PLATINGPROCEDURES

BARREL PLATING

by Raymund SingletonSingleton Corp .• Cleveland

Barrel plating typically involves a rotating vessel that tumbles a contained, bulkworkload. The barrel is immersed, sequentially, in a series of chemical process tanks,including plating baths, while tumbling the workload. Utilizing interior cathode electricalcontacts to polarize the workload, dissolved metals are attracted out of solution onto theindividual workpieces. Effectively, the workload becomes part of the plating equipmentduring processing because the individual pieces function as bipolar electrical contacts to theother pieces in the workload. This bipolar contact is a significant contributor to the highefficiencies of barrel plating because the entire surface of the workload, in the current path atany time, is in cathode contact.

USES OF BARREL PLATING

Barrel plating is used most often for bulk finishing. It is the most efficient method forfinishing bulk parts and any pieces that do not require individual handling. According to the"Metal Finishing Industry Market Survey 1992-1993,"· there are approximately 6,750 platingfacilities in the United States. Of these, 37% exclusively provide barrel-plating services, andan additional 32% provide both barrel and rack plating; therefore, approximately 69% of allplating facilities employ the advantages of barrel plating in providing their services.

Plated finishes generally are usually used to deliver the following three functions (singlyor in combination): (1) corrosion protection, (2) decoration/appearance, and (3) engineeringfinishes (for wear surfaces or dimensional tolerances). Barrel plating is used most often forcorrosion protection. Because of the surface contact inherent in the tumbling action duringprocessing, barrels are not often used for decorative or engineering finishes.

AdvantagesAlong with the high efficiency already mentioned, the advantages of barrel plating are

many and interrelated:

I. The relatively large cathode contact area yields faster, larger volume production, inthe presence of ample current, when compared with rack-type plating.

'Copyright 1994 by the Metal Finishing Suppliers Association and National Association ofMetal Finishers. Used with permission.

346

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2. A barrel-plating system occupies less floor space and requires a lower investment forequipment than a rack- or other-type plating line of similar capacity.

3. Barrel plating is labor efficient because it is not necessary to handle, rack, load, orunload individual workpieces,

4. The work usually remains in the same vessel for other operations, including cleaning,electrocleaning, rinsing, pickling, chromating, or sealing. A more recent innovation inbarrel equipment is drying of the work while it remains in the barrel. The eliminationof handling and some work transfer enhances efficiency.

5. Barrel plating is very versatile because of the variety of parts that can be processedin the same equipment. It is the predominant method for finishing fasteners, metalstampings, and similar bulk work. It has been said that "if a part can fit through thedoor of a barrel, it can be barrel plated." This is an oversimplification. Most often, thepart configuration, end use, and finish type determine the applicability of barrelplating.

6. Conversely to barrel operations, rack plating often requires special part carriers orfixturing and other purpose-built equipment. This can include special contacts such asformed anodes based on the individual part type and shape. Barrel plating does notusually require these items, although there are special purpose contacts available.

7. Barrel rotation causes the workload to tumble in a cascading action. This, in additionto the bipolar electrical activity from individually contacting parts, usually producesa more uniform plated finish than rack plating.

8. Agitation of the tank solutions by barrel rotation inherently eliminates stratificationand produces homogeneous baths. Additional agitation equipment is usually notrequired, although certain tanks and operations are equipped with spargers (airagitation pipes).

OriginsBarrel-plating methods originated in the post-Civil War era, with equipment readily

adapted from available wooden barrels, kegs, or baskets. Equipment was constructed of woodbecause it was probably the most economical and available material that was not a conductorof electricity.

Subsequent advances in the knowledge of chemistry, electricity, and material sciencesenabled the evolution of barrel-type metal-finishing equipment for bulk finishing. Thisevolution culminated in the third or fourth decade of the Twentieth Century with now familiarbasic designs.

Today, the submerged portions of barrel-plating equipment are constructed, as much aspossible, of nonconductive, chemically inert materials that can be utilized in various acid andalkaline solutions. Great advances in plating-barrel performance, capability, and longevitywere the result of plastic materials newly available after World War II. Prior to that time,plating barrels were known to be constructed of more primitive plastic or phenolic materials.

