Identifying the root causes contributing to defects in order to minimize scrap

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1 Identifying the root causes contributing to defects in order to minimize scrap Zanele Mpanza Transport Systems and Operations CSIR Pretoria, South Africa Abstract The case study company was established in A needs analysis was conducted at the company and a high scrap rate was identified. The company made a financial loss about R in 2013 and also in 2014 due to scrap. The company still faces a high scrap rate in spite the quality management system in place. Defects are still prevalent in the manufacturing industry despite the attempts by companies to implement measures to eliminate them. A contributing factor to defects is the absence or poor utilisation a quality management system. Measuring the effects these defects on productivity and their contribution to losses in monetary value is likely to assist the company to use the tools in the quality management system to eliminate the root causes these defects. An investigation into the root causes was conducted using a root cause analysis technique. Pareto analysis, Fishbone diagram and 5 Whys were some the tools used to uncover the root causes defects leading to scrap. The identified recurring defects were analysed to eliminate their root causes. Keywords Quality management, Quality control, Production, Manufacturing, Root Cause Analysis. I. INTRODUCTION Defects are still prevalent in the manufacturing industry despite the attempts by companies to implement measures to eliminate them. A contributing factor to defects is the absence or poor utilisation a quality management system. Measuring the effects these defects on productivity and their contribution to losses in monetary value is likely to assist the company to use the tools in the quality management system to eliminate the root causes these defects. Once the failures are identified, it is best to learn from them and avoid the recurrence using continuous improvement techniques. The case study company was established in The company conforms to all legal requirements as per the laws the country and is one a handful foundries that are registered in terms the Atmospheric Pollution Prevention Act 1995 (Act ). In June 2004, the company obtained ISO 9001:2000 quality accreditation and it was re-certified in 2010 to ISO 9001:2008. The company exports some its products to Europe and the Middle East. The company uses the investment casting process to manufacture its products using aluminium, ductile iron, copper, stainless steel and other ferrous and non-ferrous metals. The metal is melted by using gas-fired and induction furnaces. The company manufactures castings for the aerospace and defence, automotive, mining, and medical industries. Some their products include components for military vehicles, field guns, missiles, automatic weapons, rockets, safety equipment, automotive products and general engineering. The company is a job shop production plant meaning that they only make to order (MTO). The challenge the company faces is a high level scrap despite having a quality management system in place. In 2013 and also in 2014, about R worth products were scrapped annually due to quality defects. Sometimes the production line produces 10% more to compensate for scrap, resulting in overproduction. According to lean manufacturing principles, overproduction is one the seven wastes [1]. Therefore, the root causes for defects need to be eliminated to reduce overproduction and thereby reduce waste. The defects affect the quality castings which sometimes leads to customer dissatisfaction. The defects also affect the lead time because re-work that has to be done and thereby causing delays. The company sends out customer satisfaction surveys to all its customers. The customer surveys are analysed and the quality products and lead time are identified to be one the leading complaints from customers. The aim the project was to reduce scrap by identifying the root causes contributing to defects. 1139

