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Technology Focus

Plastic Sheets: Converting a Different Animal

by Jeff Peterson

February-March, 2007
The conversion of plastic sheets, particularly foil stamping and diecutting, is becoming increasingly popular for a variety of applications, including direct mail credit cards (primarily cards for promotional purposes) and plastic cartons/packaging (versus paper board cartons). Handling sheets made of plastic is an entirely different animal than working with paper stock – an ‘animal’ that presents new challenges to the stamper and diecutter which must be addressed if the converter is going to produce a successful job.

Choosing the Right Plastic Sheet
The most commonly used sheet for promotional credit cards and packaging applications is PVC. PVC plastic is relatively inexpensive and very versatile. It is suited for many different applications, ranging from the siding on a house, heavy-walled piping, and work gloves to clear and colored sheets for packaging and other applications where the sheet is decorated utilizing off-set printing, foil stamping, screenprinting, and diecutting. Although PVC is currently the most popular plastic, other plastics such as Polypropylene (PP) and Polyethylene (PE) are becoming more prominent in the global marketplace, especially in Europe where using PVC has been discouraged due to environmental reasons.

Probably the most important element when working with plastic substrates is to make sure the sheets are properly treated before they are decorated. Most sheets are corona treated right after they are extruded. During the extrusion process, additives (also referred to as plasticizers) are added to the plastic to help it flow better and faster or to achieve a desired characteristic. This process is of great help in the manufacturing process, but not so great when the sheet must be decorated with ink or foil. This is why the sheet must be treated before printing or stamping. It is recommended that the converter or printer source a well-respected supplier of plastic sheet when working with plastic applications. It is not worth a few dollars in savings to try to utilize a less expensive plastic that may or may not be properly treated.

Another important point to remember is that the corona treatment process does not last forever. Plasticizers can begin to reappear in the plastic sheet after only a few weeks. So a sheet that foil stamped perfectly three weeks ago may not accept foil today. It is important that the printer or converter work with a fresh, good quality sheet and not let the sheets sit around for any length of time.

When the plastic sheet is treated after extrusion, it is in a roll form. It must then be sheeted, stacked, re-stacked for custom orders, and then re-stacked again during the printing and converting process. This continuous stacking and re-stacking can build up friction between the sheets and create static. The static can reduce the effectiveness of the corona treatment and attract particles and dirt to the sheet. Although printers and converters deal with static on paper sheets, the static electricity created with plastics can be much worse. To compensate for this, static eliminators and neutralizers must be used on the press. Higher gloss or smoother finishes will cause more static problems. If a textured plastic sheet with a matte finish can be used, there will be less friction between the sheets and they will feed much more easily through the printing or foil stamping/diecutting press. Special coatings also exist that can be used to reduce static, but these coatings can affect the printability/stampability of the sheet. Before using these types of coatings, careful testing is recommended.

Another important fact to remember is that the environment in which the printing and/or converting takes place will affect the amount of static between the sheets. The winter months are typically the worst due to a lack of moisture in the air. The addition of moisture can help control and regulate static, which is why humidifiers or “misters” are often installed in conversion areas to help control static problems.

Handling Plastic Sheets
Plastic sheets are much more rigid than standard paper stock and will produce sharper corners and edges, so the handling process can be a challenge. The best recommendation when working with plastic is to utilize smaller stacks than one would with paper. In addition, a load/pile turner can be a great asset, especially if printing and/or foil stamping is being done to both sides of the sheet.

To keep the sheets from being scratched or damaged during the foil stamping or diecutting process, it is important that the press is thoroughly cleaned. The application of Teflon Tape to any friction points as the sheet moves through the press also is recommended to minimize scratching or damage to the sheet. In addition, the ability to float the sheet with as much air as possible will help control any imperfections on the sheet.

Thickness variations also can exist with plastic sheets. Depending on the gauge of the sheet, the percentage will vary. Although in the printing process the press blankets can compensate for this in most cases, it must be watched carefully when foil stamping. A change in the thickness during the stamping process may require a change in the makeready to compensate for any variations.

Foil Stamping Plastic Sheets
As pointed out earlier, treating the plastic is the most important part of ensuring a quality foil stamped image. As a rule, the dyne level must be 40 or above to achieve a proper stamp. The sheet should be treated to a level of 45 to 50 if at all possible.

Just as there are different hot stamping foils for various types of paper stock, specific foils for stamping plastics also exist. These must be low gassing, low temperature foils. Plastics like PVC can begin to degrade at temperatures as low as 200 degrees Fahrenheit, so utilizing a foil that can release at a low temperature is a must. Utilizing a foil supplier that manufacturers hot stamping foils specifically for plastic applications is highly recommended. A graphics foil will not be a good choice for these types of jobs.

Because plastic can cause more gassing than many paper stocks, using a cylinder-style foil stamping press is an excellent option as well. Gassing especially can become an issue with larger stamped image areas. Applying the foil through a cylinder-style press lays the foil across the plastic sheet versus stamping the image into the sheet as a platen or clamshell press would do. This helps decrease the possibility of the plastic melting too much when the image is applied.

Additionally, using a fairly hard makeready when flat stamping on plastic is recommended to keep the image from penetrating too far into the sheet. And because the plastic sheet can cause gassing problems, “doming” the makeready to help squeeze the air and gasses out of the stamped image also is suggested. The dome makeready can be built up by utilizing makeready tape or tissue paper - laying a larger piece down first and building it up with slightly smaller pieces on top of one another to create a domed effect.

Diecutting Plastic Sheets
The last process when working with plastic is diecutting the sheet. As with foil stamping, several different aspects must be considered when diecutting plastic versus paper. For example, an extremely sharp, sleek cutting edge is needed. A ground steel rule edge is usually recommended over the more popular shaved or skived edge. Not only is the ground edge sharper, its machine lines (grinding lines) run in the same direction as the cutting process. The machine lines of a skived edge run in the exact opposite direction of the cutting action, which can cause challenges when cutting plastic. A sleek, ground edge can reduce cutting pressures and produce cleaner, smoother edges.

For creasing and scoring various plastic sheets, a saw tooth-shaped microperf rule has been proven to be very effective. Through a great deal of research and testing, Zimmer Industries has developed 100TPI (teeth per inch) rule with a .006” (.15 mm) depth that has proved to be an excellent choice for cold scoring and creasing of many types and thicknesses of plastics, creating an excellent balance between folding and strength. In addition, this type of rule also provides an extremely “fine” creased edge feel and appearance.

Diecutting and scoring plastic sheets with a heated rule is often going to provide a cleaner, faster process and if available, is the best choice. However, if a quality ground edge rule is used, diecutting the sheet cold can be achieved. In addition, the right microperf rule discussed above also can achieve quality cold creasing and scoring.

Working with plastic sheets certainly creates an entirely new landscape for the foil stamper/diecutter. However, opportunities continue to exist for converters in this arena. Those that choose to do their homework and make a commitment to do it right can reap the benefits of a growing portion of the marketplace.

References:
Michael Dolan, Nakai International Corporation, (800) 445-2684, www.nakaiintl.com

Greg Zimmer, Zimmer Industries, Inc., (800) 225-0108, www.zimmerindustriesinc.com