EQUIPMENT TYPES

Available barrel equipment varies widely" but generally conforms to two majorconfigurations: (I) horizontal barrels and (2) oblique barrels. Horizontal units are the mostcommon, being adaptable to a greater variety and capacity of work (see Fig. I).

Horizontal barrels also vary by size and are grouped into three major categories: (I)production barrels, (2) portable barrels, and (c) miniature barrels.

Production barrels, the largest units, usually have a capacity in the range of 1.5 to 17 fe.They handle the majority of the work.

Portable barrel units are so named because of their generally smaller size (capacitiesrange from 0.1 to 1.5 ft3

) and their ability to be manually transferred from one operation to

348

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Fig. 1. Typical horizontal barrel and superstructure assembly showing inverted V-typecontacts.

the next, sometimes without the aid of an overhead hoist. Portable barrel units are used forplating smaller parts, smaller lots, delicate parts, and precious metals work (see Fig. 2).

Miniature, or minibarrel, units are used for many of the same reasons as portable barrels.Minibarrels range in capacity from 6 to 48 in.3 Minibarrels are used to process the smallestand most fragile loads and work. Also, miniature barrels are often used for lab work such asproduct or process development (see Fig. 3).

Whereas rotation about a horizontal or inclined axis is common to different types andstyles of barrel-plating equipment, there are many diverse construction features andcomponents available that enhance capabilities and improve versatility. Examples of thesebarrel features are as follows:

1. Cylinders with maximized load volumes (see Fig. I) within the dimensional clearancelimits of associated equipment

2. Special-diameter and/or special-length barrel assemblies for use in nonstandardizedinstallations such as rack tanks

3. High-eapacity electrical contacts (allowing plating operations with individual barrelassemblies handling as much as 1,400 A per station)

4. Automatic operation for handling, loading, and unloading to reduce labor require­ments (see Fig. 4)

5. In-the-barrel drying equipment to dry the work while it remains in the barrel, whichreduces parts transfer and handling operations

6. Up-rotation apparatus to minimize contamination and carryover (drag-out) of solutionto adjacent process tank stations

350

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Fig. 2. Portable barrel assembly with self-contained drive, dangler contacts, and clamp-styledoor.

7. Special apparatus to spray rinse work while it remains inside the barrel to reducewater usage and ensuing treatment costs.

The previous examples are representative. There are other barrel and system enhance­ments that increase production and reduce cycle times, drag-out, and maintenance require­ments. Optional equipment types are many, including the examples of barrel assembliesmanufactured to operate in existing rack-plating installations shown in Figures5 and 6.

Another type of production barrel is the horizontal oscillating barrel. These often utilizebarrels that are open on top and have no doors or clamps. The technique is to limit barrel

Fig. 3. Ministyle barrel assembly with self-contained drive and integral-mesh, moldedbaskets.

352

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motion to a back-and-forth (usually less than 180' of arc) rocking action about the horizontalaxis. rather than 360' full rotation. The motion is more gentle for very delicate parts and canbe a plus when treating parts that tend to nest. tangle. or bridge badly inside the barrel.Because agitation and tumbling are not as vigorous as full rotation. the plater must take careto avoid nonuniform plating (particularly for parts that tend to nest). Processing is generallylimited to smaller loads with these barrels to avoid spillage and loss because of the alwaysopen door.

Fig. 5. Barrel assembly equipped for use in a rack plating line.

354

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Oscillating barrels are not utilized as much as they were in the past. This is becauseplaters can use variable-speed drives to produce slower rotational speeds on full-rotationbarrels to obtain equivalent results. Many older oscillating barrel installations have beenconverted to full-rotation operation.

The second major barrel equipment style is the oblique barrel. It can be pictured as anopen-top basket that rotates around an axis tilted to a maximum 45' from the vertical.Capacity diminishes beyond a 4S' axis tilt.

