INTRODUCTION
Problem statement
With the trend of shifting to renewable energy and reducing dependence on fossil energy, energy and charge storage is very interested and increasingly booming Mobile sectors especially electric vehicles will drive the demand for batteries, with Lithium-Ion batteries dominating the future market share Demand for batteries is forecast to grow rapidly by up to 25% per year, and hundreds of plants will be built to meet demand by 2030 (Jacky, 2022)
Research and Market has published its global battery market report for 2020-
2027 in 2021 According to this report, in the context of Covid-19, the global battery market is still quietly growing, reaching 120.4 billion in 2020, expected to increase to 279.7 billion USD in 2027, a compound annual growth of 12.8% in the period 2020-2027 The US market is estimated at $ 32.5 billion and China is estimated at $ 61.1 billion by 2027, other markets such as Japan and Canada are forecast to grow by 9% and 11.1%, respectively [1]
The battery industry in our country is currently on the rise because most devices and explosive machines need to use battery systems And batteries are one of the products with a long lifespan and today battery products are increasing Batteries are also a key part of the electric vehicle industry and are considered the "next generation growth engine" On the other hand, the price of raw materials (lithium, cobalt) is on a downward trend, contributing to the reduction of production costs Thus, the battery manufacturing industry has great potential to promote development
Of course, the fast pace of development also comes with strict competition Battery manufacturers are struggling on many aspects including sustainability; stable and competitive production process; safety; effectiveness
In the trend of global economic transformation, the pressure is increasing on companies, factories and enterprises due to competition and constant change In which, TTI (Techtronic Industries) is also a battery manufacturer in Vietnam that is
2 influenced by competition and constant changes in the battery industry And in order to improve competitiveness and respond to continuous development, TTI company as well as many businesses are always interested in the important factors such as production cost, product quality, and delivery time The goal of manufacturers is always to reduce the cost of creating products but still meet the expectations of customers, in parallel revenue and profit will be optimized
The problems in production that TTI enterprises, where students are working, can be mentioned as follows and will be described in chapter 3
- Poor quality problems, high fail rate in the production line, high return rate greater than 0.5%
- The problem of productivity and output, which does not meet the production demand Productivity meets only 88% of demands
- Waste in production, low performance
With the problems encountered in production, it is necessary to use solutions to overcome and improve the current situation Six Sigma is frequently used to approach and solve problems in manufacturing
Six Sigma is a system of process improvement methods to minimize failure, causes of failure, and variation in the process Thereby improving customer satisfaction, increasing market dominance and corporate profits and many other benefits Although Six Sigma is an approach to improving quality and processes commonly used in manufacturing, it is now also used in many other sectors such as banking, hospitals, financial services, aviation, utility services and many others [2] [3]
From the benefits of applying 6 Sigma to manufacturing activities to improving quality and improving product productivity and urgency from the difficulties of the business, the topic: " Application of Six Sigma Methodology in Battery Assembly
Line to Improve Quality and Productivity " was carried out
Research objectives
The study uses a Six Sigma method to approach battery assembly lines to improve quality and productivity.
Thesis objectives
- Analyze the current state of quality in battery assembly line, identify problems and find the causes
- Analyze the current state of productivity of the line and find the causes
- Deploy 6 Sigma by using DMAIC cycle for battery assembly line
- Record and analyze the results achieved from the Six Sigma methodology
Thesis structure
The content of the dissertation consists of 5 chapters, specifically as follows:
➢ Identify research issues and objectives
➢ Content and scope of research
- Chapter 2: Theoretical and Methodological Foundations
- Chapter 3: Analyzing the Current Situation
➢ Analyze the status and identify problems
THEORETICAL BASIS AND LITERATURE REIVEW
Definition
The International Organization for Standardization (ISO) defines, "6 Sigma is a statistically based business improvement approach that seeks to find and eliminate defects and their causes from an organization's processes, focusing on outputs that matter to customers."
