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Barrel plating
PLATING PROCEDURES BARREL PLATING by Raymund Singleton Singleton
Corp.,
C/eve/and
Typically, barrel plating involves a rotating vessel, immersed in a plating bath, which tumbles a contained, bulk workload. The barrel interior uses cathode electrical contacts to attract metals out of the solution onto the work. The workpieces effectively become part of the plating equipment during processing because they function as bipolar electrical contacts to the other pieces in the workload. This bipolar contact greatly contributes to the high efficiencies in barrel plating since the entire surface of the workload, in the current path at any time, is in cathode contact. Barrel plating is a mass finishing operation, It is, therefore, the most effective method for treating bulk parts and any pieces that do not require individual handling. According to the Meral Finishing Industry Marker Survey 1992-1993,’ there are approximately 6,750 plating facilities in the United States. Of these, 37% are barrel plating facilities. Of the remainder, 32% of the facilities use both barrel and rack plating. In other words, 69% of all plating facilities use barrel plating. Of the three functional uses of plated finishes, barrels are used most often applying finishes for corrosion protection. Decorative or dimensional-plated finishes are not generally applied in barrels because of the surface contact inherent during processing.
ADVANTAGES Along with high efficiency, major advantages of barrel plating are many and interrelated. The relatively large cathode contact area allows faster, larger volume production, in the presence of ample current, when compared with a rack-type plating line. A barrel plating equipment system occupies less space and requires lower equipment costs than for a rack or other type plating line of similar capacity. Barrel plating is labor efficient because it is not necessary to handle. rack, load, or unload individual workpieces. Also, 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 dryin, c of the work while it remains in the barrel. The elimination of additional handling or work transfer enhances efficiency. Barrel plating is very versatile because of the variety, type. and size of parts processed in the same equipment. It is relatively universal for finishing fasteners, metal stamping?., and similar bulk work. There is a saying 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. 330
Fig. 1. Typical contacts.
horizonral
barrel
and superstructure
assembly
showing
inverted
V-type
Rack plating often requires special part carriers, contacts, and other purpose-built equipment. This can include special formed anodes based on the individual part type and shape. Barrel plating does not usually require these items. The workload tumbles in a cascading action due to the rotation of the barrels. This tumbling, plus bipolar electrical activity from individually contacting parts, usually produces a more uniform plate than does rack plating. Barrel rotation provides agitation of tank solutions. This produces homogeneous baths and generally eliminates stratification. Generally, additional agitation equipment is not required, although certain tanks, baths, and cleaning operations are equipped with spargers (air agitation pipes) when required.
ORIGINS
AND
EQUIPMENT
Barrel plating methods originated in the post-Civil War era with equipment adapted from readily available wooden barrels or kegs. Subsequent advances in the knowledge of chemistry, electricity, and materials science enabled the evolution of metal finishing and related barrel-type mass finishing equipment. This evolution culminated with now familiar basic designs in the third or fourth decade of this century. Barrel equipment is constructed, as much as possible, of 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 construction materials available after World War II. 332
Fig. 2. Portable bar) *el assembly with self -contained drive, dangler contacts, and clamp-style door.
Equipment Types Available barrel equipment varies widely, but generally conforms to two major configurations: horizontal barrels and oblique barrels. Horizontal units are the most common, being adaptable to a greater variety and capacity of work (see Fig. 1). Barrel equipment also vanes by size. Sizes are grouped into three major categories: (1) production barrels, (2) portable barrels, and (3) miniature barrels. The largest units handling the majority of the work are production plating barrels. Next in size are portable barrel units, so named because of their generally smaller size and ability to be- handled manually, sometimes without the aid of an overhead hoist system. Portable barrel units are used for plating smaller parts, smaller lots, delicate parts, and precious metals work, which is too expensive to be done with large tanks or baths (see Fig. 2). Miniature, or mini, barrel units are used for many of the same reasons as portable barrels. Miniature barrels are used to process the smallest and most fragile loads and work. Also, miniature barrels are often used for lab work or process development (see Fig. 3). While rotation about a horizontal or inclined axis is the norm, many features of construction, components, and capabilities improve barrel versatility. Examples of major
Fig. 3. Ministyle barrel assembly with self- contained dl *ive and integral-mesh, i?lOlt ded baskets.