2 II. MATERIAL AND METHODS Direct observations were conducted; quality documents and process interaction sheets were studied to understand the process and procedures. The defects data were obtained from the quality control documents for the year NonConformance Reports (NCR) were also studied to understand some the customer complaints. This exercise was done to identify the most common complaints and the action taken by the company thereafter. In order to address this issue, a Root Cause Analysis was performed. The RCA follows five simple steps being: Define the Non-Conformity Investigate the Root cause Create proposed action plan and timescales Implement proposed action Verification & monitoring effectiveness RCA is complemented with other problem solving tools. For this study the tools used include a Pareto Analysis, a Fishbone diagram, and the 5 Whys. After establishing the possible causes, we ask 5 Whys. The 5 Whys are asked until meaningful conclusion is reached. This method is used for the three defects identified to have a high significance in financial losses. The financial value the scrap per month was analysed. A Pareto analysis (or ABC analysis) was performed to identify the most significant causes scrap. The following steps were followed in performing the Pareto analysis: A method for classifying the data was determined and in this case the number products scrapped for different defects was used. The next step was to choose a unit measure. For this study two units measure were used. These are the cost scrap per defect type and the frequency occurrence the defect type. Two separate Pareto analyses were done based on these two measures so that results could be compared. Data were then gathered for an appropriate time period which, in this case, was the whole The data were then summarised by ranking the items in descending order according to the two selected measures in Step 2. The total cost was then calculated as well as the total number occurrences in the second case. The percentage for each scrap defect was then calculated. A bar graph showing the percentage each item was finally constructed for the two cases. After this the results the Pareto analysis were interpreted and the most significant defects were identified. Other techniques for root cause analysis were then employed in order to fully understand why the problems occur and to give recommendations to address them. III. THEORETICAL FRAMEWORK Kaizen is a Japanese term for continuous improvement which emphasises that all employees in an organisation participate in improvement and that they perform their tasks a little better each day [2]. The problem solving and improvement process has a number steps in which various methods and tools are used to better understand the problem and to determine the best solutions. The steps include: Identifying the problem Describing the current and revised processes Generating ideas for process improvement Achieving consensus among team members Evaluating and monitoring results In Pareto analysis, items interest are identified and measured on a common scale and then are ordered in descending order, as a cumulative distribution [3]. This technique is sometimes referred to as the rule. Typically 20 percent the ranked items account for 80 percent or more the total activity. For example, 20 percent mistakes account for 80 percent 1140

3 defects, 20 percent defects account for 80 percent financial losses. The greatest effort is concentrated on the few jobs that produce most the problems. It allows the separation the significant few from the trivial many so that efforts and resources are used in the best possible way and the biggest gains are attained. Root Cause Analysis (RCA) is a method that is used to address a problem or non-conformance, in order to identify the root cause the problem [4]. It is used to correct or eliminate the cause, and prevent the problem from recurring. Root cause analysis requires the investigator to look beyond the solution to the immediate problem and understand the fundamental or underlying causes the situation and correct them, thereby preventing re-occurrence the same issue. This may involve the identification and management processes, procedures, activities, inactivity, behaviours or conditions. The 5 Whys method Root Cause Analysis requires questioning how the sequential causes a failure event arose and identifying the cause-effect failure path [5]. Why is asked to find each preceding trigger until the root cause the incident is identified. The 5 Whys method helps to determine the cause-effect relationship in a problem or a failure event. The 5 Whys method uses a Why table to sequentially list the questions and their answers. It can start with a statement the situation and ask why it occurred. Then the answer to the first question can be turned into a second Why question and so on [6]. The fish bone diagram, sometimes referred to as the cause-and-effect diagram or Ishikawa diagram, was developed by Pressor Kaoru Ishikawa in the 1960 s. It was originally developed as a quality control tool but it can be used as a problem solving technique or a brainstorming tool. It is a very useful tool for identifying root causes [1]. IV. RESULTS AND DISCUSSION A. Defining the Non-confirmity In order to fully understand the results this section a table showing the recurring defects and their definitions was developed. The identified recurring defects are as shown in Table I together with the possible causes for each defect. Table I: Recurring defects and their definitions Defect Name Definition Dull run Metal not full in the mould Mould not patched causing leakage Mould not fired Mould not heated enough in the oven Ceramic inclusions Ceramic stuck on metal during pouring Metal dirt left during melting Ceramic getting inside mould during handling Slag Not dipped correctly First dip peel Not dipped correctly Broken mould Broken during knock out (ceramic removal) Mould reaction Inadequate chemical mix Inadequate mould temperature during pouring Over fettled Not fettled correctly Bent or damaged Material too st Bent during cutting or when thrown on the floor Cut f damage Gauge not accurate Wrong cut f Broken wax Broken during dipping 1141

