The printability of paper in gravure printing is depending on a number of properties of the paper, such as the condition of the surface, the dynamic compressibility, the wet ability and porosity of the paper. Measurement of only one of these physical characteristics does not permit prediction of the printability with certainty. With the Heliotest, which integrates all these parameters, the forecasting of printability is possible, even for papers with similar properties. The Heliotest, developed by CTP at Grenoble, France, can be used for all grades of paper for gravure printing. In France the method is standardized in NF Q 61-002.
Principle
The Heliotest attachment consists of an engraved printing disc, a doctoring system and a special, not quick drying gravure ink. Some drops of the ink are put on the printing disc, the surplus of ink is wiped off and a print is made on the substrate, which has been attached to the sector. The disc contains three types of engraving:
• A variable halftone screen area. In this area the distance from the beginning of the print till the twentieth missing dot is measured. The longer the distance, the smoother the paper. This part of the disc is the most important part.
• A conventional screen area. This part is used for visual assessment and is of low importance.
• Four lines of dots. In these lines the total number of missing dots is counted. This is done in case the distance measured in the variable halftone screen area is too small, that is when the paper is very rough. The more missing dots, the rougher the paper.
Method of operation
• It is recommended to execute the test in the standard atmosphere; to most standards it is 23.0 ± 1.0 °C (73.4 ± 1.8 °F) and 50 ± 2% rh.
• For the operation of AIC2-5T2000 and Global Standard Tester follow the instructions of the manual, IGT information, leaflet W100 and the display accurately.
• Handle the samples carefully.
Assessment
VISUALLY:
1.1. Starting from the side with the largest dots in the variable screen area count the missing dots until the 20th one.
1.2. Measure the distance in mm from the 20th missing dot to the beginning of the variable screen area.
1.3. In the case the distance in point 2 is very small (only some mm’s) count the total number of missing dots in the four dotted lines on both sides of the variable half tone.
1.4. Repeat points 1 and 2 or 3 for each test strip.
1.5. Calculate the average and if required the standard deviation. In some cases it may be useful to mention the highest and lowest value as well
1.6. If needed, assess the printing quality visually in the conventional screen area.
IMAGE ANALYSING:
2.1. Measure the printing result with the PTS DOMAS Image Analysing System, module Heliotest.
2.2 Repeat points 2.1 for each test strip
2.3 Calculate the average and if required the standard deviation. In some cases it may be useful to mention the highest and lowest value as well.
