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Determining Optimum Exposure

(March 2011) posted on Tue May 03, 2011

Discover how proper stencil exposure influences quality and productivity throughout the screen-printing workflow.


By Wim Zoomer

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Stencil life and print quality are directly related to a screen printer’s ability to determine optimum exposure time in prepress. Fortunately, calculating proper exposure time can be a simple process. It starts with mesh selection. The mesh count and the mesh type the screenmaker chooses depend on the image. An image with large solid areas calls for a coarse mesh that will allow more ink to transfer to the substrate. The screenmaker would choose a finer mesh for a highly detailed image or one composed of halftones.

Inks also influence mesh selection. UV-curable, solvent, water-based, conductive, and other inks have their own requirements. For example, UV-curable inks contain 100% solid matter, so the wet ink deposit and post-cure ink deposit after are practically the same—practically, because UV-ink shows some shrinkage during curing. Solvent-based ink may contain 70% solvent, for example. In other words, 30% of ink in its solid state remains on the substrate after printing and drying using high temperature. Obviously, these properties affect mesh selection.

Once the appropriate mesh and ink are selected, correct stencil-exposure time will keep required print quality consistent until the end of the print run. However, incorrect exposure is one of the primary and most frequent causes of stencil failure. Let’s look at the screenmaking process and related parameters that affect print quality before we discuss how to determine correct exposure time.

Emulsion
Many screenmakers use direct emulsions based on hybrid (dual-cure) systems or capillary film. These two-pot emulsions contain the diazo sensitizer. Some screenmakers use emulsions based on photopolymer systems. The exposure time of photopolymer emulsions is shorter compared to hybrids. It is not surprising that the exposure latitude of photopolymer systems is quite small. The light-sensitive components in the emulsion relate to a certain wavelength range in the ultraviolet light spectrum. In general, the light sensitivity of direct emulsions and capillary films is in the range of 350-500 nm. The emulsion will start to polymerize upon exposure to light in this wavelength range.

Mesh fabric
Mesh fabric is commonly woven of polyester threads and is available in numerous mesh counts and thread diameters. Screenmakers use relatively fine fabrics when printing fine text, halftones, linework, or thin ink deposits. They apply a thin coat of emulsion (or a thin capillary film) onto the fabric. Emulsion thickness after drying is often approximately 5 μm.


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