HMI Screen Printing

Screen printing is a meticulous process; whether you’re screen printing t-shirts, newspapers, or electronics, precision is expected. Of all the things that are screen printed, electronics are one of, if not the most critical. Screen printed conductive traces must meet customer width and resistance requirements, graphic overlays must look crisp and perfect. From the first run to the last, it is critical that the image being screen printed maintain uniformity and quality. Reproducibility is key!

There are 3 factors that are at play to ensure reproducibility: mesh selection, screen tension, & emulsion.

 

Mesh Selection

The two most common mesh types are polyester and stainless steel, they each have their pros and cons.

While stainless steel can be stretched to a higher tension than polyester mesh, polyester can be stretched to a high enough tension to successfully print tight tolerance jobs.

Stainless steel can withstand more pulling force than polyester thread, however,

Stainless steel threads are slightly thinner than polyester threads, this creates more “open area” on stainless steel mesh. More open area allows more ink to flow through the screen. For this reason, a stainless steel mesh with a higher thread count could be used and result in the same ink deposit as polyester mesh with a lower thread count – this would come into play with thin traces (fine line printing), or the need for better edge definition.

Stainless steel mesh is expensive compared to polyester, as are the screens required for the stainless steel (not to mention much heavier than the frames used for polyester).

Though stainless mesh doesn’t rip as easily as polyester, it does dent. Care must be used when handling.

Stainless steel might seem like the standout option, but polyester mesh performs very well and is used for the majority of our current products. Not only is it more cost effective than stainless, but we can typically attain the required edge definition and ink lay down. The use of stainless steel is design driven.

 

Screen Tension

Screen tension plays a big role in the quality of screen printed products.

• High tension (tight screen) gives better control over consistently maintaining the proper image size, meaning, the printed image is less likely to stretch during the print stroke
• High tension lends to better register one print to another print pass (to achieve this, both print passes should have screens with almost the same exact tension).

Mesh suppliers will give recommended tension levels for each of the different mesh counts; tension meters are used to monitor the mesh during the stretching process to ensure the recommended tension is achieved.

Polyester mesh is stretched to a high tension, then glued to the metal frame. Steel mesh is not glued to a metal frame, the frame would collapse due to the tension and strength of the mesh. Frames used for stainless are self-tensioning – mesh is laid over the frame, the edges of the mesh are inserted into channels within the frame. The channels are then pulled to apply tension to the mesh.

If the tension drops out of specification during the printing process, the channels on stainless steel screens can be pulled tighter to get the screen back to the proper tension. However, if the tension drops during printing on a polyester screen, because the mesh is glued, a new screen would be necessary.

 

Emulsion

Emulsion is a photo-sensitive coating placed on the mesh to ensure that the image being screen printed is the only thing that is printed. Once the emulsion is applied to the screen, a UV light is used to cure (aka harden) the areas around the image. Then, once the screen is being used on the printing press, the ink floods the screen and is pushed through the areas without emulsion.

There are two types of emulsion:

• Direct emulsion – liquid state; this is applied to the mesh using an automatic coating machine.
• Capillary film – solid state; this is applied to the screen in sheet form. Capillary film typically provides a more consistent thickness of emulsion than the direct method, which can lead to better edge definition in the printing process. However, this labor-intensive process of manually adhering the capillary film to the mesh is expensive compared to direct emulsion.

 

Reproducibility is extremely important and can be achieved if proper screen printing set up, and processes are followed, ensuring that every part printed is identical, from the first to the last. Contact us today if you are in need of a supplier that takes screen printing very seriously, every step of the process.