Faster and Better Exposures

Screenprinting is a manufacturing process. In the end, it really should be about what is most efficient, effective, and economically viable (commonly described as the “3-Es”) to both the organization and the clients.


Bill Hood, ASDPT Fellow

Maximum productive efficiency requires producing goods and services at the lowest possible economically viable cost that effectively meets the client’s expectations. Producing at the fastest speeds possible reduces waste in terms of workflow and labor cost while producing the quality that will build the organization’s reputation with the market audience.

Those three factors are the primary performance measures for any operating system. To effectively manage a system, it is necessary to have feedback and take action to improve.

It is about investing in the right items to make your organization capable of printing the highest quality in your target market, of getting screens to the press faster, printing more pieces per hour so that you keep a full schedule of jobs on press year-round.

Never fall prey to the common misconception of exposure. Exposure works by UVA energy passing through the negative space (clear) areas of the film positive and cross-hardening the emulsion on a stencil. To be efficient, effective, and economically viable any process must meet or exceed these primary performance measures. Currently, the screenmaking area is responsible for the greatest number of variables in the screenprinting process and possibly holds the record for the least understood principles.

Exposure Time

The first step is in assuring that you are using an efficient, effective, and economically viable exposure lamp. By measuring the performance level of the exposure unit, we can ensure that we are in a good place. One only needs to invest in an $8 Stouffer T2115 Transmission Guide[1] to fully understand and resolve exposure problems on every stencil manufactured. It is the most accurate and least expensive guide on the market and should be in every screenroom.

The Stouffer Guide will show you the proper exposure needed for hardening the emulsion. Most screenmakers, that are attempting to hold minuscule details, are underexposing their stencils because they are using other devices that only allow for an understanding of resolution and not true exposure. Those screenmakers only printing spot colors are most likely overexposing their stencils. Either way, few are properly exposing every stencil.

Begin your experiment on a bright sunny day with no clouds to be seen overhead. Using two identical screens with the same mesh count, mesh color, emulsion over mesh ratio, and tension level expose the first screen with the correct exposure based on your exposure lamp and distance using a Stouffer Guide. Make another exposure with direct sunlight for 20 seconds using the same Stouffer Guide on the second screen using a compression exposure unit as shown below.

Save the exposure unit for those days when you lose power but want to use the downtime to expose stencils for use when the power comes back.

After developing the two stencils, the difference seen on the Stouffer Guide is that the first screen received whatever UVA energy your lamp is providing at the moment. The amount of UVA energy, of course, is reduced as the lamp ages and will never remain uniform. The second screen was exposed by the sun which on a bright, sunny, cloudless day was exposed with a 420nm UVA light source. The solid step 7 will vary between the two, but in using the guide supplied on the packaging sleeve with the Stouffer Guide you can find the correct exposure time for both that provides for a fully cross-hardened exposure.

The closer these two times are together the higher both the efficiency and effectiveness of the exposure lamp. The goal, of course, is to get these two times as closely as possible. The chart below provides an illustration of the different scenarios you may find in your organization. The upper right box is the place to be. If an organization finds itself in another box, the chart provides a path for improvement.
I think we can all agree that we do not want to spend an excessive amount of time and energy to complete any task. What we need is an exposure unit that will provide extremely fast exposures, i.e., the faster the exposure, the less time the light has to pass through the black areas of the film. And, the more transparent the clear film positive is the greater the contrast between the two.

5 Steps to Take

  1. If you are currently using a film positive material that is not optically clear, you can greatly reduce your exposure times by switching to a more transparent, hopefully, an optically transparent, film positive material.
  2. Invest in a faster-exposing emulsion. There have been many innovations in emulsion technologies that have resulted in shorter exposure times.
  3. Invest in a higher UVA output lamp, emitting in a spectral range of 350 to 420nm, and preferably a single point light source to provide the fastest exposure times.
  4. There are differences in lamps in emitting UVA energy, the metal halide lamps work much better than halogen lamps, and emit less heat.
  5. Assure that the glass in your exposure unit is “low-iron” or “iron-free”, which contains approximately 1/10 the iron oxide of conventional float glass. The glass should not exhibit a color cast when viewed from the edge. Most low-iron or iron-free glass transmits up to 45-percent more UVA energy and will drastically lower exposure times.

Economically, your investments will be repaid quickly with faster exposures and higher quality stencils with greater detail.

References

  1. The Stouffer T-2115 is available from http://www.stouffer.net/TransPage.htm. Note that the scale can degrade over time, especially if left out and unprotected. It is wise to order several to save on shipping costs and keep the spares safely in their packaging until needed.



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