Types of Metal Casting Surface Finish

Foundries smooth cast parts with finishes such as as-cast, shot-blasted, tumble-deburr, machined, ground, polished, bead-blasted, powder-coated, anodized, and electroplated. Each step lowers roughness—from 200-500 µin Ra in sand cast to ≤4 µin Ra after precision polishing—while adding corrosion or cosmetic benefits and improving dimensional fidelity.

Mechanical Surface Modification Processes

Abrasive Blasting Techniques

• Shot Blasting and Shot Peening: Shot blasting propels steel shot or grit at high velocity against the casting surface to remove scale, rust, and contaminants. The process creates a uniform matte finish with typical surface roughness values between 125-250 microinches Ra. Shot peening uses the same equipment but focuses on inducing compressive stress in the surface layer, which increases fatigue resistance by up to 300%.
• Sandblasting: Sandblasting uses compressed air to spray abrasive particles like aluminum oxide or silicon carbide onto the casting. This technique removes surface defects and creates texture for better paint adhesion. The resulting casting surface finish ranges from 50-125 microinches Ra.

Surface Refinement and Finishing

• Grinding and Lapping: Grinding removes material using rotating abrasive wheels to achieve dimensional accuracy within 0.0001 inches. The process eliminates casting imperfections like parting lines and gates. Lapping takes precision further by using fine abrasive slurries between the workpiece and a lapping plate, achieving surface finishes as smooth as 1 microinch Ra.
• Polishing and Buffing: Polishing uses progressively finer abrasives to create mirror-like finishes on castings. The process typically starts with 220-grit compounds and progresses to 1200-grit or finer. Buffing follows polishing and uses soft cloth wheels with polishing compounds to achieve reflectivity levels above 90%.
• Vibratory Finishing and Tumbling: Vibratory finishing places castings in a vibrating tub with ceramic or plastic media and chemical compounds. The continuous motion deburrs edges and smooths surfaces uniformly. Tumbling rotates parts in a barrel with similar media, ideal for batch processing of smaller castings under 6 inches in diameter.

Chemical and Electrochemical Surface Treatments

Conversion Coatings

• Phosphating: Phosphating creates a crystalline phosphate layer 0.0001-0.0005 inches thick on ferrous castings. The coating improves corrosion resistance and provides an excellent base for paint adhesion.
• Chromate Conversion Coatings (Chromating/Alochroming): Chromate conversion produces a thin chromium oxide layer on aluminum and zinc castings. The golden or clear coating thickness measures 0.00001-0.00004 inches.
• Black Oxide Coating: Black oxide forms a magnetite (Fe3O4) layer on ferrous castings through immersion in alkaline salt solutions at 285°F. The coating adds only 0.000001 inches to dimensions while providing mild corrosion resistance.

Electrochemical Surface Enhancement

• Anodizing
  • Type I (Chromic Acid Anodizing) Type I anodizing creates coatings 0.00002-0.0002 inches thick on aluminum castings. The process operates at 70°F and produces minimal dimensional change.
  • Type II (Sulfuric Acid Anodizing) Type II represents the most common anodizing method, producing coatings 0.0002-0.001 inches thick.
  • Type III (Hardcoat Anodizing) Type III creates dense coatings 0.001-0.004 inches thick with hardness values reaching 60-70 Rockwell C.
• Passivation: Passivation removes free iron from stainless steel casting surfaces using nitric or citric acid solutions. The treatment enhances the natural chromium oxide layer without adding coating thickness.
• Electropolishing: Electropolishing removes a controlled layer of metal (0.0001-0.001 inches) through electrochemical dissolution. The process preferentially removes high points, reducing surface roughness by 50%. Electropolished stainless steel castings achieve Ra values below 10 microinches.

Metallic Coatings

• Hot-Dip Galvanizing: Hot-dip galvanizing immerses iron or steel castings in molten zinc at 840°F. The metallurgical bond creates coating thicknesses of 0.002-0.006 inches.
• Autocatalytic (Electroless) Plating: Electroless plating deposits metal through chemical reduction without electrical current. Nickel-phosphorus coatings achieve uniform thickness within ±10% even in blind holes and internal passages. The process produces hardness values of 48-52 Rockwell C as-plated, increasing to 68-70 Rc after heat treatment.

Plating and Metallic Deposition

• Electroplating (Galvanic Plating): Electroplating uses direct current to deposit metals like chrome, nickel, or zinc onto conductive castings. Chrome plating achieves thicknesses from 0.00001 inches (decorative) to 0.020 inches (hard chrome).
• Electroless Plating (Chemical Plating): Chemical plating deposits uniform coatings without electricity, relying on autocatalytic reactions. Electroless nickel provides excellent corrosion resistance and can be applied to non-conductive surfaces after proper activation. Coating rates typically reach 0.0005 inches per hour.
• Pickling and Chemical Etching: Pickling removes oxide scale and contaminants using acid solutions like hydrochloric or sulfuric acid. The process prepares casting surfaces for subsequent treatments by creating a chemically clean substrate. Chemical etching selectively removes material to create decorative patterns or improve paint adhesion.

Coatings and Platings

Organic and Polymer Coatings

• Liquid Painting: Liquid paint systems include primers, topcoats, and clear coats applied by spray, dip, or brush methods. Two-component epoxy paints achieve dry film thicknesses of 0.003-0.008 inches.
• Powder Coating: Powder coating electrostatically applies dry powder to castings before curing at 350-400°F. The process creates uniform coatings 0.002-0.005 inches thick without runs or sags.
• Electrophoretic Deposition (E-Coating): E-coating uses electrical current to deposit waterborne paint uniformly on complex casting geometries. The process achieves complete coverage in recessed areas with coating thicknesses controlled within ±0.0002 inches.

Case Hardening Processes

Carburizing

Carburizing introduces carbon into the surface of low-carbon steel castings at temperatures of 1650-1750°F. The process creates a hard case 0.020-0.060 inches deep with surface hardness reaching 58-63 Rockwell C. Gas carburizing offers precise control of carbon potential and case depth uniformity within ±0.005 inches.

Nitriding

Nitriding diffuses nitrogen into casting surfaces at 950-1050°F, below the transformation temperature of steel. The process produces extremely hard surfaces (up to 70 Rockwell C) with case depths of 0.004-0.030 inches. Nitrided surfaces exhibit minimal distortion since no quenching is required.