PLATING PROCEDURES

BARREL PLATING

by Raymund Singleton Singleton Corp., C/eve~and

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

USES OF BARREL PLATING

Barrel plating is used most often for bulk finishing. It is the most efficient method for finishing bulk parts and any pieces that do not require individual handling. According to the "Metal Finishing Industry Market Survey 1992-1993, ''1 there are approximately 6,750 plating facilities in the United States. Of these, 37% exclusively provide barrel-plating services, and an additional 32% provide both barrel and rack plating; therefore, approximately 69% of all plating facilities employ the advantages of barrel plating in providing their services.

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

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

many and interrelated:

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

~Copyright 1994 by the Metal Finishing Suppliers Association and National Association of Metal Finishers. Used with permission.

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2. A barrel-plating system occupies less floor space and requires a lower investment for equipment 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, or unload 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 in barrel equipment is drying of the work while it remains in the barrel. The elimination of handling and some work transfer enhances efficiency.

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

6. Conversely to barrel operations, rack plating often requires special part carriers or fixturing and other purpose-built equipment. This can include special contacts such as formed anodes based on the individual part type and shape. Barrel plating does not usually require these items, although there are special purpose contacts available.

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

8. Agitation of the tank solutions by barrel rotation inherently eliminates stratification and produces homogeneous baths. Additional agitation equipment is usually not required, although certain tanks and operations are equipped with spargers (air agitation pipes).

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

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

Subsequent advances in the knowledge of chemistry, electricity, and material sciences enabled the evolution of barrel-type metal-finishing equipment for bulk finishing. This evolution culminated in the third or fourth decade of the Twentieth Century with now familiar basic designs.

Today, the submerged portions of barrel-plating equipment are constructed, as much as possible, of nonconductive, chemically inert materials that can be utilized in various acid and alkaline solutions. Great advances in plating-barrel performance, capability, and longevity were 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 major configurations: (1) horizontal barrels and (2) oblique barrels. Horizontal units are the most common, being adaptable to a greater variety and capacity of work (see Fig. 1).

Horizontal barrels also vary by size and are grouped into three major categories: (1) production barrels, (2) portable barrels, and (c) miniature barrels.

Production ban'els, the largest units, usually have a capacity in the range of 1.5 to 17 ft 3. They handle the majority of the work.

Portable barrel units are so named because of their generally smaller size (capacities range from 0.1 to 1.5 ft 3) and their ability to be manually transferred from one operation to

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

the next, sometimes without the aid of an overhead hoist. Portable barrel units are used for plating 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 smallest and most fragile loads and work. Also, miniature barrels are often used for lab work such as product or process development (see Fig. 3).

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

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

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

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

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

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

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

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

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

The previous examples are representative. There are other barrel and system enhance- ments that increase production and reduce cycle times, drag-out, and maintenance require- ments. Optional equipment types are many, including the examples of barrel assemblies manufactured to operate in existing rack-plating installations shown in Figures 5 and 6.

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

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

3 4 6

Fig. 4. Fully automatic load~unload system with integral door barrel assembly for hands-off operation.

motion to a back-and-forth (usually less than 180 ° of arc) rocking action about the horizontal axis, rather than 360 ° full rotation. The motion is more gentle for very delicate parts and can be 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 care to avoid nonuniform plating (particularly for parts that tend to nest). Processing is generally limited to smaller loads with these barrels to avoid spillage and loss because of the always open door.

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

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

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

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 its rotational axis shaft to a position that dumps the workload. Similar to 180 ° horizontal oscillating barrels, this results in relatively small workloads and reduced tumbling action. Today, platers can take advantage of fully automatic doors on full-rotation horizontal barrels to 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 in various 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 ample current and continuous contact are available (gentle abrasion of the part surface is not a problem). One can infer from the previous example that a barrel's value and versatility depend on its capability to (1) plate a particular finish and (2) function properly in system solutions and temperatures. This capability is determined by the materials, construction, and detail features incorporate d into the barrel unit.

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

Fig. 6. Special-length barrel assembly for plating elongated parts or for use in a rack plating line.

