Myth: you can guess the perfect exposure time
Some printers are surprised to hear that exposure times are NOT linear. By that, I mean if a screen is under exposed the emulsion and image stencil will break down or wash away. And if a screen is over exposed, nothing will washout and you will be left with a solid emulsion stencil without any image. Lastly, if the image develops into a stencil, then your exposure time is PERFECT! Sadly, this is a myth that has caused some screen printers headaches in loss production and profits.
The examples of over and under exposure listed above are indeed part of the exposure scale. But they represent the far extremes end of a bell curve. Each end of the scale is extremely under exposed on one end of the scale and extremely over exposed on the other end.
At the top of curve is the perfect exposure time, surrounded on each side starting at slightly over or under exposed then increasing to moderate and finally to extreme over or under exposure, where we can see the extreme variances of image washing out or completely disappearing.
Max Under Little Under Perfect Little Over Max Over
What does this mean?
Just because you have a screen stencil of your entire image, it doesn’t mean you have a great stencil. We can an expose and develop an entire image in our emulsion screen stencil with no visible imperfections and still be substantially over or under exposed.
Why is this Important?
Problems directly linked to UNDER exposure.
· Stencil breakdown on press
· Pinholes
· Poor edge definition
· Scumming in the image area
· Decreased EOM / stencil thickness
· Increased chance of moire pattern
· Dot gain
· Poor color reproduction in process printing
· Difficulty reclaiming screens
Problems directly linked to OVER exposure.
· Difficulty developing image area
· Loss of detail
· Undercutting, where image shrinks
· Poor registration
· Poor edge definition
· Difficulty reclaiming screens
Types of Exposure Calculators
Step and repeat exposure calculator
They are Free and you download the file and then print to create your own film positive. You can download one here.
Advantages
Free
Accurate as it includes density from your printer
Features fine details and halftones
Disadvantages
Step and repeat takes longer to use.
Requires multiple exposures in increments
Saati 21 Step
Achieve the correct exposure every time with the SAATI 21-step exposure guide. Easy to use, just count steps after developing. Find the right exposure with one easy test, 100% guaranteed. Shop Now
Advantages
Cost effectiveness
Only requires one exposure
Easy to use & read
Small format can be used next to an image for QC.
Disadvantages
Doesn’t showcase fine lines and halftones in stencil
Kiwo Exposure Test Film
The RESOLUTION FILM in combination with the EXPOSURE TEST FILM is ideal for quickly finding the appropriate exposure range for a given screen making process. The neutral density filters on the EXPOSURE TEST FILM allow the user to perform 1 overall exposure, resulting in 10 areas with varying degrees of exposure.
Advantages
Only requires one exposure
Easy to use & read
Exposes fine lines and halftones in stencil
Disadvantages
Expensive
Checkout our blog post on Film Density and hot it impacts screen exposure,