Several readers have asked about the challenges of coating screw threads. As newer coatings are developed for other improvements, has coating threads also changing?
Some of the same questions being posed now are similar to those that were asked more than a year ago when I provided a response in the September 2002 issue of Products Finishing (www.pfonline.com/articles/clinics/0902cl_ecoat1.html). A portion of that response still applies and is as follows:
Female threads are susceptible to possible “air pockets” and “puddles,” which results in inadequate coating or excess coating as the coating bath collects in the threads. Blind threaded holes in castings or machined components should be avoided or positioned to trap air (with no coating). Blind holes also can cause contamination of pretreatment or e-coat baths due to machining fluids and/or pretreatment chemicals being carried over. A smaller through-hole or tapping after coating can solve blind hole problems.
My experience with threaded fasteners is that e-coat film thickness has little effect on threads 1/4-20 or larger. This is probably due to the tolerances on both male and female threads and the uniform thin film thickness of the e-coat. Fasteners smaller than 1/4-20 may experience some difficulty during assembly. Trial-and-error testing is a good first step. If inconsistency is found, then checking the threads with a “go/no-go” thread gage before and after e-coat will tell you if you need to specify under (male) or over (female) sized threads on the fasteners. Using assembly tools with variable torque settings can also help if the e-coat seems to cause an interference fit.
As newer coatings are being developed toincrease dry film thickness (DFT) and/or uniformity of the DFT, threaded components become more challenging. Measuring DFT on threads is almost impossible due to the peaks and valleys. Accurate thickness can only be obtained through photomicrographs, which are very time consuming and require sophisticated and expensive equipment. Most paint suppliers have this type of equipment and probably have typical pictures of threaded parts.
As film thickness increases and more uniform coating (more on the peaks) is possible, interference may be found. The use of a “go/no-go” thread gage may detemine if you need to investigate undersized fasteners before coating and/or masking one of the two components.
As with most improvements in coating technology, some compromises are necessary. The coating of threaded parts is probably one of these compromises. Get your fastener and coating suppliers to work with you.