coating materials and application methods

DIP COATING

BY THOMAS C. JONES

HENKEL SURFACE TECHNOLOGIES, MADISON HEIGHTS, MICH.

Dip application of a protective coating involves simply immersing a workpieceinto a suitable tank containing the coating material, allowing the part to drainafter withdrawal, and force drying or baking the wet coating to achieve thefinish. Dip coatings are used in many industries for both primer and one-coatfinishes.Thorough cleaning of parts is essential prior to dipping. For optimum quality,a phosphate conversion coating is also recommended.

BENEFITS

Simplicity: Manpower and equipment requirements are minimal. The process iseasily automated.Low Cost: Paint utilization should be relatively high (e.g., greater than 90%transfer efficiency) on properly operated systems, since nonused paint (drainage)is mostly recovered and returned to the system.Ease of Control: Minimally skilled operators can maintain solids, viscosity, andother factors for acceptable application properties.Good Coverage: Except for air bubbles or pockets, all contact areas are coated.Close racking of parts is possible.Consistency: Similar parts receive coatings similar in appearance and filmthickness (i.e., the process is independent of the operator).

LIMITATIONS

Nonuniform Coatings: “Wedges” (thin films on upper surfaces, thicker on lowersurfaces) tend to form on vertical surfaces. Flow lines around holes or openingscan also occur. “Beads” on bottom edges are inherent defects, although properviscosity control can minimize this effect.Part Design and Hanging: Improperly racked parts can bucket paint, leading towaste and potential blistering in the puddled areas. Entrapped air pockets canprevent access of paint, with resultant bare areas. It may be necessary to designdrain/access holes into some workpieces to allow for immersion application.An attempt should be made to rack a part so that drainage occurs from a singlepoint. Oscillation during immersion can sometimes remove air pockets.Solvent Washing: Entrapped solvent during the curing process can resolubilizean already dried film, resulting in bare areas.Product Change: A change from one formulation to another requires eitherextensive cleaning and recharging of a single tank or the availability of multipledip tanks. Thoroughness of clean-out is especially important when switchingincompatible materials (e.g., replacing a solvent-borne system with a waterbornesystem).Flammability: The potential for fire is always present when solvent-borne dipprimers are used. With waterborne systems, this problem is greatly reduced.Foam: Undesirable foam, which usually originates in the paint recirculationsystem, can produce voids or craters in the final finish.This problem is moreprevalent with waterborne paints.Sticking: Small objects, such as fasteners processed in baskets or trays, canfuse together during cure. Processes, such as autodeposition or electrodeposition,that utilize water rinsing following the coating tank generally do notproduce this effect.Viscosity: Control is critical. High viscosity gives thick films and excessiveconsumption. Low viscosity produces thin films.

TYPICAL COATINGS USED

Selection of a coating system (i.e., resin type, pigment color) is directly relatedto the performance intended for the finished parts. Although any formulationwith the appropriate viscosity for acceptable transfer efficiency (i.e., greater than90%) can be used, properties of appearance, quality, cost, and other factors mustbe taken into account.There is a trend toward the use of waterborne formulations because theseare both fire resistant and ecologically desirable. Defoamers are often requiredto control foam in waterborne systems; however, silicone-containing materialsmust be avoided. Waterborne paints are often more aggressive toward equipmentthan solvent-borne formulations.

EQUIPMENT REQUIREMENTS

If high-volume throughputs are desired, a continuous conveyor for work transferis usually employed in contrast to a manual or programmed hoist. Circulatingpumps are required to maintain uniform viscosity and constant paint composition.The “bead” that characteristically forms on the lower most edge of a drainingpart is sometimes removed by ultrasound, by electrostatic detearing, or byair jets; the latter is generally the least expensive technique.A controlled withdrawal rate is useful in controlling coating thickness. Thisis more readily varied with a hoist system.

Tank/Agitation System

A tank lining chemically resistant to the coating employed is required. Epoxy-typecoatings with a 15-mil minimum thickness are commonly employed. The tank andpiping for circulation can be mild steel for solvent-borne coatings but should bestainless steel for waterborne systems.Tank volume is dependent on work package size and the transport system inuse, with conveyorized systems requiring longer tanks than a hoist system. Thework package should be submerged at least 3 in. below the surface and at least6 in. above any circulation piping at the bottom of the tank. Overflow weirs, toremove floating debris, are usually located at the entrance end of the tank and/or along the sides.Agitation can be either from an eductor positioned on the bottom of the tankbisecting the longest dimension, which tends to minimize parts falling from therack, or by eductors directing flow along the bottom of the tank, which results in acircular motion. Centrifugal- or turbine-type pumps are normally used for solventsystems; however, where shear sensitivity and/or foam are potential problems (aswith waterborne paints), double-diaphragm pumps, with an accompanying surgesuppressor, are recommended. As with other metal components, stainless steel isrecommended for wetted pump surfaces when waterborne systems are used.Pump wear can be reduced by a filter on the suction side. Self-cleaning filtersreduce maintenance times.In some installations, the dip tank is complemented by an additional collectiontank (for return of the drippings) and a circulation tank (for isolatedadditions of paint, solvent/water, and other additives). Paint flows from thecollection tank to the circulation tank (for adjustment) and then to the dip tank.Care must be taken not to introduce foam along the way. Covers (removable) onthe collection and circulating tanks elevate humidity and help to reduce foam.

Drip/Flash-Off Zone

The area following the dip paint tank allows the recovery of paint by means ofa pan that returns drippings to the tank. Depending on formulation, air-flowregulation and temperature control may be required. Some waterborne paintsrequire humidity control.Too rapid solvent loss will result in a rapid increase in coating viscosity on thesurface that tends to “fix” runs or sags, with a resultant decrease in the qualityof appearance.A minimum of 3 min is recommended for dwell time to allow 90% plus paintrecovery and enable optimum flow/leveling. In general, high air velocities shouldbe avoided.Some installations provide a controlled heat input, either from the cure zoneor from some other related source (e.g., pretreatment) to prepare the film forthe final cure.

Curing Zone

The time and temperature parameters for cure are dependent on the polymerand cross-linking polymer used. Although a forced-air convection oven is usedmost often, infrared technology has also been employed.Energy can be consumed by bottom entry and exit from the oven.The exhaust system should result in an oven under negative pressure comparedto the shop.Forced air can be used following the oven to facilitate cooling.

MAINTENANCE

Floating residues must be removed from the paint tank to prevent clinging ofthe material on withdrawn workpieces. This is usually accomplished by periodicskimming.Overhead conveyor systems require lubrication for maximum life. However,any lubricants used should be pretested as potential contaminants in the paintbecause some materials can cause cratering and other defects in the final product.Racks must be periodically stripped of dried paint. Either thermal stripping(molten salt bath or high-temperature oven), cryogenic stripping (exposure toliquid nitrogen, followed by physical removal of the embrittled paint), or mediablasting (sand, steel shot) can be used.Cleaning of the drain-off area must occur on a regular basis. Care must betaken to avoid getting dried paint into the circulation system, with resultant damage to filters, pumps, and nozzles.