Troubleshooting web lateral instability
The Roll Testing Facility and Tapio equipment were mainly developed and used for these types of grades, but were also used with other types of web material, such as aluminum foils and plastic films.
As PackagingFilms magazine is for plastic films extrusion and converting, we have selected three case studies to illustrate how non-uniformity of plastic films (or product composed of one layer of plastic film) may cause lateral instability on printing and converting equipment. In some cases, web lateral instability was severe and led to common issues such as print mis-registration, web shifting and wrinkles. Whenever possible, web manufacturers found solutions to improve web uniformity and reduce lateral instability.
Case study 1: Lateral instability leading to wrinkles
One mill producing coloured-sheet extruded plastic films had wrinkle issues with some rolls, which could no longer be used by some of their customers. At one customer where the rolls were problematic, the lateral stability of rolls coming from different suppliers was evaluated. Figure 3 compares web lateral stability on press for one rejected roll and one good roll from another supplier.
The problematic rolls clearly show higher web lateral instability on press. These rolls also displayed print mis-registration as well. The lateral instability being random, it was not possible for the lateral control of the press to react to these sudden changes. Formation of wrinkles was mainly observed just after unwinding, likely caused by the lateral displacement of the web.
The problem with these rolls visually appeared to be a lack of tension on one of their sides (or baggy edge). Basic web properties analysis (caliper as example) did not explain this drop in tension. Figure 4 shows the CD tension profiles, as measured on the RTF, of one rejected roll (red curve). The roll shows a very non-uniform CD tension profile (with slope), indicative of bagginess and loose web towards the left side. This is a very typical case seen by FPInnovations on several web materials. If the film producer could make the CD tension profiles of the rolls uniform, it would likely solve the wrinkles and miseregistration problems.
Following this analysis, the manufacturer applied modifications to their process in order to correct the CD tension profile. Figure 4 shows the corrected CD tension profile and compares it with the previous rejected profile. No detail was provided by the manufacturer on modifications applied to their process.
The corrections applied to get uniform CD tension profiles were tested afterward at the same customer. The improvement in the CD tension truly contributed to improve web tracking on press, and significantly reduced wrinkles and print mis-registration. Figure 5 shows the lateral stability of several rolls, including problematic rolls before and after corrections brought to the process. It also includes one competitor roll, known to run well on the printing press. The corrections made on the machine reduced the lateral instability of the rolls to a level comparable to the competitor roll.
Case study 2: Excessive web weaving on converting equipment
One manufacturer producing blown polyethylene films experienced web weaving and wrinkles issues during laminating. he web weaving was mainly problematic at the end of the process, where lateral movements reached up to 3–4 mm. The manufacturer sent one problematic roll and one good running roll from a competitor for analysis (both sent directly from customer).
The rolls were tested with FPInnovations’ RTF, with the focus on quantifying the web weave. In this case, the measurements at converter were not conducted, since offsite testing of the rolls provided the appropriate information to resolve the problem.
Figure 6 shows the web weave measured on the roll tester equipment for the rejected roll and for the good running roll. The rejected roll shows a very noticeable periodic pattern in its web weaving behaviour (in MD).
A wavelength of 355 meters was obtained by Fourier analysis. The competitor’s good running roll did not show any significant variation in the web weave, which remained low. Subsequent analyses done on the problematic web put the emphasis on finding a similar wavelength, in MD, in one of the properties. Identifying such a wavelength would point to the cause of the web weave. With the TAPIO test, a very similar wavelength of 350 meters was detected in MD caliper, with fairly high amplitude (up to 6 micrometers).
As seen in figure 7, the MD caliper (shown here for a good part of the rolls) show periodic variations for the rejected roll, while no significant variation was found for the competitor’s good running roll.
Analysis of the CD properties showed adequate and non-problematic profiles. Winding and roll density were affected by the non-uniform MD caliper (Figure 8), amplifying the web weaving as the non-uniform wound-out-tension and density would probably lead to tension variations during rolls unwinding.
Following these findings, the manufacturer had to investigate its process operations to find where the caliper variation of 350 meters could come from. Corrections brought to some process control loops were successful in reducing caliper and web weaving variations to the level of the competitor rolls. No roll was tested on Roll Testing Facility and Tapio after corrections, as the problem was solved.
Case study 3: Lateral instability leading to print mis-registration
A web packaging manufacturer producing two-layer food grade was having web lateral instability on a 4-colours commercial press. The web product was composed of a base layer of paperboard and a layer of polyethylene film. Measurements taken on press with the laser sensors showed that the web weaving had a strong correlation with print mis-registration. Print miregistration was measured on printed samples as the distance between un-superposed colours, and this showed a strong correlation with lateral movement of the web.
A FFT analysis of the web weave data further enabled to detect a periodic variation, with a wavelength of 16 meters in MD. With such a short wavelength, press operators were unable to control or reduce the weaving.
Laboratory analyses were conducted on the final product to evaluate web properties uniformity. CD tension and product properties were rather uniform. The process of adding polyethylene at the surface of paperboard did not show any variation that could explain the issue. Since a MD variation with a wavelength of 16 meters had been found in the web weave data measured on the press, focus was put on finding a similar variation in the properties in MD.
Ultimately, the same periodic variation of 16 meters was detected in the fibre orientation (TSO Angle more specifically), as seen in figure 9. It was concluded that changes in fibre orientation taking place over 16 meters modified the web tracking on press, generating the web movement.
The cause for the web weave now known to be related to TSO angle variations in MD, means that the mill could focus on finding its source. In the end, the TSO angle variations were caused by headbox pulsations. Headbox pressure was recorded at the mill and running a FFT analysis on this data generated the exact same wavelength as that found for TSO Angle and web weave. The headbox pulsations were generated by the fan pump, and its replacement allowed a significant reduction in TSO variations in machine direction.
Figure 10 shows the power spectra of the headbox pulsations before and after the fan pump replacement. This improved the web weaving on the printing press to an acceptable level. In the present case, the application of polyethylene film was uniform and did not explain the weaving issue. The base layer composed of paperboard was the one with periodic variations in one of its properties.
In this empirical study, we have attempted to find the impact of non-uniform web properties on web lateral stability on printing and converting equipment. Measurements using portable positioning edge sensors were completed to quantify web lateral instability. Web materials were also tested with sophisticated laboratory equipment to characterise properties, in both CD and MD. Correlations between the web weaving data collected on printing and converting equipment and the web properties were made.
In many cases, lateral instability of the web did relate to non-uniform CD profiles of one of the web property. In other cases, it is periodic variations found in MD web property that caused the lateral instability. Case studies with a particular focus on plastic films were presented, for which the analysis allowed the web manufacturers to identify which property was the root cause of the weaving. When possible, web manufacturers did the necessary changes to their process to reduce the web weave issue.
Editor’s note: An interesting comment was made on this article from Thomas Gray, president of INKnet Systems. He said, “A private study on web tears shows that ink consumption plays a significant role in weakening a paper web. alone with that as the web is brought up to speed there are various harmonics that are generated. typical findings on a high speed web press heat set inks there is a major color shift at the 4th hour run”.
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