334
Fig. 4. Fully automatic with integral door barrel ofloperation.
loadiunload system assembly for hands-
barrel features that can provide increased produciivity include: (1) cylinders with maximized load volumes (see Fig. I) to function within the dimensional limits of associated equipment; (2) special diameter and/or length barrel assemblies to work in nonstandardized installations; (3) high efficiency and/or 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 requirements (see Fig. 4); (5) equipment to dry the work while it remains in the barrel saving transfer and handling operations; (6) “up rotation” apparatus to minimize contamination and carryover (drag-out) of solution to adjacent process tank stations; and (7) special apparatus to spray-rinse work inside the barrel to reduce water usage and ensuing treatment costs. The above 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 horizontal barrel assemblies manufactured to operate in existing rack plating installations (see Figs. 5 and 6). It is worthwhile to mention the horizontal oscillating barrel. This technique most often utilizes barrels that are open on top and have no doors or clamps. Motion is limited to a 180” arc back-and-forth 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 or bridge badly; however, 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 top. 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 rotation barrels to obtain equivalent results. Many older oscillating barrel installations have been converted to full rotation operation. The second major barrel equipment style, the oblique barrel, can be pictured as an open-top basket. It incorporates a rotation mechanism that generates a tumbling action around an axis tilted to a maximum 45” from the vertical (capacity diminishes beyond 45”). A feature of oblique barrels is the elimination of doors or other closure devices. Since the top is open, unloading consists of tilting the rotational axis of the barrel assembly to the necessary position,
336
Fig. 5. Barre/
assembly
equipped jtir use in a rack plating
line.
which 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. Included are 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 chromium where ample Fig. 6. Special length barrel line.
338
assembly jbr plating
elongated
parts or jtir use in a rack plating
current and continuous contact is available (and gentle part surface abrasion is not a problem). The capability of equipment to plate a particular finish, and to function in required solutions and temperatures, determines its usefulness. 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. A single line, producing more than one finish type, must control cross-contamination of the different plated metals in stations used for rinsing, sealing, chromating, and cleaning. WORKLOAD The barrel plater must evaluate the following items to decide if the desired finish for a particular part can be barrel plated: (1) finish function (relative to use of the part), (2) part configuration, (3) part size, (4) part weight, (5) calculated part surface area, and (6) total workload volume and square foot surface area. The workload capacity is usually 40-50% of the total barrel volume. The maximum barrel workload is usually determined based on total square foot surface area of the load and the capacity of the chemistry and equipment to plate. Plated finish functions are of three basic types: (1) engineering applications to attain (add material) or maintain a dimensional requirement and/or as a bearing surface; (2) decorative for appearance, which also enhances the value of the base material; and (3) corrosion protection to increase the useful service life beyond performance of the unplated base material. There are plated finishes that need to perform combinations of the above three basic functions. Engineering finishes are not usually applied by barrel plating. Decorative finishes are successfully barrel plated when surface effects from part contact are controlled to an acceptable level. Barrel plating is most commonly used to finish parts for corrosion protection. Part configuration affects the ability of work to be successfully barrel plated. Generally, parts that weigh
Fig. 7. Barrel inferior showin8 disk- andstrip-type c(mtacts.
EQUIPMENT
DESIGN
Horizontal, oblique, or other design barrel equipment must contain features to maximize productive capability. Minimum labor requirements and ease of maintenance should always be factors of well designed components and systems.
Barrel
Construction Barrel construction must be of materials that are unaffected by use in the applicable plating process. It is also important that the barrels be capable of operation in excess of maximum bath temperatures in the plating system. A plating barrel may expand and contract as much as j/x in. in overall length due to temperature changes in a plating cycle. Good design and construction will eliminate stresses that cause barrels to go to pieces due to temperature changes. Temperature changes acting on the different construction materials with different coefficients of expansion cause barrels to work apart. When barrels are constructed of all plastic or do not use metal fasteners, stress points are eliminated. These barrels can expand and contract at a uniform rate that greatly extends their useful service life. High temperature capability promotes barrel integrity and long life. It 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 structural member, to reduce manufacturing costs, should be avoided. Examples are: (1) trunion (hub) bearings molded as one piece to the barrel hanger-arm supports, and (2) cylinder ring, or bull, gears that function as the barrel head (end). These items perform adequately when new, but when the wear part needs replacement, the entire member or barrel must be replaced. This can be a very costly design for the user.
Barrel
Features For the majority of plating processes, the flat-sided barrels are best. Flat-sided barrels produce pumping action as a benefit of rotation. Pumping action constantly replaces metal-depleted solution inside the barrel with fresh solution from the rest of the tank. Flat-sided barrels also tumble parts more effectively. This is most effective when the interior surface of the barrel is ribbed, or grooved, to further assist tumbling of the work and eliminate the sticking of parts to the panel surfaces. 340
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 aid the tumbling of the work.
Perforations A plating barrel’s use must be considered when specifying the perforation sizes. Job shops generally obtain barrels with smaller openings to accommodate the widest range of potential workpieces. Captive shops most 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 (due to the pumping action mentioned previously), and less drag-out (carryout) contamination of adjacent tank solutions. This is because larger perforations minimize 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 barrels with larger holes. The barrels with smaller holes arc 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 the sides of the barrel, times the individual area of each hole, divided by the total panel area of the barrel. Since 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. Barrels with the same open-area ratio can be used, regardless of hole size, without the need to readjust rectifier settings or current density. Most barrels are or should bc manufactured with a 23% open area, which experience indicates optimizes barrel strength and performance. There are other types of barrel perforations available to the plater. These include herringbone (right-angle holes to prevent small diameter straight pieces from passing out through the barrel sides), screen, mesh inserts, and slots. These special hole types can be utilized for barrel plating straight and small diameter pieces (i.e., nails, pins, etc.).
