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Print quality is in the eye of the observer

'WHAT YOU SEE is what you get'. This saying explains the emphasis on print quality improvements in the PaperWeek Friday morning session of Paper Mill Aspects, sponsored by the Printing and Graphics Ar...


March 1, 1999
By Pulp & Paper Canada

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‘WHAT YOU SEE is what you get’. This saying explains the emphasis on print quality improvements in the PaperWeek Friday morning session of Paper Mill Aspects, sponsored by the Printing and Graphics Arts Committee.

What you see was very important for the first study presented. A technique for subjective measurement is introduced. The Proscale “is a computer-based research technique along with a manual describing how to collect fast, efficient judgements of paper and print quality from as many as seventy samples, and as many as fifty observers” the authors note.

As background information, Don Donderi, McGill University, spoke about psychologists’ efforts to take subjective measurements. He said that in the 1970s, S.S. Stevens proposed that you can find out what people experience by asking them, and asking them to rate it numerically. This method relies on the fact that people can use a number scale. Proscale is based on this method.

Basically you take a sample, mount the sample on a neutral background, get a big table and set all the samples out on the table. This allows the observer simultaneous comparison of all the samples.

The observer looks at the sample and rates it numerically. A larger number is accorded to a sample for which the observer has a greater preference. After the samples have been rated, the observer is asked what is the lowest rating, the smallest number that you would accept, and purchase. “You can ask people to estimate through observation as long as they understand what they are looking for,” said Donderi.

Donderi said that the advantage of having all the items out together is that the adaptation effects become minimal. People see all the options and choose the one that they truly like, not just the best one between two extremes. (He used the example of Goldilocks choosing from three bowls of porridge.)

Proscale was used to study print quality in a wet sheet with water-based ink. Standard paper measurements and image analysis measurements were used. All of the data analysis was handled by the Proscale software, and the data were obtained from 21 Paprican observers with a minimum of difficulty, in a test session that lasted about one-half hour per observer.

It was not the effects of wetting that had James Maclaren Industries looking at print quality improvements. The Masson, QC, newsprint mill changed the #1 backing forming fabric on its Bel Baie III machine. The objective of the fabric design changes, said Roger Danby, JWI Group, was to “improve print quality while maintaining paper machine efficiency.” Previous attempts had resulted in lost efficiency and no improvements in the print quality.

The print quality evaluation of newsprint sheets involved doing: split sheet image analysis; fibre length analysis of the printed sheet; fibre length analysis of headbox samples, and; forming fabric surface definition.

“Image analysis defined the problem,” said Danby. A computer program, said Danby, was used to calculate the top surface support and drainage characteristics of each fabric used in a given time period.

The tests showed that the sheet micro- as well as macro-uniformity are essential for good print quality on both sides of the sheets, and micro-density differences in the surface of the sheet will directly affect print quality.

“Image analysis is a great tool to understand density print quality relationship,” Danby noted in his presentation. “Sheet surface density is the key to uniform print quality, not the density throughout the whole sheet. The engineered understanding of each forming fabric’s top drainage surface is important in arriving at a satisfactory engineered solution.”

Gravure print quality of uncoated mechanical printing papers was discussed by Joe Aspler, Paprican. His report was the last in a series presenting the results of commercial scale printing trials on a variety of grades of paper and board, using a variety of printing processes. The ultimate goal of these trials, as noted in the paper, has been to find the common thread among surface requirements for different printing grades and processes, and to determine which are the most appropriate predictive testing methods.

Findings include: The correlation between subjective rankings and physical properties are better on the top side of the sheet. Subjective rankings of print quality correlated well with Print Surf Roughness and with unprinted gloss. There was excellent correlation between the printed and the unprinted gloss, unlike the much more complicated case of coated papers.

Overall print quality is the big picture, but Don McDermid, Econotech Services, concentrated on the specific property of fracture toughness in his presentation.

Fracture toughness, he explained in his paper, is the ability of a paper to resist further crack propagation. This property is important for the runnability of printing paper grades.

For this experiment, done to TAPPI standards and procedures, bleached pulps were refined in a PFI to various freeness levels. Handsheets of 60 g/m2 basis weight were prepared in a standard British sheet machine. Burst, tear, tensile and bulk properties were evaluated.

Results show that an increasing amount of softwood chemical pulp in a chemical/mechanical pulp mixture improved the fracture toughness, thereby resulting in fewer web breaks. When a filler is added to the paper, the fracture toughness decreases.

Fracture toughness was found to correlate well with runnability.