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Hanging Profitably

June/May - 2009
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Developing sensible and practical hanging and hanging methods are some of the most important tasks associated with coating a product, but are often given only secondary consideration. Emphasis should be put on maximizing output and productivity.  Organization and planning is essential and with some effort, the coating line can be a profit center and no longer a liability.
The following concepts and ideas are presented to the coating professional in order to aid in the process of selecting the most appropriate hanging method for the particular application.

Hook and rack construction can take many forms.  However, to simplify things it is best to break it down into the following categories; dedicated versus modular design, material and rack construction, hooks and product holder types.

Dedicated vs. Modular  Dedicated Racking Systems are designed to hold a specific part or parts.  Dedicated racks are typically of welded metal construction with permanent product holders (such as hooks, clips, pins, wedges, fasteners or magnets).  Often the product holders can be made to be removable so they can be replaced as they wear which will reduce rack repair costs. However, the frame and product holders will most often be set in position.
Modular Racking Systems are designed to be re-configured to efficiently coat a variety of parts.  Many modular racks have removable crossbars and product holders which allow for flexibility in hanging several sizes and styles of products.   

Dedicated racking is typically best for high volume similarly sized and similarly fixtured parts, where you can utilize one or a few racks to handle the greatest percentage of coated product. Modular racking should be considered when coating small to medium volume parts with varying sizes and geometry.  However, the decision whether to use dedicated or modular racking in your coating operations depends upon many of those factors discussed in the previous section, namely; the quantity of parts that will be run, the frequency that the parts will be run, plant floor space, working window/density, budget, etc.

Material and rack construction  Most hanging products for Powder, Liquid or E-Coat applications are typically are produced from either carbon steel or stainless steel materials.  Stainless steel constructed racks will typically last longer than carbon steel racks, especially if rack is to be cleaned thermally or by “burn-off”.   Thermal stripping typically exposes the rack or hanger to temperatures between 800 degrees F and 1000 degrees F.  At these temperatures, stainless steel composition remains relatively stable.  That is not true of steel materials, because the carbon within the steel begins to break down at high temperatures.  The amount of breakdown and reduction of carbon content will be determined by the frequency and temperature of thermal cleaning process and the time required.  As carbon steel breaks down it becomes soft, making it more likely to distort, stress and consequently, bend.  As the rack deforms, then coating quality and production output will likely diminish and the rack or hanger will need to be repaired or replaced more often.  Stainless steel material cost is typically, two times to two and a half times the cost of carbon steel.  Sometimes hardened steel or spring steel can be substituted for stainless steel to provide longer life to hanging and rack components and for a much lower cost.

Material hardness  There are several grades of either steel or stainless steel that exhibit many different properties, however we will only discuss two; mild and spring tempered. The differences in the two classes amount to the carbon content and draw down.  Spring steel typically has higher carbon content than milder materials. In addition, higher hardness is also derived from the draw down during the manufacturing process.  Draw down is essentially the amount of compression the material goes through during manufacture.  Hardness is important for holding product and weight capacity.  From a fixturing or product holding standpoint, if the product to be coated needs to be held tightly to prevent movement, floating, premature removal, drainage, then spring or hardened material may be the best choice. Spring tempered or hardened materials also provide higher weight load capacity than mild steel. 

Hooks Part Holders come in a variety of shapes and materials.  The simplest and most common are hooks.  Whether with sharp bends or radius bends, hooks are often the hanging method of choice.  A hook can either be hung from the coating system individually or be part of a modular or welded rack system. Hooks can be made from virtually any suitable material, i.e. round wire, square wire, or flat stock.  Round wire hooks are the most common and are used for parts with suitable hanging holes.  Square wire has gained traction in the last few years.  When bent on the edge, square wire reduces the contact area and coating buildup between the hook and the part.  The smaller contact area reduces the “hook mark” left on the product and provides better grounding for the next part to be hung. Hook geometry and configuration can also play an important roll.  For example, additional material or bends can be added to the hook to prevent the product from floating or falling off the hanger or rack. Orientation of the hook on the rack or coating line can also improve density.  For instance, hanging relatively flat “ware” product at 45 degrees relative to the rack will increase density 1.5 times versus hanging parallel to the rack or hanger.  In addition, hanging product on both sides of the rack or hanger will likely double the amount of product that a rack or hanger can hold.  Finally, nesting hook/crossbar configurations or staggering hook/crossbar configurations may also substantially increase product density.

