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Monday, March 25, 2013

Sheffield Roughness

With the introduction of digital Sheffield testers in the late 1980’s, the Sheffield method maintained its prominence for certain grades, at least in the USA. There are many regions of the world where the Bendtsen test is used; however, the correlation between Bendtsen and Sheffield for these grades is excellent. There are many grades where the Sheffield method gives more uniform results than Bendtsen; those grades being the higher basis weight and stiffer grades, where the Bendtsen deadweight is not heavy enough to fully flatten the sheet under test.

Sheffield Measurement Head
The Sheffield test subjects the paper to loading pressures of 0.09 mPa at zero Sheffield units, and 0.154 mPa at 400 Sheffield units. The reason for the nonconstant loading is related to the design of the air system, and the “hovercraft effect” of the variable pressure between the measurement lands. A rougher surface causes higher airflow; therefore these instruments are designated as roughness testers.

Xerography Processes:
There are many reasons why the manufacturers of photocopy machines have target ranges for Sheffield roughness. A xerographic machine needs optimum paper surface properties for reliable sheet feeding, image transfer, and image fix. The fix level decreases as the Sheffield roughness increases, as it affects toner adhesion. Print density loss is observed as roughness increases. There also can be image problems with papers that are too smooth. Toner particles can be flattened and appear as larger dots, thus increasing the perception of the background. Rougher papers produce less background. With regards to paper handling, smoother papers are less stiff for a given basis weight. Smoother papers increase “electrostatic tacking” in the image transfer process. The coefficient of friction decreases with increasing roughness, a factor that is important in sheet transfer operations. The Sheffield roughness properties are carefully specified for the electrostatic copier printing process.



PROFILE/Plus Roughness Tester
Some of the other reasons for testing roughness are related to converting processes. The die-cut sheet feed in an envelope machine requires only one sheet at a time to be picked up and transferred, whereas multiple sheets will cause paper jams. The coefficient of friction between plies has a high correlation to Sheffield measurements.

There are some applications where metallized films are applied to the surface of paper. The reflectance properties of the film can expose wire marks on the base sheet. This is another example where the gentle loading force of Sheffield test better replicates end use properties of the paper, as compared to the PPS test.

Many plastic films are packaged in reams, like paper, for use in a photocopier to produce overhead projector transparencies. When the surfaces of the films are extremely smooth, there are static forces and cohesive forces that interfere with single sheet feeding. The manufacturers of such films generally create rough surfaces that enable an air film to exist between sheets. It is common to use Sheffield test results to control the process that generates the rough surface. Again, the PPS test would have measuring head loading that is excessive for this test.
When selecting a test instrument for paper, it is important to understand the relationship between the end-use of the product and the physical test parameters of the instrument. A further requirement is to use a test where process control settings on a paper machine (or plastic web processing equipment) can be modified to optimize the final product for its intended end use. The old adage was “If you can’t control it, why measure it?” In today’s marketplace, the customer will be able to find a supplier who makes the product he wants.

Learn more about Sheffield and PPS Roughness at the Technidyne Website

Monday, March 18, 2013

What do you need from your vendors?

Most industries have experienced reduced manpower in recent years. The Pulp and Paper Industry is certainly no different.Years ago we worked with R&D Centers to develop new technology to assist companies in making better quality product. Now, there are few R&D facilities and people who devote their time to research.  We are being asked to provide this service.  This is just one example of how the industry has changed.

So, I ask the basic question, what do you need for your vendors?

Here are some responses:
  • Basic scientific research
  • Research on new technologies
  • Research on cost savings
  • Consultation to provide expertise that mills don't have
  • Consultation to provide ideas from outside the organization
  • Application consultants
  • Repair and preventative maintenance providers
Technidyne constantly works to be responsive to its customers and their needs.

Therefore, I ask, what do you need from us?

Email your thoughts to me at toddp@technidyne.com.

Monday, March 11, 2013

What is a "starred" paper roll?


Starred Rolls

One example of a roll winding defect is illustrated by a “starred” roll. In the image to the right this defect is illustrated. A roll that becomes starred was produced with low wound in tension when the roll was started on the winder or soon after the roll was started. When rolls are produced with low roll density or low wound in tension followed by rings or bands of higher roll density, or higher wound in tension, there is a risk of the higher density bands collapsing the lower density interior of the rolls resulting in the “starring effect.” This type of defect is more common on older winders that do not have rider rolls to help provide a uniform, hard, dense start to the roll.


In one example of a starred roll, as the roll diameter became larger, more weight (from the roll itself) and the resulting higher wound in tension on the outside wraps of the roll caused a larger diameter roll being rewound (without a rider roll) for an envelope converter to have higher density (higher wound in tension) on the outside wraps of the roll. While the rolls looked good coming off the winder, pressure from clamp trucks picking up the rolls caused enough stress to collapse the softer wound (lower density) interior of the roll.


In the winder of a modern paper machine, crews have tension controls, brakes, and rider roll pressure to help control the roll density. Uniform roll density translates into uniform roll hardness. This image illustrates roll hardness, indicated by the deceleration measurements from the Tapio RQP, from the core to the outside diameter of the roll. By cutting slabs off the roll and retesting roll hardness at descending diameters, a graph sometimes known as a plot of “wound-in tension,” can be created. Understanding roll density and the WIT characteristics of a roll becomes a powerful diagnostic tool for evaluating winder performance.


By using a WIT (Wound-In Tension) analysis with a Tapio RQP, the winder crew was able to modify tension and braking (even without a rider roll) to deliver rolls with more uniform hard starts and more uniform roll density. Along with subtle winder modifications, the clamping pressure on clamp trucks was limited, eliminating two factors that resulted in starred rolls and eliminating the defect.


These images are intended to show one of the more advanced applications for a roll hardness test, or roll quality profiler (Tapio RQP).  This example was contributed by Technidyne's Manager-Technical Services, Jon Saatvedt.  Please visit the Technidyne website or the Tapio RQP website for more information.

Wednesday, March 6, 2013

What does Roll Hardness tell you? (Part 2)


Technidyne has experience selling and servicing the Tapio RQP in North America. Here are some more illustrations of what the device can do.

Parent reel before and after calendaring


Paper roll testing

Customer rolls are cut from the same parent reel measured before the winder.
 











Roll Side Measurement

Roll side measurement correlates well with the tension inside the roll.  Higher roll hardness relates to higher tension.


For more information email Rodger Segelstrom or see the Tapio RQP website.