Folding Carton Surface Performance

Making a Problematical Prediction

In this age of rocket science, why can't we predict surface performance on folding cartons?

It is a safe bet that the number one issue in folding carton coatings involves surface performance as measured by slide angle and COF (Coefficient of Friction). This certainly has to be true for cartons subjected to high-speed forming, filling and sealing. The demanding nature of these packaging operations underscores the need for surface specifications that are reliable and predictable. Yet today, with all the technical resources available to board mills, coating manufacturers and the carton converting industry, the issue requires constant attention with no little frustration for everyone involved. This article attempts to shed light on the subject of folding carton coatings, and to provide some useful information.

The Process Model

Every manufacturing process consists of what is called I-T-O or Input-Transformation-Output. Using this concept, we can separate the components of the folding carton manufacturing process into basic segments. (Figure 1)

 

 

In discussing the importance of surface coating performance, however, the process doesn’t stop there. Indeed, the output, or end result of the folding carton manufacturing process will become an input to another process—that carried out by the end user. This may be a dry foods producer, a powdered soap manufacturer, a beverage line, or any of a number of high-volume, high-speed forming, filling, sealing and casing processes. Thus, the diagram expands. (Figure 2)

 

 

As one of many possible examples, consider a folding carton, designed and manufactured for retail beverage distribution. In the Printing and Converting Process, the transformation of several inputs (paperboard, inks and coatings, plates/cylinders, films, adhesives) produces folding cartons. These then become inputs to the Beverage Operation as they are formed, filled, sealed, cased and shipped. In turn, the cased cartons of soft drinks become an input to the Distribution Process as the packaged beverage is transported, distributed to retail outlets, displayed and then purchased by the consumer. The process diagram expands again. (Figure 3)

 

 

The Role of the Carton Surface

At every point in the expanded process, the surface of the folding carton must meet certain levels of performance. In the printing process, a converter needs a specific range of surface performance (SP), typically measured as a slide angle or COF (Coefficient of Friction) in order to cut and finish (glue) the printed sheets. In end user operations, the SP must enable the filling operation to optimise speeds and minimise waste. In the distribution process, the SP must resist the rigors of package movement that could rub or scratch. In displays, the SP must contribute to easy stacking while minimizing slip.

 

In addition to these functional attributes, the coating is expected to provide additional and subjective graphic enhancement. In general, the primary function of an overprint varnish (OPV) or coating (aqueous, UV/EB, or solvent) is to protect the underlying inks and provide the necessary SP for the various processes. Additionally however, the packaging must convey the appropriate graphic message and its appearance must support the marketing objective of a quality product with high consumer appeal. For an OPV, this usually includes a specified level of gloss that is measured by calibrated test equipment and conforms to common descriptors such as High Gloss, Matte or Satin.

 

Coating manufacturers can contribute to evaluation of Surface Performance through testing methods and the use of sophisticated equipment. In some cases, for example, the coat weight may create a film that does not cover the “highs” and “lows” of a substrate with a rough surface. Cross-section photos taken by an electronic microscope (Figure 4) can show the interaction between coating and substrate. The challenge for the analyst is to correctly interpret the image and recommend formula changes to achieve desired results.

 

 

In this photo, the correction fluid defines the upper edge of the applied coating, permitting measurement of the coating; the epoxy preserves the surface for storage. The Scanning Electron Microscope used in this work was acquired by Flint Ink under Grant #EAR-96-28196 from the National Sciences Foundation.

 

Coating Technology Today

 

Today’s formulators of graphic arts coatings work with diverse materials to achieve products that provide ideal surface performance at the lowest cost. In developing a new coating they must consider more than a dozen factors:

 

1. Monthly or annual volume requirements
2. Substrate
3. Inks
4. Application equipment
5. Drying/curing equipment/method
6. Application speed
7. COF specs
8. Gloss specs
9. Preferred chemistry
10. Cost targets
11. End use requirements
12. Rub requirements
13. Odor restrictions
14. Wet block requirements
15. Glue block requirements
16. Gluing restrictions
17. Customer testing methods
18. Environmental considerations at press side and storage

 

Fortunately for the folding carton industry, basic coating solutions for general applications have evolved as the industry has matured. Proven raw materials and design models are now available for most substrates and application methods.

How then can we explain the curious unpredictability of the surface performance of an OPV? The fact is, that while the basic chemistries vary widely among the water, solvent, and energy curable (UV/EB) overprint varnishes used today, there are problems of Surface Performance common to all.

