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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

To manufacture a suitable screen printed job, a large number of variables, such as mesh selection, frame selection, and EOM, must be controlled. Print conditions such as mesh, frame, and snap-off distance influence dimensional accuracy, an important contributor to print quality.

Dimensional accuracy is less important when printing a single-color job than it is when the application is a multicolor job. However, when embossing and diecutting follow printing, image dimensions become more relevant.

Dimensional accuracy is a relative term in screen printing and depends on a number of process variables the printer can manage: mesh selection, mesh tension, frame selection, squeegee parameters, and snap distance.

Mesh selection
Stainless-steel mesh supports high dimensional accuracy. Due to its mechanical characteristics, stainless-steel mesh allows a high mesh tension without residual deformation. A high mesh tension allows a close snap-off distance to minimize image distortion, assuring dimensional stability and consistency.

Mesh tension
Static mesh tension refers to screen tension when the mesh is affixed to the frame. During screen printing, the squeegee pushes the mesh in contact with the substrate to transfer the ink. During this operation, the mesh stretches. Simultaneously, the mesh’s tension increases. Due to the additional tension, the dynamic tension, the mesh immediately releases from the printed ink deposit after the squeegee has passed. Consistent, high mesh tension yields a sharp print. However, the degree of the mesh’s deformation directly affects the dimensional accuracy of the printed image.

The mesh’s elasticity determines the maximum mesh tension. Stainless-steel mesh is substantially more rigid than high-tension polyester mesh. On the other hand, high-tension polyester mesh is more rigid than normal polyester mesh. A tensile test machine slowly elongates mesh. The tension proportionally increases until the line branches off. Any tension beyond this level permanently deforms the mesh. This level is called 0.2% yield point. The deformation is called plastic deformation. During tensioning and printing, we should not go beyond the 0.2% yield point.

Elasticity
The screen-printing mesh’s elasticity is a material characteristic, depicted by the E-modulus or Young’s modulus. It describes the tension required to proportionally deform the mesh, using the linear part of the curve.

Frame selection
The frame must be rigid and dimensionally stable, such as a cast iron or stainless steel. A frame with slope profiles is extra dimensionally stable to minimize image distortion.


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