2.1.2 Benefits of Six Sigma method
- Help businesses deliver goods on time
- 6 Sigma makes it easy for businesses to expand production
Six Sigma is a well-structured improvement method, which is an improvement method characterized by a 5-stage DMAIC process such as Figure 2.1 a D – Define
As the first stage of the DMAIC cycle, the "Define" Phase consists of the following main steps:
- Identify customers (internal, external) and find out the voice of customers to find products, services, processes with Critical to Quality characteristics (CTQ)
- Set up a SIPOC chart with information about the process's suppliers, inputs, processes, outputs, and customers
- Clarify each process item in the overall process
The goal of the "Define" phase is to present the problem to be solved, the scope and objectives of the project b M – Measure
This stage will measure the current performance of the process, detecting the factors that affect the process The goal of the "Measurement" phase is to quantify and understand the current state of the problem The "measurement" phase includes:
- Identify specific performance requirements related to CTQ characteristics
- Create process maps related to defined inputs and outputs Each step of the process must show the relationship between input to output
- Set a comparative milestone in terms of process capacity
- Identify areas where failure during measurement may occur of the process
- Conduct measurement and data collection of input, middle and output agents of the process c A – Analyze
This "Analyze" phase will analyze the data that has been collected in the previous phase to identify the potential root causes of the problem Several tools are used to identify the root cause and effect on the problem, such as Pareto charts, cause- and-effect charts, "5 why" methods, FMEA etc The outputs of this stage demonstrate the root causes are more likely to affect the process d I – Improve
The improvement phase includes innovative improvement and selection of optimal or near-optimal solutions for the best results and the most powerful
7 efficiency There are many tools used for the improvement process Four steps are required to carry out the improvement phase
- Evaluation and selection of solutions
During this period, guaranteed improvements need to be standardized, revised, documented The new process is monitored through indicators and measurement tools to assess the capabilities of the process over time
The sigma level is used to indicate defects per million opportunities, the purpose of six sigma is to aim for a minimum of defective products If process reach 6 sigma, it means that there are only 3.4 defects per million chances of a defect The higher the sigma level, the less defects that equate to the cost of production, higher profitability, higher quality
The DPMO estimation is used for evaluating the process performance measured in terms of attribute data such as nonconforming units or defective or defects, especially when there are multi processes with complexity of data such as continuous, attribute data in the overall process
DPMO calculation formula (Defects per million opportunity) as shown in equation 2.1
Number of defects found 1000000 DPMONo of unit No of defect opportunites per unit (2.1)
From the DPMO index, it is possible to rely on the following Table 2.1 to convert to sigma level or process yield performance It is noted that the sigma level when retrieve in short term will be shift 1.5σ in long term [4] An example is that if the sigma level of process is 6σ in short term, it could be only 4.5σ in long term
2.1.5 Quality management tools a Fishbone diagram
Fishbone diagram, also known as ISHIKAWA chart The chart is used to examine cause-and-effect relationships or analyze causes and problems and are a popular tool for improvement projects At the same time, it is also an approach and
Table 2.1: Convert DPMO with sigma level
9 set of opinions (Brainstorming), input parameter values in the multiple regression or experimental design with x – variable or parameter and y – result variable
To construct a chart as shown in Figure 2.2, the following steps should be performed:
- Step 1: Identify the problem/consequences
- Step 2: Create an analysis team
- Step 3: Draw the consequence box and centerline
- Step 4: Identify the main causes
- Step 5: Identify and classify possible causes
- Step 6: Rank the causes according to the order of causes that affect the most b Pareto chart
Pareto charts are used to show how often defects occur, used to prioritize problems that need to be addressed Focusing on addressing the most frequently occurring failure will result in the greatest improvement in results To build a Pareto chart as shown in Figure 2.3, the following steps should be performed:
- Step 1: Distribute data attributes sorted by type
- Step 2: Sort in descending frequency order from left to right
- Step 3: Help detect the most common failure
- Step 4: Distinguish the most common error from the most important one c Histogram
Histogram charts are used to measure the frequency of a certain problem, clearly showing the change and fluctuation of a data set Example on the image Figure 2.4 in terms of product diameter dimensions
The principle of quality control is to capture the causes of quality fluctuations and to manage those factors For this purpose, it is necessary to know the fluctuation (distribution) of specific data properly Through the layout of the data on a distribution chart, we can understand the whole objectively
The purpose of setting up a distribution chart is to:
- Knowing the distribution shape in the form of a graph makes it easier to understand
- Know the process capacity compared to standards (technical regulations)
- Analyze the process and manage it
