). For ordinary rust layers, an angle below 70° is adopted (for cutting and scouring). Impact at 90° causes rebound and counteraction, reducing rust removal efficiency.
2. Jet velocity and distance: Air resistance consumes the kinetic energy of abrasives. Kinetic energy decreases by approximately 10% for every 1-meter increase in distance. Scale cannot be effectively removed when velocity is lower than 50m/s. Shot blasting is suitable for close-range operation with fine abrasives.
3. Inner diameter and length of blast hose: Excessively small pipe diameter and overlong pipeline cause pressure loss, so blast hoses should be kept as short as possible.
## V. Quality Control of Shot Blasting (Sand Blasting)
Two indicators are used for quality evaluation: the removal degree of rust and contaminants, and workpiece surface roughness.
1. Surface roughness: Directly affected by abrasive grain size, material and jet parameters. It can be inspected with roughness testers or by comparison with standard reference blocks. Surface roughness directly affects the quality of the subsequent hot-dip galvanized coating.
2. Rust removal quality grades (GB/T 8923.1—2011)
Sa1 Light blast-cleaning, Sa2 Thorough blast-cleaning, Sa2½ Very thorough blast-cleaning, Sa3 Blast-cleaning to visually clean metal; there are also manual rust removal grades St2, St3 and flame cleaning F1. Evaluation is performed visually under good lighting by comparing with standard photographs.
3. Grade selection: Coating processes require high rust removal grades, while hot-dip galvanizing has lower requirements. Both technical performance and cost shall be considered during selection.