The major feature of oblique barrels is the elimination of doors or other closure devices.Because the top is open, unloading consists of raising the barrel about a pivot at the top of itsrotational axis shaft to a position that dumps the workload. Similar to 180' horizontaloscillating barrels, this tesults in relatively small workloads and reduced tumbling action.Today, platers can take advantage of fully automatic doors on full-rotation horizontal barrelsto achieve the same advantage with greater ease and higher production.

FINISH TYPES

All common types of plating are done in barrels, including zinc (alkaline and acid invarious chemical systems), cadmium, tin, copper, precious metals (such as silver and gold),and nickel (both electrolytic and electroless). Barrels are used to plate chrome where amplecurrent and continuous contact are available (gentle abrasion of the part surface is not aproblem). One can infer from the previous example that a barrel's value and versatility dependon its capability to (I) plate a particular finish and (2) function properly in system solutionsand temperatures. This capability is determined by the materials, construction, and detailfeatures incorporated into the barrel unit.

Some barrel equipment lines have the capability to produce more than one plated metalor finish type. Most plating lines are dedicated to one finish type. Elimination of drag-out ina plating line that produces more than one finish type is a primary concern. Drag-out orcross-contamination of the different plated metals in stations used for rinsing, sealing,chromating, and cleaning can be minimized by incorporating an up-rotation sequence in thebarrel operation. Up-rotation is discussed in the section "Hoist systems, tanks, and ancillaryequipment."

Fig. 6. Special-length barrelassembly for platingelongated parts orfor use in a rackplatingline.

356

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WORKLOAD

The barrel plater needs to evaluate each of the following items to decide if the desiredfinish on a particular pan can be barrel plated: finish function (relative to use of the part), partconfiguration. part size. part weight, calculated part surface area, and total workload volumeand square foot surface area.

The workload capacity is usually 40 to 60% of the total interior barrel volume. Themaximum workload volume is usually determined based on total square foot surface area ofthe load and the capacity of the bath chemistry and electrical equipment to plate. Other factorsare the weight of the individual workpieces and their propensity to damage the finish orserviceability of other parts in the load. Damage of this type is usually the result of the weight,configuration, or edge characteristics of the parts as they tumble in the barrel.

As designated in the section about the uses of barrel plating, plated-finish functions areof three basic types: corrosion protection to increase the useful service life beyondperformance of the unplated base material; decoration for appearance. which also enhancesthe value of the base material; and engineering applications to attain (add material) ormaintain a dimensional requirement and/or as a bearing surface.

There are requirements for plated finishes that need to perform more than one of thepreviously mentioned three basic functions. Barrel plating is most commonly used to finishparts for corrosion protection. Decorative finishes are successfully barrel plated when surfaceeffects from part contact are controlled to an acceptable level. Engineering finishes are notusually applied by barrel plating.

Configuration of the workpieces affects the ability of work to be successfully barrelplated. Generally, parts that weigh less than I Ib each and are less than 25 in.' each in volumecan be barrel plated successfully. A simple shape is obviously easiest to barrel plate. Barrelplating is usually the most successful, cost-effective way to plate threaded pans and fastenersproperly. The tumbling action of the barrel makes and breaks the electrical contact throughoutthe workload, yielding the most even coverage on the root, mean diameter. and crest of thethreads.

Part material must not be adversely affected by any baths required in the totalplating-process cycle. A trial load is a useful tool for evaluating which barrel equipment andtechnique can be utilized for plating a particular pan.

Long workpieces and entangling parts, such as rods, bars, or tubes, can be successfullybarrel plated. Methods used to plate these parts include long barrels; longitudinal and radialcompartments; rocking motion; and various. special stationary contacts (see Fig. 4). Specialextra-length barrels allow long pans to fit, whereas compartmented barrels confine movementof long parts and entangling parts. helping to eliminate bridging or entanglement. Limitedbarrel oscillation or rocking motion (usually 180' of rotation or less) accomplishes the sametask by minimizing pan movement. To do this a reversing switch. or contactor, along with anadjustable control timer can be installed on the barrel drive to rotate altemately the cylinderin each direction.