Barrel Cathode Electrical
Contacts
The type of interior cathode electrical contacts a barrel has determines the variety of work the barrel can process. The flexible dangler (cable-type) contact is most common. 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, Best plating contact and results are achieved when the danglers remain submerged in the workload. Submerged danglers maximize contact and minimize or eliminate arcing, sparking, or burning of the work. Dangler length should result in the contact knob touching the bottom of the barrel I/4-‘/3 of the barrel length from the end of the barrel. For short barrels or stiff dangler cables, the danglers can be crossed in the middle of the barrel to ensure that they remain submerged in the load. Disk, cone, center bar, strip, and button contacts will usually do a better job barrel plating rods, long parts, and delicate parts. These types of interior cathode contacts are referred to as stationary because they are affixed to the barrel and rotate with the load. They are, therefore, stationary relative to the load. Stationary contacts are less abrasive to the work and generally exhibit fewer problems with entanglement. A plate-style contact is usually utilized in oblique barrel equipment.
Barrel Doors There Clamp-style
342
are several available styles and fastening methods for plating barrel doors. doors have predominated over the years. This is because they are both quick and
~
pg.
~
non door partition.
8.
Knob-style
with center
two-sec-
bar and
easy to operate. Knob-style doors are also greatly utilized. The threaded components of knob doors must be designed for efficient operation and useful service life to minimize replacement requirements. Divided doors, which are one-half the total barrel length, can be furnished for ease of handling and to work with partitioned barrels (see Fig. 8). There is, as in all things, diversity in barrel equipment and operations. Many shops use and prefer clamp-style doors. Others operate successfully with knob-style doors. Many shops use more than one style of barrels and doors. Because the effective security and operation of barrel doors is critical to operation of the entire line, much attention is given to this area. Some recent designs secure the workload with the door retained within capturing edges of the door opening, rather than being mounted from the outside. With some designs, the workload causes the door to seal tighter. This is good for very small parts or work with edges that pry and cumulatively wedge into crevices. When additional labor is needed to operate a door fastening system, the benefits of the door function must be evaluated relative to the extra labor necessary. There have been recent innovations for barrel operation and door fastening methods that eliminate manual door handling and labor for attendant operations. Automatic barrel operation translates into system automation, which can greatly enhance efficiency and eliminate costs. Barrel systems and related material handling equipment can be installed to automatically load, unload, and operate the barrels (see Fig. 4). This also benefits platers by maximizing worker safety.
Component Features Particular equipment features can substantially affect system performance and serviceability. It is 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 potential for accumulated or trapped hydrogen gas problems. Barrel rotation causes a cascading action of the workload inside the barrel. Because of this, the workload center-of-gravity is shifted to one side of the barrel assembly. Tank-driven, 344
horizontal barrel assemblies equipped with an idler gear, offset the center of gravity of the cascading workload to the proper side to best maintain good electrical contact between the barrel assembly contacts and the cathode contact saddles of the tank. Another positive feature is hanger arms made of plastic or other nonconducting materials. Nonconducting hanger arms eliminate treeing and possible loss of plating current efficiency. Design simplicity and efficiency of barrel equipment are important for ease of maintenance, particularly for components operating below the solution line. The use of alloy fasteners that are nonreactive to the chemical system in use is especially important for acidand chloride-based plating systems.
