Think Ahead, Treat it Right

“Success depends upon previous preparation, and without such preparation there is sure to be failure.” The profound words of Confucius couldn’t better explain the importance of corona surface treatment prior to printing on plastic materials.

With corona treating, or any surface treatment system, the goal is to increase the material’s surface energy to provide wettability and adhesion. But, treating a substrate can be ineffective when the system is not properly run and maintained. For this reason, converters should be aware of how to effectively process their materials.

Over/Undertreating

When a corona treating system transfers too much or too little energy to a substrate, a host of problems can occur. “Attempting to obtain satisfactory printing results on undertreated materials can result in the use of excessive amounts of ink in an effort to try to make up for the inadequate treatment level, and doing so can have its own set of problems,” says Bruce Stobbe, president of Corotec Corporation. “Overtreatment can result in damage to the material itself, as well as problems with blocking of the material.”

Poor adhesion, low dyne levels, and inadequate wettability can also occur, says Rob Hablewitz, sales manager of Pillar Technologies. He recommends “[establishing] a beginning point for the power level and [working] your way up on power until the anticipated dyne level is achieved; this being done through quality assurance checks of the film. Once the power level is established for the given product at the given speed, note the power level, re-use it next time, and you are assured of desired dyne repeatibility.”

Tom Gilbertson, VP of application engineering for Enercon Industries, agrees that converters can achieve proper treatment levels through trial and error. “Test protocols which include adhesion and bond strength measurements at a variety of power levels should be used to determine the acceptable power level for each substrate employed,” he says. “Material type, material thickness, and even material suppliers are all variables which can impact the appropriate power level. Once determined, the appropriate power setting should become a permanent part of your job specification.”

Jeff Opad, VP of sales for Jemmco, says the best way to ensure accurate treatment is to train operators how to measure watt density. “The formula for calculating watt density is web width (in feet) x line speed (fpm) x number of sides treated (using the same power source) divided into the actual amount of power being used to obtain the desired dyne level,” he says. “Once this concept is understood, then the user can develop a watt density curve using the formula above, for each of the material formulations they run. These watt densities, once identified, will produce the same treat level every time.”

Web tension

Accurate treatment depends on web tension, which, if not controlled properly, can hinder the process. With too much tension, the material can wrinkle or snap; whereas, a lack of tension creates air gaps between the material and the roll, says Opad. In both cases, the material may be unevenly treated, resulting in backside treatment.

Materials that do not make close contact with the roll under the electrode or the backup roll can affect the treatment process, says Stobbe. “Wrinkles in the material or air trapped under the material will result in some treatment of the backside of the material which will reduce the treatment level in the respective areas on the top side, and may also cause the material to stick together, or block,” he says.

So, what can be done to avoid complications associated with web tension? “Consider nip rolls when running at speeds greater than 1,000 fpm, as this will eliminate concerns associated with developing an air boundary layer between the ground roll and the web,” Hablewitz says. “Also, in some cases spreader rolls/crowned rolls are used to eliminate wrinkles.” Gilbertson also suggests using nip rolls, as well as a driven or spreader roll for high-speed applications, to help control the web as it passes through the treater. “And of course, although you never want to hear this, slowing down the line will minimize your web handling problems,” he says.

“The decision to drive the treater roll,” he continues, “hinges on a number of primary parameters which can be influenced by still more secondary considerations. The simple rule of thumb on this question is any web (substrate), whether film or foil, that is one mil or less and will be run at 500 fpm or more should be treated on a driven roll.” Stobbe points out that converters should also be aware of baggy edges or a baggy center to ensure the material is consistent and free of defects. These conditions can contribute to wrinkles or trapped air between the material and backup roll.

Maintenance

As simple as it sounds, maintaining surface treating equipment goes a long way. “One of the biggest causes of downtime on a corona treater is due to improper cleaning. Many materials tend to leach slip and other additives during the corona process,” says Opad. “Corona treaters incorporate the use of exhaust blowers. Newer style treaters are often more open style designs, so that they tend to pull in the air, along with dirt from the surrounding area. All of this can get deposited on the electrodes and rollers.” He explains that this dirt buildup can also contribute to producing unwanted backside treatment, by causing high and low spots on the treater roll, enabling air gaps to be created.

“The three most important things are clean, clean, and clean. Clean [equipment] often and keep them clean, especially during the humid months,” says Stobbe. “This is the one thing that converters can do that will have a very significant impact on the reliability and function of their corona discharge treating equipment, and it is the most often overlooked, until problems occur.”

In addition to inspecting the cleanliness of ground rolls and electrodes, Hablewitz recommends keeping up on scheduled maintenance. “[E]liminate broken web or loose substrate near the exhaust capture ports of the station. Inspect all high-voltage connections,” he says. “Assure proper exhaust/cooling air to eliminate issues associated with fugitive ozone emissions.”

Opad advises to keep the treater roll properly grounded and to replace the ground brush regularly, to prevent rollers and bearings from failing prematurely. In addition, he says, “Checking the wiring and high-voltage connections should also be [done regularly]. Power is often lost due to power bleeds caused by HV wire proximity to ground sources, and insulation breakdowns.”

With a little care, “properly designed and maintained treaters should last a long time,” says Opad. “The application, use, and care of a treater will be the largest influencing factor on its performance from day to day and year to year.” pP

Resources

AAA Press International www.aaapress.com

Arcotreat Corp. www.arcotreat.com

Corotec www.corotec.com

Enercon Industries www.enerconind.com

Jemmco www.jemmco.com

Pillar Technologies www.pillartech.com

QC Electronics www.qcelectronics.com

Solo Systems 972-475-5569