What are the considerations for electrocoating tubular products in a custom coating environment? R.B.
In a custom coating operation it is much more difficult to control the product design. Part design is the best way to deal with tubular components. Part or product design usually does not give much consideration for the coating operation. Since electrocoat always includes immersion coating and quite often immersion pretreatment, tubular parts that do not fill and drain readily are always a problem. Drainage can also be a problem for pretreatment prior to liquid or powder coating, but to a lesser degree than electrocoat.
Part design for OEM’s or manufacturers who use a custom coater should give more consideration to the coating operation. Specifically for electrocoat, part design (in order of being “coating friendly”) should include one or more of the following design criteria:
- The tubular section should completely fill and drain easily and quickly. Leaving the ends of all tubing open does this best. With electrocoat’s exceptional throwing power, corrosion protection can be improved by coating the inside as well as the outside.
- Any tubular section that does not have holes in it should be welded or crimped watertight (and probably airtight). If airtight welding can be accomplished, remember that the air space trapped by welding will displace a like volume of liquid (electrocoat) and could create a floating problem. Airtight welding is usually necessary because the first few stages of pretreatment are heated and the air inside the tubular section attempts to expand, pressure testing the part. Since the final stages of pretreatment and the electrocoat bath are at ambient temperature, the welded tubular sections, if not watertight, act as a vacuum, drawing liquid into the air space as the part cools. If the openings are only pinholes, the liquid that is “sucked in” will probably not drain out by gravity. This trapped liquid will probably boil out during the baking process or stay inside and act as a heatsink causing part of the coating to be uncured or undercured.
- Tubular products or sections of parts that do not have open ends or are not welded tight should be given other consideration(s). Additional holes in the tubular section to allow for liquid to flow in and out may reduce “boilout” or “bleedout” problems. However, unless the holes are quite large, contamination and/or dragout between stages of pretreatment or the electrocoat tank and post rinse system will result. To determine if floating, drainage or air entrapment are going to be a problem, submerse the part in a clear container of water, watch for air bubbles and measure the length of time for the part to drain completely. Determining the proper hanging method and/or the effect of additional drainage holes can also be accomplished by this water test.