Hoist
System, Tank, and Associated Equipment The features of hoist systems and associated tanks are important to barrel plating capabilities. Most tank systems are designed to maintain the solution level approximately 5 in. below the top rim of each individual tank. At this operating level, the plating barrels should run fully submerged. One benefit is to eliminate the chance of excess hydrogen gas accumulation (for some plating types) in barrels that are not run totally submerged. Fully submerged operation reduces the potential for problems due to excess hydrogen accumulation. Operating with a solution level higher than 5 in. below the top rim of a tank can cause solution to be splashed out during barrel entry or exit, resulting in problems. Solution loss and adjacent-tank drag-out contamination can also be minimized by equipping the barrel hoist system with up-barrel rotation. A drive mechanism on the hoist tumbles the barrel and load in the overhead, above-tank, position. Up-barrel rotation facilitates drainage, especially when finishing cupped or complex-shaped parts. Greatest current densities for the workload are ensured by locating the plating tank anodes, including anode baskets or holders, in the closest proximity to the barrel exteriors without allowing mechanical interference. For horizontal barrels, anodes that are contour curved to just clear the rotational outside diameter, can result in a 1620% increase in current density. Tank-mounted barrel drives for horizontal barrels should be adjusted vertically to optimize engagement of the gears. Drives that are too high will carry the weight of the barrel assembly and workload resulting in excessive loads on the drive-shaft bearings. Excessive weight causes premature wear and failure of the gears, bearings, and other components. Reducer oil leakage is a directly related problem. When the weight of the barrel unit and workload is concentrated on the gear and drive shaft, rather than on the tank electrical contacts, proper contact between the tank saddles and barrel contacts is not possible. If the 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 barrel assembly gear hops across the tank drive gear and the unit does not turn. This situation not only results in premature gear wear, but also poor plating, cleaning, and processing of the work. It is best not to rotate all process station drives in a barrel plating line in the same direction. The advantage of having an approximately equal number of drives rotate the barrels in the opposite direction is to ensure or optimize even wear on all drive components (bearings, gears, etc.) and electrical contacts (i.e., extended dangler life.) Alternate drive rotations also minimize replacement requirements and downtime. Tank drives for barrels can have provision to change barrel rotation speed to allow for change of workload type or plating finish. For example, a lower speed of rotation is much better for very delicate or heavy parts. A faster rotation speed may be used to produce a more uniform plate, 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. 345
Certain tank drives provide for speed change with multiple sheave belt pulleys on the output shaft of the drive motor and input shaft of the speed reducer. Many present-day systems use directly coupled C-flange motors bolted directly to the reducer. The speed change adjustment capability on directly coupled units is accomplished electrically through the control panel. For a long time it was thought that tanks with more than three stations should be avoided. This is because separate or duplicate tanks with the same function allow the line to continue in operation if a bath needs to be replaced or one tank requires maintenance. Separate tanks for the same bath can be piped to each other to share the bath for uniformity. Each tank can be isolated with valves when necessary for maintenance; however, important as this consideration may be, many platers prefer to use single-unit, multistation tanks because of the homogeneity of the bath solutions and temperatures.
New
Developments There have been at least two notable developments in barrel system capabilities. As the industry moves toward minimizing water usage and treatment costs, rinsing and drying are receiving attention as operations that can be modified to provide savings. h-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 the line is equipped to dry 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. Also, the work may be tumbled in a dry condition, which can negatively affect some parts and finish types. With the benefits to be received from minimizing water usage and wastewater treatment costs equipment suppliers have participated by designing equipment that uses less water in the barrel system. One development is to connect separate rinse tanks from different parts of the line together, in sequence, to optimize the rinsing capability of the water before it is sent through the filtration and treatment process. In other words, the water is taken advantage of for more turns. This means less water is added to the rinse tanks in total since it is utilized for more repetitions. Of course, not all rinse tanks can be handled together this way; however, where it is practicable, the water savings are tremendous. For example, acid rinse baths can be subsequently utilized for the post-cleaning rinses, since 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 that a bath-type, immersion rinse. Spray station equipment has been furnished rather than the typical immersion bath. Some equipment is also available that can spray-rinse work inside the barrel.
RATE OF PRODUCTION Reasonable production may be maintained with total workload surface areas ranging 6&100 ft2 per single barrel. Current draw varies with the type of plating and may vary greatly. Most production barrel platers operate in the IS-40 A/f? range. Nickel plating can go as high as 50 A/f?. Actual current density is higher since only the exposed surface of the barrel load, in the direct path of the current at any time, is platin g. The exposed surface is much less than the total surface of the entire load. All surfaces eventually receive the same relative exposure due to the tumbling action. Barrel tanks generally draw higher current than do still (rack) tanks of the same capacity. It is, therefore, important to equip barrel tanks with greater anode area, usually in a 2:l ratio to the total surface area of the workload. Barrel anodes corrode faster; however, the production is much greater. There are procedures explained elsewhere in this Guidebook that permit 346
estimating the time required to deposit a given thickness for many types of plating. also information for selecting proper current densities and total cycle times.
There is
RECORDS Proper operation of a barrel plating line requires the maintenance of records for each part and plating specification done in the shop. These can be entered on file cards or in a computer database. The information may be used to construct graphs. Since many of the thickness, time, area, and current relationships are relatively linear, one can use previous job information to 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.
Reference 1. Metal Finishing tion, Westmont,
Industry Market Ill. and National
Survey 1992-1993, Metal Finishing Suppliers Association of Metal Finishers, Chicago; 1994
Associa-
Gold Plating Technology by F.H. Reid and W Goldie 630 pages SlS5.00
centrifugally, eliminating c rejects I caused by spottina! !
rhis is a critical review of the litera:ure with over 900 references cited. The 42 chapters, written by 23 authorties, cover everything from historical >ackground to applications. A worthwhile investment.
