Based on the information given by the flowchart and by analyzing the available data regarding waste in the corrugating process it is possible to understand the two major waste factors are the
"Rotary Shear" and the "Non-conformance at converting" as is proven by a 6 month summary analysis of waste prior to this project shown in figure 5.1.
Figure 5.1: 6 month summary of waste data
5.1.1 Unplanned Stops
5.1.1.1 Impact of unplanned stops
In order to understand the impact of the unplanned stops, the unplanned waste has to be calculated.
The unplanned stops contribute to the waste attributed to the "Rotary Shear" since this is the step in which non-conformant WIP is considered waste and disposed. The following figure 5.2 shows a random sample of 7 days taken in order to quantify the impact of those stops.
For this sample it is possible to extract that, on average, approximately 61% of the waste registered in the column "Rotary Shear", is unplanned waste. The value obtained for this sample
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Figure 5.2: Waste due to unplanned stops
can be assumed to be representative of the day to day reality of the process. This is a clear problem that needs solving. Knowing this, the focus turns to finding out what is causing the unplanned stops, which can be found in the next subsection.
5.1.1.2 Causes for unplanned stops
Figure 5.3 shows a more condensed version of the PC-Topp unplanned stops output for February of 2021 and turns the focus to the top 15 stops according to frequency, the most common causes for unplanned stops. These stops are usually a result of poor raw material conditions, operator error or simply preventable.
Figure 5.3: February Top 15 stops according to frequency
Despite being a useful summary of the problems that perturb the corrugating process, further information was needed that was only available in the computer where the information is registered by the shift leader. From piecing together the information shown in figure 5.3 with the shift leader comments, it was possible to reach some deeper causes for the stops.
The problems concerning the stackers and the order change problems in the rotary shear were solved, since they were reoccurring and mechanically fixable.
Wet reels and paper breaks are a result of the paper warehouse not being properly thermally isolated. This leads to a very undesirable situation since paper is a hygroscopic material, meaning it will pick up moisture from the environment. Wet paper reels are not appropriate to be fed to the single facers, since they will most likely break when force is applied pull the stretched paper
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through the single facer. The longer a paper reel is stored in the warehouse the more likely it is to absorb moisture, to be damaged and to be susceptible to breaks. One of the causes is then, the storage conditions of the reels.
Paper jams, missed splices and flute profile changes can be a result of human error. The process of preparing the paper reels that are going to be fed to the single facers in the paper reel stands, entails some steps that are operator driven. Some examples are: "treating" the reel if partially damaged, aligning the paper and putting tape in order to allow for the splicing of reels and adjusting pressure levels in the single facer. If one of these actions is not done correctly, unplanned stops are likely to occur. This reveals that maybe not every part of the process is being done in a correct manner which implies that there is a need for better training or work instructions.
The mechanical problems attributed to the splicers and part of the paper jams are related to machinery issues. The paper jams can be a result of uneven application of pressure to the single faced web, originating from the unevenness of some of the pressure rolls that affect the single faced web. The splicers mechanical problems are quickly fixed by the maintenance department, not being able to be solved by the corrugator workers. One of the causes might then be a lack of preventive maintenance and training workers to spot signs of possible failure.
These causes will further be explored in the Ishikawa diagram section of the project.
5.1.2 Non-conformance of board
5.1.2.1 Impact of non-conformances on waste
As a result of the changes made during the application of the methodology, the assessment of the most significant non-conformance was possible. Using the data available for April of 2021 it was understood that the contributions for total waste were approximately: Warp= 36% ;Not glued
= 8% ;Other= 56%. Looking at these results, it might seem that the "Other" grouping of non-conformances is the biggest problem. The reality is that a significant amount of red papers were not correctly filled in, more often than not just including the information that was needed in the previous version. This led to a less than ideal situation, since it meant that some information was being lost. Despite that, focusing only on "Warp" vs "Not glued" it is clear to observe that the
"Warp" non-conformance had the biggest impact on the total waste of this section.
5.1.2.2 Causes for warp
In order to be able to combat the warp non-conformance, its root causes were explored, as con-templated in the methodology.
The main causes for the occurrence of warp are: Moisture difference between top and bot-tom linersandtension difference between top and bottom liners. These are highlighted by DS Smith, having a big board detailing the causes that lead to several types of warp, from the amount of adhesive used to the speed of the process.
For each cause there are several possible explanations:
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1. Moisture imbalance: It is particularly well know in the corrugator department that the board tends to warp to side with more moisture. Using papers with high humidity, excessive or irregular application of adhesive and parallel misalignment of the preheater rolls can lead to differences between top and bottom liners and therefore to warp.
2. Tension imbalance: It is related with the differences in pressure applied to the board. Usu-ally caused by rolls not turning properly and not having a correct parallel alignment.
Generally speaking, many of the problems that cause warp are either related to machinery components or to the parameters of adhesive, tension and moisture. In order to attack the root causes, a better control of those parameters in the process and a more frequent and attentive look at the corrugator components might lead to a significant reduction of warped corrugated boards.