After identifying the waste creation factors, a gemba walk was conducted in order to obtain the workers’ views regarding the process. The gemba walk consisted in observing the process more closely and asking questions to the employees about their job, establishing how they do it, the existence or not of standardized work, common problems and improvement suggestions.
The answers were then summarized in order to present the most significant information for the identification of root causes for waste. This step was one more contribution of information to completely define the root causes for waste in the corrugating process.
Finally, an Ishikawa diagram was designed with the purpose of more clearly exposing all of the root causes for waste found during the application of the methodology. By doing this, it is easier to understand the most significant causes of inefficiencies in the corrugating process and consequently it becomes easier to develop solutions to combat them.
Using all of the information gathered from the application of the methodology, improvement solutions were designed with the intention of reducing the effects of the root causes for waste.
These measures were explained in detail, specifying what problem they combat, in which step of the process they should be applied to and why they should be applied.
In parallel, a greedy algorithm was being developed to plan the picking of paper reels. This step explored the structure of the algorithm, explaining in further detail its variables, its inputs, how it functions and its outputs.
4.2 Identification of waste creation factors
The first important step is to represent the process using a flowchart. This allowed to understand where inefficiencies might be coming from and to have a clearer vision of the process workflow.
Since it is a long process, a part of the flowchart is shown in figure represented in A.1 in annex A.
Figure 4.1: Partial representation of the process flowchart
By analyzing the process flowchart there can be identified five main areas in which waste can occur. The first one is the paper warehouse, where the waste corresponds to damaged reels that are beyond repair and not suitable to be used in the corrugating process. The second revolves around the single facers. In this step of the process, the creation of waste was attributed to the cleaning of wet reels (including cutting the damaged paper in the roll), the splicing process (entails cuts to
18 Methodology
the paper roll) and the reel leftovers (reels that were partially used but were not large enough to be stored again in the paper warehouse and were sent to the shredder). The third was the "Rotary Shear" in which non-conformant WIP in the corrugating process, if identified, is considered waste and leaves the process. The fourth is associated with the "Slitter Scorer" and it is the "Trim". In this step the corrugated board is trimmed along its width to the dimensions specified in the production order, being the excess board disposed of and considered waste. The fifth area is related with the non-conformances identified in the final product. The waste can be attributed to "non-conformance at exit" of the corrugator when the quality check identifies corrugated board not corresponding to specifications and is sent to shredder. It can also be attributed to "non-conformance at converting"
when the quality check is not effective and non-conformant board is sent to the converting process, but it is identified as non-conformant before being fed into the converting machines.
The data analysis allowed for a later deeper knowledge of the process and root causes for waste, supported by the expertise of the corrugator manager and the "Maintenance & Engineering"
manager.
The data collected by the company to measure the inefficiency of the corrugating process was the weight corresponding to the waste. Also relevant to this project is the fact that the waste values were divided in different areas of the corrugating process in which waste occured, more specif-ically: "Trim" (only value represented as a percentage), "Reel cleaning", "Splicers", "Reel left-overs", "Rotary Shear", "Non-conformant at Exit", "Non-conformant at converting", and "Paper Warehouse Waste". Each one of these sections has an objective value represented by a percentage.
The real percentual value of waste is calculated by taking the weight of waste from the section in that day and dividing it by the weight of the paper and adhesive used for that particular day as represented in equation (4.1)
Waste(%) = Waste(kg)
Paper+Adhesive(kg) (4.1)
The analysis of this data was enough to understand, at least at the surface level, the most relevant contributors to the total waste created in the process. This step focuses the rest of the methodology according to the areas which create more waste.
