What is a Film Positive?
Once digital artwork has been color separated for screen printing, each color must be printed onto what is called a film positive. A film positive is a clear transparent film that is fed through an inkjet printer (must be certain models) that prints only black ink onto the film so that it can then be used to burn a screen. Inkjet film printers do all of the hard work nowadays, but before computers a film had to be cut by hand using vinyl and an exacto knife. In order for a film positive to work well for burning a screen the artwork must be very dark and opaque with zero translucency. If a film is not opaque enough, then the artwork cannot be successful burned and washed out of the screen. In order to get the most opaque possible film it is a good idea to use a printer that only uses black ink to create the film, often referred to as an “all black max” system. It is also important that you make sure you are using a high quality waterproof film that prints well with the ink from the printer.
Because each print color in a screen print design must be printed one at a time using seperate screens each screen must have a film printed. Printing multi-color designs is much more difficult than a single color because they must be registered on the press. To make registration easier registration marks (bullseyes) are typically printed on the top and bottom of the film positive so that each screen can locate a film on the same spot to make registering the artwork on the press easier. Then, on the actual printing press one of the films is often used to register the actual screens using the same bullseye registration marks. However, successfully registering a multi color design typically takes more than just registration marks and some help from the artist in charge of preparing the artwork. When printing a multi-color design we need a little bit of “play” that allows each color to more easily align with one another. To create more play the person in charge of preparing the artwork and printing the films will often need to add either a “stroke” or “choke” each color of the artwork. “Stroking” a color adds an outline around the entire artwork so that it is just slightly bigger. The thickness of the stroke can easily / should be controlled as too much of a stroke can cause problems with registration. “Choking” a color is the opposite of a stroke, in which the outline of the artwork is made smaller. Every piece of artwork is different and often takes experience/digital artwork skills to know when and how much of a stroke to add to certain aspects of a design.
How to Create a Film Positive for 4-Color Process (full color) Artwork
Most screen print jobs are printed through a method called spot-color printing, in which each color in the artwork is printed separately one at a time using a specific ink color. The second most common method of screen printing is called 4-color process-printing in which Cyan, Magenta, Yellow, and Black (CMYK) ink colors are printed to create a full color design, similar to how a desktop inkjet or laser printer works. 4-Color process printing is much more difficult/advanced compared to spot color printing when it comes to burning & aligning screens, but most importantly printing perfect film positives. 4-Color process printing requires Raster Image Processing (RIP) software that can breakdown full-color artwork into CMYK channels and print “halftone dots” of the artwork. RIP driver software typically costs around $500 and acts like a plugin to help give your film positive printer a lot more control and capabilities.
Halftone Dots look a lot like pointillism artwork, but their purpose is to control how much ink is laid down when the screen is printed. The amount of ink that is laid down will affect the shading and even the overall color. 4-Color process prints have thousands of halftone dots that allow just the right amount of ink to pass through the screen onto the product being printed. When all of the CMYK inks are properly prepared in the digital artwork program, RIP software, and exposed without losing detail the 4 colors come together to create beautiful 4-color designs. There are several other variables that come into play when it comes to printing films for 4-color process such as the line count, frequency angle, and the dot size & dot gain control that must be learned in order to be able to successfully print 4-color process films. Each color has to be properly configured to be printed as a film positive or the finished product will not look like it should.