Powder Coating / Shot Blasting Plants

Material cost: Powder costs ₹150-400/kg (standard colors) to ₹600-1,500/kg (specialty metallics, textures). Transfer efficiency (powder adhering to part vs overspray) 50-70% for manual guns, 70-85% for automatic. Coating 1,000 sq.m/day at 80 µm thickness requires: 1,000 m² × 80 µm × 1.4 g/cm³ × (1/0.65 efficiency) = 175 kg powder/day. At ₹250/kg = ₹43,750/day. Overspray quantity: 35% overspray × 175 kg = 61 kg/day overspray. Without recovery: 61 kg/day wasted = ₹15,250/day × 300 days = ₹45.75 lakh/year! Plus disposal cost ₹20-40/kg = ₹3.6-7.3 lakh/year. With 98% recovery: Only 1.2 kg/day lost = ₹90,000/year waste. Savings: ₹45 lakh - ₹9 lakh = ₹36 lakh/year! Recovery system cost: Cyclone + cart ridge filter + fan = ₹8-15 lakh. Payback: 3-5 months. Additional benefits: (1) Environmental: Prevents powder release meeting <1 mg/m³ emission limits. (2) Workplace cleanliness: No powder accumulation on floors/equipment reducing housekeeping. (3) Quality: Prevents contamination—recovered powder filtered before reuse removing dirt/oils. Recovery challenges: Color contamination (white powder contaminated with 2% black = gray, unusable). Solution: Dedicated recovery per color or multi-color optimization (dark to light sequence minimizing change-overs). Powder degradation: Overspray powder heat-cycled in booth (from radiant heat of freshly coated parts) can partially cure reducing reactivity. Solution: Blend 70% recovered + 30% virgin maintaining quality.

Orange peel (texture): Powder doesn't flow smoothly forming dimpled surface like orange skin. Causes: (1) Oven temperature too low—powder melts but doesn't flow (increase 10-15°C). (2) Part temperature non-uniform—thin sections overheat, thick sections under-cure (improve oven circulation). (3) Film thickness excessive—thick powder insulates preventing heat penetration (reduce application, target 50-80 µm). Pinholing (small holes): Gas bubbles trapped during cure erupting through surface. Causes: (1) Surface contamination (oil, moisture) vaporizing during cure—trapped gas escapes creating holes (improve pretreatment, ensure dry surface). (2) Outgassing from substrate (MDF, castings with porosity releasing absorbed moisture/gas—preheat substrate or slow cure allowing gas escape). Cratering (fish-eyes): Circular defects from silicone or oil contamination. Cause: Silicone lubricant spray, oils from handling contaminating surface—powder won't wet, crawls away forming crater (eliminate silicone in facility, use lint-free gloves). Poor adhesion (flaking/peeling): Coating dewetting from substrate. Causes: (1) Inadequate conversion coating (chromate/phosphate)—electrostatic attraction alone isn't enough, chemical bond needed (verify pretreatment quality, test film weights). (2) Under-cure—insufficient crosslinking reduces bonding (increase temperature or time). (3) Contamination preventing bonding. Color variation: Causes: (1) Oven temperature variation (±10°C causes shade shift—tighten control to ±3°C). (2) Film thickness variation—thicker appears darker (control application, 60-80 µm target). (3) Powder batch variations (supplier quality—use certified powder). Air system contributions: Booth airflow uniformity preventing powder settling unevenly, oven circulation ensuring temperature uniformity, proper recovery preventing contamination from dirty powder recycling.

Principle: Charged powder particles attracted to grounded (earthed) part, sticking via electrostatic force until cured. Charging methods: (1) Corona charging (tribo-charging rare): Gun has electrode at -60 to -100 kV DC creating corona discharge (visible purple glow) ionizing air. Powder particles passing through corona field acquire negative charge (-). Part grounded (0V potential) attracting negative powder. Transfer efficiency: 65-85% (higher than liquid spray 30-50% because charged particles wrap around edges reaching recesses). Application process: (1) Fluidization: Powder in hopper fluidized by compressed air making it fluid-like. (2) Powder feed: Venturi eductor or dense-phase pump conveys powder through hose to gun (15-100 g/min). (3) Atomization: Powder released from gun nozzle as cloud. (4) Charging: Particles pass corona field acquiring -20 to -50 µC/g charge. (5) Attraction: Charged powder accelerates toward grounded part (electric field force 10-100× gravity). (6) Adhesion: Powder adhering via electrostatic force + van der Waals forces. Challenges: Faraday cage effect: Deep recesses, inside corners shielded from electric field—powder can't penetrate (solution: tribo guns generating charge by friction vs corona penetrate better). Back-ionization: Thick powder layer insulates part from ground reducing attraction—further powder repelled (limit to 50-100 µm per coat). Humidity: High humidity (>70%) reduces powder resistivity causing charge leak-off reducing adhesion (dehumidify booth air <60% RH). Grounding: Poor grounding (rusty hooks, paint on hangers) prevents charge transfer—powder won't stick (maintain clean electrical contact).

