Resurfacing, Recoating, and Refinishing Services Information
Image credits: REM Surface Engineering | Products Finishing, Inc.
Resurfacing, recoating and refinishing services alter or restore damaged coatings or surfaces to a usable condition. Services involve various methods for washing, cleaning, refinishing, and recoating surfaces.
When a damaged or worn part or product needs surface repair, the manufacturer will contact the specialized shop with material and dimensional specifications required to correctly alter the part. Parts are delivered to the job shop where the appropriate procedure is done with minimal turnaround time.
When selecting a resurfacing, recoating, and refinishing company, it is most important to find one that can perform the needed services for the client's products.
Surface Refinishing and Treatment
There are a number of methods a company may use for surface refinishing and treatment of different materials.
Abrasive blasting services removes all visible rust, mill scale, paint and contaminants from a substrate's surface by the use of power spraying or tumble chamber. Sandblasting is a type of abrasive blasting used to create a roughened surface that improves the adherences of coatings.
Sandblasting. Image credit: Raf-Bud
Sandblasting operation. Video credit: blastitclean.co.uk / CC BY-SA 4.0
In the abrasive flow machining (AFM) process, a mixture of abrasive grain in a high viscosity carrier media is passed through the inner diameter or internal openings of the part. AFM processes are used to deburr, polish and generate controlled radius geometry in components. Orbital AFM processes are used for external finishing and geometry control. MicroAFM processes are used to radius, deburr and improve the surface finish of orifices in nozzles, fuel injectors, spray tips or other parts with very small or micro-sized holes.
Anodizing is an electrocoating process that involves the use of negatively charged particles to create a protective oxide coating. In the process, the target metal is submerged in a charged electrolytic solution and acts as the negatively charged anode. This creates a buildup of metal-oxide particles on the metal's surface. This oxide layer creates a more corrosion and wear resistant surface, and allows paint primers and glues to adhere easier than to bare metal. Anodizing is most commonly done to aluminum, but titanium, zinc, magnesium, niobium, and tantalum metals can also be anodized.
Electrocoating process. Image credit: Electrophoretic.com
Polishing, buffing, and belting processes are metal smoothing operations that change the surface appearance of the metal. These operations can be for aesthetic and/or functional purposes.
Some finishing services may refine a product's surface using one of a number of chemical processes. Isotropic superfinishing (ISF) machines utilize chemicals to accelerate material removal and finishing. Shops offering chemical finishing may provide a coating produced by chemical or electrochemical treatment of a metal surface. These functional coatings provide corrosion resistance and improved adhesion of primers or paints. Electropolishing is an electrochemical process which removes cosmetic surface flaws using a combination of rectified current and a blended chemical electrolyte bath.
Corona treatments are low temperature corona discharges which are applied to enhance bonding between a surface and polymer coatings or adhesives. Corona treatments can be applied to plastics, cloth or paper to enhance surface energies.
Deburring or radiusing processes round sharp edges or corners. Deflashing processes remove excess material (flash) or parting lines from forged, cast, or molded parts.
Electropolishing is often referred to as a "reverse plating" process. Electrochemical in nature, electropolishing uses a combination of rectified current and a blended chemical electrolyte bath to remove flaws from the surface of a metal part. The resultant surface is clean and bright.
Image credit: Master Finish
Honing and super finishing are precision finishing process for generation of very flat, smooth or low surface finishes. They can maintain much greater control over geometry or flatness compared to handheld buffers, offhand polishers and tumblers or mass finishing machines. These processes involve scrubbing parts with an abrasive stone to improve its geometric form.
Mass finishing processes are methods for bulk processing of parts. Tumbler, disc, drum or vibratory finishing equipment is used with abrasive media and compounds. The movement of the media and compounds against the parts imparts the desired surface finish.
Oxygen cleaning removes combustible oils, greases or other materials that could cause an accidental fire or explosion when in contact with oxygen. Oxygen cleaning may also involve the removal of excess rust. Oxygen cleaning is performed on factories with furnaces, autoclaves, aerospace facilities, refineries, pressure gauges, pressure transmitters, regulators, fittings, thermowells, gold mines, hospitals, tubing, pipe, hoses, filters, valves and other assemblies and plants.
Passivation removes "free iron" contamination left behind on the surface of stainless steel during machining and fabricating. These contaminants may drive premature corrosion and ultimately result in deterioration of the component if not removed. The passivation process also facilitates the formation of a thin, transparent oxide film that protects the stainless steel from corrosion and preserves the appearance of the metal.
In the shot peening process, smooth glass beads or metal shot are blasted at a surface. They impart compressive residual surface stress, which removes residual tensile surface stresses. Shot peening improves fatigue strength or performance under high number of load cycles. Shafts or other components exposed to high number of load cycles often require parts or materials with higher fatigue strength.
Pickling and chemical deburring processes use etchants, acids, acid pickles, to etch or chemically remove a layer of surface material or sharp edges.
Sanding and grinding processes area used for rough to fine surface finishing operations. Abrasive discs, belts or grinding wheels are used to remove surface irregularities, clean and/or produce the desired surface finish.
Recoating and Additive Processes
There are also a number of different recoating and additive processes performed by service companies.
Conversion coating includes any process involving chemical or electrochemical treatment of a metal surface. The coatings provide corrosion resistance and prepare a surface for better adhesion of primers and paints. These are not intended to be decorative coatings. Chemical finishes include black oxide, chromate conversion, and phosphate coatings.