-or Ideal for barrel
type batch processing. Fan impeller draws lectrically heated air through the dryer while centriiu al force spin dr 9es parts. all in 1 W sq. t 01 space. Reliably used bv ilnishers for more ihan 30 year;. basket
tmd Orders co: METAL FINISHING rhree Univrrsiry Plaza, Hackensack,
‘or faster service, call
(201) 487-3700 ZAX your order IO (201) 487-3705
Model CD-100
348
Winscolt
Dryer
NJ 07
III book orders musr be prepaid. NY, NJ and riA residents add appropriare sales tax. Please nclude $5.00 shipping and handlrng for delivery )f each book via UPS IO addresses in rhe U.S.; 58.00 for each book for Air Parcel Post shipment o Canada; and $20.00 for each book for Air ‘arcel Post shipment co all ocher countries.
Corp.,
C/eve/and
Typically, barrel plating involves a rotating vessel, immersed in a plating bath, which tumbles a contained, bulk workload. The barrel interior uses cathode electrical contacts to attract metals out of the solution onto the work. The workpieces effectively become part of the plating equipment during processing because they function as bipolar electrical contacts to the other pieces in the workload. This bipolar contact greatly contributes to the high efficiencies in barrel plating since the entire surface of the workload, in the current path at any time, is in cathode contact. Barrel plating is a mass finishing operation, It is, therefore, the most effective method for treating bulk parts and any pieces that do not require individual handling. According to the Meral Finishing Industry Marker Survey 1992-1993,’ there are approximately 6,750 plating facilities in the United States. Of these, 37% are barrel plating facilities. Of the remainder, 32% of the facilities use both barrel and rack plating. In other words, 69% of all plating facilities use barrel plating. Of the three functional uses of plated finishes, barrels are used most often applying finishes for corrosion protection. Decorative or dimensional-plated finishes are not generally applied in barrels because of the surface contact inherent during processing.
ADVANTAGES Along with high efficiency, major advantages of barrel plating are many and interrelated. The relatively large cathode contact area allows faster, larger volume production, in the presence of ample current, when compared with a rack-type plating line. A barrel plating equipment system occupies less space and requires lower equipment costs than for a rack or other type plating line of similar capacity. Barrel plating is labor efficient because it is not necessary to handle. rack, load, or unload individual workpieces. Also, 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 dryin, c of the work while it remains in the barrel. The elimination of additional handling or work transfer enhances efficiency. Barrel plating is very versatile because of the variety, type. and size of parts processed in the same equipment. It is relatively universal for finishing fasteners, metal stamping?., and similar bulk work. There is a saying 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. 330
Fig. 1. Typical contacts.
horizonral
barrel
and superstructure
assembly
showing
inverted
V-type
Rack plating often requires special part carriers, contacts, and other purpose-built equipment. This can include special formed anodes based on the individual part type and shape. Barrel plating does not usually require these items. The workload tumbles in a cascading action due to the rotation of the barrels. This tumbling, plus bipolar electrical activity from individually contacting parts, usually produces a more uniform plate than does rack plating. Barrel rotation provides agitation of tank solutions. This produces homogeneous baths and generally eliminates stratification. Generally, additional agitation equipment is not required, although certain tanks, baths, and cleaning operations are equipped with spargers (air agitation pipes) when required.
ORIGINS
AND
EQUIPMENT
Barrel plating methods originated in the post-Civil War era with equipment adapted from readily available wooden barrels or kegs. Subsequent advances in the knowledge of chemistry, electricity, and materials science enabled the evolution of metal finishing and related barrel-type mass finishing equipment. This evolution culminated with now familiar basic designs in the third or fourth decade of this century. Barrel equipment is constructed, as much as possible, of 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 construction materials available after World War II. 332
Fig. 2. Portable bar) *el assembly with self -contained drive, dangler contacts, and clamp-style door.
Equipment Types Available barrel equipment varies widely, but generally conforms to two major configurations: horizontal barrels and oblique barrels. Horizontal units are the most common, being adaptable to a greater variety and capacity of work (see Fig. 1). Barrel equipment also vanes by size. Sizes are grouped into three major categories: (1) production barrels, (2) portable barrels, and (3) miniature barrels. The largest units handling the majority of the work are production plating barrels. Next in size are portable barrel units, so named because of their generally smaller size and ability to be- handled manually, sometimes without the aid of an overhead hoist system. Portable barrel units are used for plating smaller parts, smaller lots, delicate parts, and precious metals work, which is too expensive to be done with large tanks or baths (see Fig. 2). Miniature, or mini, barrel units are used for many of the same reasons as portable barrels. Miniature barrels are used to process the smallest and most fragile loads and work. Also, miniature barrels are often used for lab work or process development (see Fig. 3). While rotation about a horizontal or inclined axis is the norm, many features of construction, components, and capabilities improve barrel versatility. Examples of major
Fig. 3. Ministyle barrel assembly with self- contained dl *ive and integral-mesh, i?lOlt ded baskets.