Powder coating cure is thermosetting chemical reaction (crosslinking) requiring precise time-temperature exposure. Cure chemistry: Powder contains resin + hardener + pigment + additives. At 160-200°C (depending on formulation): (1) Powder melts (glass transition 50-80°C). (2) Flows out wetting substrate and leveling (viscosity drops). (3) Hardener reacts with resin forming crosslinked network (polymerization—irreversible). (4) Viscosity increases as crosslinking proceeds. (5) Final hard, durable coating. Under-cure consequences: Insufficient crosslinking → soft, poor chemical/solvent resistance, poor adhesion, low hardness (fails pencil hardness test), reduced gloss. Product fails in field (scratches, stains). Over-cure consequences: Excessive heat → polymer degradation, yellowing (especially whites), brittleness, reduced gloss, outgassing/blistering. Temperature uniformity: ±5-10°C variation across oven causes: (1) Hot zones over-cure (yellowing). (2) Cold zones under-cure (soft film). = parts fail inspection despite "correct" setpoint. Temperature control requirements: Setpoint accuracy: ±3°C of target. Spatial uniformity: ±5°C across all zones measured without product. ±10°C with loaded product acceptable if doesn't cause defects. Air velocity: 200-400 FPM around part ensures convective heat transfer. Too high (>600 FPM) causes surface defects (powder blows before setting). Achieving control: (1) High-velocity recirculation: 8-15 ACM turbulent flow preventing stratification. (2) Multi-zone burners: 3-6 burner zones independently controlled trimming temperature across oven length. (3) Proper loading: Don't over-pack—blocks airflow. (4) Calibration: Regular thermocouple calibration (± accuracy degrades over time).

Pretreatment purpose: Prepare metal surface for maximum coating adhesion and corrosion protection. 3-stage vs 5-stage vs 7-stage wash: 3-stage: (1) Alkaline cleaner removing oil, grease. (2) City water rinse. (3) Conversion coating (phosphate/chromate/zirconium). 5-stage: Adds 2nd rinse after conversion + DI water final rinse reducing contamination. 7-stage: Adds pre-clean rinse + post-conversion seal improving quality. More stages = better but higher capital/operating cost. Alkaline cleaning: Hot (50-70°C) spray or immersion in alkaline detergent (pH 10-13) saponifying oils. Surfactants emulsifying fats. Typical 3-8 minutes. Concentration 2-10% by volume. Phosphating (most common): Iron or zinc phosphate conversion coating. Part immersed in phosphate solution (pH 2-4) for 2-5 minutes. Chemical reaction deposits crystalline phosphate layer 1-5 micron thick on metal. Benefits: Micro-rough surface improving mechanical adhesion, corrosion barrier (phosphate slows rust even if coating scratched), promotes organic coating wet-out. Typical crystal weight 1.5-3.5 g/m². Chromate (declining): Hexavalent chromium (Cr6+) conversion coating. Excellent corrosion protection but toxic, carcinogenic—EU RoHS restricted. Being replaced. Zirconium/titanium (modern): Environmentally friendly alternative forming zirconium oxide film. Thinner (<100 nm) vs phosphate but adequate adhesion, faster processing (30-90 seconds), no sludge (phosphate generates sludge disposal issue), extended bath life. DI rinse: Deionized water final rinse (<10 µS conductivity) preventing water spotting. City water leaves mineral deposits interfering with cure. Drying: Air blow-off + 10-15 minute oven dry (100-120°C) ensuring zero moisture before coating (moisture causes blistering). Air handling: Wash booth exhaust (500-2,000 CFM) removing chemical mists, dry-off oven fan circulating warm air.

Airflow patterns: (1) Downdraft: Air enters ceiling, flows downward through grated floor to collection plenum below. Advantages: Uniform velocity across height, operator not in airflow path (comfort), excellent powder capture. Disadvantages: Requires pit or elevated floor (construction cost), floor grating maintenance. (2) Crossdraft/backdraft: Airflows horizontally from front to rear exhaust wall. Advantages: Simpler construction (no floor pit), lower cost. Disadvantages: Operator in airflow (discomfort, cold), non-uniform velocity (varies with distance from exhaust). (3) Combination: Downdraft extraction + side supply achieving benefits of both. Velocity requirements: 80-150 FPM face velocity or average booth velocity capturing powder. Too low = powder escapes, deposits outside booth. Too high = excess air volume (energy waste), turbulence disturbing application. Booth sizing: Rule of thumb: CFM = Face opening area (sq.ft) × velocity (FPM). Example: 8×10 ft opening × 100 FPM = 8,000 CFM. Recovery system integration: Cyclone-first: 85-95% powder removed in cyclone (reusable), remaining 5-15% to cartridge filter (contaminated filter powder often discarded or sold cheap). Advantages: Low filter loading = long filter life (18-36 months), low pressure drop (80-120 mmWC), energy efficient. Filter-first (no cyclone): All powder to cartridge filter. Disadvantages: Rapid filter loading = frequent cleaning (pressure drop spikes), shorter filter life (6-12 months), energy penalty from high pressure drop, recovered powder mixed (harder to reuse if multi-color). Cartridge filter selection: Polyester media rated 70-90% initial efficiency, 99.5% after cake formation. Surface area: 40-60 m² per 10,000 CFM. Pulse-jet cleaning every 30-120 seconds maintaining <150 mmWC pressure drop. Energy efficiency: Booth + recovery fans consume 15-50 kW. VFD control reducing speed during idle (no coating) saves 40-60% vs constant speed. Payback <18 months.

