Pin holes in your stencil may be evident immediately after washing out your screen, or they may develop on press during the print run. Either way, the time spent touching up pins holes is time wasted.
The Following are the Most Common Causes of Pin Holes:
- Screens not degreased: The single largest component in screen printing emulsion is water and we all know about oil and water right. If improper degreasing is the culprit, you will see small fish eyes in the emnulsion coating. All screens must be degreased using a degreaser designed for screen printing. While common detergents like Dawn have degreasing properties, they also typically have other ingredients like lanolins that can contaminate the mesh. Degrease both sides of the screen using a soft brush to work the degreaser into a lather. Rinse thoroughly. Water should sheet down a properly degreased screen without streaking.
- Dirty exposure glass: Nobody likes cleaning exposure glass, it is even worse than cleaning screens, but those bits of dust, dirt, and ink block light just like your film and wind up being exposed into the emulsion. Glass should be thoroughly cleaned at least daily, and more in busy shops.
- Emulsion not allowed to rest after mixing: You should allow 30 minutes for emulsion to rest after mixing. This practice helps eliminate air bubbles caused by mixing and gives the diazo that you mixed in a chance to dissolve thoroughly. Air bubbles and small specs of diazo can create week spots in your stencil and break down on press.
- Insufficient emulsion coating: A properly coated screen will have emulsion proud of the mesh on the print side of the screen. Too thin of an emulsion layer can break down on press as mesh knuckles break away from the emulsion. To achieve proper coating, coat the print side of your screen until emulsion fills the mesh as viewed from the squeegee side. Coat one time on the squeegee side. Dry screen horizontally with squeegee side up.
- Dust in emulsion coating: Dust in your emulsion coating can leave a weak spot that breaks down on press. To avoid dust, degrease your screens and dry them in a clean area protected from dust. Avoid blowing a fan on the screen as fans can kick up dust and deposit it on the mesh. Coat the screen and dry horizontally in an area protected from dust and light, such as a drying cabinet. Again avoid the use of fans. When handling emulsion, avoid scraping emulsion back into the bucket with cardboard or other materials that can leave particle contaminants in the emulsion.