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WORKLOAD

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

The workload capacity is usually 40 to 60% of the total interior barrel volume. The maximum workload volume is usually determined based on total square foot surface area of the load and the capacity of the bath chemistry and electrical equipment to plate. Other factors are the weight of the individual workpieces and their propensity to damage the finish or serviceability 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 are of three basic types: corrosion protection to increase the useful service life beyond performance of the nnplated base material; decoration for appearance, which also enhances the value of the base material; and engineering applications to attain (add material) or maintain a dimensional requirement and/or as a bearing surface.

There are requirements for plated finishes that need to perform more than one of the previously mentioned three basic functions. Barrel plating is most commonly used to finish parts for corrosion protection. Decorative finishes are successfully barrel plated when surface effects from part contact are controlled to an acceptable level. Engineering finishes are not usually applied by barrel plating.

Configuration of the workpieces affects the ability of work to be successfully barrel plated. Generally, parts that weigh less than 1 lb each and are less than 25 in. 3 each in volume can be barrel plated successfully. A simple shape is obviously easiest to barrel plate. Barrel plating is usually the most successful, cost-effective way to plate threaded parts and fasteners properly. The tumbling action of the barrel makes and breaks the electrical contact throughout the workload, yielding the most even coverage on the root, mean diameter, and crest of the threads.

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

Long workpieces and entangling parts, such as rods, bars, or tubes, can be successfully barrel plated. Methods used to plate these parts include long barrels; longitudinal and radial compartments; rocking motion; and various, special stationary contacts (see Fig. 4). Special extra-length barrels allow long parts to fit, whereas compartmented barrels confine movement of long parts and entangling parts, helping to eliminate bridging or entanglement. Limited barrel oscillation or rocking motion (usually 180 ° of rotation or less) accomplishes the same task by minimizing part movement. To do this a reversing switch, or contactor, along with an adjustable control timer can be installed on the barrel drive to rotate alternately the cylinder in 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 between the workpieces and the contacts. As a result, the work cascades over or around the stationary contacts, and less abrasion or edge contact takes place, minimizing the potential for damage to the work (see Fig. 7). Disk, center bar, cup, strip, button, hairpin, and chain are some types of stationary contact. Certain types of stationary contacts, such as strip contacts, assist tumbling of the work.

Parts that are flat or lightweight should be plated in barrels with uneven interior surfaces that 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.

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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 to the 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 plating must be acceptable.

BARREL EQUIPMENT DESIGN

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

Barrel Construction Barrels 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 capable of 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 the different bath temperatures in a plating line. Changes in temperature cause stresses that can work a barrel to pieces. This is particularly critical for barrels cons~ucted of materials with different coefficients of expansion. The effects of the temperature changes can be minimized with good design and quality construction. When barrels are fabricated of a single type of plastic and joined by a plastic weld or fusion process, stress points are eliminated. Barrels made this way can expand and contract at a uniform rate, which greatly extends their useful service life. The use of metal fasteners for assembly is a less desirable method because of stress points and the possibility of loosening. Minimizing the effects of temperature changes promotes barrel integrity and long life. T h e capability of a barrel to be used in higher temperature baths can, as an added benefit, aid faster plating.

Good equipment design will reduce maintenance and replacement part costs. Costs are reduced 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 reduce manufacturing costs, should be avoided. Examples are (1) trunion hub-bearing surfaces molded as a component of hanger-arm supports and (2) cylinder ring or bull gears that are also the barrel head. These perform the same as other equivalent parts when new, but when the

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

Detail Features For 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 bath caused by rotation of the flat-sided barrel. Round barrels do not produce pumping action as efficiently. Pumping action helps constantly replace metal-depleted solution from inside the barrel with fresh solution from the rest of the bath.

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

Perforations The 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 accommodate the widest range of potential workpiece sizes. Captive shops often have the luxury of using - barrels with larger holes because they can more easily predict their minimum part size. Larger perforations usually exhibit faster drainage, more efficient exchange of metal-depleted solution, and less drag-out (carryout) contamination of adjacent tank solutions. This is because larger perforations minimize the negative effects of liquid surface tension.

Many shops maintain extra barrel assemblies that have the smallest perforation sizes that will be needed. In this way, the line can be operated, the majority of the time, using larger hole barrels. 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 open area ratio, regardless of perforation size. The open area ratio is defined as the total number of holes in a barrel panel multiplied by the individual open area of each hole and divided by the total area that contains the included perforations.