The barrel interior can be equipped with stationary cathode contacts to plate small,delicate, or nesting parts (for example, small electronic components with projecting fingers).Stationary contacts rotate with the cylinder so that there is little relative movement betweenthe workpieces and the contacts. As a result, the work cascades over or around the stationarycontacts, and less abrasion or edge contact takes place, minimizing the potential for damageto the work (see Fig. 7). Disk, center bar, cup, strip, button, hairpin, and chain are some typesof stationary contact. Certain types of stationary contacts, such as strip contacts, assisttumbling of the work.

Parts that are flat or lightweight should be plated in barrels with uneven interior surfacesthat are not flat and smooth. A convoluted or uneven barrel interior surface, such as grooved.ribbed. or dimpled, promotes tumbling and eliminates much of the sticking of flat workpieces,

358

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Fig. 7. Barrel interior showing disk- and strip-type contacts.

When finishing recessed or cupped parts, other smaller parts, which are to be plated tothe same specification, may be mixed in with the load to provide contact into recessed areas;however, the cost of the time spent to separate the smaller parts from the others after platingmust be acceptable.

BARREL EQUIPMENT DESIGN

All designs of barrel equipment, including horizontal and oblique, should includefeatures to optimize productivity. Reduction of labor requirements and improved ease-of­maintenance are important factors for well-designed components and systems. Some of theseimportant features are discussed in the following sections.

Barrel ConstructionBarrels should be made of materials that are chemically and physically inert to use in

each bath or piece of equipment in the plating line. It is important that the barrels be capableof operation in excess of maximum bath temperatures in the entire system.

A plating barrel may expand and contract as much as 3/8 in. total in length due to thedifferent bath temperatures in a plating line. Changes in temperature cause stresses that canwork a barrel to pieces. This is particularly critical for barrels constructed of materials withdifferent coefficients of expansion. The effects of the temperature changes can be minimizedwith good design and quality construction. When barrels are fabricated of a single type ofplastic and joined by a plastic weld or fusion process, stress points are eliminated. Barrelsmade this way can expand and contract at a uniform rate, which greatly extends their usefulservice life. The use of metal fasteners for assembly is a less desirable method because ofstress points and the possibility of loosening. Minimizing the effects of temperature changespromotes barrel integrity and long life. The capability of a barrel to be used in highertemperature baths can, as an added benefit, aid faster plating.

Good equipment design will reduce maintenance and replacement part costs. Costs arereduced significantly when it is possible to replace individual wear parts and components.Wear parts that are manufactured as an integral piece of a larger components, to reducemanufacturing costs, should be avoided. Examples are (I) trunion hub-bearing surfacesmolded as a component of hanger-arm supports and (2) cylinder ring or bull gears that are alsothe barrel head. These perform the same as other equivalent parts when new, but when the

360

wear part needs to be replaced, the larger piece. of which the wear part is a component. mustbe replaced. This can be a very costly for the user.

Detail FeaturesFor the majority of plating, flat-sided barrels are best. Flat-sided barrels produce

pumping action as a benefit of rotation. Pumping action is the inherent agitation of the bathcaused by rotation of the flat-sided barrel. Round barrels do not produce pumping action asefficiently. Pumping action helps constantly replace metal-depleted solution from inside thebarrel with fresh solution from the rest of the bath.

Flat-sided barrels tumble parts more effectively. This tumbling is optimized when the flatinterior surfaces of the barrel are not smooth. They can be ribbed. grooved, or dimpled. Thevarious types of uneven surfaces also minimize sticking of parts to the panel surfaces. asmentioned previously. Additional tumbling ribs, cross bars, or load breakers of various typesare usually needed only for round-plating barrels. They can be added to flat-sided barrels fordifficult situations. Most oblique type barrels incorporate uneven, stepped, bottoms to producethese same effects.

