Paper Substrates Forgive, Plastic Ones Do Not.
Much has happened since we last wrote about printing on plastic substrates. Compared to a few years ago, many more printers are printing on plastic substrates and the variety of available substrates has increased, as has interest in printing on these materials, the result of innovative new applications.
One thing that has not changed is the fact that printing on plastic substrates is different than printing on paper. Plastic substrates do not absorb as paper does. Therefore inks, varnishes, and coatings will lie on the surface of these nonabsorbent substrates, and not be absorbed. Furthermore, the surface of plastic materials lack in bond sites for printed/coated products and foil stamping adherence.
There are many applications that simply are better in plastic than paper. Among these are such products as credit/debit, gift, phone, membership, and I.D cards, binders, and menus. Other applications include, packaging labels, packaging, folding carton, point of purchase, tags, banners, outdoor signs, etc.
Applications on plastic substrates include both rigid and flexible films. Newer or future applications include printed circuits, batteries and solar collector panels based on thin films. Plastics being used in printing are, Polyvinylchloride, Polystyrene, Polypropylene, Polyethylene, Polycarbonate, Polyester, synthetic papers, and new exotics.
Surface Energy is Critical.
Most likely the most critical factor in printing on plastic substrates is its surface energy. Surface tension is measured in terms of dyne level. When printing on plastic substrates, it is critical that one understand this property and its implications at different levels. The surface energy of a plastic is the ability of its’ surface to attract a liquid and allow it to wet out the surface.
Experienced printers of plastic substrates understand that the dyne level must be between 38 and 50, with 40 thought of as ideal in order to expect acceptable printing results. If the dyne level is below 38 the inks won’t adhere. At the other end, if the dyne level is over 50 there will be handling problems relating to static electricity. Plastic substrates with a dyne level measured below 38 may be printed acceptably, but only after adequate surface treatment.
Corona Treatment of Plastic Substrates.
The majority of plastic substrates are corona treated immediately after being formed by extrusion. Plasticizers are typically added during the extrusion process as flow aids, and to provide final flexibility properties. These plasticizers want to migrate to the surface of the extruded substrate and as a result lower its’ dyne level. A remedy to this lowered dyne level printing compromise is additional corona treatment just before printing/coating or foil stamping. Corona treatment will not last forever. Therefore, it is critical that the dyne level of a plastic substrate be checked, and if necessary retreated, just before printing, coating or foil stamping.
Another phenomenon that occurs with plastic sheet is that handling induces a static charge. Plastic substrates are in corona treated roll form after the extrusion manufacturing process. Rolls are processed further by sheeting (cutting), and stacking. During further converting, the sheeted goods are again stacked and restacked. All of this handling (friction) increases the static electricity charge, which will attract particulate and dirt matter, and can also reduce the corona treatment effectiveness.
Countermeasures to be used involve the use of anti-static devices in the printing/converting process. These devices may be of a passive or active type. Passive types would be static bars, string or tinsel. Active devices would be ionized air blowers or curtains. It is well to remember that low humidity attenuates the static problem so that it is especially important to utilize misters or humidifiers to maintain an acceptable humidity level. This is of great importance during winter months.
Printing/Coating - UV Is Attractive
Printers of plastic sheet have in many cases adopted UV decorating as a viable approach. Multi-color sheet fed and flexo presses have the capability, when UV cure equipped, to run high volumes of production effectively. 100% solids UV chemistry cures rather instantly so that sheets are dry immediately off press, and may be handled in further converting processes. This is not the case with conventional offset inks that require short lifts (2 to 3 inches) to minimize weight and prevent offsetting during the typical drying time of 24 to 48 hours. It is also recommended that large 50 micron spray powder be used to better aerate the sheeted pile. Remember conventional offset inks have the problem of fountain solution not being absorbed due to the non-absorbency of plastic substrates. These inks are recommended to be formulated for low oxygen and high moisture exposure. Simply, printing on plastic substrates is definitely a different ballgame.
Sheeted plastic substrates are very rigid compared to paper substrates. The stock is handled better in smaller stacks. Many plastic substrates will scratch easily. Sharp edges and corners as well as any dirt particles can produce scratching. Sheeted goods can also be damaged as the material moves across the press and other equipment surfaces. Rub and friction points are best covered with protective smooth Teflon tape.
Finishing - Foil Stamping
A quality stamped foil image is dependent on an acceptable dyne level, like printing/coating. Generally, the dyne level must test to be 40 or above with a target in the 45 to 50 range. Specific foils are made for use on plastic substrates with the requirement is that they be low gassing and low temperature activating. It is said that a cylinder type foil stamping press performs better than a platen press when foil stamping on plastic substrates.
Die cutting is another tricky process when converting plastic substrates. An extremely sharp cutting edge is required for achieving repetitious precise cuts. A ground steel knife edge is recommended to produce a clean smooth cut edge. Finally, it is said that a heated die cutting and scoring rule has the best chance of producing a cleaner cut or score.
In summation, printing/coating, foil stamping and converting plastic substrates is in a different league than printing on paper. The converter must be on his toes at all times to effectively control the many variables that come with plastics as a material. Not to say that paper as a substrate is not a challenge, but plastics as a class of materials bring many more variables to the converting party.
Cork has experience in coatings especially formulated for plastic substrates. Give us a call for you next project and get our team on your side.