334
Fig. 4. Fully automatic with integral door barrel ofloperation.
loadiunload system assembly for hands-
barrel features that can provide increased produciivity include: (1) cylinders with maximized load volumes (see Fig. I) to function within the dimensional limits of associated equipment; (2) special diameter and/or length barrel assemblies to work in nonstandardized installations; (3) high efficiency and/or 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 requirements (see Fig. 4); (5) equipment to dry the work while it remains in the barrel saving transfer and handling operations; (6) “up rotation” apparatus to minimize contamination and carryover (drag-out) of solution to adjacent process tank stations; and (7) special apparatus to spray-rinse work inside the barrel to reduce water usage and ensuing treatment costs. The above 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 horizontal barrel assemblies manufactured to operate in existing rack plating installations (see Figs. 5 and 6). It is worthwhile to mention the horizontal oscillating barrel. This technique most often utilizes barrels that are open on top and have no doors or clamps. Motion is limited to a 180” arc back-and-forth 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 or bridge badly; however, 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 top. 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 rotation barrels to obtain equivalent results. Many older oscillating barrel installations have been converted to full rotation operation. The second major barrel equipment style, the oblique barrel, can be pictured as an open-top basket. It incorporates a rotation mechanism that generates a tumbling action around an axis tilted to a maximum 45” from the vertical (capacity diminishes beyond 45”). A feature of oblique barrels is the elimination of doors or other closure devices. Since the top is open, unloading consists of tilting the rotational axis of the barrel assembly to the necessary position,
336
Fig. 5. Barre/
assembly
equipped jtir use in a rack plating
line.
which 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. Included are 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 chromium where ample Fig. 6. Special length barrel line.
338
assembly jbr plating
elongated
parts or jtir use in a rack plating
current and continuous contact is available (and gentle part surface abrasion is not a problem). The capability of equipment to plate a particular finish, and to function in required solutions and temperatures, determines its usefulness. 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. A single line, producing more than one finish type, must control cross-contamination of the different plated metals in stations used for rinsing, sealing, chromating, and cleaning. WORKLOAD The barrel plater must evaluate the following items to decide if the desired finish for a particular part can be barrel plated: (1) finish function (relative to use of the part), (2) part configuration, (3) part size, (4) part weight, (5) calculated part surface area, and (6) total workload volume and square foot surface area. The workload capacity is usually 40-50% of the total barrel volume. The maximum barrel workload is usually determined based on total square foot surface area of the load and the capacity of the chemistry and equipment to plate. Plated finish functions are of three basic types: (1) engineering applications to attain (add material) or maintain a dimensional requirement and/or as a bearing surface; (2) decorative for appearance, which also enhances the value of the base material; and (3) corrosion protection to increase the useful service life beyond performance of the unplated base material. There are plated finishes that need to perform combinations of the above three basic functions. Engineering finishes are not usually applied by barrel plating. Decorative finishes are successfully barrel plated when surface effects from part contact are controlled to an acceptable level. Barrel plating is most commonly used to finish parts for corrosion protection. Part configuration affects the ability of work to be successfully barrel plated. Generally, parts that weigh
Fig. 7. Barrel inferior showin8 disk- andstrip-type c(mtacts.
EQUIPMENT
DESIGN
Horizontal, oblique, or other design barrel equipment must contain features to maximize productive capability. Minimum labor requirements and ease of maintenance should always be factors of well designed components and systems.
Barrel
Construction Barrel construction must be of materials that are unaffected by use in the applicable plating process. It is also important that the barrels be capable of operation in excess of maximum bath temperatures in the plating system. A plating barrel may expand and contract as much as j/x in. in overall length due to temperature changes in a plating cycle. Good design and construction will eliminate stresses that cause barrels to go to pieces due to temperature changes. Temperature changes acting on the different construction materials with different coefficients of expansion cause barrels to work apart. When barrels are constructed of all plastic or do not use metal fasteners, stress points are eliminated. These barrels can expand and contract at a uniform rate that greatly extends their useful service life. High temperature capability promotes barrel integrity and long life. It 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 structural member, to reduce manufacturing costs, should be avoided. Examples are: (1) trunion (hub) bearings molded as one piece to the barrel hanger-arm supports, and (2) cylinder ring, or bull, gears that function as the barrel head (end). These items perform adequately when new, but when the wear part needs replacement, the entire member or barrel must be replaced. This can be a very costly design for the user.
Barrel
Features For the majority of plating processes, the flat-sided barrels are best. Flat-sided barrels produce pumping action as a benefit of rotation. Pumping action constantly replaces metal-depleted solution inside the barrel with fresh solution from the rest of the tank. Flat-sided barrels also tumble parts more effectively. This is most effective when the interior surface of the barrel is ribbed, or grooved, to further assist tumbling of the work and eliminate the sticking of parts to the panel surfaces. 340
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 aid the tumbling of the work.