PerforationsThe type of work being processed in a barrel must be considered when specifying the

perforation sizes. Job shops generally use barrels with smaller perforations to accommodatethe widest range of potential workpiece sizes. Captive shops often have the luxury of usingbarrels with larger holes because they can more easily predict their minimum part size. Largerperforations usually exhibit faster drainage, more efficient exchange of metal-depletedsolution, and less drag-out (carryout) contamination of adjacent tank solutions. This is becauselarger perforations minimize the negative effects of liquid surface tension.

Many shops maintain extra barrel assemblies that have the smallest perforation sizes thatwill be needed. In this way, the line can be operated. the majority of the time. using larger holebarrels. The smaller hole barrels are used only when necessary.

It is very important that all barrels used in a single production line have the same openarea ratio. regardless of perforation size. The open area ratio is defined as the total number ofholes in a barrel panel multiplied by the individual open area of each hole and divided by thetotal area that contains the included perforations,

Open Area Ratio = (No. of holes) X (Open area of each hole)!Total area for included perforations

For example, if you count 133 holes, 3/32 in. in diameter (0.0069 in.2 each), in a 4 in.2

area, the calculation would be as follows:

Open Area =133 X 0.0069/4 =0.23 or 23%

Interestingly, there is a convenient geometric relationship between hole size. centerdistance from hole to hole, and open area. When the distance between centers of givendiameter holes is twice the diameter of the holes (in a staggered center pattern that has sixholes equidistant all the way around). the open area ratio is 23%. Consequently. I/M in.diameter holes on 1/4 in. centers, V'b in. diameter holes on .y,. in. centers. and '/16 in. diameterholes on I/M in. centers are all 23% open area ratio patterns. Experience indicates the 23% openarea ratio optimizes barrel strength and plating performance.

Because the open area of any barrel determines the access of the plating current to theWork, the plating performance is directly related to the percentage of open area; therefore,barrels with the same open area ratio can be used in the same plating line regardless of holesize. Because the access of the plating current to the work will be the same, there is no needto readjust rectifier settings or current density. Most barrels are or should be manufacturedwith a 23% open area.

361

Fig. 8. Cross-section ofherringbone-style perforations to keep small-diameter, straight parts

inside barrel.

There are other types of barrel perforations available to the plater. These includeherringbone, screen, fine mesh, and slots. To produce herringbone perforations, the barrelpanels are drilled halfway through each panel at a 45" angle to the inside and outside panelfaces (see Fig. 8). In this way, the holes intersect at the middle of the panel in a 90' angle.Small-diameter, straight workpieces, such as nails, pins, etc., cannot pass through theperforations because the holes are not straight. Plating solution and current can pass throughthe perforations, although at a reduced rate.

Barrels with fine-mesh panels with very small openings are generally made ofpolypropylene and are used to plate very small or delicate work. Larger workpieces will tear,gouge, or wear through the mesh in an unusually short period of time.

Cathode Electrical ContactsThe type of interior cathode electrical contacts in a barrel significantly determines the

variety of work the barrel can process. Flexible-cable dangler-type contacts are the mostcommon in barrel plating (see Fig. 9). Dangler contacts are dynamic relative to the workloadbecause the workload rotates with the barrel and tumbles over the danglers. The danglersremain fixed to the barrel support assembly as this occurs. Other types of dynamic cathodecontacts are hairpin and chain.

The best plating results are achieved when the danglers remain submerged in theworkload. This is because submerged danglers maximize contact and minimize or eliminatearcing, sparking, or burning of the work. The contact knob end of each dangler should touchthe bottom of the barrel one-fourth to one-third of the inside barrel length from each barrelend. To determine proper dangler length, measure the total distance from the point that thedangler contact knob should touch the inside bottom of the barrel, continuing through thebarrel hub to the outside mounting point of the danglers. For short barrels or stiff danglercable, the danglers can be extended beyond the midpoint of the barrel to provide contact atthe opposite end of the barrel to insure that they remain submerged in the load.

Special dangler contact knobs have been developed to help maximize performance whena standard configuration is not totally adequate. Custom knobs that are heavier can bespecified to help ensure they remain submerged in the workload. Also, special knobs withlarger contact surface area are available where improved conductivity is important.

362

Fig. 9. Knob-style, two-section door with center bar and partition.