4 Defect Name Definition Slurry too heavy Metal penetration Metal penetrating through the mould Metal in bore/slot Mould not dry outside or inside causing leakage Shell too hard Ceramic/ mould too hard Rough surface Metal not mixed correctly with chemicals Wrong material Using wrong material for casting Dimensional Wrong dimensions a die Over produced Items produced when they were already available Shrink Feed metal not available to compensate for shrinkage as the metal solidifies Gas Solid form the material cannot hold large amounts dissolved gas If the air could not escape it then forms bubbles Cracked Might crack during metal pouring Metallurgical Chemical mixing not balanced or other metallurgical characteristics Faulty wax Wrong wax from the factory Broken ceramic core Mould broken Broken ingate Wax breaking during dipping Based on defect data for the year 2014, a Pareto Analysis was carried out and the results are as shown in Table II. The table shows that the Dull Run defect is the highest in terms occurrence. Based on the definition the Dull Run defect, which is metal not full in the mould and mould not patched causing leakage, it means that a lot moulds are not fully filled with metal and a lot them have leakage. A total 661 defects are caused by Ceramic Inclusions defect. Ceramic Inclusions occur when foreign articles are stuck inside the mould, indicating that the mould was not properly cleaned. The lowest occurring defect is Bent or damaged, contributing a total 2 out a total 2505 defects. Table II : Pareto Analysis for defects (Frequency Occurrence) Reason for Scrap Total Percentage Cumulative percentage Dull Run Ceramic Inclusions Shrink Cracked Metal in bore /slot Broken wax Mould not fired Broken ingate Cut f damage 20 1 Broken mould 4 0 Bent or damaged 2 0 TOTAL

5 The results from the table were then plotted onto a graph in order to visualise the significant few from the trivial many. The main types defects were identified using the Pareto Analysis. Figure 1 shows the company s main causes scrap based on frequency occurrence. Figure 1: Pareto Analysis based on Frequency Occurrence The figure shows that the three dominant defects in this case are: Dull Run Ceramic Inclusion Shrink In order to verify the initial results the Pareto analysis above, another Pareto analysis was carried out but now based on the cost scrap per defect type as opposed to the frequency occurrence. Table III below shows the analysis. The table shows that the defect causing highest losses is the Ceramic Inclusions with R (South African Rands). Dull Runs closely follows with R93 519, contributing about 26% the total money lost. The total amount money lost is R , approximately R in just one year. Table III: Pareto Analysis for Defects (Cost in Rands) Reason for Scrap Total Percentage Cumulative percentage Ceramic Inclusions Dull Run Shrink Metal in bore /slot Cracked Broken wax Mould not fired Cut f damage Broken ingate Bent or damaged Broken mould TOTAL

6 Based on the information from Table III, the bar graph (Figure 2) was constructed. This is done in order to identify the most significant types defect experienced at the company based on cost. Figure 2: Pareto Analysis base on Cost per defect type Again it shows that the main types defects are Dull Run, Ceramic Inclusions and Shrink. These three scrap defects are responsible for 78% all the scrap costs incurred by the company. This means that the concentration should be on eliminating these defects in order to reduce money lost due to scrap. After the identification the main defects leading to scrap, the next step was to look into the root causes the defects. Root causes leading to these defects need to be investigated to eliminate them. B. Investigating the Root Cause First, a Fishbone diagram was used to identify possible causes and secondary causes as shown in Figure 3. Figure 3: Fishbone diagram The causes these defects need to be investigated to find out why they occur, and implement preventive measures so they can be eliminated. From the Fishbone diagram, the possible causes, primary and secondary, are identified and then for each possible cause, a root cause is identified. It can be deducted from the definition the three significant defects that the most common contributors are man, methods, control, and machinery. The top four contributors are analysed to obtain their root cause by asking the 5 Whys as shown in Table IV. The question Why? is asked five times until a root cause is identified. From the table, it can be deducted that the most common reasons are 1144

