If we know that things are going to get worse, then let’s work now to prevent problems. While I’ve developed a checklist of thing that I’ve done to help prevent Winter Static, I’m interested in things that you’ve done.

What do you do to help prevent problems from Winter Static?

From a lean manufacturing perspective, all energy and resources should be focused on generating customer value. Resources and energy that are not creating customer value are waste. Here are seven forms of waste:

1. Transportation is moving product without transformation. This is factory lay-out and material flow issue where static plays no role.

2. Inventory is stuff sitting around before it is sold like “work in progress” or WIP rolls and spare parts inventory. The spare parts inventory should be just enough. Spare static bars and power supplies that are never used are wasteful.

3. Motion of material, workers or equipment should create customer value. If not, it is waste. Maintenance of the static control system and static measurements should be efficient and eliminate any unnecessary motion or activity. Having a static control system that is too complex or that does not work well is a waste of time.

4. Waiting for a problem to be solved or for an operation to be restored to reliable operation is wasteful. To solve a static problem quickly and minimize wait time, have a plan, have access to key data, and have the essential knowledge. Training can create customer value by minimizing waiting.

5. Over-processing is doing more that is necessary such as rework. Having to rewind a roll to neutralize static is over-processing.

6. Over-production is making more than enough because we “know” that we’ll have to throw some away. Any over production caused by static problems is wasteful.

7. Defective product caused by static is waste. High static that causes failures in customer operations is defective product. Producing product with static levels exceeding acceptable limits is wasteful.

Note that static plays a role in 6 of the 7 forms of waste. This realization is an “eye opener” for me!

Please feel free to ask questions and offer your perspectives on static. I’ll be writing about static issues and offering my reviews and opinions on meeting that I attend and articles that I read.

I’ll be at the AIMCAL Web Coating Conference from 10/23-26/2011 talking about controlling static on unwinding rolls.

Stay tuned … more to come.

Cheers, Kelly R.

Here is my understanding. Electrical resistivity is one of the most widely varying physical properties in the universe. Well, that is a big claim. Here is some data.

The electrical resistivity of silver is about 2 E-8 ohm-m.

The electrical resistivity of PTFE exceed 1 E+22 ohm-m.

This is an astonishing large range of almost 10^(+30) … 30 decades!!!

It is natural for humans to categorize or group similar things. Conductors are materials with resistivities at the low end of this range. Insulators are materials with resistivities at the high end of this range.

Well, this grouping is not really very useful. Here is a better way to think about insulators, conductors and static dissipative materials. Static charge on the surface of materials will dissipate with a characteristic time constant; the charge relaxation time. The charge relaxation time is proportional to resistivity. Charge on conductors dissipates very quickly while charge on insulators persists for a long time.

So, what is a short time and what is a long time? Well, it depends on the application. Think about the time it takes for a physical or mechanical phenomena to occur; the clock time in an integrated circuit, the time it takes for you to walk across the carpet during the winter months, or the time that a static charged balloon will stick to the ceiling.

For “conductors” in an integrated circuit, charge must move very quickly relative to the clock speed of the circuit.

For “static dissipative” shoes, charge must dissipate before you can walk across a room and touch a door knob.

For “insulators,” charge must persist on the surface of the balloon long enough to get a good laugh.

If charge dissipates from a material much faster than the time needed for the physical or mechanical process, then the material is a conductor. If charge persists on the surface of materials for times much longer than the physical or mechanical process, then the material is an insulator. And, if charge dissipates from a material just a little bit faster than the mechanical of physical process, then it is static dissipative.

I know that this is unsettling, because we must think about the application to decide if a material is “static dissipative.” However, this is exactly the point. Materials might be static dissipative for one application, and insulating for different, faster application.

This is why the term “static dissipative” is so general and hard to define.