Danglers can be ordered with contact knobs made of stainless steel, titanium, or othermaterials. This is important when the mild steel knobs of standard danglers would benegatively affected by the type of plating chemistry used. Be aware that the alternate materialswill probably exhibit lower conductivity.

Other cathode contact types such as disk, cone, center bar, strip, and button contacts willUsually do a better job of plating rods. long parts and delicate parts. These types of cathodeCOntacts are referred to as stationary because they are affixed to the barrel Itself and rotate withthe load. They are, therefore, stationary relative to the load. Stationary contacts are lessabrasive to the work and generally exhibit less problems with entanglement. A plate-styleCOntact is usually utilized in oblique-style barrel equipment.

Barrel DoorsThere are several available styles and fastening methods for plating-barrel doors.

Clamp-style doors have predominated over the years. This is because they are both quick andeasy to operate. Knob-style doors are also greatly utilized (see Fig. 10). The threadedcomponents of knob doors must be designed for efficient operation and useful service life tominimize replacement. Divided doors can be furnished for ease of handling because they aresmaller, being one half of the total barrel length each. Divided doors are used with partitionedbarrels that have a transverse divider in the middle for compartmentalization.

There is, as in all things, diversity in barrel equipment and door operations. Many shopsuse and prefer clamp-style doors. Clamps are efficient because of quick installation andremoval. Others operate successfully with knob-style doors. Many shops use more than onestYle barrel and door.

Because barrel-door security for part retention and efficient mounting, fastening, andOpening of barrel doors is critical to operation of the entire line, much attention is given to thisarea. Some recent door designs secure the workload within capturing edges of the doorOpening, rather than from the outside. With this type of design, the door carries the weight ofthe workload on the capturing edges. rather than the retaining clamps or knobs. This type ofdesign is good for very small parts or workpieces that cumulatively pry and wedge intocrevices.

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Fig. 10. Dangler-style interior barrel cathode contacts.

Recent innovations to automate operation of plating barrel doors can be utilized toeliminate manual labor for opening, loading, and closing. In addition to the labor savings, thesafety of the overall environment of the finishing operation is increased. Automatic barreloperation translates into system automation, which can greatly enhance efficiency andeliminate costs. Automated barrels, hoist systems, and related material handling equipmentcan be configured in which the equipment automatically sizes and weighs workloads, loadsthe barrels, closes the barrels for processing, opens the barrels, unloads the finished work toconveying equipment for further processing or drying (see Fig. 4). This is the ultimateevolution of a barrel-finishing system.

Detail ComponentsThere are important equipment features that substantial1y affect plating system perfor­

mance and serviceability. It is very important to consider these items and their benefits whenselecting barrel-plating equipment.

Horizontal barrel assemblies equipped with an idler gear will result in fully submergedoperation of the barrel, ensuring maximum current access to the work. Fully submerged barrelplating also minimizes any potential for problems with accumulated or trapped hydrogen.

Barrel rotation causes a cascading action of the workload inside the barrel. Because ofthis, the center of gravity of the workload is shifted to one side of the barrel assembly.Tank-driven, horizontal barrel assemblies equipped with an idler gear offset the center ofgravity of the cascading workload to the proper side to best resist the tendency of the rotatingtank drive gear to lift the barrel contacts from the-tank contact points; therefore, use of an idlergear on the barrel assembly helps maintain good electrical contact between the barrelassembly contacts and the cathode contact saddles of the tank. Conversely, a barrel assemblywithout an idler gear promotes poor electrical contact because the center of gravity of theworkload is shifted to the opposite side and works against holding down the contacts.

Another positive feature is hanger arms made of nonconducting materials such as plastic.Nonconducting hanger arms eliminate treeing, stray currents, and possible loss of plating­current efficiency. (Treeing is the accumulation of deposited metal on the barrel or acomponent because of stray currents.)

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Design simplicity and efficiency of barrel equipment are important for ease ofmaintenance, particularly for components operating below the solution level. The use of alloyfasteners that are nonreactive to the chemical system in use is especially important foracid-based plating systems such as chloride zinc.

