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