3.1. Company Analysis
The analyzed enterprise produces precision steel pipes of various dimensions for upholstered furniture. In 2020, the company produced 10,000 tons of metal products with a total value of EUR 8 million. Currently, the plant employs 82 people. The production in the enterprise takes place in a one-shift system and is divided into two lines:
Metal slitting line cutting line—work takes place 5 days a week, and the shift lasts 8 h (8:00–16:00); during the shift there are two breaks, one for 30 min and one for 10 min;
Bending and electric welding line—work takes place 7 days a week, and the shift lasts 12 h (8:00–20:00); there are four breaks during the shift: Two 30-min and two 15-min.
The implementation of such a work system results from the specificity of the operation of devices installed on the lines. Another reason is that the cutting operation progresses much faster than in the welding operation.
The raw material for the production of pipes in this plant is cold rolled steel rolls. The headquarters and warehouses of the main supplier of steel rolls are far away from the analyzed enterprise. The average time of order fulfillment for a material in a metallurgical plant is three months. Therefore, the company prepares annual demand forecasts, which are reviewed every six months. Orders for steel rolls are placed once a month.
The company’s customers place orders for the next month no later than on the 25th of the previous month. Verification of orders is possible every day. The estimated monthly demand for the plant’s production is about 700 tons, while deliveries to customers are made daily by trucks with a capacity of 20 tons
Production orders for individual departments are generated based on the received customer orders, the level of inter-operational stocks, the level of stocks of finished products, and the expected level of shortages. The company uses the world-famous concepts of Kaizen, 7 Strat Sheet, 5S, and others.
3.2. Description of the Production Process
The production process of steel pipes for upholstered furniture consists of seven stages (
Figure 2). The production of steel pipes can be divided into two key stages and is carried out on two production lines—a metal slitting line and a line for bending and electric welding of steel pipes.
The raw material in the form of steel rolls with appropriate parameters is transported to the plant by road transport. The rolls in their original packaging are unloaded using an overhead crane and then sent to the warehouse.
The process begins on the slitting line of the metal. With the help of an overhead crane, the right steel roll goes to the place where the packaging is removed. The roll is then fed onto a line where the metal is cut into strips hereinafter called strips. Finally, the strip is wound on the shaft to form circles. With the help of an overhead crane, the tied strip is transported to the inter-operational stock warehouse, where it awaits the next stage of production.
Then the worker of the bending and electric welding line takes the correct strips using an overhead crane and feeds it to the double-sided line unwinder. After unfolding the first strips, the next one is fed.
The first end of the tape roll is combined with the beginning of the second continuous strips by means of welding machine tungsten electrode in an argon atmosphere. The strip is then fed to the linear forming section, where it is rolled into a round tube blank by means of rolls. A continuous pipe blank with unclosed metal edges is fed to the high-frequency welding section. Thereafter, the blank is compressed by rolls on three sides, followed by the contact of the molten edges with each other. At this point, it can be concluded that the pipe is welded. The pipe is then immersed in a bath with a cooling emulsion, and then directed to the calibration section. The final diameter of the pipe is precisely shaped in the calibration section. The pipe weld is subjected to an ultrasonic test on a stand, and the quality of the weld is also checked. The next step is to shape the round pipe to a specific profile (oval, flat, rectangular, square, and others). The length of the pipe is measured automatically, and the circular saw cuts the continuous pipe into lengths of specified dimensions according to the orders.
In the packing section, the pipes are placed manually by the worker. The outer surface of the pipe is visually inspected. The first tube that goes into the pocket is intended for testing. The pipes are packed in accordance with the specified quantity, depending on the customer’s preferences, and as a result, welds are formed. Using an overhead crane, the seal with the pipes is placed on the stand in an inclined manner to allow the remaining emulsion to drain from the inside of the pipe. A manufacturer’s label is attached to each package. In the final step, the finished pack of dried pipes is transported by an overhead crane to the finished products warehouse.
