What are the advantages or disadvantages of positive plates compared to negative plates?
Advantages of using positive plates:
Film stripping is easier in positive type because operators feel more comfortable due to "what you see is what you get".
Evaluation of plate quality after the development is easier due to the same reason.
Disadvantage of using positive plates:
To make the plate become more resistant against scratches or chemical substances, it is necessary to heat the plate in an oven for a certain period of time at a high temperature. This creates higher costs by the necessary oven equipment and longer preparation time for the printing plate.
In case of low ink coverage on an image, it is more difficult to obtain a high quality printing plate because it is very difficult to realize absolutely clean conditions in the prepress department where the plates are exposed. Little dust particles on the glass frame of the exposure equipment can easily be copied onto a positive plate and therefore be reproduced in white areas on the sheet / card.
Advantage of using negative plates:
A heat treatment of the plate surface after the plate development is not necessary. The scratch resistance is good and durability of the plate is higher than of positive plates.
Less costs due to faster preparation time and no investment costs for an oven equipment.
Dust particles on the glass frame of the exposure equipment will not be copied onto the negative plate and therefore white areas on the sheet / card will remain clean.
Price for negative and positive printing plate is identical.
Disadvantage of using negative plates:
The film stripping of negative films is a little more difficult because the printing areas are transparent on the film and therefore a little harder to be aligned and evaluated.
Conclusion:
The advantages of the negative plate type outweigh by far and are therefore the better solution for the majority of waterless offset printers (with or without UV drier).
