Four Rules for Static Control in solvent coating and gravure printing

Four Rules for Static Control in solvent coating and gravure printing

Today minimizing electrostatic charges in the material being processed has become a priority to avoid downtime and ensure production quality

In Gravure printing and solvent based coating static electricity continues to be a major problem. There are more plastic webs being coated or printed at higher speed. Paper based webs are dryer with more synthetic coatings increasing their affinity for static charging. Many look at the problem of static as being mainly a static charge to be controlled on the moving web. This alone is not the answer and could lead to false sense of optimism. On the contrary, the web charge is only one part of the problem. The static charge on the web gets most of the attention because it is the most obvious.
You can feel it with the hair on your arm.
When you place the grounded Static String, proposed by Bardo (www.bardosrl.it), across the web the static goes away.
This paper examines the 5000 volt rule as well as three other rules for static protection which were developed from a variety of sources including the NFPA Handbook 77, Static Electricity, and from observations.

The Four Rules for Static control in solvent coating and gravure

1) Properly ground and Bond all conductors including people.
2) The 5000 Volt Rule: Maintain the static charge density on the web below 5000 volts.
3)Prevent capacitive storage of static charge.
4)Establish Procedures and Training.

Rule 1: Properly ground and Bond all conductors including people

In early 1999, a flash fire occurred at a Rexam Release site, involving the coating head of a silicone coater. The fire resulted in a very serious injury to a machine operator, who sustained second and third degree burns over 70% of his body.
The accident itself involved the transfer of a flammable coating from a 5 gallon pail to an open pan at the gravure coating head.
The investigation, which was conducted by Rexam staff, local and state fire marshals, our insurance carrier and underwriter, concluded that "static was the most likely cause of ignition, but the exact source was not known."
Initially, an audit for flammability was conducted at all our US locations. All deficiencies were identified and action plans for improvements were started.
Efforts included grounding and bonding, use of less flammable solvents, clothing and shoes, electrical review, signage, transfer of solvents and humidification for static reduction. These engineering controls focused on classified areas, defined by the National Fire Protection Association(NFPA) as areas n the plant where flammable liquids are used, stored, processed, dispensed or used in cleaning."
Proper Grounding is a positive continuous conductive connection between conductors to the earth.
Proper Grounding of persons is essential because they are conductive and can store static charge energy and discharge to other conductors. Also, a static charge can induce a charge on them from a charged web or covered roller.
Careful attention must be paid to the pathway to ground from a person’s skin to conductive or static dissipative shoes or heel grounders to conductive or static dissipative flooring surfaces to ground..
It is not well known that concrete may be sufficiently static dissipative to lower the static charge from a person to a safe level. Concrete can be cleaned of non-conductive coatings and checked with a Megohm meter per ASTM standards using 100 volt test level. Readings of 1000 megohms are sufficient to dissipate static from a person through an esd safety shoe or a properly worn grounding heel strap.
The conductive pathway from a person cannot be assumed but should be checked daily for each person entering the static controlled area using a shoe/heel grounder checker.
Protective Transfer of flammable coating, inks and solvents. Static control for these
operations are described in detail in NFPA bulletin No. 77 and others and include bonding and grounding of conductors during transfer of flammable liquids. A common failure to bond and ground under this rule is caused by non-conductive liners in containers and ungrounded persons can discharge to open conductive containers.
Myth: Static charge on a machine can be controlled by proper grounding of the machine.
Plastics and plastic webs are non-conductive and thus they cannot be grounded.
Not only can they not be grounded but they can become charged by contacting grounded metal rollers.
Rule 2: The 5000 Volt Rule Lower static charge on the non-conductive web and maintain below 5000 volts

The 5000 volt rule in flammable gravure and coating area if often quoted as a safe standard.
The 5000 Volts Rule is equivalent to keeping the surface charge density below one tenth of the discharge value" This is a defining paper on the subject and it gives us a great deal of information about the typical measurement technique using a static field meter. Taking the measurement on a free span of web is the typical area where reading are taken away from all grounds and Al Seaver explains how this can be done fairly accurately.
Dr. Seaver's conclusion regarding the "5000 volts Rule: "Thus, the 5000 Volts Rule is equivalent to keeping the surface charge density below one tenth the discharge value."
This provides a higher safety margin than many have previously estimated. 5000 volts is one tenth of 50,000 volts. This means that the charge density level on a free span of non conductive web would have to reach nearly 50 KV to produce charge density high enough to discharge with enough heat to ignite flammable vapor.
This certainly bust the following myth: "Static charge on nonconductive web surfaces needs to be controlled to near zero levels to prevent static discharge hot enough to ignite flammable vapors."