The photos in
Figure 3 show the manufactured products, individual stages of the production process, and the company’s machine park in order to improve the processes taking place in the enterprise. The use of photos in the article has been agreed with the plant management.
3.3. Mapping the Current State in the Steel Pipe Manufacturing System
VSM allows one to identify the root causes of problems that arise along the entire value chain. It is an analytical tool that cannot actually solve the problem. Therefore, once problems are identified, it is necessary to use other methods or tools to fix them.
Based on the obtained data, analyses of the entire production process, and consultations with the company’s engineers, a final map of the current state of the steel pipe manufacturing system was developed and the map legend (
Figure 4). The map (
Figure 5) reflects all operations performed in the analyzed enterprise by operators, their duration, and indicator values. It gave a full picture of the company’s situation at the time of the research.
The top part of the map shows the information flows starting from the customer, through internal company communication and ending with suppliers. The material flow is visualized in the lower part. Below it there are tables with individual indicators, where:
Value-Added Time is just 100 min. The total order fulfillment time is 35 days. After conversion, it gives a 0.002% share of Value-Added Time in Lead Time. This means that practically the entire production time of products is Non-Value-Added Time. In
Figure 4, all activities are divided into three groups:
Non-value Added—observed activities that do not create added value of the product, such as idleness, conversation between operators on the slitting line;
Necessary Non-Value Added—activities that do not directly create added value, but are necessary from the point of view of the continuity of the production process, such as checking the width of a steel roll, entering data into the workflow report, segregating the generated waste or cleaning;
Value Added—activities that add value to the final product, which contribute to its direct transformation and give it features for which the customer is potentially willing to pay, for example fixing the roll on the decoiler, marking each roll of stripper, setting the production line, etc.
Figure 6 reflects the time distribution into value adding activities and redundant for the slitting line. We chose this line as an example because it is not fully automated (like a welding line), so we have more to do with Non-Value-Added activities, for example it has manual retooling by operators.
Table 2 presents the collected data and the described metrics regarding the initial state of the analyzed process.
The longitudinal cut line has been included in the critical areas. It was taken into account due to the large number of NVA activities that have a negative impact on the production process. This fact is due to the greater proportion of operators’ manual labor required to unload and load the rolls onto the line, more frequent changeovers, and the waiting time for the machine to finish. The various activities of the NVA prove that the line work is poorly organized, and the working time fund of the line workers is not fully used.
During the observation of the working day, some imperfections were detected in the first seconds of the retooling operation. The essence of the retooling operation is the correct replacement of knives and spacing and mounting them on the shaft in order to achieve the desired cutting parameters. The operation of preparing the line for the production of piping with the desired parameters is performed manually in a manner where an employee at a height of 1.5–2 m replaces the necessary parts. Under current conditions, the average time of line changeover is about 60–80 min. The average weight of one knife is about 10 kg. Visually, it was possible to assess the low comfort of work by an employee at height and a fairly high degree of accident probability. The observed situation provoked the willingness to propose possible changes in the line retooling operation in order to improve the efficiency of work at a given position.
3.4. Future State Map and Proposed Improvements
The constructed map of the current state allows for the identification of problem areas. In the chapter below, we will propose a map of the future state with marked areas for improvement and improvement actions under the KAIZEN philosophy. As part of the development of the future state map, the authors decided to focus on those production areas of the enterprise in which the greatest number of Non-Value-Added activities occur.
The specificity of the steel pipe manufacturing technology is determined by the layout shown in the drawing (
Figure 7) below the workstations and their location in the production hall. Production takes place on two production lines, which are connected by a supermarket to store Work-in-Progress (WIP) inter-operational stocks.
The finished goods warehouse is located at the end of the production chain. Any attempts to reorganize the physical flow of materials and products are not appropriate due to the design and size of the devices and the technology used in the plant.
