For papermakers today, it is very important in today’s market to be in tune with market demand. One of the most important factors in the demand for your product is quality, and furthermore, consistent quality. One of our goals at NPA is to supply accurate print quality evaluations for our paper clients. One way we do this is by offering cost benefit quality control programs designed around each of our clients needs.
Paper mills have always been leaders in the area of routine quality control testing of their products. Most mills have research departments to some capacity that perform the necessary tests to improve/check quality. Over the past 22 years at NPA we have taken the fundamentals of paper testing to a new and exciting level with Ink on Paper testing. These specialized tests have expanded the areas of the print demands on a paper or board stock, and have made us a leading source for print testing knowledge.
- How fast does ink set on a papers surface?
- Will the paper’s surface pick on press?
- Does the paper’s surface have the ability to accept fountain solution?
- Is the paper’s surface uniform in its ability to absorb ink?
- What happens to gloss levels when ink is applied?
- Can this stock be printed with conventional inks, or EB or UV formulations?
- All of these questions can be answered with our series of Ink on Paper tests.
When run on a routine basis throughout the year, NPA can help the paper and board manufacturers in guiding them through formulation changes buy measuring how changes at the mill will affect the printability of the stock. These programs also include handling customer complaints, by testing inks, fountain solutions and other materials that can be involved.
Paper & Ink Stability Test:
This test measures the ability of a paper to absorb the thin oils from ink and allows us to simulate a printed ink film passing through additional printing units As the oils leave the ink film, the ink sets and after time, eventually dries. If the ink contains heavy oxidation-type oils, it sets slowly. If the ink is formulated with hydrocarbon oils, it will be fast setting. If the ink is an EB or UV formulation, there is no interaction with the paper or board stock.
As the TestPress and load cell assembly measures the splitting force of the ink, repeated printing impressions every 7 (seven) seconds generate a profile of printing forces. This is a “setting curve”, and the slope of this curve is reported as the rate at which the ink and paper combination “set”. The values are reported in grams (forces) per centimeter )paper width) per second (time). We use a special standard “Fast Set” cyan ink for testing all papers and bard products. However, the best feature of this test is that any ink can be used. Therefore when troubleshooting printing problems from your clients, we can use their inks as well.
The paper is printed for as many as 10 printing passes (elapsed time is 56 seconds) or until the press operator sees coating pick, or failure of the paper or board surface on the blanket. The final force recorded is the print strength of the substrate. This unique characteristics of the P&I test enables the coating formulator to determine if a paper is picking on press because it is weak, or because it is fast setting.
Unlike the IGT and Prufbau, special inks are not required. This testing system prevents the ink from changing during the time of the test, so even fast setting inks and varnished can be used.
The Water Sensitivity Test:
During this test, we simultaneously apply fountain solution and ink to the paper sample. In offset printing (conventional, EB and UV) the ink is surrounded with a thin fil of fountain solution (water). The surface of the paper or board must be able to absorb the water quickly and enable the ink to transfer.
This test tells us whether or not the paper’s surface can accept enough fountain solution to print without snowflaking or poor solids. A densitometer is used to calculate the % of ink transfer in the “water area” and also records the % of wet coating pick, if any, on the sample. This test is particularly useful when testing coated papers, since differences in wet pick can be due to drying conditions (amount and degree of cure of the binder), coating weight, surface porosity, or the % of starch in the binder system.
The Mottle Test:
We always evaluate the paper/board’s uniformity of ink absorption, since mottle on a job is cause for rejection. We run this test using two inks, and print them in reverse tack order. If the paper is non-uniform in its absorption to ink on the surface, the first down ink will build tack non-uniformly, and will trap the second ink unevenly. This mottle is rated visually from a “1” being the best to a “5” being poor. Mottle on press is dependent on this test together with the results of the P&I Stability and Water Sensitivity tests.
Gloss can be measured at a 20o, 60o, or 75o angles. For print gloss, the wet ink film thickness we use is measured at 0.05 mils. (This thickness remains constant throughout most of our test procedures.) For this test, gloss readings are taken and recorded from the white stock then, using our Test Press, we apply ink to the paper. After the ink has been dried, either in a simulated lift for 24 hours (sheet fed), or through out Heat Set Tester (heat set web offset) or passed through our UV curing system, gloss readings are taken from the print. We can also apply water based or UV gloss & dull coatings and flexographic inks as well.