Using the Alkemi Exposure Calculator

Introduction

Incorrect exposure is one of the primary and most frequent causes of stencil failure. The new Alkemi Exposure Calculator provides quick, accurate determination of exposure times, with all photostencil systems It can also be used as a printing aid to optimize print quality, or as a means of process control.

Description

Factor Light Transmission
1.0 (no filter)
0.7
0.50
.33
0.25 (darkest filter)        
100%
70%
50%
33%
25%

The Alkemi Exposure Calculator is a film positive comprising five columns, each with a resolution target, a set ,of· half-tone tints and lines of text. Each column is marked with a factor number and four columns are backed with a grey neutral filter of different density.

This format enables five different exposures to be made simultaneously

How to use the Alkemi exposure calculator

  1. Estimate the correct exposure time, using the guidelines supplied with all Alkemi photostencil materials, then double it.

  2. Expose the photostencil to the Calculator in the normal way, washout and dry thoroughly. Note For indirect stencils dry the film unmounted.

  3. Examine the stencil to determine the correct exposure time. Interpretation of correct exposure depends on stencil system.

Reading the Results of the Exposure Test

I. Direct Stencils

The correct exposure for capillary films, direct emulsions and direct/indirect stencils is the point at which the entire thickness of exposed stencil is fully hardened by the UV light This process of exposing/hardening is accompanied by a color change in the stencil.

EXAMINING THE STENCIL

  1. The Stencil will show variations in color from one factor to the next. Follow the color change from the lightest to the darkest until it stops.
    The factor where the color change stops is the column that represents optimum exposure

  2. Once the correct factor has been chosen. multiply the factor by the test exposure time. This gives the correct exposure time (or number of units) for that particular stencil/mesh/light source combination. Example:

    Correct Factor  X  Test Exposure  =  Correct Time

    .7               X   10 Minutes     =  7 minutes 

  3. Underexposure If there is still a color change between Factor 0.7 and Factor 1. this indicates an underexposed stencil. Double the original test exposure, and repeat the test.

Underexposure can result in a weak stencil with poor mesh adhesion and reduced resistance to printing inks, wash up solvents etc. It can also make the stencil more difficult to reclaim.

Overexposure leads to loss of fine detail. but will not compromise mesh adhesion. or solvent resistance.

II. Indirect Stencils

  1. Process the stencil as normal but dry unmounted.

  2. There are two ways to read the results:

    1. Carefully cut out solid areas of the emulsion from the columns. peel them from the base and measure with a micrometer.
      Overexposure will lead to reduced mesh adhesion and loss of fine detail.
      Underexposure will produce a weak stencil.

    2. An alternative method is visual inspection of the resolution of the five columns. Inspect the centers of the targets for filling in of the image. As the exposure time increases. resolution will decrease. At optimum exposure. the films have different resolving powers:

    Note: Resolution capability is diminished by the use of multi-point light sources (e.g. fluorescent tubes). or shorter than recommended lamp to glass distances.

Film Micron Resolution         Inches Resolution
Alpha Star
Microplus
Five Star        
50
50
75-100
        .002
        .002
        .003-.004

The Calculator as a Printing Aid

Definition Targets

These can be used to assess printed ·edge definition (straightness/clarity of the printed edge) and print resolution (fineness of detail achievable) The target is designed to allow the user to select the best angle to position film positives/ stencils in order to avoid 'sawtooth' and mesh interference. Resolution is checked by assessing the degree of 'filling-in' at the center of the target.

Halftone Tints

The 10% and 90% areas can be used to gauge the degree of highlight dot loss and flooding of shadow areas respectively.

The use of a square dot configuration means that the 'corners' of the dots just meet at 50%. Examination of a print will show at a glance whether there is 'dot loss' or 'dot gain' in the midtone.

Failure of the 'corner' to meet indicates 'dot loss'. This may be due to undercutting, caused by overexposure or poor light geometry, drying in of ink, or the choice of a low resolution photostencil

The merging of dots, or thickening of the join between dots, indicates 'dot gain'. This is due to flooding, which may be caused by over-thinning of ink, or by use of a photostencil with poor edge definition.

Before starting a print run with half-tones. it is always recommended that a test strip is printed, which contains a full range of tone values in the selected dot count. Positives with square dots are not recommended for fine half tone printing.

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