Rubber platemaking has been around since the early 1900s. For many decades, rubber plates were the standard for flexographic printing. Even today, some printers are using rubber for special applications where extreme durability is mandatory. This form of platemaking is becoming more rare, but it still has a place in modern flexography.
The process of making rubber plates is fairly complex when compared to sheet photopolymer platemaking. An etched metal master plate (usually made of magnesium or zinc) is used to impress the image, under heat and pressure, into matrix board (board coated in bakelite). The matrix board is then used as a mold for the rubber, which is vulcanized using heat and pressure into the master mold. Each step of this process must be carefully controlled by an experienced operator, to ensure the finished plate has the proper relief and is properly vulcanized. The result is a finished rubber plate that is ready for press.
The rubber platemaking process as a whole has not changed much over the years, and the process of etching the metal master plate is no exception. A 20% nitric acid, 80% water mixture is used to etch the metal master plate. Extreme caution and personal protective equipment must be used when handling and mixing nitric acid, as it is a very corrosive substance. The CDC's National Institute for Occupational Safety and Health states that nitric acid has "serious local effects through all routes of exposure" and regarding repeated or long term exposure, "Lungs may be affected by repeated or prolonged exposure to the vapour. The substance may have effects on the teeth , resulting in teeth erosion." For more detailed information, see the CDC web page referenced here: https://www.cdc.gov/niosh/ipcsneng/neng0183.html.
Although metal etching is the industry standard for master plates, there is an alternative. Flint Nyloprint WA photopolymer is engineered to withstand the heat and pressure of the rubber platemaking process. This metal-backed sheet photopolymer is imaged using a film negative, and washed out in pure water. This eliminates the need for dangerous acids altogether, greatly improving safety in the workplace.
In addition to the safer, simplified platemaking process, photopolymer master plates are capable of holding finer detail as compared to etched metal. There is virtually no loss of detail from the film to the photopolymer master plate, unlike etched metal. Photopolymer users confidently plate fine type, screened images, and small line work that may not be possible with metal master plates.
APR's Technical Solutions Group is available to help with any of your flexographic needs. If you would like more information on photopolymer master plates or other supplies for platemaking, contact Catherine Green at email@example.com.