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But before we can explain how to determine the correct exposure time, we must focus on the screen-making process, including a number of parameters affecting print quality.

Emulsion
To make the screen photo-sensitive, many screen makers use a direct emulsion based on a so-called hybrid (i.e., dual-cure) system or a capillary film. These two-pot emulsions contain the diazo sensitizer. Some screen makers use emulsions or capillary films based on photopolymer systems, which have significantly shorter exposure times. It is not surprising that the exposure latitude of photopolymer systems is smaller than the dual-cure ones.

EIM and EOM
We assume that every mesh opening is completely filled with emulsion. This means that the emulsion in mesh (EIM) is 100%. The emulsion on mesh (EOM) has to be at least 5 microns to achieve the required stencil's lifetime and avoid a premature stencil breakdown during the print run and the resulting pinholes.

However, if the exposure time is insufficient, it may cause pinholes, possibly followed by a premature stencil breakdown. Sufficient EOM lifts the nodes of the fabric, allowing the ink to flow underneath, preventing the creation of a negative sawtooth.

Fabric
For convenience we assume that the screen maker uses polyester fabric.

The mesh count and the mesh type (HD, S, T, plain or twill weave) the screen maker chooses depend on the print image. If the image contains large solids, the screen maker takes a coarse fabric, allowing more ink transfer, so more ink deposit is printed on the substrate. In case of fine linework, type or halftones, the screen maker will choose a finer fabric. After all, fine details require fine meshes.

The emulsion thickness after drying is commonly at least 5 microns.

Coarse fabrics allow printing thick ink deposits, high-viscosity inks or inks containing pigments; the corresponding images will be coarse as well. The coarser the fabric, the thicker the EOM to avoid a dark edge around the image.

Commonly used fabrics are white. The white threads of the fabric reflect the UV light and therefore may cause undercutting of the UV light during exposure. Besides white, there are yellow- and orange-dyed fabrics. During exposure, dyed fabrics absorb UV light and consequently cause less reflection. Due to this characteristic, dyed fabrics cause less undercutting of the light, producing an image with fewer edge defects. The loss of quality is dramatically reduced. Dyed fabrics are more expensive than the white ones and are especially used for printing very fine details or images with high-quality requirements.

Light source
The emulsion is susceptible to a certain UV-wavelength range. The bulb, used to expose the emulsion, emits light within this wavelength range (i.e., emission spectrum). The most used bulbs are metal halide-based. These discharge bulbs, predominantly producing ultra-violet and blue light, contain mercury vapor and metal (iron or gallium) halide to exactly adjust the emission spectrum. These bulbs produce emission peaks at 385 and 420 nanometers.

Light energy
The power of the lamp (such as 1 or 5 kilowatt) substantially affects the exposure time. More power shortens the exposure time.

With frequent use, the bulb ages and its intensity reduces gradually. To compensate for this bulb instability, most exposure systems are provided with a light integrator, which measures the amount of light energy. The light integrator ensures exposure of a screen with the same amount of light energy irrespective of the bulb's age.

Distance
The exposure time is proportional to the distance squared. In other words: Increasing the distance between lamp and frame increases the new exposure time.

We should also realize that the distance of the lamp to the stencil affects the intensity distribution of the UV light. The maximum light intensity is located in the center of the stencil's print area. The light intensity decreases toward the sides of the screen.

Moreover, we will observe loss of fine details due to undercutting of the light, especially at the edges of the print area.

Increasing the distance between screen and lamp improves the light distribution across the print area on the screen but increases the exposure time. The relation between distance and exposure time is as follows:

new exposure time = (new distance/old distance)2 x old exposure time

Example:
Old distance: 60 cm New distance: 120 cm
Old exposure time: 25 seconds
The new exposure time is 100 seconds. Doubling the lamp distance quadruples the exposure time.

Standardization
Standardization is key: Restrict the number of variables during the exposure process. Use a limited number of fabric specifications. Stick to one type of direct emulsion or capillary film. Apply the same type and color of fabric.

Do not change exposure conditions. Standardization will reduce the risk of making obvious errors. For the convenience we assume we always use the same exposure system, particularly the type of bulb and the distance between screen and lamp. If a light integrator is not available, it adds a continuous variable to the determination of the required exposure time.

If we understand how the exposure time affects the most relevant emulsion's characteristics, it allows us to accept the remaining process variables - image quality and emulsion strength.

In exposure there are just three possibilities: underexposure, overexposure and the correct exposure.

To avoid getting lost in the number of process variables available, I recommend using a logbook. A logbook allows the screen maker to find the conditions under which a screen was coated, which fabric has been used for a certain job and the exposure time applied for a job with fine details and the lamp's age.

Polymerization
Using the correct exposure time is essential to ensure a good print quality and the stencil's lifetime. The UV light causes a chemical reaction between the emulsion's monomers, pre-polymers and other chemical substances, changing them into a strong and stable grid of polymers to eventually create a stencil. The emulsion's color will become darker. Unexposed emulsion is soluble in water and will be removed from the fabric while washing out the image. On the other hand, exposed, and thus polymerized, emulsion is moderately soluble in water and therefore will not be removed from the fabric during washing out.

The exposure time strongly affects the stencil's characteristics, such as the resolution, the edge sharpness, the eventual EOM, the chemical and mechanical resistance and its resulting lifetime.

