Static charge on wood

Troubleshooting and resolving static problems on freshly planed boards. October 9, 2002

We have a high static charge in our wood as it is exiting the planer. This charge is influencing the reading of our new moisture meter. We have tried several methods to remove the charge - ionizers, metal brushes - but nothing seems to work well. Has anyone run into a problem like this before? Most static eliminators are designed for films/sheets; the thickness of the wood and its superb insulating properties are making it difficult to remove at the speeds we are running and in the short distance we have to remove it. The charge seems to be coming from the cutting head and the friction from the belt. We can eliminate and/or reduce the charge from the belt but it is not enough. How can we stop the wood from gaining charge to begin with? The problem is less when the moisture content of the wood is higher and the day is humid and we run slower and we change species - but none of those options we really want to or are able to change. The hardwood is 3/4" thick and running at 400 fpm.

Forum Responses
I'm guessing the cutter head is insulated from ground. Is there a way to lay a braided cable over the shaft so that it will ground the static buildup? You would need to replace it on a regular basis but it should eliminate the buildup. There doesn't need to be a lot of pressure on the shaft from the cable.

From contributor D:
The idea above might work but I wouldn't want to keep a grounding braid on a spinning shaft. I would check the ground on the planer. I sometimes find three phase machines without a good ground. Same with your moisture meter. Then, make sure the moisture meter and planer are tied together.

A couple months ago, I put a post up that points out that you can blow up Wagners with static by sliding them around on dry wood.

My grounding pole is 8 feet long, driven in 8 feet of shell. When we have a dry spell like now, I get a lot of static off my planer and tablesaw. A guy told me to pour a bucket of water where the grounding rod is when it's dry out. Works for me.

From the original questioner:
Thanks for the input. Our electrician will be coming in to check the grounding on our planer and cutter head. I would warn anyone operating an in-line moisture meter to check out the static charge on their wood. It took us months to realize that it might be the cause of our incorrect readings. This after various attempts to correct other things that had no real affect on the meter.

Do you have metal rolls exiting the planer? These need to be grounded. The static can be eliminated by using with a high humidity or even a moisture mist. This static is not common on higher MC lumber, so your wood has been somewhat over-dried. Over-dried wood with a high speed planer will be a problem.

Gene Wengert, forum technical advisor

From contributor D:
Wood in the 5% to 8% range can contribute to static discharge. I've seen the problem with capacitive meters. But you are right about the rollers. They also should be tied to the planer and moisture meter.

From the original questioner:
We thought about introducing more moisture to the planer booth, but that adds more variables to the mix. Will surface moisture alter the readings? Will applying a mist increase surface checking? Will a moist environment cause problems with our bearings and equipment? The wood is dried between 6 and 8% moisture. The problem is more extreme the drier the wood, but it is still there when the wood is at 7%. We would love to run our product at higher moisture contents since it runs better through the plant, but our product is shipped to very dry climates and we would have quality problems if our product was manufactured at a higher moisture content than it is used in. We will work on the grounding.

We did try metal brushes that are actually drum brushes that worked well for removing small amounts of static. They have metal that runs from the brush through the handle with a loop on the top. Just attack a grounding wire and voila - an inexpensive and effective way to remove small amounts of static from the surface. Unfortunately, in about 4 inches, the static contained in the core of the wood rose back up. We have heard they have worked well in other applications. Two brushes at your local music shop are about $15.00.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Electrostatic buildup in wood is caused by triboelectric charging of two dissimilar materials. Most likely the charging has occured in the widebelt sanding processes. We have had the problem in our shop and found Static Eliminator Bars to work very effectively. Raising humidity in the building is not always the key and will usually cause more expansion/shrinkage problems.

In terms of wood "insulating" properties this is not necessarily so. It is best to think of wood as a dielectric and in this way the wood should be able to be discharged. The rate at which you discharge the wood can depend on the type of deionizer you purchased.

Your planer should not be the cause of your static buildup. To pinpoint the source you can rent an electrostatic field meter and measure the charge on the wood and trace its path throughout your facility. Then you will see exactly where the charge has buildup.