مقالات آماده سازی قطعات

finishing equipment & plant engineering

BASIC CONSIDERATIONS AND

CONCEPTS FOR RACK DESIGN

BY DON BAUER, ASSOCIATED RACK CORPORATION,

VERO BEACH, FLA.

Historically, metal finishing racks have come a long way in their design. Thecrudest form of racking is simply a wire twisted onto the part. With the productionand quality requirements of today, an expensive piece of copper wire will notsuffice. The parts must be presented to the anodes with consistency to ensurevisual and measurable quality requirements. For decorative automotive finishesof today, require custom racking. In this paper, I will review some basic conceptsfor rack design and maintenance.The most important aspect of any rack is the design. Without the properdesign, one cannot expect to obtain the optimal productivity that the processis capable of achieving.

There are many factors that come into play when designing and buildingracks. In this paper I will discuss: Equipment Manufacturers, DimensionalRestrictions, Weight Restrictions, Part Presentation and Spacing, RackMaterial, Manufacturing Techniques, Optimized Designs, and Selecting aSupplier.

EQUIPMENT MANUFACTURERS

There are several automatic equipment manufacturers in the market and eachmay have distinct methods of attaching or mounting a rack. The two mostcommon lines manufactured are the return line and hoist line. Regardless ofthe equipment, the first consideration is how the rack will hang on the cathode.Return lines are typically a “Carrousel” design. The racks will travel from stationto station in a circular movement. The racks are usually similar in dimensionsand will carry, as close as possible, the same surface area. Manufacturers ofreturn-type equipment use brass alloy castings, or hangers, to carry the currentto the rack.Hangers are different; what needs to be known to ensure a perfect fit isthe center-to-center dimension, the area where the rack hangs on the casting.Castings are mounted on the machine and the racks hang on specific notchedareas of the casting. If a rack does not hang properly onto the casting, it cannotbe assured of the required current flow, and one runs the risk of the rack nothaving the appropriate clearance as it moves from station to station.

The advantage of a hoist line is the racks can be designed to have varyingcenter-to-center locations and can vary in overall width, allowing several racksto be loaded onto a flight bar. Hoist lines typically do not use castings and theracks are simply hung onto the cathode bar.

DIMENSIONAL RESTRICTIONS

The primary dimensional restrictions (the work envelope) are the overall height,width and length of the rack and the placement of the parts. These specificsmust be known before a useable rack can be fabricated and successfully perform.The overall height is measured where the rack hangs on the hanger or flightbar to the bottom-most useable space in the process tanks. One of the mistakesmade when taking these measurements is neglecting unseen any piping in thebottom of the tank. The other common mistake is not taking into consideration

obstructions on the top of the tanks when the racks are moved from tank totank.The length is the useable left-to-right dimension. This is important on both areturn and hoist line. For a return line, typically one rack will be set into a singletank for rinsing. If the width of the rack is too wide, then rack and parts can bedamaged from hitting or scraping the walls of the tank. On a hoist line, rackswith the wrong width will not allow the maximum number of racks on a flightbar, resulting in lower overall production.

WEIGHT RESTRICTIONS

The manufacturer has designed the machine hoists systems to handle specificweights which will include the racks and the parts. This information must beknown and may restrict the number of parts that can be processed on a workbar. This is not typically a problem, but it can be if particularly heavy parts arebeing processed. Also important is the human weight restrictions in the loading/unloading segment of the process.With racks that are to be handled off line, the rack designer will need to knowwhat the weight limits will be as required by the customer’s safety and healthadministrator.Many situations require parts to be loaded onto the rack and then hand carriedto the plating line. The rack supplier must ensure that the total weight ofthe rack and parts do not exceed the maximum restrictions. Keeping the weight

manageable will avoid possible lost time, injuries and worker compensationclaims.The use of aluminum in the framework of the rack will eliminate the weightthat copper adds. (This will be discussed later.)

PART PRESENTATION AND SPACING

Most plating processes are reliant on proper positioning of the parts, particularlyduring the plating process. Consideration of the part configuration is vitalwhen planning how it is placed on the rack contacts. If a part is cup-shaped, thepart would not rack with the cup facing up or down. The desired positioningwould be such that the solutions flow freely in and out of the part when beingmoved from tank to tank.

Maximum exposure to the anodes must be achieved. For the same cup-shapedpart, the open end of the part must face toward the anodes, especially whenplating inside of the recessed area is a requirement. When plating wheels,one would not face the outside diameter of the wheel toward the anodes. It ispreferable to position the face of the wheel toward the anodes and configurethe rack to locate the rim as close as possible to the anode to allow for properthickness and plating distribution. Even with these design concerns, somerecesses might not plate properly. In these cases, an auxiliary anode must beconsidered.

AUXILIARY ANODES

Plating inside of recessed areas, such as wheels and step bumpers, is difficult andsometimes impossible to do without the use of an auxiliary anode. The determinationis at times easy to make and at other times not so simple. Experienceand knowledge of the process is normally adequate to make the decision. Attimes it is necessary to use an auxiliary anode due the part simply not platingto requirements or specifications.Auxiliary anodes should be made adjustable and easily removable to facilitateloading, unloading and replacement. The auxiliary anodes must be secured tothe rack using insulated material so the anode does not cross the cathodic currentpath with rack.

Auxiliary anodes can be made using lead, steel, stainless steel, nickel, or coatedtitanium, and the choice of anode material is process specific. For nickel/chromeprocesses, it is best to use auxiliary anodes made of titanium that is platinumplated, platinum clad niobium mesh. During the past few years, platers havehad success using an iridium oxide coating on titanium. This material is lessexpensive than platinum and normally lasts longer before recoating.

