Plate making is a science, not an art. While there is a bit of flexibility in specifications for different processes, there are defined parameters that must be followed if high-quality plates are to be made consistently.
In this article, we’ll discuss six important factors for making high-quality plates.
Relief Depth
Relief depth is the difference in height between the printing surface and the floor of the plate. We can obtain this measurement by measuring the overall plate thickness, then subtracting the floor thickness. There are recommended relief depths for different plate thicknesses, as shown below.
Imaging Quality
Whether you’re using digital or analog plates, the finished plate cannot exceed the quality of the original image carrier. If you’re making analog plates using film, confirming proper film density (great than 4.0) is critical.
With digital plates, the digital imager must be checked for correct focus and power settings periodically (every four to six weeks, or any time the laser head is contacted by a loose plate). These tests can be done yourself with the proper tools and training or by your digital imager supplier.
Exposure Conditions
Plate exposure units contain two types of UV bulbs: UVA bulbs (for back, main and post exposures), and UVC bulbs (for light finishing). For maximum plate quality, these bulbs must be monitored and replaced at the end of their useful life. While extending exposure times to make up for weak bulbs can work in a pinch, this tactic sacrifices plate quality. Longer exposure times can lead to broadening of the plate’s shoulder angle, resulting in dot gain and filling in of fine reverse detail (especially with analog plates).
In addition, the plateroom environment should be kept clean and free of dust to prevent any unwanted debris from contaminating the plate before or during exposure. New exposure technologies utilizing LED exposure are more reliable than fluorescent bulbs, yielding extremely consistent plates. They are also a greener choice, as they contain no mercury and use significantly less energy.
Polymer Saturation
Both solvent and aqueous plate making use liquid to wash away the unexposed photopolymer in the plate’s non-printing areas. This liquid, whether a hydrocarbon solvent or water, will eventually become contaminated with dissolved polymer solids. The level of saturation, or percent solids, can have a dramatic effect on both plate quality and equipment maintenance.
In solvent plate making, it’s recommended to keep the concentration of solids below 6 percent to ensure optimum plate washout and keep equipment maintenance to a minimum. If the solids are allowed to collect above this level, the result can be increased washout time, which results in a longer dwell time in solvent for the plate, culminating in a longer drying time. Extensive cleaning may also be needed to remove excess polymer buildup throughout the system.
In aqueous plate making, weekly solution changes and machine cleanings are key to trouble-free operation. Since aqueous polymer doesn’t dissolve completely in water, there is a chance small particles of polymer could redeposit onto the plate if they are not removed from the system. It is essential these machines are kept clean and proper filtration is used to manage the washout solution. The exception to this rule is thermal plate processing. Since thermal processors utilize a one-time-use wicking media to remove the uncured photopolymer, there is no polymer saturation to monitor—only the amount of wicking media remaining in the machine.
Drying
One of the most important—yet most overlooked—factors in solvent plate quality control, complete drying is crucial to consistent plate making. Incomplete drying can be caused by a number of factors, including early removal of the plate from the dryer, inadequate dryer air circulation and improper drying temperature.
To check a solvent plate for complete drying, remove the plate from the dryer and allow it to cool for five minutes. Next, measure the overall plate thickness with a micrometer. The plate should be no thicker than 0.001-in. over the original plate gauge (be sure to measure the original gauge on a sheet of raw material). When plates are not completely dry, they remain swollen from the absorption of solvent. This can lead to problems on press, including poor registration, over-impression and decreased plate life.
Plate Handling
Good plate handling and storage practices can save time and money with both press downtime and plate remakes. Plates should always be handled with care and treated as a fragile component of the printing press. Never place objects on top of plates, fold/crease plates or expose plates to unknown chemicals. Plates should only be stacked flat when foam or parchment paper is placed between them to prevent direct contact. Environmental factors that can harm printing plates include exposure to room light or sunlight, and storage near ozone-producing equipment (the most common offenders include HVAC and electrical equipment).
To ensure maximum plate life, used plates should be cleaned as soon as they are removed from press, using an approved cleaner and a soft horsehair brush. Another great option for plate cleaning is an automated cleaning machine. These simple machines make quick work of dirty plates, producing consistent cleaning results with minimal labor—streamlining the postpress workflow and making the most of valuable employee time.
By following these simple steps, you will ensure your plate making and storage is as efficient and effective as possible.
About the Author: Catherine Green has more than 12 years’ experience in graphic arts. An honors graduate of Clemson University’s Graphic Communications program, she has held positions in prepress, plate making and technical support. She brings expertise in digital plate making, prepress and process improvement to All Printing Resources’ (APR) Technical Solutions Group. She is an active member of FTA, previously serving as a judge in the Excellence in Flexography Awards and on the Flexo Quality Consortium (FQC) Committee.
For more information regarding products or procedures mentioned in this article, contact Catherine at [email protected].