In addition, most of the articles in the literature focus on the study of Lean Manufacturing elements, such as the implementation of continuous flows, the use of supermarkets, Kanban cards (finding the optimal number of cards), push and pull production systems, production leveling and differentiation, indicating the process stimulating the work of the whole, etc. In contrast, other elements of Lean Manufacturing, such as multi-shop work, reduction of production cycle time, and process improvements, etc., have not received sufficient attention. This article aims to focus on areas for improvement of individual operations along the value chain, reducing losses occurring in separate processes and increasing the efficiency of employees working time in these positions, which in turn leads to an increase in the efficiency ratio of the entire organization.
3.4.1. Future State Map
The developed map of the future state (
Figure 8) reflects the map of the current state with problem areas marked in the form of clouds. A longitudinal cut line has been included in such areas. The slitting line was taken into account because of the large amount of NVA activities with the aim of reducing the amount of non-value adding activities, and also because of the long duration of the operation to possibly reduce this time. After presenting the map of the future, descriptions of individual improvements in selected areas were placed.
3.4.2. Kaizen 1—An Improvement under SMED
During one work shift, the average number of rolls of steel cut is about six pieces. This means that in one shift, line operators perform six cutting cycles. The essence of the retooling operation is the correct replacement of knives and spacing and mounting them on the shaft in order to achieve the desired cutting parameters. The operation of retooling the strips production line with the desired parameters is performed manually. An employee at a height of 1.5–2 m changes the knives and adjusts the spacing between them depending on the decent dimensions of the strip. It is associated with low work comfort and a fairly high probability of an accident. The average weight of one knife is approximately 10 kg. Currently, the total time for line retooling is approximately 2 h during one shift.
In addition, operators have an average of 15 to 25 min of waiting time per cutting cycle to wait for the machine to finish, which is approximately 120–140 min of inefficient working time per shift for each operator.
The longest-running operation of the line is the cutting operation. The average time for this operation is about 20 min depending on the length of the raw material of the steel roll without the direct involvement of the line operators, except for the initial visual inspection of the correctness of the operation. For one employee, the total Non-Value Added (including activities necessary in the process) in the total working time is about 61%, while the share of value added activities is 39% (Value Added) from the total operating time of the cutting line. Analyzing the structure of the second employee’s working time on the metal cutting line, it is possible to notice a higher percentage of unnecessary activities caused by the longer total waiting time for the machine to work and at the end of the previous operation. On the one hand, too high a percentage of unnecessary activities proves a positive organization of the production process, because they do not result from the occurrence of disturbances or failures or the lack of necessary tools or details. On the other hand, there is a fairly high level of non-utilization of the line operators’ working time, resulting in a high labor cost.
The idea to improve the operation is to purchase an additional replaceable part of the cutting line (more precisely, an additional pair of metal cutting knives and a shaft to which the knives are attached), on which the changeover takes place. By purchasing an additional part, the retooling operation would take place as follows:
A pair of knives is mounted on a special rack with an overhead crane, where it is changed over.
After finishing the cycle of cutting metal on strips of the previous dimensions, the part of the line is disassembled, and in its place, with the help of a crane, the appropriate, already converted part is installed.
The cost of the investment is estimated at around EUR 40,000. Under current conditions, the line retooling time, with two employees, is an average of 120 min per shift. During one work shift, operators should change the line at least twice.
For a better understanding of the proposed solution, below we would like to present a potential scenario for the organization of retooling on a given line.
Now, when they come to work, the operators start retooling by replacing the knives on the line shaft and carrying out this work at a height of 1.5–2 m, which takes an average hour (no steel roll is cut during this time). After retooling, they begin the cutting cycle of steel rolls. In the cutting cycle, the machine time is about 20–25 min, as the employees are not busy with any work during this time. After cutting on three steel rolls, operators should replace the knives again, stopping the line and retooling for an hour.