ROBBERS AND SHIELDS

Robbers are used to prevent burning in the high current areas of the part beingplated. Robbers also have other names such as “thieves” and “burning bars.” Partswith sharp edges or points flush left, no indent at times need special care. Thecurrent flow to these areas is typically excessive and the parts have a tendency toburn. By placing a robber, which is connected to the cathode and positioned nearthe high current area of the part, the robber will pull the plating away from thehardware, resulting in a part that is free of excess plating and burning.Shields are another option to feather the current path to the part. In this case,a shield is used instead of a robber for the parts on the outside border of therack. A shield can be made of a non-metallic material and is used to cover uphigh current areas, shielding the plating flow to high current areas of the part.This is an effective and lower-maintenance alternative to robbers.

RACK MATERIAL

In selecting a material to build a rack, the preferred metal is copper. To carry thenecessary current throughout the rack and distribute it to the parts, nothing ismore conductive than copper.Some suppliers have been known to use steel in an effort to “price down” theirproduct. This should be avoided at all cost. Steel only carries 12% of the currentfor the same size in copper.Aluminum has been successfully used even though it only carries 60% of thecurrent as compared to the same size material in copper. Normally when weightis an issue, aluminum is a logical choice.Stainless steel is used throughout the industry as the preferred tip material,and alloys 301/302 and 316 are used. Stainless steel offers adequate currentcarrying capacity in most cases and is resistant to strippers that are used whenthe racks are cleaned.

MANUFACTURING TECHNIQUES

To protect the metallic framework of the rack from the chemistry of the platingline and to insulate the metal rack components from plating, the suppliermust coat the rack in a good PVC (plastisol). All plastisol are not created equal.Some rack suppliers cut corners by using bottom-shelf plastisol and this is notobvious when the racks are delivered. The rack looks as good as a higher-pricedcompeting rack. The problem will evolve and is noticeable when comparing thelife of the plastisol. Premature degradation of the plastisol will create cracks inthe coating, causing solutions to be transferred from tank to tank and contaminatingprocess baths. It will also cause acids to reach the copper framework andeat away at the rack material. Coating failure and damaged framework will costmore in the end due to early repair of the rack and replacement of the damagedframework. One should never purchase racks based upon cosmetics alone, asmuch can be hidden under the plastisol. Be sure that your supplier uses goodquality material from the metals to the plastisol.Building a plating rack is not rocket science. For the most part, all rack suppliersuse similar methods of manufacturing — that is, we cut, drill, punch, bend,rivet, bolt, and coat. The difference from a good supplier and an “adequate”supplier is quality of construction.The preferred method of constructing the rack frame is to bolt the pieces ofthe framework together. There are occasions when the customer requests thatthe framework be soldered at the joints. This does improve the integrity of thejoint by fusing the two pieces together, but there is a question if it improvesconductivity.Tips can be bolted or riveted in place on the rack. Either method works finefor smaller tips; larger tips will require bolts. Rivets do offer a tighter contact inoverall pounds per square inch. One may twist the bolt head off trying to matchthe “psi” of a rivet.Racks do have a life expectancy. Over time, through use, tips will break, theplastisol will break down, or your machine will crash, making a pretzel out ofyour once-square rack. When one returns a rack to your supplier for rebuilding,one should expect more than just repairs. On a rack that has been used, resulting

in the tips being broken from stress, the rack supplier should replace allof the existing contacts with new ones, thereby turning a repair into a rebuild.This normally costs a little more, but what good is it to have a few tips replacedonly to have a few more break within a month after getting the rack back afterrepair? The first tips broke due to stress, and the rest of the tips are also stressed.Finishers should never have only the broken tips replaced.One method to save money is to consider replaceable tip racks. These are madeto allow the broken or worn tips to be replaced by the customer without theexpense of sending in the frame. Extra tips can be kept on hand for use as required.

OPTIMIZED DESIGNS

Your supplier would like to sell you as many racks as he can, but in most casesthis is just not necessary. Competent and experienced rack suppliers will designthe racks to carry as many different parts as possible on the same rack. By doingthis you can minimize your inventory and initial costs.Unfortunately, you cannot always optimize the rack design. One must alwaysplan for enough in the budget to supply all the process requirements. Moneynot used can be returned to the account, but anyone who has underestimated acapital request knows that it is next to impossible to receive extra funding aboveand beyond what was previously approved by management.There is always the human element that must be considered when designingracks, and ergonomics must always be considered. Excessive contact squeezingcan cause repetitive motion injury such as Carpal Tunnel Syndrome. Gravityor push-on tips should be incorporated in contact design — when the processpermits — and this will help with productivity and keep employees happy andcontent. It is always a good idea to rotate your people to avoid repetitive injury.

SELECTING A SUPPLIER

When selecting a supplier, one should not look for the cheapest, but rather onethat offers the greatest value. In many cases you get what you pay for. After all,who goes into a tire store and asks for the cheapest tire? It may cost you less, butit will never last as long, or perform on the same level, as a better tire.Check the present customers of the company you are considering; they cangive you some insight to their capabilities. Make sure your supplier is knowledgeableof your processes and requirements. More importantly, find a companywho cares about the metal finishing industry. You will know these companiesthrough their involvement in industry-related organizations.

CONCLUSION

I have presented some basic thoughts on rack design that should help you inunderstanding the world of racks. The hidden message in all of this is: Let aprofessional take care of all of your racking needs.There is one more important message to consider: There is a secret to selectingthe perfect rack supplier. Please feel free to contact me, and I will tell youthe secret to finding the perfect supplier. You, too, can then discover the “RightStart to a Perfect Finish.”

For more information, e-mail Don Bauer at این آدرس ایمیل توسط spambots حفاظت می شود. برای دیدن شما نیاز به جاوا اسکریپت دارید, call

(772) 321-6448 or visit www.associatedrack.com.