Pins  Pins, like hooks, can be made from variety of materials.  Pins are typically welded to cross bars on a rack.  The pin material and configuration depends upon what product or products will need to be coated and where they can be fixtured from.  Like hooks, parts with round hanging areas might be best hung from square pins to minimize contact areas.  Parts may also be hung more securely by increasing the angle of pins as related to the cross bars.  In this instance you will be using gravity to provide security.  Like hooks, pin orientation and pin/crossbar geometry can also play an important role in increasing or maximizing product density.
Arrows/ Wedges  Arrows or wedges are product holders typically laser cut or stamped from flat steel and welded or secured to rack or hanger.  The tapered arrow or wedge design allows for a variety of parts with different hanging hole sizes to be securely fixtured.  The thin edges provide a “bite” into the part causing good contact between the product and the hanger and good holding retention too.  When using arrows it is always advisable to insure the base of the arrow is larger than any product hole.  This will prevent the part from contacting the crossbar and also prevent unwanted movement of the part and allow for proper drainage during the coating process.

Clamps, clips and magnets There are a variety of clamping and clipping devices that can be employed to hold product more securely or hold product with no suitable hanging hole(s).  Clips can be designed from wire or flat stock material to achieve good holding retention while minimizing hanging marks.  Sometimes magnets can be used to hold a part into place where no hanging holes.

Other rack and construction considerations  When designing racks, attention must be given to design and durability.  Unlike coated products, racks and hangers are typically mishandled and abused.  Therefore, designing a rack to handle the mishandling is probably a good idea. However, a rack can also be over engineered and carry excess weight than necessary.  Excess weight puts undue stress on the conveyor and also increased the amount of energy needed in the coating operation. Utilizing lighter materials with reinforcements like gussets and supports can remove weight, while providing good durability. In addition, laser cut profiles and crossbars can provide good hanging options without the need of machined products. 

Economics of Hanging In concluding the discussion on hanging, it is best to illustrate why all the factors aforementioned are important.  There is no better illustration than a real example of a line density calculation.  In the following example, we have a customer running a 600 ft. monorail e-coating line that runs approximately 1 shift per day, 5 days a week.  It was determined that the customer could have hung four more parts per rack with a slight modification to the racking system. 

Data Set #1 :
Line Speed = 10 ft./minute   
Line Length = 1000 ft. 
Run Time = 7 hrs./day

Calculation # 1:
Daily Passes = [60 min/hr /(1000 ft. / 10 ft./minute) /] x 7 = 4 per day

Data Set #2 :
Days Run Per Week = 5
Rack size:  44” W x 40”H
Number of Racks on line: 251
Weeks Run Per Year = 13
Price of Product = $0.164 /ea.
Parts not hung = 4 per rack
Equation:  (4 passes x 5 days x 13 weeks) x [$0.164/part x (4 missed parts x 251 racks)] = $42,812 in lost revenue.

So in this example, the coating line would have generated an additional $42,812 in revenue just by designing their coating racks to hold four more parts.  With proper analysis and careful design and development, this e-coater could have achieved maximum density from the beginning, adding an additional $43,000 to their top line for only one coated product.
Conclusion  Too often budgets for hanging products are arbitrarily determined from the beginning.  However, the budget should be determined after careful analysis of all other factors first.  Designing the most efficient racking system should be first priority.  It is most often the case that savings in labor and the increase in output from optimizing the hanging system will exceed the costs of upgraded components and systems.  

This article was written by:
Scott P. Rempala, President
Mighty Hook, Inc.