It is important to mention that the "Trim" factor was ignored for this methodology since this waste source is not a direct result of corrugating process problems. Every production order width is a standardized width superior to the result of the sum of the widths of the specified corrugated boards. The trim waste is attributed to the excess of width in the corrugated board (before the Slitter Scorer step) when compared with the production order. This happens due to the fact that paper reels only come in standardized widths, but the sum of the widths of the boards specified by the customers can be any value. As an example, if a customer orders a board, divided into three orders, with widths of 500mm, 600mm and 800mm, the planning department had to register a production order with width 2050mm since it is the standardized width closer to the sum of orders. This in turn means that there is a trim waste in weight corresponding to the 150mm that are extracted from the board to adjust its width. This can be generalized as a cutting stock problem
4.2 Identification of waste creation factors 19
which is the responsibility of the planning department that deals with the problem by following internally defined rules and so, it was not considered in this approach.
4.2.1 Unplanned Stops
In the corrugating process, there is a significant measure of planned waste, mainly related with order changes. However, unplanned waste is still very significant and is tightly associated with unplanned stops of the corrugator. This step focuses on analyzing the impact and causes of the unplanned stops.
The wet end of the corrugating process, as mentioned previously, consists in a balance between pressure, temperature and adhesive. If even one of these factors is applied in a disproportionate amount to the papers that form the corrugated board, the rest of the process and the quality of the board can be compromised. Another important factor that influences all the others is the speed in which the papers and subsequent board travel through each section. For example, a board that travels very slowly throughout the process, at a set temperature, is much more likely to become dry and brittle than a board, at the same set temperature, moving at a much higher speed. This in turn leads to a very particular characteristic of the corrugating process.
When there are unplanned stops in the corrugator, the WIP in the sections before the rotary shear is waste since its physical properties are non conformant. This is because, even though the machines stop working, the pre-heaters in the single facers are still transferring heat to the papers, the single faced webs in the bridges are not receiving enough pressure or heat to bond perfectly and the hot plates are still transferring heat to the board, meaning that every unplanned stop equates to a high amount of unplanned waste.
The next step was to dissect the unplanned stops, not only by measuring the impact on the total waste, but also by assessing its main causes.
4.2.1.1 Impact of unplanned stops
The impact of unplanned stops in the waste had to be calculated in order to understand to which de-gree it explains the levels of waste. This was achieved using the following assumptions discussed with the "Maintenance & Engineering" manager:
1. Each change of execution results in one sheet of waste 2. Each change of programme results in three sheets of waste
3. Each change of flute profile results in 75 sheets of waste (Average between 60 and 90) 4. Each sheet, in average, weighs 0.5kg/m2
5. Each sheet, in average, is 2m∗0.9m = 2.15m2
By using these assumptions and by analyzing the production orders to understand the number of changes of execution, the number of changes of programme and the number of changes in the
20 Methodology
flute profile it was possible to calculate the planned waste for a day. Comparing the calculated planned waste with the total waste, the impact of unplanned stops in the corrugating process is measured. This in turn, verified how much of a problem the unplanned stops in the corrugator were for the process efficiency.
4.2.1.2 Causes for unplanned stops
The unplanned stops that occur during the work day are registered by the corrugator shift leader in PC-Topp, choosing from a series of possible reasons already predefined including in which section happened and what caused the stop. This allows to group stops of similar nature, making easier to find out which are more frequent. Furthermore, there is a description option to include even more information about the unplanned stop. Using all of this data, it is possible to know which stops are more frequent and which are more time consuming.
PC-Topp takes in the inputs from the shift leader and organizes the stops according to the total time they lasted. Being PC-Topp the software used to control and analyze the productive perfor-mance of the plant, it also computes total work hours and what percentage of those hours were unplanned stops. This analysis is focused on the top 15 stops according to frequency, allowing for an understanding of more common problems found in the process. These stops are usually a result of poor raw material conditions, operator error or simply preventable.
This same analysis could have also been done to the top 15 stops according to stop time, but it is not going to be subject of further work. This was based on the fact that a big part of those causes were coincident with the top 15 in frequency, and those that were not, usually were rare and hardly preventable causes such as machinery problems that had to be obligatorily solved by the maintenance department.
All of the identified causes were briefly explored to understand which was the root cause.
The unplanned stops were grouped according to the root cause associated, to later be used in the Ishikawa diagram.