- Know the center and fluctuations of distribution
And from the above information the user can:
- Spot problems and set up improvement programs
- Consider which actions are effective
- Affirmation of the results of actions d Scatter Diagram
A scatter chart is used to determine whether there is a regular dispersion trend in the position of points
To draw a scatter chart as shown on Figure 2.5 needs:
1 Collect data that needs to be found (at least 30 data)
- Select data for the horizontal and vertical axes, write the ratio of the axes: When the 2 axes have a causal relationship, the horizontal axis is the cause and the vertical axis is the result Represent the data pair on the chart with a dot: if there is the same or duplicate data, mark "O" at that point
- Fill in the topic: what problem survey, survey date, number of samples, creator, how to collect data
People use scatter charts to:
- A scatter chart is used to determine whether a correlation exists between two characteristics by marking pairs of figures on the X-Y coordinate system or marking 1 characteristic on the Y axis and other characteristics on the X-axis
- Determine the level of correlation based on a scatter chart
- Based on scatter chart analysis, r-factor and regression equation, quality characteristics and process characteristics can be predicted, as well as identify factors that need to be strictly controlled to ensure product quality
2.1.6 Line balancing by Takt time a Definition
Production balance is a method of arranging and arranging work parts to ensure customer needs, using human resources and equipment with the highest efficiency b Benefit
The benefits of pass balancing are known through many practical studies, including important benefits such as:
- Minimize the number of workstations
- Maximum efficiency of resource use
- Minimum waiting time between stages
There is a lot of research on the application of six sigma solutions in improving production quality in particular and improving processes in general Some of the research related to the topic being carried out is mentioned as follows
DEFINE PHASE
About TTI company
Name: Techtronic Industries Vietnam Manufacturing Co., Ltd
Location: 9A VSIP II-A, Street 27, Vietnam – Singapore Industrial Park II-A, Vinh
Tan Commune, Tan Uyen Town, Binh Duong Province, Vietnam
Techtronic Industries is a leading global company with rapid growth in the field of power tools, accessories, hand tools, outdoor electrical equipment, self-care vehicle floor (DIY), specialized equipment for consumers and industry in the renovation and repair industry, maintenance, construction of houses and infrastructure The company is committed to accelerating the pace of industry transformation through eco-friendly wireless superior technology TTI's brands such as MILWAUKEE, RYOBI and HOOVER are recognized worldwide for their longstanding heritages of wireless product foundations with outstanding quality, optimal performance, absolute safety, high productivity and absolutely convincing innovation
TTI is a global leader in wireless technology including Machine Tools, Outdoor Electrical Equipment, Floor Care Home Appliances and Spare Parts for consumers, professional users and people working in the household, construction, maintenance, industrial and infrastructure development industries With the goal of implementing innovation throughout the company, we have achieved a leading position in the field of wireless technology with all kinds of products that are changing the way we work in these industries
Value is the foundation for us to make all decisions and a guideline for all interactions inside and outside the company We strive to ensure that values are enforced at every level of the organization, helping to create trusted relationships and long-term brand loyalty
- Treat each other with integrity and respect
- The spirit of encouraging innovation
From the very first day of establishment, Outstanding Quality has been the focus of TTI It is important for TTI's employees to understand how the company approaches quality holistically; Understanding quality delves into every element that constitutes our organization, from the design phase to product manufacturing, from the hiring process to ongoing leadership development training, and from the supply chain implementation phase to daily interactions with consumers and professionals Quality is the DNA of TTI culture
TTI's products are diverse including handheld products such as chainsaws, blowers, lawn mowers, drills, in addition to providing products such as batteries and charging accessories, etc The Figure 3.1 below provides pictures of products manufactured at the lines at TTI company
As shown in the previous section, TTI's products are very diverse from outdoor products to indoor products And the special feature is that these products mostly need battery power sources for the product to work Figure 3.2 shows the connection between the products at the enterprise and the battery product The battery product line of the enterprise is a product line that accounts for a large output and is a complementary product to other products Play an important role in the production of the company
Figure 3.1: Some products of the company
The manufacturing process of battery products is described in Table 3.1 There are several stations in the battery line with the corresponding number of operators to do the tasks The flow of production is going directly from each station to each station in the line except sub station No.4 is different There are totally 9 stations in the battery assembly process and 14 worker including the material feeder
Figure 3.2: Batteries and battery families
Sub Main Sub Phụ Chính Phụ
Functional testing Kiểm tra chức năng
Paste label into housing Dán tem vào thân pin