The proposed purchase of a spare part will allow employees to be busy with work during forced idleness and will eliminate an hour-long break in the line’s operation in the middle of the day. It is impossible to avoid the loss of the first hour of work, while later (when cutting the “first” three steel rolls) employees will be able to install new knives on a replaceable shaft, taking advantage of the interruptions resulting from waiting for the end of the machine’s operation. Once they have finished cutting on three steel rolls, one only needs to change the shafts with an overhead crane. The only condition for the profitability of the above solution is the optimal planning of the work task in advance, so that the line cuts at least three rolls of steel on one changeover.
In the case of the above solution, the waiting time for the completion of the cutting by the machine will be effectively used by the operators to retool the free replaceable part to the next parameters included in the working task. The introduction of the solution will result in a reduction of the changeover time to 1 h per shift, which has been included in the future state map. As a result, the next required retooling will coincide with the machine time and will allow 1 h to be released per shift, during which the next cutting cycle can be performed. Thanks to this change in the organization of the line operation, while maintaining the same human and time expenditure, the production system is able to process one more roll of steel compared to the current state, which means an increase in line productivity by 17%.
In addition, this solution translates into an increase in the comfort of operators’ work during the set-up operation. Working at height is completely eliminated, which also reduces the risk of an accident at work.
3.4.3. Kaizen 2—Streamlining the Packing Operation on the Slitting Line
The operation of packing each coil of already cut strips is characterized by the highest repetition frequency. Two workers are always involved in it, working in tandem. The packing of strips coils consists of several parts. First, a packing machine is taken into which a reinforced polypropylene tape is inserted. Then the strips hanging on the winder are tied one by one with tape. The average time to perform this operation on one piece of strips is 28 s. On average, there are about 10 cut strips on the coiler, which equals 280 s to complete the packing operation. Separate packing of the individual strips is required to prevent it from unrolling during transportation later in the manufacturing process. At the next stage of production, i.e., during cutting, the coils taken from the inter-operational stock warehouse go to the steel pipe production line, where they are unpacked, i.e., the tape is removed. Reusing the tape is not possible due to the insufficient length of the tip (it prevents reinserting the tape into the packing machine). Typically, the worker who performs the packing operations leaves tips about 5–6 cm long. Within a month, the waste from polypropylene tape is about 80 kg.
The main goal of streamlining the packaging operation of strips is to shorten the time of this operation and reduce waste. When analyzing solutions in companies with a similar nature of production, it was decided to use a reinforced aluminum tape instead of a polypropylene tape. When choosing a tape, we were guided by its strength level.
Thanks to this solution, the operation time will be reduced to 5 s for packing one strip, which would consequently lead to a reduction of about 25% of the packing time during one cutting cycle of a steel roll. Moreover, the waste of adhesive tape in this case would constitute a much smaller amount than in the case of polypropylene tape.
Performing a verification test on a strip with a fairly large thickness of 2 mm showed that the given solution is unacceptable to the production reality. Therefore, it was necessary to find a new improvement proposal.
Another suggestion is to reuse the polypropylene tape by planned plugging of the larger ends with a length of 20 cm. Thanks to this solution, the polypropylene tape can be used at least the second time.
Re-performance of the verification test confirmed the usefulness of this solution.
The proposed solution does not reduce the time needed to complete the packaging operation but has a large impact on financial and environmental issues. As a positive effect, after implementing the above solution, we assume that the cost of packaging will decrease by about 1.5 (one and a half) times. When leaving longer ends of the packaging material, it becomes possible to reuse it, and the positive effect estimated by us has been reduced by the probability index of the risk of wear of the belt strength parameters, which is the main disadvantage of this solution. Another positive effect of the above solution from the ecological point of view is the lower demand for the tape during the month. As already mentioned above, the monthly waste of the tape is about 80 kg and is not suitable for recycling.
As the assumed positive effect when implementing the above solution, the cost of packaging will be reduced by one and a half times. The risk of wear of the belt’s strength parameters is the main disadvantage of this solution.