Cladding is a process where another type of material is metallurgically bonded to a base metal, usually through a rolling or high pressure process. Usually the base metal is less costly, but less corrosion-resistant or wear-resistant. Stainless steel clad to carbon steel would be an example of a clad or bimetal material. Hardfacing is a process which welds materials with different properties to the substrate. Hardfacing processes are very useful for improving wear and corrosion resistance to selected areas of machinery, such as cutting edges of earth-moving machinery. Common hardfacing techniques include arc, torch, and other processes.
Plating is a process used to deposit a coating onto a metal or conductive surface using a metal salt solution of the metal coating to be deposited. Electroplating requires the application of an electric current, which turns the substrate into a cathode in order to attract positive metal ions to its surface. Electroless plating uses a chemical deposition process instead of an electrical current to deposit the metallic layer, and provides a more even plate layer, even on complex geometries. Nickel is the most common material deposited in electroless plating processes, but gold, silver, and copper can also be applied.
Sealing and impregnation coating processes use vacuum, pressure, vacuum-pressure, and/or wicking action techniques to drive resins or coating materials into parts to seal open porosity.
Vacuum-pressure impregnation (VPI) is a sealing and impregnation process that uses a combination of pressure and vacuum to drive the resin into the open porosity or surface cavity defects. VPI is used to seal porous castings, powder metal parts and transformer or motor windings. The sealing process improves integrity and the resistance of the parts to corrosive environmental elements.
In spray coating, the coating, sealant or resin is applied using a sprayer or atomizing applicator. Common applicators include pressurized nozzles or spray guns and rotary atomizers. An electrostatic charge can also be applied to reduce overspray.
Thermal spraying - A continuous coating by melting the consumable material into droplets and impinging these droplets on the substrate. The thickness of the coatings may range from 25 µm to 2.5 mm (.001 - .1 in). The thermal spray coatings compete with plating and paint coating for atmospheric corrosion resistance in water tanks, TV towers, bridges, and other large steel structures.
Powder coating - Powder coating is finishing technologies in which dry paint particles are electrostatically charged and applied to a grounded part. Electrostatic attraction holds powder particles on the part surface until heat is added to flow the powder together and cure it. Parts must be electrically conductive. Although powder coating is most widely used on metal parts, recent technological developments have broadened possible substrates to include glass, special plastics and wood.
Thin film coatings play a prominent role in the manufacturing of many electric devices. They are used to apply dopants and sealants to chips and other microelectronic parts. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are two most common types of thin film coating methods.
Physical vapor deposition involves the condensation of a vaporized film onto a workpiece surface using physical processes rather than chemical reaction. These processes include cathodic arc, electron beam, evaporative, pulsed laser, and sputter.
Chemical vapor deposition involves the reaction or decomposition of a precursor substance on a wafer or other substrate. This results in a deposited coating on the surface of the substrate. CVD processes are used to create semiconductors integrated circuits and solar cells, to coat certain metals (including as molybdenum, tantalum, titanium, nickel, and tungsten), and to produce synthetic diamonds.
Other coating methods include the following:
Dip/immersion coating - In dip coating or immersion coating processes, parts are dipped or immersed into a bath or tank filled with the liquid coating solution.
Galvanization - Galvanizing is the practice of immersing clean, oxide-free iron or steel into molten zinc in order to form a zinc coating that is metallurgically bonded to the iron or steel surface. The zinc coating protects the surface against corrosion by providing protection to the iron or steel in two ways. It shields the base metal from the atmosphere and because it is more electronegative than iron or steel, the zinc reacts with corroding agents first, providing a longer service lifetime for the part.
Painting - A coating is physically applied to the substrate via a brush or spray. Paint contains a vehicle which carries the solid and resin (film-forming) components of the coating, and evaporates to allow the coating to adhere to the applied surface. Paints can be used for decoration and/or protection.
Washing and Cleaning Processes
In addition to recoating and refinishing treatments, companies may also perform a number of surface washing and cleaning processes.
Burn-off or thermal systems use heat to remove any grease, oil, paint or other organic compounds from the surfaces of parts.
Degreasing uses a solvent or vapor to clean and remove organic compounds and greases that are insoluble in water or aqueous systems. Degreasing may also use a non-volatile cleaning agent that emulsifies oil or grease.
Spray washers use pressure washing or rinsing capabilities to direct a stream of water or a water / detergent mixture at high pressure to clean or remove surface debris (e.g. scale, rust, paint, dirt, grease, abrasive dust). Multiple jets with rotating heads are often used to cover a wider area as the surface is traversed or as parts are conveyed through the system.
In immersion tank cleaning systems or scrub tanks, parts are immersed in a tank where the cleaning bath is agitated with impellers or paddles, or the parts are scrubbed manually.
Stripping and coating removal services remove paint, coatings, plated layers using a solvent, chemical or mechanical process.
Ultrasonic cleaning processes use immersion tanks where the cleaning solution is vibrated at ultrasonic frequencies. Transducers are usually immersible or physically coupled in order to transfer their vibratory energy to the cleaning solution.
The material properties and capabilities of a client's products and parts are important to consider when selecting resurfacing services. These properties affect what type of resurfacing, recoating, and refinishing services can and should be performed on the surfaces of different parts. Treated materials include aluminum, copper alloys, iron, cast iron, nickel alloys, precious metals, steel alloys, stainless steel, or titanium. Non-metal substrates are typically made of ceramic, glass, plastic, polymer, or wood.
In addition to the types of services that coating service providers offer, the location of the company is often important to consider. In most applications, products need to be delivered from the client to the coating shop. Thus, depending on the quantity and size of the products, location can have a big effect on the total cost of the service and the total project time. Additional complications may arise when requesting services from companies located in different countries.