Painting is not the only solution for decorating plastic signfaces. For high-volume jobs, screen printing is an economical solution. In selecting an ink system, ask your screen print supply distributor for a recommendation. The technical bulletin will give you printing recommendations. The type of mesh, squeegee, stencil and thinners are a few of the variables that will determine the amount of ink deposited on the sheet, which affect the appearance of the sign, when it is illuminated.
Compared to other plastics, such as polyesters, printing on polycarbonate films should be a piece of cake. Polycarbonate has a naturally high dyne level, which means that you can achieve better ink adhesion. Unlike other plastic films, such as polyester, polycarbonate films require no surface pretreatment before printing. And, since polycarbonate withstands higher curing temperatures than polyester, the stability of these films allows the printer to maintain better registration.
Still print failures can occur. Here are some tips to improve your odds.
- Incompatibly of materials is a major cause of printing failures. Only use the inks and transfer tapes that the manufacturer recommends.
- The key to success in printing with solvent inks is to completely dry the inks so there are no residual solvents. Force drying of the inks at temperatures between 130° and 150°F is the preferred method. Some potential problems that can arise from insufficient drying, are poor ink adhesion, stress cracking and sheet curl.
- When printing with UV inks, thinner ink deposits are generally better, because the UV light can more easily penetrate and more thoroughly cure the layer of ink. If the ink deposit is too thick, the UV light will not fully penetrate the ink. This can mean that the layer of ink that is making contact with the substrate is not cured or is not fully cured. Usually this uncured or under-cured ink is too soft to bond or adhere to the substrate.
- Don’t take the manufacturer’s recommendations at face value. To ensure compatibility of the ink with the film and the transfer adhesive, conduct your own in-house testing.
- Your testing should include a cross hatch test to check the adhesion of the ink to the substrate; intercoat adhesion (the bond of one layer of ink to another) and compatibility of the tape with the ink.
NOTE: The cross hatch test consist of scoring the printed ink many times with an Xacto knife and then scoring the ink again over the first set of lines at 90° angle . Using a plastic squeegee, an aggressive tape, such as 3M Brand #600 clear tape, is rubbed over the scored cutlines. The tape is then pulled off 180° against itself, in one quick motion. If any of the ink comes off, the adhesion of the ink to the substrate is insufficient.
- Once you find a combination of raw materials that work for you, that you stick with that winning formula. You should document the details of every job, so that nothing is left to chance when duplicating a reorder. Repeatability of results depends largely on reproducing all of the variables involved in printing. In addition to screen tension, record key information on your production order, such as mesh, ink formulation and cure rate.
- Although polycarbonate films typically have a much longer shelf life than other plastics, ink adhesion can change as the plastic ages. In a perfect world you should store all of their materials in a temperature and humidity controlled environment.