For example, in 1907, Dr. Frederick G. Cottrell, a professor of chemistry at the University of California, Berkeley, applied for a patent on a device for charging particles and then collecting them through electrostatic attraction — the first electrostatic precipitator. Cottrell first applied the device to the collection of sulfuric acid mist and lead oxide fume emitted from various acid-making and smelting activities. Vineyards in northern California were being adversely affected by the lead emissions [http://en.wikipedia.org/wiki/Electrostatic_precipitator].

So, next time your cut sheets jam because the static level is too high, remember that the same static attraction is helping clean the air!

I suggest using an electrostatic fieldmeter to begin an investigation of a static problem. Voltmeter measurements are complementary and can be added to the work stream if needed to help sort things out.

Electrostatic fieldmeters are good, workhorse instruments for detecting static charge. Readings should be taken on free spans between rollers and give a good measure of static charge averaged over a relatively large area. Fieldmeter readings are very good for relative comparisons, though it is a bit cumbersome to estimate actual web charge given a fieldmeter reading.

Also, electrostatic fieldmeters are ‘blind’ to a common charge pattern on insulating webs. Often, webs have a positive charge on the top surface and an equal amount of negative charge on the bottom surface. In this case, the field meter will read zero because the sum of the charge is zero. So, fieldmeter can give a false sense of security … reading ‘low static’ even when there is a significant amount of charge … enough to cause static problems.

Electrostatic voltmeters are excellent instruments for getting specific and detailed information on web charge. Voltmeter measurements are somewhat more tedious to set-up and make because readings should be taken on the surface of the web where it is wrapped around a grounded metal roller. The voltmeter probe should be located as close to the web surface as is practical. Usually, I mount the probe using a bracket that is attached either to a rod clamped to the machine frame, or attached to a magnetic base for indicators. One vendor recommends spacing the probe 2mm plus or minus 1mm from the film surface. There is a theoretical basis for this that I will discuss in a future edition of Static Beat, my column in PFFC.

Voltmeters read surface potential over a small area … a spot with a radius that is roughly equal to the probe to film surface spacing. So, voltmeters are especially useful for detecting spots or stripes of charge with a characteristic dimension on the order of a couple of millimeters. For measurements on moving webs or films, it is important to select a voltmeter that is fast enough to see a spot of charge. For example, a voltmeter with a 50 mS rise time (about 20 Hz bandwidth) would be somewhat limited in detecting charge on a moving web. Depending on the web speed, a spot of charge can come and go before it could respond.

A voltmeter detects the relatively common charge pattern where a film has positive charge on the top surface and negative charge on the bottom surface. Since a fieldmeter does not respond to this patter of charge, voltmeter measurements are complementary to fieldmeter measurements.

Fieldmeter readings are easy to make and relatively stable because the average the web charge over a large area. And, while voltmeter readings are somewhat more difficult to make, they give detailed information of the web surface charge distribution and can separately measure the charge on the top and bottom surfaces of the web.

Thanks for the great question. Keep asking! … Kelly R.

I strongly support and encourage clients to use static meters to measure static levels in processes. As an engineer, it is my nature to quantify issues and analyze problems “by the numbers.” The first step is to document the levels of static in a process when thing are running fine. While measuring static when there is no problem looks like a “waste of time,” this is critical to establishing what is normal and acceptable. Once we have a time trend of static readings, we will be able to recognize when increases in static are objectionable to our customers.

Two key enablers are needed to get started.

(1) You need to have a static meter.

(2) You need to identify sensible locations to measure static.

Many good static meters are commercially available. And, I’ll say more about identifying sensible locations to measure static.

More soon … Kelly R.

One area of high interest is using roll-to-roll manufacturing technology to produce active electronic devices. As printed electronics develops and technology enables integrated circuits to be positioned and fixed onto moving webs, our “traditional” converting operations will be transformed into efficient, high value manufacturing operations. Good static control will be “table stakes” for the manufacture and converting of electronic products.

I invite you to turn this Blog into an interactive discussion. Please feel free to offer your comments, ask questions, and suggest topics for us to discuss.

I look forward to hearing from you. Kelly R.