Basic Procedures for Ensuring Photopolymer Plate Quality
Verifying quality of a new photopolymer plate is obviously an essential step of any successful printed conversion process. Advancements in plate reading devices now allow a QC check of a photopolymer plate to include mapping software of topology, plate shoulder width and angle to determine proper exposure and support characteristics, while digital plates with black masks are read for reflectivity. However, let's not forget the basics, and equally as important how to perform a basic plate QC properly and consistently. You may be surprised what is slipping through the cracks.
Thickness: This common QC step is used to verify the quality of raw materials and insure that the finished product is sufficiently dry. When asked, "How do you know when the plate is dry?" We often get an answer like, "It is dry after 2 hours." Or we may get a response like, "It is dry after about 90 minutes, but we keep the plates in the dryer for a couple of hours just to be safe." What we would like to hear is that the thickness of the raw material was checked with a micrometer and recorded, and the finished product was measured and found to be the same thickness after drying.
Micrometers should have larger diameter anvils, that when employed apply a very small amount of pressure in order to avoid displacement of the raw or finished material. The plate must be laying completely flat in order for the material to rest properly between the foot and the anvil and several areas should be verified. Areas checked for thickness should be at least 1" x 1" to insure adequate support.
Before the recent release of HTD dryer technology that minimizes the dry time of solvent washed polymer plates to 20 minutes, the drying process was certainly the bottle neck of most solvent wash photopolymer plates. Therefore wash out time had to be optimized to ensure a proper wash out, at the minimum time required to remove the unpolymerized material. Minimizing washout time meant less absorption of solvent into the plate, thus a shorter dry time. Therefore it is important to most plate manufacturers that dry time be the required time to evacuate solvent from the plate. You can only be certain that this has happened by knowing the actual thickness of the raw material from the beginning.
Relief: Just because manufacturers of polymer allow greater latitude for floor thickness, don't believe for a moment it isn't as important. If we could consistently produce a floor with a thickness tolerance of +/- .0005", press operators from all over would demand just that. Actually they should demand it if that were the case and with good reason. We sometimes expect works of art from the press, with dots printing with consistency, great circularity, and little or no elongation. Depending on the photopolymer material, operators are supplied with irregular floors, which in turn effects the relief across the face of the plate and their ability to consistently match the customer approved sample. First and foremost, if the printer is printing screens or process work, then they should be supplied with a plate material capable of holding an even and consistent floor.
When measuring the plate floor to determine relief, the plate must be laying completely flat in order for the material to rest properly between the foot and the anvil and several areas should be verified. Areas checked for thickness should be at least 1" x 1" and ideally 1/4" away from the land area to insure shoulders do not affect the readings.
Durometer: Durometer or hardness is measured using a Shore A gage, while harder items are measured using a Shore D gage. The hardness of plate affects the printability of the plate. Each plate material is designed for particular market segment which requires differing performance agents. In other words, your plate is designed to run within a certain durometer in order to deliver certain print qualities. Plate durometer is measured with a minimum plate thickness of .250". Therefore if we attempt to measure durometer of a photopolymer plate < .250", we can be certain we are also measuring the hardness of the area supporting the plate.
Stacking layers of photopolymer is not a good option because with each layer sheet polymer is a layer of mylar or dimensionally stable backing, which will add to the durometer. True durometer of the relief material can only be achieved by delaminating the mylar layer prior to exposure. You can then stack the material to a minimum thickness of .250" before checking for hardness. Note: This process in not common of the typical plate QC. This process would be more commonly used in trouble-shooting an ink transfer issue following verification of proper post exposure and after light treatment.
During the final stages of platemaking, the plate receives a post exposure (UV-A) and after light treatment (Detack UV-C). Unfortunately on the occasion of a press being down and waiting on a replacement plate, it is at this stage that we see corners cut. When these two processes are not completed properly, ink transfer will be affected by a plate that is still slightly tacky or even swollen due to the improper surface tension. It is also in these last steps of the plate making process that the final hardness of the plate is achieved. Therefore, the time saved in platemaking, will most likely be overwhelmed by the time spent on press trying to achieve proper ink lay-down and color.