Rack Design


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Q. We seem to be spending a lot of money on racks in order to get maximum part density and spend a lot of labor changing and repairing the racks. Do you have any secrets that would help us? A.S.


A.The one word answer to your question is “compromise.” There are a lot of considerations when designing tooling. These considerations are best reviewed in outline format since the interactions (compromise) are easier to evaluate that way:

1. Universal design vs. individual (or group of ) part(s) design
1.1. Usually depends on the number of different parts run and the frequency they are run
1.2. Universal tooling reduces the labor for changing racks and therefore storage and repair cost
1.3. The down side to universal tooling is that it usually has less than optimum part density compared to individual part rack
1.4. Another compromise is to design for a series of parts that are very similar in size and can be hung on/through/in the same pin/hole

2. Horizontal vs. vertical universal rack design
2.1. Horizontal design works well if you have a lot of long/skinny parts that vary in length
2.2. Horizontal design can also help use the third dimension:
2.2.1. Perpendicular to the line for a monorail line
2.2.2. The direction of travel for a square transfer system
2.3. Horizontal design can create some grounding and maintenance challenges since most contact points are below liquid (paint) level
2.4. Vertical design works best with smaller parts since the individual racks can be loaded off-line and transferred to the line or loaded on-line with operators at different elevations
2.5. Vertical design can usually allow for the contact points to be above the paint level which reduces maintenance
2.6. Many times a combination of vertical and horizontal design can be used on different portions of the line or alternating loadbars

3. Hooks vs. pins
3.1. Hooks are good for large heavy parts since the weight of the part create grounding contact as the part proceeds through pretreatment
3.2. Square stock gives better grounding, but is more expensive and not necessary for heavy parts with good hanging holes
3.3. Square stock formed “on edge” (diamond cross section) is better for lighter parts, but can break if formed with a tight V shape and small square stock is hard to find
3.4. Pins (round, square and wedge shaped) are great for establishing a good ground as it usually amounts to putting “a square peg in a round hole”
3.5. If pins are mounted vertically, the parts hang at an angle reducing part density but improving grounding
3.6. Pins mounted at a slight angle increase density and can be done vertically (herring bone style) or horizontally (by alternating the pins on either side of the support rod/bar
3.7. Pin length can be an aid in keeping parts from falling/floating off while being immersed whether on a monorail (slower) or square transfer system (faster)
3.8. Wedges are the best shaped pin as they allow for a wide variety of hole size and shape
3.9. Wedges work best when the base of the wedge is larger than the hanging hole size in the parts; this prevents a “contact line” where the parts rest against the support rod/bar

4. Hanging parts without holes
4.1. If parts are not susceptible to “air pockets” and “puddles”, they can probably be processed in baskets made of non-flattened expanded metal (for shape edges); usually results on multiple contact marks which might be objectionable
4.2. On square transfer systems, if these baskets can be the bottom part of some carriers, they can be used for processing parts without holes and also as a “safety net” to catch parts that may fall/float off of hooks/pins above the basket
4.3. If parts are susceptible to “air pockets” and “puddles”, the use of wedged-shaped pins mounted at the proper angle and spacing can eliminate the “air pockets” and “puddles” and provide minimum contact marks

5. Weight and design considerations
5.1. Another compromise is that the stronger the tooling is the longer it lasts and less maintenance is required, but the more “wasted weight” is involved
5.2. “Wasted weight” is excess tooling weight that requires BTUs to be heated up to cure temperatures without producing any sales or increased inventory dollars
5.3. “Wasted weight” can be reduced by:
5.3.1. Forming or welding strength into racks as opposed to heavier stock
5.3.2. Use of laser or plasma cut rack components can reduce weight without loss of strength
5.4. Weight reduction can result in tooling damage during cleaning if burn-off is your method of cleaning

6. Welded vs. “assembled” design considerations:
6.1. While welded racks are probably cheaper to make and provides good ground contact, it usually results in “wasted volume” when being stored and cleaned
6.2. “Wasted volume” when racks are not being used and require storage can be at very large expense depending on how and where the racks are stored
6.3. “Wasted volume” when racks are being cleaned can be at very large expense depending on how racks are handled, stored, and positioned during cleaning (in-house or out-source)
6.4. “Assembled” tooling includes “snap-on” hooks or interchangeable crossbars and are usually part of universal tooling
6.5. While more expensive to build, “assembled” tooling can significantly reduce “wasted volume” as the components take less space to store and better density while cleaning
6.6. The other challenge to “assembled” tooling is maintaining good ground contact at the points of assembly, but several secret design considerations have been developed that allow for easy assembly with good contact

7. Cleaning considerations
7.1. While addressed in several points above, cleaning can be a very large expense of tooling design
7.2. Burn-off (pyrolysis, hot sand or molten salt) is the most used method of cleaning E-coat tooling but requires temperatures above 500° F that can weaken tooling
7.3. Many burn-off operations find “hotter and faster” can produce quicker turn-around when cleaning racks and temperatures above 700°F are not uncommon which results in greater damage by warping or weakening tooling
7.4. Chemical cleaning does less damage but usually is slower and more expensive if disposal costs are included
7.5. Blasting (shot, grit, sand or plastic media) varies in cost and effectiveness depending on the aggressiveness of the media and labor involved
7.6. Blasting usually results in reduction of sharp edges and therefore more grounding requiring more frequent cleaning

This answer has expanded into “Tool Design 101” but hopefully provides an understanding of the opening statement that tooling design is really a series of compromises of various cost considerations. No one design fits every system and quite often several designs are used. Hopefully this outline format will allow for discussion with your suppliers of coating, equipment and tooling fabrication and cleaning to assist you in these decisions. 

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