Open Area Ratio = (No. of holes) × (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 × 0.0069/4 = 0.23 or 23%

Interestingly, there is a convenient geometric relationship between hole size, center distance from hole to hole, and open area. When the distance between centers of given diameter holes is twice the diameter of the holes (in a staggered center pattern that has six holes equidistant alt the way around), the open area ratio is 23%. Consequently, V8 in. diameter holes on 1/4 in. centers, 3/i6 in. diameter holes on 3/8 in. centers, and IA6 in. diameter holes on 1/8 in. centers are all 23% open area ratio patterns, Experience indicates the 23% open area ratio optimizes barrel strength and plating perfolrnance.

Because the open area of any barrel determines the access of the plating current to the work, 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 hole size. Because the access of the plating current to the work will be the sane, there is no need to readjust rectifier settings or current density. Most barrels are or should be manufactured with a 23% open area.

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L__ I 4 5 "

q- 6_ Panet Thickness

\

Panel VId{h

Fig. 8. Cross-section of herringbone-style perforations to keep small-diameter, straight parts inside barrel.

There are other types of barrel perforations available to the plater. These include herringbone, screen, fine mesh, and slots. To produce herringbone perforations, the barrel panels are drilled halfway through each panel at a 45 ° angle to the inside and outside panel faces (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 the perforations because the holes are not straight. Plating solution and current can pass through the perforations, although at a reduced rate.

Barrels with fine-mesh panels with very small openings are generally made of polypropylene 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 Contacts The type of interior cathode electrical contacts in a barrel significantly determines the

variety of work the barrel can process. Flexib]e-cable dangler-type contacts are the most common in barrel plating (see Fig. 9). Dangler contacts are dynamic relative to the workload because the workload rotates with the barrel and tumbles over the danglers. The danglers remain fixed to the barrel support assembly as this occurs. Other types of dynamic cathode contacts are hairpin and chain.

The best plating results are achieved when the danglers remain submerged in the workload. This is because submerged danglers maximize contact and minimize or eliminate arcing, sparking, or burning of the work. The contact knob end of each dangler should touch the bottom of the barrel one-fourth to one-third of the inside barrel length from each barrel end. To determine proper dangler length, measure the total distance from the point that the dangler contact knob should touch the inside bottom of the barrel, continuing through the barrel hub to the outside mounting point of the danglers. For short barrels or stiff dangler cable, the danglers can be extended beyond the midpoint of the barrel to provide contact at the 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 when a standard configuration is not totally adequate. Custom knobs that are heavier can be specified to help ensure they remain submerged in the workload. Also, special knobs with larger contact surface area are available where improved conductivity is important.

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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 other materials. This is important when the mild steel knobs of standard danglers would be negatively affected bythe type of plating chemistry used. Be aware that the alternate materials will probably exhibit lower conductivity.

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

Barrel Doors There 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 and easy to operate. Knob-style doors are also greatly utilized (see Fig. 10). The threaded components of knob doors must be designed for efficient operation and useful service life to niinimize replacement. Divided doors can be furnished for ease of handling because they are smaller, being one half of the total barrel length each. Divided doors are used with partitioned barrels that have a transverse divider in the middle for compartmentalization.

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

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

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

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

Detail Components There are important equipment features that substantially affect plating system perfor-

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

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

Barrel rotation causes a cascading action of the workload inside the barrel. Because of this, 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 of gravity of the cascading workload to the proper side to best resist the tendency of the rotating tank drive gear to lift the barrel contacts from the tank contact points; therefore, use of an idler gear on the barrel assembly helps maintain good electrical contact between the barrel assembly contacts and the cathode contact saddles of the tank. Conversely, a barrel assembly without an idler gear promotes poor electrical contact because the center of gravity of the workload 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 plating- current efficiency. (Treeing is the accumulation of deposited metal on the barrel or a component because of stray currents.)

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

HOIST SYSTEMS, TANKS, AND ANCILLARY EQUIPMENT

It is important to the performance capabilities of a barrel hoist and tank system to review the 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 level higher than 5 in. below the top rim of a tank can cause the solution to be splashed out during barrel 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 by equipping tile barrel hoist system with up-barrel rotation. A drive mechanism on the hoist rotates the barrel and load in the Overhead, above-tank, position, facilitating better drainage before moving to the next station. This is especially helpful whe n finishing cupped- or complex-shaped parts.