HOISTSYSTEMS, TANKS, AND ANCILLARY EQUIPMENT

It is important to the performance capabilities of a barrel hoist and tank system to reviewthe following items and include the advantageous features where possible.

Most barrel-plating tanks are designed to maintain the solution level approximately 5 in.below the top rim of each tank. At this level, the plating barrels should run fully submerged.eliminating the potential for excess hydrogen accumulation. Operating with a solution levelhigher than 5 in. below the top rim of a tank can cause the solution to be splashed out duringbarrel entry or exit. resulting in treatment and clean-up problems and wasted solutions.

Solution loss and adjacent-tank drag-out contamination can also be minimized byequipping the barrel hoist system with up-barrel rotation. A drive mechanism on the hoistrotates the barrel and load in the overhead, above-tank. position. facilitating better drainagebefore moving to the next station. This is especially helpful when finishing cupped- orcomplex-shaped parts.

Locating the plating-tank anodes (including anode baskets or holders) in the closestproximity to the barrel exteriors. without allowing mechanical interference. ensures greatestcurrent densities for the workload. Anodes that are contour curved to just clear the outsiderotational diameter of the barrels can result in 10 to 20% increase in current density.

For horizontal barrels. vertical adjustment of tank-mounted barrel drives should optimizeengagement of the gears. Drives that are adjusted too high will carry the weight of the loadedbarrel assembly on the drive gear, resulting in excessive stress on the gear, drive shaft, andbearings. This causes premature wear and failure of these components. Reducer oil leakage isalso a resulting problem.

In addition, when the weight of the barrel unit is concentrated on the drive gear and driveshaft, rather than on the plating or electroclean tank saddles, proper contact is not possible. Ifthe drive gear carries the barrel assembly. the contacts are most often lifted out of position.

When a tank drive unit is adjusted too low. poor drive-gear engagement results.Sometimes the driven barrel gear hops across the tank drive gear. and the unit does not tum.This situation not only results in premature gear wear because of abrasion but also in poorplating because of poor electrical contact.

It is best to alternate tank drive rotation in a barrel plating line in each following processstation. The advantage of having approximately an equal number of drives rotating the barrelsin the opposite direction is to ensure even wear on all drive components (bearings. gears, etc.)and greatly extending contacts dangler service life. Alternate rotation of drives certainlyminimizes replacement requirements and downtime.

The teeth of the steel gears on barrel assemblies and tank drives should be greased toenhance service life and fully engaged performance greatly. Displaced grease will notnegatively affect the tank baths because the gears are normally located beyond and below thetank end wall.

Barrel drives, whether tank or barrel mounted. can have provision to change barrelrotation speed. This is to allow for change of workload type or plating finish. For example,a lower rotation speed is often better for very delicate or heavy parts to minimize abrasion.A faster rotation speed may be used to produce a more uniform plated finish or more readilybreak up loads of nesting or sticking parts. Allowing for change of barrel rotation speedmaximizes the capability to produce the greatest variety of finishes on a larger variety of parts.

Certain tank drives provide for speed change by using multiple-sheave belt pulleys on theOUtput shaft of the drive motor and the input shaft of the speed reducer. Moving the belt ontoother steps in the pulley yields a different speed for each step. Many present-day systems use

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directly coupled C-flange motors bolted directly to the reducer. The speed-change adjustmentcapability is achieved electrically through the control panel by using adjustable pots or othertype controls.

For a long time, it was thought that process tanks with more than three to five stationsshould beavoided. This is because smaller duplicate tanks, doing the same process, will allowthe plating line to continue in operation if a bath needs to be replaced or one of the tanksrequires maintenance. Separate tanks for the same process can be plumbed to each other foruniformity of the baths. Each tank can be isolated with valves, when necessary. formaintenance. Experience has shown, however, that many platers prefer to use single-unit,multistation tanks because the bath is more homogeneous and the temperature more uniform.They schedule maintenance at downtimes and have been able to make emergency repairs ina short time, when necessary, in order not to interrupt production.

