Photopolymer Platemaking
The Light Finishing Step

Submitted By:
Charlotte Cushing
DuPont Cyrel®
Packaging Graphics

URL: http://www2.dupont.com/Cyrel/en_US/

New printing plate technology has provided the flexographic industry with a broad range of photopolymer plate types for every kind of flexographic printing application.  Does the platemaker need to be informed about the type of plate he is using?  Most definitely!

Not all printing plates are exposed and finished using the same times. Determining the correct exposure times for the products you use is essential to producing a quality printing plate. 

The light finishing step is a particularly critical area in platemaking.   Light finishing involves exposing the dried plate to short wavelength, ultraviolet germicidal lamps (220-260 nm) for a predetermined length of  time to remove the tackiness from the plate’s surface before being run on press.  If a plate is not finished properly, it will stick to other plates and be ruined.  Tacky plates can also pick up dust and lint or a buildup of ink, causing dirty printing.

A step test, similar to the one used for back exposure, is used to determine the correct finishing time.   Any imaged, processed and dried plate can be used for this test.

1. Mark off as many equal sections, at least 2 inches in width, as needed on the plate to accommodate the range of guideline times necessary to complete the test.
   (Use two minute time intervals for each step.)
    
2. Identify the time intervals on the bottom of each segment.
    
3. Light finish the entire plate for two minutes.
    
4. Place a mask over the first two-minute segment.
    
5. Expose the remainder of the plate for an additional two minutes.
    
6. Continue the masking and progression of two more minutes until you are finished with all segments.
    
7. Check each finished segment for tack.  The plate is properly finished when it is no longer tacky.

For maximum printing performance and durability, a minimum light finishing exposure (just to remove the tackiness) is recommended.  Prolonged exposure or “overfinishing” weakens the plate structure and can lead to cracking (either immediately or subsequently during use or storage) or dot chipping.  It can also lower the plate’s resistance to solvents and the environment (ozone).

There is not a standard light finishing time for all plate types.   Since light finishing times depend on conditions in your equipment, i. e., lamp intensity, unit temperature and air flow, it is important that the test be conducted at your facility.

Take the time to do a light finishing step test.  Optimizing your light finishing times will enable you, or your customer, to achieve consistent printed results.

Charlotte Cushing
Sr. Technical Service Analyst
Cyrel® Packaging Graphics Products

© 2000; DuPont Cyrel®; All Rights Reserved
 

Post your Plate Questions on our Flexo Forum