To start with, you must realize that the following variables can greatly influence ink mileage performance.
- Pigment strength
- Absorption of paper stock
- Prepress methods such as UCR
- Ink and water balance
Before starting such a test, ensure these variables are controlled by performing this test on a long print run. The following variables must be controlled during the test.
1. Ink coverage on the plates must remain the same throughout the test. If the plates become worn during the test, the results will be skewed. Ensure this variable is maintained.
2. Paper must be made by the same company with identical properties. Different papers brands, even of the same grade, can have different surface properties.
3. Ink and water balance must be maintained. Though this is harder to control, given the variables like piling, try to keep waters settings at the same level in the ink and water balance window.
4. Density must be closely monitored.
Ink Mileage Test Procedure
Here is a breakdown step by step of how the test should be done:
1. Gather your ink samples. Have them labelled well, including their exact weight in grams before the test.
2. During a long run, stop production and remove the ink completely from the ink fountain. Wash up the rollers as well.
3. Fill the fountain with ink.
4. Start the press and keep the above four variables consistent throughout the run. Run a predetermined number of printed copies. The longer the better. Then stop the press.
5. Empty all the ink from the fountain and put it into the original bucket. Clean the fountain and wash up the printing unit.
6. Repeat steps 3 to 5 for each test ink you will try.
7. Weigh each bucket again and record the grams alongside the original weight.
|Label each ink mileage test bucket well.
If you want to get a more industry standard figure, calculate your impression size and do the math to come up with a grams per meter squared figure.
Compare Papers For Ink Mileage
Without a doubt, some papers absorb more ink than others, requiring higher density. Another test could be to not change inks at all, but instead change papers after a predetermined number of copies. This would reveal which paper is more economical to print on. However since ink costs far less than paper, this test is less important.
Of course the real cost of an offset printing ink is mostly determined by its stability and the quality of the finished product. Additionally, an ink manufacturer can only increase the pigment strength so much before he starts losing other important properties of the ink. There is more to the cost of an ink than just the mileage, but this test gives you a nice place to start.