Perforations A plating barrel’s use must be considered when specifying the perforation sizes. Job shops generally obtain barrels with smaller openings to accommodate the widest range of potential workpieces. Captive shops most 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 (due to the pumping action mentioned previously), and less drag-out (carryout) contamination of adjacent tank solutions. This is because larger perforations minimize 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 barrels with larger holes. The barrels with smaller holes arc 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 the sides of the barrel, times the individual area of each hole, divided by the total panel area of the barrel. Since 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. Barrels with the same open-area ratio can be used, regardless of hole size, without the need to readjust rectifier settings or current density. Most barrels are or should bc manufactured with a 23% open area, which experience indicates optimizes barrel strength and performance. There are other types of barrel perforations available to the plater. These include herringbone (right-angle holes to prevent small diameter straight pieces from passing out through the barrel sides), screen, mesh inserts, and slots. These special hole types can be utilized for barrel plating straight and small diameter pieces (i.e., nails, pins, etc.).
Barrel Cathode Electrical
Contacts
The type of interior cathode electrical contacts a barrel has determines the variety of work the barrel can process. The flexible dangler (cable-type) contact is most common. 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, Best plating contact and results are achieved when the danglers remain submerged in the workload. Submerged danglers maximize contact and minimize or eliminate arcing, sparking, or burning of the work. Dangler length should result in the contact knob touching the bottom of the barrel I/4-‘/3 of the barrel length from the end of the barrel. For short barrels or stiff dangler cables, the danglers can be crossed in the middle of the barrel to ensure that they remain submerged in the load. Disk, cone, center bar, strip, and button contacts will usually do a better job barrel plating rods, long parts, and delicate parts. These types of interior cathode contacts are referred to as stationary because they are affixed to the barrel and rotate with the load. They are, therefore, stationary relative to the load. Stationary contacts are less abrasive to the work and generally exhibit fewer problems with entanglement. A plate-style contact is usually utilized in oblique barrel equipment.
Barrel Doors There Clamp-style
342
are several available styles and fastening methods for plating barrel doors. doors have predominated over the years. This is because they are both quick and
~
pg.
~
non door partition.
8.
Knob-style
with center
two-sec-
bar and
easy to operate. Knob-style doors are also greatly utilized. The threaded components of knob doors must be designed for efficient operation and useful service life to minimize replacement requirements. Divided doors, which are one-half the total barrel length, can be furnished for ease of handling and to work with partitioned barrels (see Fig. 8). There is, as in all things, diversity in barrel equipment and operations. Many shops use and prefer clamp-style doors. Others operate successfully with knob-style doors. Many shops use more than one style of barrels and doors. Because the effective security and operation of barrel doors is critical to operation of the entire line, much attention is given to this area. Some recent designs secure the workload with the door retained within capturing edges of the door opening, rather than being mounted from the outside. With some designs, the workload causes the door to seal tighter. This is good for very small parts or work with edges that pry and cumulatively wedge into crevices. When additional labor is needed to operate a door fastening system, the benefits of the door function must be evaluated relative to the extra labor necessary. There have been recent innovations for barrel operation and door fastening methods that eliminate manual door handling and labor for attendant operations. Automatic barrel operation translates into system automation, which can greatly enhance efficiency and eliminate costs. Barrel systems and related material handling equipment can be installed to automatically load, unload, and operate the barrels (see Fig. 4). This also benefits platers by maximizing worker safety.
Component Features Particular equipment features can substantially affect system performance and serviceability. It is 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 potential for accumulated or trapped hydrogen gas problems. Barrel rotation causes a cascading action of the workload inside the barrel. Because of this, the workload center-of-gravity is shifted to one side of the barrel assembly. Tank-driven, 344
horizontal barrel assemblies equipped with an idler gear, offset the center of gravity of the cascading workload to the proper side to best maintain good electrical contact between the barrel assembly contacts and the cathode contact saddles of the tank. Another positive feature is hanger arms made of plastic or other nonconducting materials. Nonconducting hanger arms eliminate treeing and possible loss of plating current efficiency. Design simplicity and efficiency of barrel equipment are important for ease of maintenance, particularly for components operating below the solution line. The use of alloy fasteners that are nonreactive to the chemical system in use is especially important for acidand chloride-based plating systems.
