How to Choose the Right Coating Application Method: Part One

Rack-Spray

Example parts suited for a rack-spray coating application. Many of these started off in dip-spin and came to DECC after quality issues presented themselves. Note: parts are not to scale.

DECC is almost an exclusively rack-spray coating company. This method involves hanging or placing a part on a fixture and then spraying the coating utilizing either electrostatic guns, HVLP guns, or a combination of both. Both types of guns could be either automated or used by a hand-spray technician.

Parts most suited for rack-spray are almost or completely flat components (like washers or snap rings as they nest in bulk applications), metal stampings with recessed pockets where coating could pool if bulk processed, components that can become tangled or distorted when bulk processed, heavy components, and anything that requires masking. Examples of some such parts are below.

Small fasteners, bolts, nuts, hardware, and tightly coiled springs are not very well suited for a rack-spray application. Components that require coating in areas that are not “in the line of sight,” such as hollow part geometries or parts with small or deep thru holes, are not well suited for a spray application either.

Rack-Spin

This is a relatively new application method…to North America, at least.

Rack-spin is similar to dip-spin except that no parts touch during processing. As its name suggests, components are racked to, or placed in, a fixture inside of a barrel. This barrel is then dipped into a vat of coating, completely submerging the parts.

The barrel is then lifted out of the coating, or the coating is drained from the vat, and the barrel is spun at a high RPM. Centrifugal force spins off any excess coating. The entire rack is then placed on a conveyor, cured, and if required, the process is continued until the necessary coating thickness is achieved.

This type of application method is ideal for oversized and very complex geometries, such as heavy metal assemblies with multiple welded assemblies or large frame components with hollow design features, where complete coating coverage is necessary.

The same limitations that are present for rack-spray parts on small fasteners, bolts, nuts, hardware, and tightly coiled springs would prohibit them from this application as well. Additionally, small to medium size metal stampings that are suited for rack-spray, but not dip-spin, would also not be an ideal fit for this coating method because a high-volume rack-spin price would be considerably higher when compared to a high-volume rack-spray application on the same component due to thru-put limitations.

For an overview of rack-spin coating and to gain an idea of parts suited for such a process, watch this informational video from Magni Coatings below.

Chain-on-Edge

Chain-on-edge coating systems start with placing a component on a spindle fixture of some kind. These spindles then move along a guided chain thru the coating process.

When they reach the point of coating application in the system, the spindles are engaged and the part then begins rotating as it passes in front of the spray gun, ensuring a consistent film build on the entirety of the part. Most systems have in-line curing steps after the part is coated. Below is a video of DECC’s chain-on-edge system that was utilized for coating projectiles for the military.

Based on the above description and accompanying video, it is easy to see that there are a limited number of part geometries that are suited for this application method.

Cylindrical components are really the only type of geometry that works on a chain-on-edge system, such as armatures or projectiles. Some systems can coat both the OD and ID of a component if required. Larger bolts can also be a good fit for this type of system if fixtured properly.

However, outside of the above, there really aren’t many other types of components that fit this coating application method…but it should be noted that this is an extremely effective and efficient method of coating the ones that do.