Locating the plating-tank anodes (including anode baskets or holders) in the closest proximity to the barrel exteriors, without allowing mechanical interference, ensures greatest current densities for the workload. Anodes that are contour curved to just clear the outside rotational 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 optimize engagement of the gears. Drives that are adjusted too high will carry the weight of the loaded barrel assembly on the drive gear, resulting in excessive stress on the gear, drive shaft, and bearings. This causes premature wear and failure Of these components. Reducer oil leakage is also a resulting problem.

In addition, when the weight Of the barrel unit is concentrated on the drive gear and drive shaft, rather than on the plating or electroclean tank saddles, proper contact is not pQssible. If the drive gear carries the barrel ~ssembly, 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 turn. This situation not only results in premature gear wear because of abrasion but also in poor plating because of poor electrical contact..

It is best to alternate tank drive rotation in a barrel plating line in each following process station. The advantage of having approximately an equal number of drives rotating the barrels in 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 certainly minimizes replacement requirements and downtime.

The teeth of the steel gears on barrel assemblies and tank drives should be greased to enhance service life and fully engaged performance greatly. Displaced grease will not negatively affect the tank baths because the gears are normally located beyond and below the tank end wall.

Barrel drives, whether tank or barrel mounted, can have provision to change barrel rotation 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 readily break up loads of nesting or sticking parts. Allowing for change of barrel rotation speed maximizes 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 the output shaft of the drive motor and the input shaft of the speed reducer. Moving the belt onto other 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 adjustment capability is achieved electrically through the control panel by using adjustable pots or other type controls.

For a long time, it was thought that process tanks with more than three to five stations should be avoided. This is because smaller duplicate tanks, doing the same process, will allow the plating line to continue in operation if a bath needs to be replaced or one of the tanks requires maintenance. Separate tanks for the same process can be plumbed to each other for uniformity of the baths.-Each tank can be isolated with valves, when necessary, for maintenance. 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 in a 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 industry moves t6ward minimizing water usage and treatment costs, rinsing and drying are receiving attention as operations that can be modified to provide savings.

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

Benefits from minimizing water usage and wastewater-treatment costs have caused equipment 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 line together, in sequence of descending water quality, to optimize the use of the water before it is sent through filtration and treatment process. In other words, the water is taken advantage of for more turns, and less water is added to the rinse tanks, in total. Of course, not all rinse tanks can be handled together this way because cross-contamination could affect some steps in the finishing process. For where it is practicable, the water savings can be significant. For example, acid rinse baths can be further utilized for the cleaning rinses, as the next step after the cleaning stations is normally the acid dipping or pickling. Also, the acid rinses have a neutralizing effect on the cleaning rinses.

Another technology to minimize water usage is the application of spray rinsing equipment rather than an immersion rinse. Water manifolds with spray nozzles directed on the outside 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. This method is not effective for all types of work, an example being cupped parts or convoluted workpieces. Another type of spray rinsing equipment incorporates an interior manifold in the barrel and water connection equipment on the outside of the barrel to spray directly onto the work inside the barrel for rinsing. Again, water conservation is the goal for which this equipment has been designed.

RATE OF PRODUCTION

Reasonable production may be maintained with total workload surface area ranging between 60 and 100 square feet per single barrel. Amperage settings can vary substantially with the type of plating. Most production barrel platers operate in the 15 to 40 A/ft 2 range. Nickel plating can vary to 50 A/ft z. Take note that actual current density is higher because

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only the exposed surface of the workload in the direct path of the current at any time is plating. The exposed surface is much less than the total calculated surface of the entire load. All surfaces eventually 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 1 ratio to the total surface area of the workload. Barrel anodes corrode faster than rack-type plating anodes; however, the production is much greater than for a rack-type line.

There are references located elsewhere in the Metal Finishing Guidebook that permit estimating the time required to deposit a given thickness for many types of plating. There is also 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 part and plating sl~ecification done in the shop. The data can be entered on file cards or in a computer database and used to construct graphs or tables for thickness, time, area, and current relationships. Using the graphs or tables, a plater can make reasonably accurate initial judgments for processing new or unfamiliar work. Suggested items to record for each job include material, part surface areas, part weight, finish type, thickness required, current, and voltage used, as well as load size and plating time.

SUMMARY

Barrel plating has distinct advantages: the ability to finish a larger variety of work and producing 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, the capacity and capability of a barrel finishing production line can be optimized.