 

Understanding the Variables

 

The I-T-O Model described previously assumes that the output is predictable to the extent that the inputs and the transformation remain constant. This is valid in theory. However, as those of us involved in manufacturing know all too well, the real world involves variability. This variability is inherent in both the input and transformation segments. Only when these variables can be controlled to a specified range, can the output be predictable within a similar specified range. Thus, in order to determine the potential for variation in output, the effect of input and transformation variation for each component must be measured and evaluated.

 

For example, suppose the predictability for COF (output) of a UV overprint varnish on paperboard is affected by both the coat weight applied and the water content of the substrate (input). Yet the measurable effect on COF (output) of varying the coating weight may be 10 times greater than the effect of a similar variation in the water content of the substrate. The challenge for everyone concerned with controlling the process is to identify the input and transformation variables for each phase of the entire process and then, through experimental design, measure the effect of changing the variables. Once this is done, we can concentrate on minimizing fluctuation of critical input variables and begin to achieve an acceptable and predictable range of variation in the output.

 

Time, Circumstance, and Test Methods

As scientific as we all hope to be in determining and predicting manufactured product output, some demons intervene. First, because we work with paper and chemistry, time plays a role. Although the typical life cycle of a folding carton is relatively short (a few days to several weeks), time may change the basic composition of the components. What we must determine is the effect of time on every desired output and, in turn, its effect on input for the subsequent process.

 

Second, circumstances change over time. As soon as cartons complete the printing/coating/drying process, the cutting and finishing processes begin to effect the SP. Circumstances continue to play a role as the converter phase ends and the end user phase begins. As the cartons move through the operation, the SP is affected by conveyer belts, forming equipment, palletisers, etc. In order to measure the changes in slip or COF, the researcher must work to isolate the variables and minimise multiple causal effects. All data must reflect both time and circumstance variables.

 

Third, test methods and protocols for conducting slide angle and COF measurements are a virtual ‘slippery slope’ for researchers, converters, and suppliers. Equipment manufacturers and testers themselves have established different procedures. Two common methods are ASTM test D 4521-96 and the TAPPI test method 815 om-95. Yet it is obvious that if data are to be useful, test results must correlate between the laboratory and the field or between converting locations. The key to correlation of slide angle and COF test data is to eliminate variation. This means that the test method must be identical, the equipment identical and calibrated, the samples identical, and the conditions identical. Even under the most controlled circumstances, however, a test can show some variation.

 

One example of variation came to our attention last year while qualifying for new business. Some samples were coated on test equipment with a high-gloss UV coating. Several different locations were then given the task of measuring and reporting the COF using Thwing-Albert sled testing devices. All locations used the same equipment, sample pieces, and protocols but the results were very inconsistent. After considerable retesting, it was determined that the coat weight varied across the sheet, side to side. The coater applicator was set unevenly and this created the anomaly! A great deal of time was wasted on critiquing test method variation, when the problem was found in application variation.

Surface Performance of aqueous coatings is inherently less predictable than that of their UV- and EB-cured cousins. The reason is obvious: water, and its absorption by paperboard, creates a set of variables all its own. Recent studies have shown that the minimum measurable range of slide angle variation is 5 degrees, from time of application to the time when the packaged goods are placed on the shelf. Additional studies have shown that retesting the same sample will not produce further change. Other studies have confirmed that the effect of relative humidity is a critical variable.

 

In the end, the converting industry has learned that to satisfy the many demands of the downstream processes, compromise is essential. For example, the additives necessary to maximise rub resistance will negatively affect gloss readings as coat weights increase. Further, the wax or silicones used to improve the rub resistance will create a lower COF or slide angle, so as rub resistance increases, COF normally decreases. The formulator must therefore know the relative importance of the desired SP attributes (rub or COF) in order to proceed.

 

Closing the Gap

 

Our collective involvement in folding carton manufacture and the many industries served is actually only sixty years new. Following WWII, consumer demand dramatically increased and producers filled the gap with everything from TVs to running shoes. Technology has kept pace, but there is still room to grow, and always a search for a better mousetrap. In the grand scheme of things, the Surface Performance of coatings on folding cartons isn’t rocket science, but it is nonetheless complex. Through continued scientific research, experimental design and pressroom testing, we will close the gap and provide the necessary -- and desired -- predictability.