A full and complete exposure of your screens is critical for a variety of reasons. Let’s delve into what these are and how you can determine the perfect screen exposure time, thereby reducing a host of problems.
Screen Exposure Time: Overexposure vs Underexposure
When it comes to screen exposure time, the exposure latitude that determines whether an emulsion is under or over exposed will vary depending on the type of emulsion used. Pre-sensitised emulsions (SBQ) have the smallest exposure latitude, while Diazo-dual cure emulsions have the widest latitude. We advise you err on the side of over exposure rather than under exposure as this is likely to present you with far less problems overall.
Possible problems relating to under-exposure of a screen
Here are some of the issues you may encounter when your screen exposure time is too low.
- The screen could be difficult to reclaim when it is under exposed, since the under exposed emulsion can react with the screen reclaiming chemicals.
- The printed image could have poor edge definition.
- The stencil could break down during a print run.
- The stencil could break down during washout of the screen.
- The screen could suffer from pinholes.
While some of these issues may also be caused by other factors, incorrect screen exposure time is of great importance. By comparison, the main problem associated with over-exposure is that the screen may not wash out properly and fine detail will be lost.
How to determine the correct screen exposure time
While manufacturers do provide guideline exposure times, it is rare that their guidelines are for the exact make and model of your screen exposure unit, or indeed for a suitably coated screen with high emulsion over mesh (EOM), which means that trial and error is the order of the day. It is therefore important that you have a procedure in place to test all your emulsions to ensure correct screen exposure time is used at all times.
Option 1: Exposure calculator
An exposure calculator consists of two parts – artwork and a graduated light filter. The different areas of the filter allows differing amounts of light through, allowing you to do a test exposure that encompasses a wide variety of times in just one session. Once the result is read after washing the screen out, a quick calculation tells you exactly what the optimal exposure time is. This makes an exposure calculator very convenient, but they also tend to be expensive and may be cost prohibitive to smaller shops.
If you are not sure how to obtain the correct screen exposure time from an exposure calculator, you may want to join the Screen Printing A to Z class with Douglas Grigar as this is one of many points that is covered in detail.
Option 2: A step test
This is a very inexpensive but slightly more laborious way of testing for correct screen exposure time without using an exposure calculator. You will need to create some artwork of your own that represents a variety of features like halftones, different line thicknesses, different point sizes of text, reversed out areas and so on, or download our free step test artwork at the bottom of this article by clicking the green button. The document is a PDF. Once you have the artwork you follow the following steps –
- A: Roughly estimate the screen exposure time you believe it will take to expose correctly (seconds). You can base this on past experience or manufacturers guidelines or a combination of the two, but be realistic. If the manufacturer used 1+1 coats of emulsion and you are using 3+2, your screen exposure time must be longer to expose properly. Estimating it too low won’t help you and may require you to do multiple step tests.
- B: Double the screen exposure time estimated in step A.
- C: Divide the total number you now have by 10.
- D: Cover all apart from row 1 with some aluminium foil or another totally opaque material.
- Expose the screen with only row 1 exposed to the light for the amount of seconds you calculated in step C.
- Move the aluminium foil down after the first exposure to allow row 1 and 2 to be exposed and expose for the same amount of time as calculated in step C.
- Keep moving the opaque material (aluminium foil) down one row at a time and expose the screen for the number of seconds calculated in step C until you have done all 10 rows.
- Wet the emulsion thoroughly and wait for it to start dropping out of the open artwork areas before you wash the screen as normal.
- In order to read the screen exposure time, it is important to keep the screen wet.
- Carefully look at the colour of the emulsion. You will see differences where each step was exposed. The point where the colour change is permanent is your optimal screen exposure time.
A worked out step test example
Let’s use an example of the full process of doing a step test to determine correct screen exposure time to make things clearer. Let’s assume we are using a diazo dual cure emulsion where the manufacturer suggests an exposure time of 130 seconds based on a 1+1 coat using a 3Kw lamp at 90cm. You are using an LED exposure unit which exposes as fast as metal halide if not better, so you decide to start by using the manufacturers guidelines of 130 seconds.
- Step A: Estimate 130 seconds
- Step B: 130 seconds x 2 = 260 seconds
- Step C: 260 seconds divide by 10 = 26 seconds
- Step D: You commence your step exposures and have the following times at the end –
- Row 1: 26 seconds
- Row 2: 52 seconds
- Row 3: 78 seconds
- Row 4: 104 seconds
- Row 5: 130 seconds
- Row 6: 156 seconds
- Row 7: 182 seconds
- Row 8: 208 seconds
- Row 9: 234 seconds
- Row 10: 260 seconds
After washing the screen you simply decide which row has the optimum exposure based on the colour change in the emulsion as described previously and use that time for future exposures for that specific emulsion. It could be useful to keep a record of your exposure tests in the event that you need to refer back to it again in future.
Perfect results every time
A bit of time spent at the start will ensure that your future exposures are correct every time and you waste no time when a job gets to press. If you have any further questions, please email us. You can download our free exposure calculator step test artwork by using the green button below to save having to create your own artwork. The button downloads a PDF document. Happy testing!