Hoist
System, Tank, and Associated Equipment The features of hoist systems and associated tanks are important to barrel plating capabilities. Most tank systems are designed to maintain the solution level approximately 5 in. below the top rim of each individual tank. At this operating level, the plating barrels should run fully submerged. One benefit is to eliminate the chance of excess hydrogen gas accumulation (for some plating types) in barrels that are not run totally submerged. Fully submerged operation reduces the potential for problems due to excess hydrogen accumulation. Operating with a solution level higher than 5 in. below the top rim of a tank can cause solution to be splashed out during barrel entry or exit, resulting in problems. Solution loss and adjacent-tank drag-out contamination can also be minimized by equipping the barrel hoist system with up-barrel rotation. A drive mechanism on the hoist tumbles the barrel and load in the overhead, above-tank, position. Up-barrel rotation facilitates drainage, especially when finishing cupped or complex-shaped parts. Greatest current densities for the workload are ensured by locating the plating tank anodes, including anode baskets or holders, in the closest proximity to the barrel exteriors without allowing mechanical interference. For horizontal barrels, anodes that are contour curved to just clear the rotational outside diameter, can result in a 1620% increase in current density. Tank-mounted barrel drives for horizontal barrels should be adjusted vertically to optimize engagement of the gears. Drives that are too high will carry the weight of the barrel assembly and workload resulting in excessive loads on the drive-shaft bearings. Excessive weight causes premature wear and failure of the gears, bearings, and other components. Reducer oil leakage is a directly related problem. When the weight of the barrel unit and workload is concentrated on the gear and drive shaft, rather than on the tank electrical contacts, proper contact between the tank saddles and barrel contacts is not possible. If the 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 barrel assembly gear hops across the tank drive gear and the unit does not turn. This situation not only results in premature gear wear, but also poor plating, cleaning, and processing of the work. It is best not to rotate all process station drives in a barrel plating line in the same direction. The advantage of having an approximately equal number of drives rotate the barrels in the opposite direction is to ensure or optimize even wear on all drive components (bearings, gears, etc.) and electrical contacts (i.e., extended dangler life.) Alternate drive rotations also minimize replacement requirements and downtime. Tank drives for barrels can have provision to change barrel rotation speed to allow for change of workload type or plating finish. For example, a lower speed of rotation is much better for very delicate or heavy parts. A faster rotation speed may be used to produce a more uniform plate, 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. 345
Certain tank drives provide for speed change with multiple sheave belt pulleys on the output shaft of the drive motor and input shaft of the speed reducer. Many present-day systems use directly coupled C-flange motors bolted directly to the reducer. The speed change adjustment capability on directly coupled units is accomplished electrically through the control panel. For a long time it was thought that tanks with more than three stations should be avoided. This is because separate or duplicate tanks with the same function allow the line to continue in operation if a bath needs to be replaced or one tank requires maintenance. Separate tanks for the same bath can be piped to each other to share the bath for uniformity. Each tank can be isolated with valves when necessary for maintenance; however, important as this consideration may be, many platers prefer to use single-unit, multistation tanks because of the homogeneity of the bath solutions and temperatures.
New
Developments There have been at least two notable developments in barrel system capabilities. As the industry moves toward minimizing water usage and treatment costs, rinsing and drying are receiving attention as operations that can be modified to provide savings. h-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 the line is equipped to dry 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. Also, the work may be tumbled in a dry condition, which can negatively affect some parts and finish types. With the benefits to be received from minimizing water usage and wastewater treatment costs equipment suppliers have participated by designing equipment that uses less water in the barrel system. One development is to connect separate rinse tanks from different parts of the line together, in sequence, to optimize the rinsing capability of the water before it is sent through the filtration and treatment process. In other words, the water is taken advantage of for more turns. This means less water is added to the rinse tanks in total since it is utilized for more repetitions. Of course, not all rinse tanks can be handled together this way; however, where it is practicable, the water savings are tremendous. For example, acid rinse baths can be subsequently utilized for the post-cleaning rinses, since 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 that a bath-type, immersion rinse. Spray station equipment has been furnished rather than the typical immersion bath. Some equipment is also available that can spray-rinse work inside the barrel.
RATE OF PRODUCTION Reasonable production may be maintained with total workload surface areas ranging 6&100 ft2 per single barrel. Current draw varies with the type of plating and may vary greatly. Most production barrel platers operate in the IS-40 A/f? range. Nickel plating can go as high as 50 A/f?. Actual current density is higher since only the exposed surface of the barrel load, in the direct path of the current at any time, is platin g. The exposed surface is much less than the total surface of the entire load. All surfaces eventually receive the same relative exposure due to the tumbling action. Barrel tanks generally draw higher current than do still (rack) tanks of the same capacity. It is, therefore, important to equip barrel tanks with greater anode area, usually in a 2:l ratio to the total surface area of the workload. Barrel anodes corrode faster; however, the production is much greater. There are procedures explained elsewhere in this Guidebook that permit 346
estimating the time required to deposit a given thickness for many types of plating. also information for selecting proper current densities and total cycle times.
There is
RECORDS Proper operation of a barrel plating line requires the maintenance of records for each part and plating specification done in the shop. These can be entered on file cards or in a computer database. The information may be used to construct graphs. Since many of the thickness, time, area, and current relationships are relatively linear, one can use previous job information to 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.
Reference 1. Metal Finishing tion, Westmont,
Industry Market Ill. and National
Survey 1992-1993, Metal Finishing Suppliers Association of Metal Finishers, Chicago; 1994
Associa-
Gold Plating Technology by F.H. Reid and W Goldie 630 pages SlS5.00
centrifugally, eliminating c rejects I caused by spottina! !
rhis is a critical review of the litera:ure with over 900 references cited. The 42 chapters, written by 23 authorties, cover everything from historical >ackground to applications. A worthwhile investment.
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