7 lack training, lack supervision and lack standardization. The 5 Whys analysis method shows that there is very little training given to employees. This leads to mistakes and inadequate techniques being employed, resulting in defects. The other common reasons are lack supervision. This indicates that there is usually no supervisor or factory manager on the shop floor. Operators then do not follow the proper procedures; it could be because they do not see the importance following the procedures. Monitoring and follow-up which is one the common root causes, also forms part lack supervision. Standardizing the working methods or techniques could also reduce defects because it will create order and uniformity. Adequate tools are also required in order to reduce defects. Table IV: 5 Whys Man Lack training Time constrains training new employees Employees needed to start working as soon as possible Large amount work and short lead times Fatigue High demand Heavy components handled manually Lack handling equipment Lack supervision Methods Failure procedures to physical follow Procedures not seen as important No enforcement procedures Inadequate techniques employed Each operator using a technique they are familiar with to perform operations No defined/ documented technique for performing operations Lack standardization Poor handling methods Handling methods inadequate for operations performed Lack handling equipment/ tools Lack capital finance to buy equipment/ tools Lack quality control No inspection operation quality each Operators not equipped with doing their own quality control Lack training incorrect Lack discipline by operators No mechanism for enforcing rules No monitoring and follow-up Cutting f incorrect size job Specifications/ dimensions not adhered to Job incorrect Jig for holding job not available Breaking casting while knocking f Tool used inadequate for the job Use temperature Machinery after strong positioning Tool not available Capital investment not available C. Proposed Action Plan The corrective action includes: Develop a skills development plan for training employees. A measuring device is needed to ensure that the mould is full and there are no gaps in the mould. Another mechanism is needed for checking whether the mould is completely dry inside and outside before it leaves the drying room. Stronger drying equipment must be installed instead relying on small fans. An appropriate handling procedure must be defined and documented. The company can invest in a correct handling tool. 1145

8 Quality control must be done by each individual. Strong quality control must be enforced. The metal casting department must check if the moulds are thoroughly cleaned. V. CONCLUSIONS This project focused on improving quality for the company by identifying the root causes defects. The company was producing 10% more than required to compensate for scrap. In 2013 and 2014 alone, about R worth products were scrapped. This indicated that the quality management tools that have been put in place were not being utilised. Different problem solving tools were used to investigate and analyse the root causes defects. Three significant defects were identified, which are Dull Run, Ceramic Inclusions and Shrink. The three defects combined contribute about R to the total losses incurred by the company due to scrap. This is about 78% the total costs all scrap. It was evident that in order to reduce the costs losses, the concentration needed to be directed to these three defects. The results showed that the most common contributors to these defects were man, methods, control and machinery. Some the significant root causes identified were the lack employee training, poor quality control and lack supervision. The root causes were then analysed using the 5 Whys method. Once the root causes were analysed, a corrective action plan was developed. The plan included the responsible person and the implementation start date. Some the implementations will highly depend on the availability funds. This paper showed that by utilising problem solving techniques, manufacturing companies can identify root causes to the problems. They can be able to eliminate these root causes and thereby improve quality and productivity. These improvements do not require high capital investment. ACKNOWLEDGMENT The author would like to thank Livison Mashoko for contributing to this research. REFERENCES J. Stevenson, Production/Operations management. 4th ed., New York: McGraw-Hill, K. Ishikawa, Guide to quality control, Japan: Asian Productivity Organization, F.R. Jacobs and B.R.Chase, Operations and supply management: The core, New York: McGraw Hill International Edition, F.R. Jacobs, W.L. Bery, D.C. Whybark and T.E. Vollman, Manufacturing planning and control for supply chain management. 6th ed., New York:McGraw Hill Irwin, [5] W. Hugo, J., Badenhorst-Weiss and E.van Biljon, Supply chain management, Pretoria: Van Schaik, [6] APICS, Basics supply chain management,participant Workbook, APICS CPIM, [1] [2] [3] [4] BIOGRAPHY Zanele Mpanza is an Industrial Engineer at the Council for Scientific and Industrial Research (CSIR). She studied Industrial Engineering at the University Johannesburg. She is also a part time lecturer at the University Johannesburg in the Department Mechanical and Industrial Engineering Technology. Her research interests include supply chain management, logistics, transport, and optimization. 1146