ARTICLES

Proper Screen Drying and Prevention of Contamination


 
A technical information article
by John Rafferty and Douglas Grigar

Screens are the most critical part of the screen printing process. Proper drying and prevention of contamination are important steps in preparing screens for printing.

This article will focus on the importance of contamination free drying after cleaning and after coating. The article will also cover the use and construction of racks and cabinets for drying needs.

Before applying capillary film or coating a screen with liquid emulsion, the mesh must be dry free of contaminants. New mesh and new screens can be contaminated by storage, shipping and handling and will also need to be cleaned before use.

Cleaning steps include:

  1. All ink is removed from the screen and frame.
  2. Emulsion remover applied and emulsion removed with a pressure sprayer.
  3. Screens should be scrubbed lightly with a clean brush and a suitable degreaser.
  4. Degreaser foam rinsed with a low pressure flush of clean water.
  5. Screen frame and mesh must be dried completely before coating.

Starting at stage 4. inadvertent contamination problems first start. Quick removal of as much moisture as possible from the mesh is imperative. There is a shop vacuum head available made just for screen mesh.

Using fans will blow dirt and lint directly on the screens. The best method for drying is one that filters the air, raises the temperature and removes moisture saturated air.

A complete seal on a cabinet will prevent contamination, but will also concentrate air in a small area causing moisture saturation. Once air is saturated with moisture it can no longer carry moisture away from the frames. Heated air will carry more moisture, but too high of a temperature will cause accelerated premature exposure. Filtered air flow into and out of the drying room or cabinet will eliminate air saturation and aid in drying.

Coating and drying of liquid emulsion:

  1. Liquid emulsion is best applied in a light safe room with high humidity. Higher humidity levels will help prevent the liquid emulsion from drying too quickly and thickening in the trough causing inconsistency.
  2. Liquid emulsion from the container is 60%-70% water. Liquid emulsion must dry forming a solid area sealing the mesh. To prevent symptoms of underexposure and develop an acceptable sensitivity to light emulsion must be thoroughly dried. Most manufactures recommend a moisture content below 30%.
  3. Emulsion manufacturers recommend drying liquid emulsion in air that is heated to 90-105 deg. F. with a relative humidity of 50% or less. To reach this level you often have to drag filtered air from an air conditioned room or install a dehumidifier into your cabinet or storage room.

A quick note on storage of dried screens: once dried coated screens are best stored in a cool, dry and dark area to extend their shelf life. Unexposed emulsion will reabsorb moisture from the air. Storage in high humidity will require that the screens be dried again before exposure.

There are many storage and drying cabinets available on the market. If you feel up to the task you can construct a cabinet that is functional and will speed the drying process. Many user constructed drying cabinets are far superior to available drying cabinets.

Drying cabinet concepts:

  1. Temperature of 80-105 deg. F. and a humidity level of less than 50%.
  2. Filtered air circulating across the surface of the coated screens.
  3. Sufficient air movement speed to prevent hot spots.
  4. Air movement not more than a complete exchange each 2-5 minutes. The CFM (Cubic Feet per Minute) rating of each fan is printed on the box. Calculating the cubic area of the inside of the box is depth times width times length.
  5. Positive pressure can be created by having a smaller vent opening than intake. Positive pressure prevents contamination entering the cabinet past the door seals and expels air out when opening door.
  6. Screens should be stacked so that air can circulate on all sides. Air inlets and outlets should be placed so that good air movement will occur in every corner of the cabinet.
  7. Air can be circulated only within the cabinet requiring a dehumidifier and a heater.
  8. Air can be introduced entirely from the outside requiring a heater and filter. Air from an air conditioned room provides lower humidity, but will require the venting from the cabinet directed outside of the room. (Fig. 1)
  9. The most desirable cabinet design would be a combination of dehumidifier, heater and filtered outside air with an adjustable intake and vent providing positive pressure. (Photo 1, 3 and Fig. 3)
  10. Cabinets with removable racks on wheels can be used for storage and movement.
  11. Racks can be constructed with parallel PVC pipe as the rack shelves. (Fig. 2) Pegs of PVC pipe or dowels set at an angle can provide support without touching the drying emulsion. (Photo 2)
 

Stay consistent and you will be able to predict your results with greater accuracy. Your goal should be consistency, predictability, and repeatability.

Proper Screen Drying and Prevention of Contamination

Photo 1 (Dehumidifier, Heater, Inlet fans)

 
Proper Screen Drying and Prevention of Contamination

Photo 2
Proper Screen Drying and Prevention of Contamination

Photo 3
 

Proper Screen Drying and Prevention of Contamination

Fig. 1

 

Proper Screen Drying and Prevention of Contamination

Fig. 2
Cabinet with air introduced entirely from outside.
Requires heater, filter and air from an air conditioned
area. Requires external venting.

 

Proper Screen Drying and Prevention of Contamination

Fig. 3
Dehumidifier, heater and filtered outside air
with an adjustable intake and vent providing
positive pressure.