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Powdercoat Spits - Do you know causes and cures?

We have had several problems with the spits. We have had reviewed and tested all the equipment: nozzles, pipes, guns etc as well the process conditions like air pressure. Some body know if thre is another potential causes and actions to eliminate the spits? We eare thinking that meybe is the powder, but we don't know how to demonstrate it?

Thanks in advance

Miguel Segoviano
Powder Coating - Querétaro, Quer´taro, Mexico
2004

Dear Miguel,

You specifically use the word "spits" and not "surging" which implies that you are using a powder to which mica or aluminium has been added to give you the desired surface effect. Certain mica colours and aluminium if they are incorporated above a certain percentage into the powder may cause spitting and this is a thing you will have to put up with if you want to retain the finish you have now.

If you mean surging then I will leave that to those more expert in application problems than myself to advise you.

Terry Hickling
Birmingham, United Kingdom
2004

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2004

I have experienced this phenomenon many times at various customer facilities in the past. Major culprits have included:

Environment/Formulation:

High humidity in the application area will kill the transferability of many formulations, particularly whites, lights and low cost formulas that are loaded with high SG materials such as TiO2, calcium carbonate and barium sulphate. This effects fluidization causing "rat-holing" in the feed hopper, and also promotes build up in the pick-up tubes, hoses, venturis and gun tips which restricts the even flow of air and powder.

Mechanical:

Many automated triggering systems have a Voltage/Pressure regulator (sure the name differs across manufacturers)located inside of the actual gun controller. This is a small diaphragm that regulates flow/atomizing air during the spray phase. This item is lubricated, and when the lubricant dries up and this part begins to stick, which it will over time, you will experience spitting due to uncontrolled air pressure.

Poor programming on automatic triggering. Purge cycle was happening too soon, splurging paint all over the trailing edge of the part.

Hoses kinked or too short. On a reciprocating gun bar, the top of the stroke may be stretching or kinking the transfer hose, causing a temporary restriction then a sudden burst of air pressure.

Just a few quick ideas based on my experience, hope it helps.

Andy P.
- Grand Rapids, Michigan

Ed. note: thanks for the excellent ideas, Andy; full name next post please.

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2004

Here are a few more things to check-

1)Air leaks- the gun spraying action is actually a vacuum. Make sure all of your connections are tight, and none of the hoses have cracks or holes in them.

2)Fluidizing- sometimes the hopper can look like it's fluiding correctly, but if you have a large or old hopper, or lots of humidity you might want to reach in the hopper and feel what is actually happening with the paint at the bottom, don't forget that this is where the pick-up tube is getting the paint that is going through the gun. 3)Gun out of adjustment- When I train a painter I always tell them to start with the air adjustment to be half as much as the paint, then fine tune it from there. The painter should have a smooth even flow of paint coming from the gun when it is adjusted right. Spitting or surging is sometimes caused by too much paint and not enough air.

Good luck.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina

I also have a severe problem of gun spitting in Nordson Auto Excel Booth. We replaced numerous parts as per Nordson recommendations, adjusted the guns cloud, controlled the fluidization but could'nt get it finished.

Can somebody help me as I have no further clue of what to do and I have a deadline of 01/30/05 to rectify this problem. I urge you to advise in detail.

Thanks!

Kaleem Muhammad
- Brampton, ON, Canada
2005

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2005

To Miguel and Kaleem,

I discovered this site quite accidentally and see it as a good means to gather experience. I shall endeavor share some practical experience with you.

PLease remember that electrostatice powdercoat will work exactly as the manufacturer promises provided that we cover the basics:

A) Environment for the powder storage and powdercoat operations must be the same and conducive to an electrostatic process. This is why that you find that the powder and paint equipment will be stored in same room for many processes. Proper environmetal control will eliminate the effects of clumping powder ( heavy moisture ). The degree of environmental control will be dictated by the powder that you use. Shoot for settings that will protect your most sensitve powder. Go and grab a hand full of powder and close your hand. If you can make a foot ball that practically stays together, there is too much moisture in your environment.

B) Make sure that hoses don't exhibit bends and loops. Try to give the powder a path that is free of excessive bends and turns. Otherwise, you will find yourself constantly revising your nonminal settings due to impact fusion which constricts the flow of powder in the hoses.

C) Be certain that you have a good conductive ground.

I feel that everyone has given you fellows some good advice considering that we have very little first hand knowledge of your actual environment.

Good luck and keep up the good work.

Lucius Woodson
- Mishawaka, IN, USA