Beyond the Densitometer

Improve Pressroom Efficiencies – Part I

While handheld densitometers have already been adopted in many flexographic pressrooms, it’s important to remember that these devices do not actually measure color and are therefore incapable of reporting the accuracy of brand or spot colors on press.  The best that desitometers can do is to measure and report the densities (ink film thickness) of solid color patches and the tone value increase (TVI) of halftone dot areas.  While these are both very important print metrics and can absolutely be used to establish some level of process control in the pressroom, the problem is that many brand customers have begun to specify color accuracy using measured CIELab values along with deltaE values as the acceptable tolerances regarding color accuracy.  Therefore, press operators armed with only densitometers cannot know if brand color specifications have been achieved or the precise color changes that are necessary to get within these specifications.  In these situations, with only with a densitometer, operators must still rely on their eyes both during make-ready to match colors and during the production run to ensure color consistency.  The danger here is longer make-ready times, color that does not ever hit the specification, and inconsistent color throughout the run which can all lead to customer rejections, job re-makes, increased waste, and, ultimately, lower profitability for the printer.  
 
So, what can a printer do to improve their efficiencies in the pressroom and reclaim their profitability?  Well, the answer is no different than how printers have increased their productivity & profitability in other parts of their workflow over the last few decades.  Simply put, packaging printers must adopt newer tools that employ the use of science & technology to remove the color “guesswork” for press operators to ensure a much more predictable, accurate, and repeatable color print manufacturing process.  This is exactly the subject of this 3-part blog series that will look at “Good”, “Better” and “Best” technology options that are being deployed today in packaging pressrooms:

• • Good: Replacing densitometers with spectrophotometers
  • Better: Adding press-side color quality software
  • Best: Automating color measurements with an inline spectrophotometer

 
This week, we look at the “Good” solution to increase color accuracy, shorten make-ready times, and allow packaging printers to reclaim their profitability.  
 
Good: Replacing Densitometers with Spectrophotometers  
Overview:  
Quite simply, densitometers do not measure color and are incapable of reporting a color’s CIELab values.  This also means densitometers cannot objectively describe how a color is perceived by the human visual system or quantify the perceived differences between two shades of colors using a ∆E formula.  For this reason, switching from densitometers to spectrophotometers is an absolute must for packaging printers.  Otherwise, press operators cannot know when contractual brand specifications have been achieved or what color changes are required to meet them.  
 
Making this shift is relatively easy for press operators because spectrophotometers can still report all of the familiar densitometric functions like solid ink density and TVI, which are important process control metrics, and the instruments are operated and handled in the same way as densitometers.  However, in addition to reporting densitometric and colorimetric values for measured samples, spectrophotometers have many more features and functions that can help guide operators to faster and more accurate color matches.  
 
Workflow:  
The workflow using a spectrophotometer to check color accuracy is extremely similar to that of using a densitometer.  On a flexo press, the press is stopped and a sample is cut from the roll while on a sheetfed press, a sample is simply pulled from the output stack.  The printed sample is then placed on a flat surface where it can be measured with the spectrophotometer.  Now the press operator selects the measurement mode on the instrument (Density, TVI, CIELab Opacity, etc.), aligns the measurement aperture over the sample to be measured, initiates the measurement, and the results will be shown on the instrument display.  If the user wishes to see other measurement information from this same sample, they will then need to change the display mode on the instrument.  
 
Key Benefits:  

• Spectrophotometers can store thousands of reference colors (including Pantone libraries) inside the instrument and color differences (∆E’s) between a printed sample and a reference or brand color can be automatically displayed
• Spectrophotometers can recommend precise density adjustments to achieve the lowest ∆E on press for a particular color. This is a valuable feature that can dramatically reduce pressroom make-ready times because the operator is shown the precise density value that is required for each color in the job to achieve the lowest ∆E.  In some cases, this may indicate that an anilox roll must be changed and in other cases it may indicate ink problems that cannot be fully overcome with density adjustments alone.  In either case, this is valuable information for the operator and can save considerable time and materials.  Consider the example below where the instrument is indicating a 0.18 density increase will greatly reduce the current ∆E of 10.2 down to a ∆E of only 0.9 for this match to Pantone 647 C: