One of the main causes of plate defects in the exposure process is pinholes. These pinholes appear in conventional platemaking when dust particles are trapped between the film negative and plate. The static charge apparent in the film negative attracts dust particles to both the film negative and the photopolymer material.
The same static charge is also apparent in digital platemaking through the removal of the protective coversheet of the raw photopolymer. Therefore it is safe to say that dust particles pose a threat to both conventional and digital plates. Minimizing this static charge can be addressed through ionizers and humidity control within the plateroom. However, it is also important to make sure that the dust particles are not drawn into the room.
Ensuring that the plateroom has positive air pressure will help maintain a clean plateroom. A simple test to check your plateroom for positive air pressure is to stand outside the plateroom entrance and only slightly open the door to the plateroom. You should feel air exiting the plateroom. In more sensitive cases you can hold a tissue to the gap in the door to see if the tissue blows outward.
Positive air pressure occurs when the air supply to the room exceeds the exhaust flow from the room. While a Positive Isolation Cleanroom in a hospital requires a minimum of 15 air changes per hour, most photopolymer plate manufacturers suggest a positive air pressure plateroom with 8-12 air changes per hour.
One air change occurs when the quantity of air equal to the volume of the room is supplied and exhausted. Air change rates are calculated to determine how well a space is ventilated compared to recommendations, published standards, or codes. Air changes per hour (ACH) is the volume of air exhausted or supplied every hour divided by the room volume (measured in cubic feet). Airflow is measured in cubic feet per minute (CFM) and is multiplied by 60 to determine the volume of air delivered per hour.
- In the case where a positive pressure room is not feasible or to further help eliminate the attraction of dust, here are a few simple practices to help eliminate the attraction of dust during plate exposure.
- While air returns should be through the platemaking equipment and low to the plateroom floor to evacuate solvent odor, fresh air supply is typically through ceiling vents. Air filtration at the supply will reduce dust particles entering the plateroom. Careful placement of platemaking equipment is required; assuring that the equipment is not located directly under the air supply to the plateroom.
- Place an anti-static rubber mat in front of the exposure unit for the operator to stand on. This will help eliminate static build-up while removing the coversheet in addition to providing less stress for the operator. It is also recommended to use 3M Clean-Walk Mats at entrances to platerooms to remove dust from shoes. These same mats can also be placed under equipment in tough to reach areas to clean as they will actually collect dust for easier removal.
- While removing the coversheet from the photopolymer, the operator should keep a portion of one hand on the raw polymer, again helping to eliminate static buildup.
- In non-digital platemaking, always remove the coversheet just prior to laying down the film negatives. Also, the use of an anti-static wand can be used in this process for air bubbles and dust removal.
- Keep the staging area for the film negatives free of dust and dirt. A static eliminating wand is recommended. Wipe both sides of the negatives with a static wand just prior to placing them on the polymer. Wipe the emulsion side down last.
- Apply the negative to the plate as quickly as possible.
- Wipe the kreen film from side to side with a lint free towel while rolling it over the plate to pull down the vacuum.
- The use of an anti-static airgun can also be a useful tool to help eliminate dust.
All Printing Resources Diagnostic Team uses Hygro-Thermo-Anemometer Technology to map airflow, measure air supply and exhaust, along with ambient room temperatures and humidity readings. These advanced diagnostic assessments offer solutions to minimizing the attraction of dust particles and minimizing solvent odors, while maximizing photopolymer plate quality and throughput.