Challenge: Different colors require cleaning to prevent contamination (2% black in white makes gray). Color change methods: (1) Manual cleaning: Blow out booth and recovery system with compressed air, vacuum accumulated powder, wipe surfaces. Time: 15-45 minutes depending on thoroughness. Labor-intensive. (2) Compressed air purge: High-volume air blast through guns, hoses, hoppers, cyclone, filters blowing out residual powder. Time: 5-15 minutes. Effective but generates powder cloud requiring containment. (3) Dedicated systems per color: Separate booth + recovery for each color (white, black, custom colors). Advantages: No change-over time, maximum uptime, perfect color purity. Disadvantages: High capital (₹15-35 lakh per booth), space requirement (need 2-6 booths), justifies only if high volume + limited colors. (4) Quick-change cartridges: Color-specific powder hoppers on quick-connects, swap entire hopper for color change. Hose/gun still require purge but faster. Production scheduling by color sequence: Light to dark: Coat whites first, then light colors, finish with dark minimizing cleaning. Trace white in black less visible than reverse. Example sequence: White → Beige → light gray → Red → Blue → Black. Allows faster purge vs random sequence. Batch production: Accumulate week's orders by color, run each color in campaign (Monday white, Tuesday black, etc.) minimizing changes from 10× daily to 1× weekly saving 15-30 minutes × 9 changes/day × 5 days = 11-23 hours saved. Economics: Single-booth shop spending 45 min/color change × 2 changes/day = 1.5 hrs = 19% of 8-hr shift lost to non-productive cleaning. Improved sequencing or second booth dramatically boosts throughput: 19% capacity recovery = ₹40-80 lakh/year additional revenue potential for medium shop. Justifies investment in second booth or better scheduling discipline.

Mil definition: 1 mil = 0.001 inch = 25.4 microns (µm). Powder coating typically applied 1.5-4.0 mils (40-100 µm). Thickness impact on performance: Too thin (<1.5 mil / <40 µm): Inadequate coverage, substrate shows through (especially on rough surfaces), pinholes from incomplete coverage, poor hiding of surface imperfections, reduced corrosion protection, fails salt spray testing. Optimal (2-3 mil / 50-75 µm): Good coverage, proper cure, excellent performance, cost-effective (minimizes powder usage while meeting specs). Too thick (>4 mil / >100 µm): Excessive powder consumption (+50-100% vs optimal = wasted material), sagging/runs on vertical surfaces (thick layer gravitates before ge gelation), outgassing/blistering (trapped volatiles can't escape through thick film), orange peel (excessive thickness prevents proper flow), longer cure required (thick film insulates—interior under-cures), edge coverage loss (thick buildup on flat peels at sharp edges from stress concentration). Thickness measurement: Wet mil gauge (not applicable): Powder is dry, thickness pre-cure ≠ post-cure. Destructive: Cut cross-section, measure under microscope. Accurate but destroys part. Non-destructive: (1) Magnetic gauge: For coating on ferrous metal. Magnet attracted to substrate, separation distance = coating thickness. Accuracy ±5 µm. Cost ₹15,000-60,000. (2) Eddy current gauge: For non-ferrous (aluminum). Measures via electromagnetic induction. Accuracy ±3-5 µm. Cost ₹25,000-1,00,000. Controlling thickness: (1) Application technique: Gun distance 6-12 inches, overlap 50%, consistent speed. Too close = thick, too far = thin/waste. (2) Powder output: Adjust gun powder feed (valve or pump speed). Lower for thin coats. (3) Multiple passes: Two light coats vs one heavy achieves better hiding + less sagging. (4) Automatic systems: Reciprocating guns with programmed speed/pattern achieving ±10 µm uniformity vs ±20-30 µm manual. Specification: Most powder coated products specify 2.0-3.0 mil (50-75 µm). Outdoor architectural (high corrosion environment) may require 3-4 mil. Always measure and document—coating too thin fails warranty, too thick wastes ₹lakhs/year in excess powder.