Proven advice for industrial applications and other situations where delivering less-than-flawless prints isn’t an option.
By Mike Young
A department supervisor concluded a hands-on training session in screen making with a smile and, in a raised voice, told her team: “It’s not the frame size – it’s the image-to-frame ratio, stupid!” Abruptly put, perhaps, but that’s what it comes down to: Is there sufficient mesh clearance between the image and frame to lessen troublesome image distortion and related issues? As squeegee pressure is applied, the mesh stretches outward due to off-contact distance and, if used, peel-off. More importantly, the closer the image and/or squeegee is to the frame’s inside edge, the worse the distortion will be due to the mesh’s severe angle of deflection. (See Figure 1.)
Other than guessing, the three most commonly used methods to establish the maximum image size for a given frame are by measurement, ratio, or proportion. The first approach uses a minimum dimension from the frame’s inside edge for placement of the image (such as 6 x 8 inches) regardless of the frame’s size; the second involves an image-to-frame ratio (IFR) that meets each job’s specific requirements; while the proportional method requires the frames to be physically twice the squeegee stroke length and triple its width, for example. The proportional method places a greater limitation on image size than going by the IFR, which I personally prefer for more stringent applications. Whatever system is used, the frame must be large enough to adequately support the whole image within the tolerance required by the application. Critical jobs are more likely to be doomed from the start – or at least made exceedingly more difficult – if frames are undersized.
The advantage of the IFR method is that it doesn’t assume all jobs are the same. Figure 2 shows different types of printing specialties loosely grouped together and at different general image sizes, providing a reasonable IFR that could be adopted as a benchmark or starting point. Each job may have different challenges to overcome. For example, the customer’s specs for one particular job required an IFR of 30 percent, while a similar job, size, and ink for another customer demanded the IFR be reduced to 22 percent – simply because the second job had tighter electrical specs.
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