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Digging into Dimensional Accuracy

(October 2013) posted on Tue Dec 03, 2013

Learn how to dial in some of the variables that can compromise image quality on press.


By Wim Zoomer

A very important factor is the ratio between the frame dimensions and print-image size. In fact, the length of the squeegee stroke and the squeegee blade’s length are responsible for the mesh deformation. These two parameters may strongly affect printed image deformation. Preconditions are:

• Print image is centered within the frame on the stencil
• The squeegee blade’s length is just a fraction larger than the image size. A longer squeegee blade causes more image distortion.

To print with high dimensional accuracy, the image must be exposed on a properly coated and highly tensioned screen that is adhered on to an oversized print frame.

This precondition is not always applicable and, in practice, may be very costly. One production job or application allows more or less tolerance compared to the other. To simplify things, consider the following equation, where multiplication factor F refers to the ratio between the dimensions of the screen printing frame and the image size. A larger factor F means a higher level of accuracy. Factor F reduces image distortion and prevents premature mesh fatigue. Frame dimensions are defined as length (L) and width (W).

Frame dimensions (L x W) frame = F * (L x W) image

Example: Suppose print-image size is 400 mm long and 320 mm wide. The category of the application is high graphic quality, represented by F= 2.5; therefore, frame dimensions (L x W) frame = 2.5 *(400 mm x 320 mm) = 1000 mm x 800 mm.

Hybrid screens
Conventional tensioning means to adhere the mesh straight on the frame. A hybrid screen is the solution for jobs that require high dimensional accuracy. A hybrid screen is a system that includes a stencil that is completely surrounded by polyester mesh. The polyester mesh’s flexibility absorbs the mesh’s deformation by the squeegee, leaving the stencil practically unaffected.

Snap-off distance
Snap-off should be adjusted based on the way the mesh releases from the printed ink deposit after the squeegee has passed—and the resulting print quality. If the definition is insufficient, it is quite common to increase snap-off distance. The consequence of a distance increase will be noticed immediately. The dimensional accuracy decreases. Every doubling of the snap-off distance decreases the dimensional accuracy by a factor of four! Although a rule of thumb for polyester mesh’s snap-off distance is 0.004 x screen-frames width, always apply the smallest possible snap-off distance.

More rules of thumb
Investments of time and money may be necessary to improve dimensional accuracy, but quality improvements, especially in high-resolution applications, justify these expenses. In summary, consider the following advice:
• Use sufficiently rigid screen printing frames.
• Optimize static mesh tension.
• Minimized dynamic tension.
• Critical screen jobs require a rigid screen-printing frame.
• Minimize snap-off distance. The mesh is supposed to separate from the printed ink deposit just behind the passing squeegee. After all, every doubling of the snap-off distance results in a deterioration of the print accuracy by a factor of four.
• Tension the mesh just before reaching the maximum allowed level (0.2% yield point).
• Check the mesh tension using a mesh-tension gauge.
• Make sure that the print image is always positioned in center of the mesh.
• The squeegee should protrude the print image by just a couple of millimeters (rather than centimeters)
• Keep static mesh tension consistent across all screens in a print job.
• Start printing with the lowest possible snap-off distance. Increase the snap-off distance step by step to maximize print quality.
• Minimize squeegee pressure.
• Apply higher mesh tensions of subsequent screen if mesh marks continuously turn up.
 


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