NEW DEVELOPMENTS

There are at least two notable developments in barrel-system capabilities. As the industrymoves toward minimizing water usage and treatment costs, rinsing and drying are receivingattention as operations that can be modified to provide savings.

In-the-barrel drying eliminates labor needed for transfer of the work from the barrel tothe dryer basket, loading and unloading of the dryer. When equipment is provided to dry thework in the barrel, work flow is more efficient. The plater must, however. consider the typeof workpieces because some do not lend themselves well to in-the-barrel drying. Adequate airflow through the load may not be possible for some types of work. This is particularly truefor workpieces that tend to nest together. reducing air circulation. Also, some parts and finishtypes can be negatively affected when they are tumbled in the dry condition.

Benefits from minimizing water usage and wastewater-treatment costs have causedequipment suppliers to develop an apparatus to use less water in the barrel system.

One development is to connect separate rinse tanks from different parts of the linetogether, in sequence of descending water quality, to optimize the use of the water before itis sent through filtration and treatment process. In other words, the water is taken advantageof for more turns, and less water is added to the rinse tanks, in total. Of course, not all rinsetanks can be handled together this way because cross-contamination could affect some stepsin the finishing process. For where it is practicable, the water savings can be significant. Forexample, acid rinse baths can be further utilized for the cleaning rinses, as the next step afterthe cleaning stations is normally the acid dipping or pickling. Also, the acid rinses have aneutralizing effect on the cleaning rinses.

Another technology to minimize water usage is the application of spray rinsingequipment rather than an immersion rinse. Water manifolds with spray nozzles directed on theoutside of the barrel wash the barrel and contained workload. Sometimes the barrel is rotated,tumbling the work, while being sprayed. It is expected that water usage is reduced. Thismethod is not effective for all types of work, an example being cupped parts or convolutedworkpieces. Another type of spray rinsing equipment incorporates an interior manifold in thebarrel and water connection equipment on the outside of the barrel to spray directly onto thework inside the barrel for rinsing. Again, water conservation is the goal for which thisequipment has been designed.

RATE OF PRODUCTION

Reasonable production may be maintained with total workload surface area rangingbetween 60 and 100 square feet per single barrel. Amperage settings can vary substantiallywith the type of plating. Most production barrel platers operate in the 15 to 40 Nft2 range.Nickel plating can vary to 50 Nft2

• Take note that actual current density is higher because only

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the exposed surface of the workload in the direct path of the current at any time is plating. Theexposed surface is much less than the total calculated surface of the entire load. All surfaceseventually receive the same relative exposure due to the tumbling action in barrel plating.

Barrel tanks generally draw higher amperages than still (rack) tanks of the same capacity;therefore, it is important equip barrel tanks with greater anode area. usually in a 2 to I ratioto the total surface area of the workload. Barrel anodes corrode faster than rack-type platinganodes; however. the production is much greater than for a rack-type line.

There are references located elsewhere in the Metal Finishing Guidebook that permitestimating the time required to deposit a given thickness for many types of plating. There isalso information for selecting proper current densities and total cycle times.

RECORDS

Proper operation of a barrel-plating line requires the maintenance of records for each partand plating specification done in the shop. The data can be entered on file cards or in acomputer database and used to construct graphs or tables for thickness. time. area. and currentrelationships, Using the graphs or tables. a plater can make reasonably accurate initialjudgments for processing new or unfamiliar work. Suggested items to record for each jobinclude material, part surface areas, part weight. finish type. thickness required. current. andvoltage used. as well as load size and plating time.

SUMMARY

Barrel plating has distinct advantages; the ability to finish a larger variety of work andproducing a greater volume of work for a specified time period over a rack-type finishing line.By incorporating as many aspects of the previously mentioned information as possible, thecapacity and capability of a barrel finishing production line can be optimized .

Thickness Testing of Eleetrodeposited and Related Coatingsby G.P Ray 169 pages $105.00

This book covers all thickness measuring methods commonly utilized in the indu strytoday . Each method is covered in its own chapter. with references to ASTM/ISO orother specifications and articles.

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