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Monday, December 23, 2013

What's a reasonable color tolerance?

A customer recently asked, "What are visually perceptible color differences in L* a* b* coordinates?" The answer is not so easy.

Generally speaking in the Paper Industry we are normally dealing with near-white colors. In that case, we often use the rule of thumb that +/- 0.3 in L*, a* or b* is visually perceptible. However, when we start to look at more saturated colors the question becomes much more difficult.

L*a*b* color tolerance
Color tolerance vs. Visual acceptability
If we set a tolerance based on L* a* b*, the color space we are looking at is cubical. However, when we plot actual visual acceptability it is more ellipsoidal-shaped.  See at the right, the black shading represents numerical acceptance, but visually unacceptability.

Obviously, there is a difference here. If we look at the ΔEcmc tolerancing which is based on ellipsoidal tolerances, this does a much better job of matching visual and numerical acceptability.  Looking at a particular cross-section of the a* b* space (below), we can see that the visually acceptable ellipses vary in size depending on the position in color space. The ellipses in the orange area of color space are longer and narrower than the broad and rounder in the green area. The shape of the ellipses are larger as the color increases in chroma (away from a*=0, b*=0).
This means that visually acceptable differences in L*, a* and b* differ depending where we are in color space.  A Δa*=0.5 would be noticeable on a near-white, but Δa*=5.0 on a red may not be noticeable.

If you have more questions about this topic, contact me at toddp@technidyne.com.



Monday, December 16, 2013

Thursday, December 12, 2013

Technidyne Christmas Lunch 2013

December 11, 2013, Technidyne had its annual Christmas lunch.  All employees were treated to a buffet lunch.  After lunch the appropriate department manager recognized employees celebrating significant anniversaries as employees at Technidyne.
There were four employees recognized:
  • 15 years -Micky
  • 15 years - Robin
  • 20 years -Rocky
  • 30 years - Gary
We are very proud of the fact that our average tenure is 17 years for all employees.  Several employees who at one time or another left for various reasons have returned to a great US-based, family owned company.

It takes time and effort to find the right people, but when we do they stay for a long time and contribute to our DNA: Technidyne's passion for customer satisfaction drives us to be the best in the world at developing economical and creative solutions.

Sunday, December 8, 2013

Paper Roughness (Smoothness): Part 7 - Applications

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 metalized 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.

Related posts include information on the relationship between paper roughness (smoothness) and the following items:
  • Papermaking Process
  • Printing processes
  • Formation
  • Parker Print Surf Test
  • Sheffield Test
  • Applications

Wednesday, December 4, 2013

Video: Calibrating Your Technibrite Micro TB-1C

We have been working on some new videos to add to our Technidyne YouTube Channel.  One of the latest videos is the calibration of the Technibrite Micro TB-1C which can be seen HERE.

 

We are posting product videos, calibration videos and maintenance videos. 

If you have ideas of other videos that would be helpful to you, please let me know by sending me an email at toddp@technidyne.com.

Sunday, December 1, 2013

Paper Roughness (Smoothness): Part 6 - Sheffield Test

The Sheffield test:
The xerography and inkjet processes are common examples of processes where the PPS tester does not replicate the loading factors. 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 non-constant loading is related to the design of the air system, and the “hovercraft effect” of the variable pressure between the measurement lands. When the PPS instrument is set to measure at the lowest loading pressure, it is still about 4 to 5 times higher than the Sheffield loading.

With the introduction of digital Sheffield testers in the late 1980’s, the Sheffield method maintained its prominence for these 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.

Related posts include information on the relationship between paper roughness (smoothness) and the following items:
  • Papermaking Process
  • Printing processes
  • Formation
  • Parker Print Surf Test
  • Sheffield Test
  • Applications