Typical Levels of static on
plastic webs

- 3000 volts: Dust attraction begins;
- 5000 volts to 10,000 volts: Static cling begins
- 10,000 volts to 50KV:
Increasing dust attraction, static cling;
- 50 KV plus:Active Discharging from the web.

Common Sense rules for flammable coating and gravure printing

Monitor web surface static charge levels.
Keep static voltage levels below 5000 volts on non-conductive webs.
Between 5 KV and 20 KV make adjustments.
Over 20 KV, stop and correct problem.

Rule 3. Eliminate and prevent high static charge storage

When a non-conductive plastic coating or sleeve is placed on the surface of a metal roller that is grounded it forms a capacitor which can store static charge energy and discharge to a conductive part of the machine or to a person.
Examples of these are: Teflon Covered rollers, Plastic Printing Sleeves, Taped or painted metal rollers, Conductive rollers that loose their conductivity.

Teflon Covered Metal Rollers

Teflon coverings on metal rollers are used to make cleanup of coating and ink residue simple and easy. A common accidental ignition of solvent vapor is caused when the person is cleaning ink or coating residue from such a plastic covered metal roller and the increased charge density discharges to the person or charges up the person and ignites the solvent vapor, around his gloved hands as he touches metal. Also, because there is voltage suppression on the plastic covering because it in close proximity to the metal roller surface, it does not show a high static field reading.
Non-contact static eliminators are ineffective at eliminating this charge because it is voltage suppressed.

A Non-Conductive Printing Sleeve

A non-conductive printing sleeve is installed over a metal roller forming a capacitor.
As it is charged by friction from the plastic web and a high static charge density is stored in the capacitor.
When the printing roller is lowered and very close to the metal back up roller there is a discharge between them igniting solvent vapor.

A Conductive Rubber Backup Roller loses surface conductivity

A conductive rubber backup roller lost carbon (conductive) particles from its surface over a period of use in a solvent atmosphere.
The non-conductive layer resulted in charge density storage in the capacitor.
As the printing head was lowered into contact, there is a discharge and ignition of vapor. This was the cause of repeated fires at a film coating company until it was discovered. It was discovered by checking the rollers that were out of service by rubbing their surface and taking readings with a static meter very close to the surface.
Roller sleeves can also be checked this way but it is better to check them with a meg-ohm meter, (100V).

Covered Concrete Flooring

Covering concrete flooring with non-conductive coatings such as, epoxy or coatings can form a storage capacitor for static charge because concrete is static dissipative and sometimes conductive.
I recently had a report of a concrete floor with metal filings that was coated with a sealer resulting in capacitive storage. Persons walking across such surface become highly charged and can discharge to metal and other persons. In another company, the floor was painted with epoxy paint.

Teflon Tapes

Other examples include covering metal rollers with plastic tapes and plastic polymer coatings and paints forming the capacitive storage.

Recommended

The material for printing sleeves, roll covers and tapes should be specified to be static dissipative or conductive so that it can be bonded or grounded to the metal roll core. Floors should be conductive or static dissipative in flammable areas.
If necessary, the Static String or a conductive brush can be placed in light contact with the sleeve or non-conductive covering to minimize the charge until it can be replaced with static dissipative material.

Rule 4: Establish Procedures and Training

- Are your staticcontrol procedures written?
- Are the material specifications written
- Including electrical resistivity for rollers, sleeves, liners, tapes, floors, etc?
- Are there handbooks for training?
- Does the training include management and operations persons?
- Is there awareness and responsibility for the program within the flammable areas?
- Are the areas controlled so that everyone entering is tested for grounding?
- Are Static Control Signs posted
- Are all persons ground tested before entering controlled areas?
- Are you monitoring and documenting the system?