4.2.2 Non-conformance of board
The corrugated board that needs to go through more transformative steps, is either sent to the WIP warehouse or directly to the converting area of the plant. The waste created by non-conformances is represented as "non-conformant at exit" or "non-conformant at converting". The latter, equates to the boards that are detected, before being transformed by a converting machine, as non con-forming to specifications or not adequate to be converted by a certain machine. This type of waste is particularly prejudicial to the company since it not only means wasting all the inputs from the corrugating process, but also might create the need for rework. This type of waste is tightly related with the "non-conformant at exit" waste since, if the board does not match specifications, it should be detected at the exit of the corrugator and consequently be labeled as "non-conformant at exit"
waste, instead of being detected at the beginning of the converting process.
4.2 Identification of waste creation factors 21
By conferencing with the corrugator manager, it was possible to understand that there were three major flaws in the corrugated board that explained it being rejected at the beginning of the converting process:
1. Warp: The corrugated board is not totally plain, displaying accentuated curves, also known as warp. For different clients there are different thresholds for acceptable warp, but if the value is too high, most of the converting machines are not able to correctly transform the board.
2. Not glued: The adhesive put in the corrugated board has not properly bonded, causing the several layers of the board to be very easily separated, causing problems when the board goes through the converting machines
3. Other: A grouping of other possible defects that are rarer. Namely, boards damaged with tears from transportation or due to being in the bottom of a stack stored in the WIP ware-house pressured against the pallet and poorly trimmed boards.
Even though these were the major reasons for waste, the company, when registering the weigh of the waste in this section, would not differentiate between these three types of causes. According to the corrugator manager, warp was the most considerable non-conformance.
To verify this empirically and reach a possible solution, there was a need to more accurately measure the origins of the waste corresponding to "non-conformant at converting" and to identify the causes for the most significant type of non-conformance.
4.2.2.1 Impact of non-conformances on waste
In order to differentiate the impact of each non-conformance on the total waste in the "non-conformant at converting" section some changes had to be made.
Before being moved to the shredder, the stacks of waste are categorized using red papers with no information filled in. Red papers are used instead of white ones to make it visually clear that those stacks are to be moved to the shredder. The forklift driver picks up the pallet in which the boards are set on and uses an industrial scale to weigh the waste, taking out the weight of the pallet. After this the forklift operator fills in the paper with several information, namely, in which converting machine it happened, if it was as an input or an output and the weight of said waste. This version of the paper could only be used to differentiate if a non-conformance happened before or after the converting process. Needing more information about the most relevant non-conformances, a new version of the red paper was introduced as shown in figure 4.2.
This paper included, along with the weight of waste and as input or output from the previ-ous version, the possibility to differentiate which was the type of non-conformance and if more than one is identified, what weight corresponds to each non-conformance. The three options are
"Empenado", "Descolado" and "Fundo da pilha" meaning respectively, "Warp", "Not glued" and
"Bottom of the pile". The third option does not correspond to the aforementioned "Other" as a
22 Methodology
Figure 4.2: Red paper for waste information
shift leader of the converting department of the plant suggested the option "Fundo da pilha" since it was more intuitive to the workers.
Along with this change, the file in which waste was registered was also changed to include three columns, each one for a non-conformance, which summed up, would result in the daily waste for the "non-conformance at exit" section.
4.2.2.2 Causes for Warp
Warp can take many shapes being up warp and down warp the most common and S-warp (one of the sides curving up and the other curving down) and diagonal warp (curving along the diagonal of the board) the most difficult to unwarp. Due to the moldable nature of the corrugated boards, even if the warp non-conformance is identified at the exit of the corrugator, it is possible to be corrected afterwards. The process of unwarping is done by applying pressure to the board in the opposite direction of the warp until the level of warp falls between the accepted values, usually below 3%.
This step of the methodology analysis the causes for warp since it is the most common non-conformance, looking into each one in more detail, allowing the extraction of root causes to be presented in the Ishikawa diagram and combated through the presented proposals.