Exposure/Stencil-System Compatibility: It pays to be compatible

by Douglas Grigar, Master Screen Printer

(A version of this article originally appeared in the April 2006 issue of PRINTWEAR.)

 

There’s a balancing act when choosing an exposure unit to best fit your stencil system of choice. Stencil selection, exposure time, and the actual exposure unit are, if not the most critical decisions in a screen printing shop, easily in the top three critical variables that determine printing success in any production scenario. So let us examine a few of the factors that should influence your pairing of stencil system and exposure parameters.

The Light Needed

To start with, let’s review a few terms relating to the subject of electromagnetic radiation. We are concerned with three types of light radiation: visible light, ultraviolet light, and actinic light.

Visible light is assigned an artificial range of 400 to 700 nanometers (nm); this is considered an artificial designation because some humans can see lower or higher than this wavelength range of electromagnetic radiation. Ultraviolet (UV) light is simply light radiation past the blue/violet human-visible range measured by the wavelengths in the range of 100 to 400nm. UV light is broken into three sections of wavelength: UV-A, often called black light, is in the 380-315nm range; UV-B, known as medium wave, is in the 315-280nm range; and UV-C, often called germicidal UV, is 280nm or less. Actinic light is simply light radiation that produces observable or measurable change when it interacts with matter, particularly reactive chemicals. Actinic light was first commonly used with the advent of photographic science; it starts in the UV-A range of 315nm and progresses into the lowest of the visible light range, up to 480nm. For exposing stencils, we are looking for high output of actinic light, the wavelength range to which photo-reactive emulsions are most sensitive.

Emulsion Exposure Curve

Emulsions used for screen printing stencils have a nonlinear response to light; that is, when graphed, they have a “curve” in the graph where chemical linking shows changes in speed of reaction due to radiation penetration (see Figure 1). Calculating a correct exposure time is essential for finding that often thin slice of time for a perfect exposure. The volume or intensity of actinic light also makes a large difference in the correct exposure time, as the chemical reactions are controlled not only by time but also by the intensity or volume of correct light: the greater the intensity, the shorter the exposure time. Many emulsions show a change in exposure curve when exposed to higher-intensity light. An example would be an emulsion that will have slow diazo-like exposure time when using a low-power black-light multi-bulb unit, but have faster SBQ-like exposure time when jumping to a high-power metal-halide exposure unit.

[Figure 1: An exposure-curve graph is typically divided into three portions: the toe, the center, and the shoulder. The shape of curves cay vary, often drastically, depending upon light volume and emulsion characteristics.]

 

Emulsion Chemical Linking Targets

The three emulsions most common to screen printing have different exposure characteristics and demand slightly different wavelengths of radiation for optimum chemical linking. Emulsion manufacturers have several of each type of emulsion available and often in several formulations for specific applications. Choosing an emulsion becomes an exercise in matching the product to the products you need to print, along with your ink and cleaning chemicals; it is of utmost importance to carefully choose emulsions and chemicals for compatibility to prevent on-press breakdown of the stencil and reasonable reclaiming performance.

SBQ or one-part photopolymer emulsions are generally reactive to lower actinic ranges from 340 to 360nm, are fast to expose, but suffer from small exposure latitude, demanding fairly accurate exposure times to prevent under- or over-exposure. Diazo is most reactive in the 360 to 370nm range for linking, while dual-care emulsions (SBQ and diazo combinations) are reactive in the 360 to 400nm range. Knowing the approximate necessary wavelengths for the emulsions you intend to use can help in the choice of an exposure unit.

Practical application of the products follows time-tested knowledge in the industry. Advanced scientific measurement of the processes adds little value once a practical, useful point has been reached. An example of this is that repeating an exact light-reactive test is more difficult than it first seems because light sources literally change in output as they are used; even high-dollar single-source lights drop in actinic output progressively, from full output to no output in only 700 hours of operation. Light-volume loss is measurable on an accelerating but slightly inconsistent curve, making exact measurement difficult to repeat. Furthermore, once testing for the practical use of the product has been conducted, more advanced testing produces information that has little useful application. The entire process of exposure is a juggling act of adjusting the time to match the diminishing output of the radiating source.

Power Loss

Output of exposure lamps, regardless of type, decrease in intensity or volume of actinic light as they age; unfortunately, lamps often do not simultaneously lose large portions of their visible light, and so do not give a dependable visual indication of this degradation. All lamps will eventually lose almost all of their ability to produce actinic light and will be unable to expose emulsion effectively.

To back up what your salesman tells you, the most expensive exposure systems are the richest in actinic light and expose screens quickly with the largest volume of light radiation. Price can often follow the speed at which exposure units are able to work, with the singular exception of units using quartz-halogen lamps.

Market Choices in Exposure Units

While the article “Exposure Units…Exposed” (Printwear, April 2005) reviews four exposure lamp types, there are only three of the four lamps generally available from manufacturers that serve the screen printing industry.

Starting with the longest exposure times, we need to talk about entry-level exposure units, often called multi-source or multi-bulb. Users can expect full and complete exposure times in the 5 to 15 minute range for SBQ emulsions, 15 to 30 minutes for dual-care emulsions, and 20 to 30 or more for diazo emulsions.*

Multi-bulb systems include multiple side-by-side rows of fluorescent tubes, multiple quartz-halogen bulbs, and quartz-halogen fluorescent-tube combinations; all are multi-bulb systems and all have extended exposure times (see Figure 2). Single-source quartz-halogen exposure units will also have exposure times similar to or longer than multiple, unfiltered black-light bulbs.

Because of extended exposure times, often the best choice for entry-level exposure units (regardless of price) is fast-exposing SBQ-photopolymer emulsion. The common use of toner-based positives output on vellum, combined with entry-level exposure units, compounds the problems inherent with low-power units by providing less-than-opaque dark areas, which forces users to under-expose in order to produce useable screens.

*Using the recommended 15% to 20% emulsion-over-mesh (EOM) ratio covered in “Coating Screens for Maximum Effect” (Printwear, June 2005). The recommended EOM of 20% is 10 to 20 times the thickness of coating common in the industry, the exposure times listed are much higher than common, and severe under-exposure is seen in most plants.

[Figure 2: Simple exposure systems are considered multi-bulb if there is more than one light source.]

 

[Figure 3: High-power metal-halide lamps lose light output quickly, and without a light integrator, require daily testing to prevent under-exposure.]