A correct exposure polymerizes the emulsion for 100%, producing an exact copy of the print image on the fabric. The emulsion's adhesion is excellent both during washing out and printing. The chemical and mechanical resistance and lifetime of a correctly exposed stencil are good, whereas the stencil's reclaimability leaves nothing to be desired.

Underexposure
Polymerization starts at the top of the emulsion (i.e., print side), which is at closest distance to the light source, and proceeds through the emulsion coat toward the bottom (i.e., toward squeegee side) where the emulsion is connected to the fabric.

If an emulsion is underexposed, it will not completely encapsulate the fabric's threads by polymerization (i.e., hardening out).

Any unexposed emulsion is soluble in water, and as a result, will be removed during washing out. The adhesion between emulsion and fabric will be reduced. The resulting thickness of emulsion coat (EIM) in the mesh declines and the definition deteriorates. As a consequence of the decreased chemical and mechanical stencil's strength, a reduced lifetime will affect the print run length. If the exposure time decreases even more, the emulsion separates from the fabric due to more emulsion being removed during washing out. The stencil's strength and the emulsion's reclaimability decrease.

Overexposure
Is overexposing perhaps a better option than underexposing?

Exposing too long does not create adhesion problems between emulsion and fabric. Additionally, the overexposed stencil will be strong enough to survive the print job. However, the image quality declines with the exposure time.

During exposure of the emulsion through the film positive, the UV light will refract and cause undercutting, an unwanted exposure of the emulsion underneath the black part of the film positive. This undercutting effect, combined with reflection caused by white fabrics, deteriorates the edge sharpness. (By the way, we have just learned that dyed fabric causes less reflection.) A longer exposure time increases undercutting, causing narrower open lines in the stencil. This means that the line width (resolution) is affected negatively as well. The very fine open details will be blocked first and do not appear during printing. Quality of the line (width) is strongly affected by exposure time. This undercutting effect does not appear, or is hardly visible, if the correct exposure time is being used.

Correct exposure
We consider the graph representing the relative exposure time vs. stencil quality. Relative quality is expressed as 0 = very weak to 1.0 = excellent.

The green curve represents the stencil's strength (i.e., chemical and mechanical resistance). We see that the stencil's strength improves after increasing the exposure time.

From exposure time = 7 units, the strength hardly increases and stabilizes at maximum level. From exposure time = 7 units, the polymerization (through cure) is completed.

The red image quality curve, however, goes through an optimum. Applying an exposure time longer or shorter than 4.5 units means a decrease of the image quality (definition, resolution).

The optimum exposure time is the intersection of both curves, representing the compromise between resistance and image quality, resulting in an optimum exposure of 6 units.

Exposure calculator
Most emulsion manufacturers supply a very convenient tool to determine the correct exposure time. These exposure calculators require only one single exposure!

The exposure calculator consists of two parts, namely a film positive with (often 5) the same images and secondly a film carrying (often 5) filters with different greyscales. The size of the greyscales corresponds with the size of the images on the other film.

Different greyscales transmit different amounts of light. The factors indicating each individual area represent the relative amount of light transmitted. "Factor 1" area transmits 100% incident light. Factor 0.25 is the darkest filter. This greyscale transmits only 25% of the incident light. In this way a succession of areas of increasing light transmission factors has been made, such as: 0.25, 0.33, 0.5, 0.7 and 1.0.

Suppose we expect an exposure time of 100 units. Double it and conduct the exposure test with this value (200 units). The different areas on the exposure calculator are now being exposed by respectively 50, 66.5, 100, 140 and 200 units.

The expected exposure time is in center, allowing the screen maker to compare the different areas. After development (washing out) and drying, we determine the emulsion's color change. The factor where the color change stops represents the optimum exposure. Different elements on the exposure calculator, such as text, linework and halftones, enable the screen maker to fine-tune the exposure time. The criteria are resolution and edge sharpness. Suppose area factor 0.6 provides the best result. The resulting optimum exposure time is 0.6 x 200 units = 120 units

Available exposure calculators
The exposure calculators are provided with a separated number of test images and the corresponding number of light transmission filters. Most common ones have factor 0.5 in center. Some exposure calculators are equipped with extra tools, such as halftone reproduction targets or definition targets to assess print resolution and edge definition. Distinguish the resemblance and differences between just two of them: the Ulano Exposure Calculator and the MacDermid Autotype Exposure Calculator.

Located in the Netherlands, technical author Wim Zoomer manages a consulting and communication business considering all aspects concerning screen printing technology for both graphic and industrial (such as printed electronics) applications. Writing advertorials, application stories and success stories for various companies, he also writes editorials for publication into several (screen) printing, industrial technology magazines as well as on their corresponding websites.

Zoomer wrote the book on the decoration processes of architectural glass entitled, "Printing Flat Glass."

He was consultant of the European Screen Printing Manufacturers Association (ESMA), and serves as board advisor of the USA magazine iSP (industrial + Specialty Printing) magazine. In 2010, Wim Zoomer was inducted into the prestigious Academy of Screen and Digital Printing Technologies.

This article appeared in the SGIA Journal Garment Edition, Winter 2018 Issue and is reprinted with permission. Copyright 2018 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.