Why Every Printshop Should Own, and Use, an Exposure Calculator

How many times during a print run, do you experience problems such as premature stencil breakdown, pin-holing, and poor mesh adhesion from your emulsion?

Unfortunately, for some printers this can be a regular occurrence, resulting in screens having to be remade, patched or 'spotted out'; finished garments being scrapped; and the stencil making department becoming a bottleneck for the factory. Why do these problems occur?

In all likelihood it's because your stencil maker does not periodically use an exposure calculator as part of their quality control process. In fact, it is estimated that only a half of all the UK's print shops actually own an exposure calculator; and for those
that do, the exposure calculator is, more often than not, locked away in a drawer! The 'problem' is that few stencil makers (or printshop owners) appreciate the importance of using an exposure calculator as a calibration device for their exposure unit: without one your exposures could be 'all over the place'. The resulting premature stencil breakdown or compromised durability of your screens could be costing you money, not to mention the downtime and headache of having to remake those screens.

There are many different exposure calculators on the market – every major emulsion or capillary film manufacturer offers their own – but they all have the same common goal; of helping you to determine the correct exposure time for a stencil with the print characteristics that you require for each print run.

What is an exposure calculator?
On all exposure calculators, you will find either a step wedge or filters with varying densities that allow/block, a specific amount of light. These enable you to quickly carry out multiple exposures on one 'test' screen in order to precisely calculate the optimum exposure. Emulsion thickness, mesh count, mesh colour and lamp age all have an impact on the correct exposure time, so being able to calculate your exposures quickly and accurately will increase both quality and efficiency throughout your entire production

The CPS Exposure Calculator is simple and easy to use. It is a film positive comprising five columns. Each column has a resolution target, a set of halftone tints and various lines of text. It is marked with a 'factor' number, and four of the columns are backed with
a grey neutral filter of different densities. These allow more or less light to pass through during exposure. This format enables five different exposures to be made simultaneously on the same 'test' screen. Calculating correct exposure times using the CPS Exposure Calculator involves the production of a 'test screen' and the evaluation of that screen. To produce the test screen perform the following three-step process:

Step 1 Estimate the correct exposure time using the guidelines supplied by your chosen emulsion manufacturer, and then double it.

Step 2 Use the exposure calculator as your film positive and expose the >stencil in the normal way; wash out the screen and dry thoroughly.

Step 3 Examine the stencil to determine the correct exposure. For direct emulsions and direct stencils this is the point at which the entire thickness of the exposed stencil is fully hardened by the UV light from the exposure unit. This process of exposing/hardening is accompanied by a colour change in the stencil. Once you have produced your 'test screen' you can determine the correct exposure time for your set-up. This is a relatively straightforward two step procedure, as follows:

Step A The stencil will show variations in colour from one factor to the next. Follow the colour change from the lightest to the darkest until it stops. The factor where the colour change stops is the column that represents the optimum exposure. Direct stencils are fully and correctly exposed where no colour change occurs between the arrows.

Step B Once the correct factor has been chosen, multiply the factor by the test exposure time. This gives the precise exposure time (or number of light units) for that particular stencil/mesh count/light source combination. For example, if the 'correct factor' is 0.7 and the 'test exposure time' was 10 minutes, then the 'correct exposure time' will be 7 minutes (0.7 x 10 minutes = 7 minutes). Bear in mind that a colour change between factor 0.7 and factor 1 on the test screen
indicates an underexposed stencil. In this event, double the original test exposure time, and repeat the test. Underexposure can result in a weak stencil with poor mesh adhesion, along with reduced resistance to printing inks and screen cleaning solvents and so on. It can also make the stencil more difficult to reclaim. Conversely, overexposure leads to loss of fine detail, but will not compromise mesh adhesion or solvent resistance.

An exposure calculator also contains definition targets and halftone tints, which can be used to assess edge definition, print resolution, dot loss and dot gain. The booklet that accompanies the Autotype/CPS Exposure Calculator explains in detail how to perform these additional tests .

Remove the guesswork
Incorrect exposure is the single biggest cause of premature stencil failure. Periodically using an exposure calculator to provide quick, accurate confirmation of correct exposure times will remove the guesswork and greatly assist you in understanding the performance of your photostencil. The exposure calculator is a key piece of equipment that any quality conscious printshop should never be without.


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