Paste label, packing battery into box
Kiểm tra ngoại quan, Dán tem và Đóng gói
Print code on battery cover and material handler
Khắc code cho vỏ pin và cấp nguyên vật liệu
Tin bead inspection; performance test Kiểm tra hiệu năng PCBA
Stick foam; put in the battery cover Dán xốp, lắp cụm pin vào vỏ
Balance test; battery installation; cover battery holder
Kiểm tra cân bằng/Lắp pin vào khung
Install the connecting piece; install the battery to the welding fixture Lắp miếng kết nối/Hàn tự động
PCBA Kiểm tra mối hàn/Lắp PCBA
Process no Process Description Mô tả quy trình
The dimension of line is not so big with the size of 8.6 meters length and 4 meters width The detail of layout can be referred in the in Figure 3.3 The layout follows U shape of process
3.1.5 Battery assembly process a Station 1: Sorting
The batteries will be fed into the machine for inspection as shown in Figure 3.4 The output is batteries with an internal voltage and resistance level that is within the standard The good batteries from the machine will be assembled into battery-free trays as shown in Figure 3.5 then locked 4 screws b Station 2: Assemble the strap and operate the welding machine
The battery assembly will be fitted with nickel metal pieces on both sides such as Figure 3.7 and assembled into jigs to be put into the automatic welding machine Figure 3.7 The automatic welding machine with a high current of 3.5KA in about some milisecond will tightly weld the metal piece and the battery to create a bond for the battery assembly and connecting pieces
Figure 3.5: Assemble the cell into carrier
Figure 3.7: Attach the metal pieces and put them into the automatic welding machine
24 c Station 3: Welding check and install PCBA
The battery assembly after welding from the automatic welding machine will be checked for weld quality, then attach the PCBA and continue to put into the automatic lead welding machine as shown in Figure 3.8 and Figure 3.9
Figure 3.8: Install the PCBA to the cells
At this station, the battery case is engraved with the characteristic characters of the product from another machine as Figure 3.10 And at the same time workers at this station will supply materials to all stations in the line e Station 5: Performance test
After soldeing station Solder joints will be inspected by workers and remove solder balls The battery assembly will then be fed into the performance tester on Figure 3.11 The purpose is to test the performance of the battery assemblies, the performance of the PCBA and the voltage and resistance standards of the battery block after welding
26 f Station 6: Attach foams, gasket sealing
The pack will be attached the foams and sealing gasket, then cover with housing as shown in Figure 3.12 g Station 7: Function test
The battery assembly will continue to be fed into the functional tester as shown in Figure 3.13 to conduct a series of functional measurements and evaluations from the machine Products undergoing this test can be used with full functionality as required
Figure 3.12: Attach foams and gasket seal
Products are labeled around as shown in Figure 3.14 i Station 9: Appearance check
At this station, the product will be checked for bonding and some of the basic functions, which will then be packaged
As showed in Figure 3.15, the SIPOC diagram show all related elements of the battery production Based on the diagram, we can easily see how the process serve the customer with outputs from inputs and suppliers
IMPLEMENT SIX SIGMA DMAIC
Measure
The measurement plan was set up and the data is recorded By having data, DPMO chart is built from May to July 2022 in Figure 4.1 to have general view of the process performance There was average of 7197 DPMO from May to July 2022 The DPMO is calculated by using the equation 2.1 as described in chapter 2 To have the sigma level, there is positible to use sigma conversion table described in Table 2.1, the other way to calculate the sigma level is described in the equation 4.1 The sigma level is 3.95 in May-July [10]
Figure 4.1: Internal DPMO (May-July 2022)
The pareto chart is set up based on the raw failure data then By having the pareto drawing in the Figure 4.2, we can find the top 2 defects with highest quantity of defects Those are welding failure and pack voltage failure Those 2 top failures accounted for 71% of total failures And it is obviously proved that solving those top failures will help to get a high reduction of defects.
Analyze
To analyze the top issue, a set of tools in Table 4.1 would be used to find the potential root causes and solve the causes
Fail Pack voltage Fishbone, Brain storming, 5 why, Capability Analysis
Welding failures Fishbone, Brain storming, 5 why, DOE
Figure 4.2: Pareto of top defects
The below Figure 4.3 shows the appearance issue of welding failure After welding, by having the inspection from operator, the failure will be captured and on hold for investigation Those symptoms are over welding, under welding, burn appearance, missing welding and the welding joint is easy to peel off This welding application is very critical to the battery since it play a role to make sure the cell pin and straps are connected well and perform the conductive material for the electricity battery system
Fist thing to do is to have all members in production line contributing the brainstorming for potential causes After that, ideas are collected and described in the first fishbone diagram as shown in Figure 4.4
Figure 4.4: Fishbone diagram for welding failures
The next step is to narrow the interested area or factors by have some more analysis Some causes are eliminated because of explanation shown in Table 4.2
Causes Explanation for narrowing potential cause
Environment - Dust, humidity, particle are controlled in factory by ventilation and close area
People - Operator is certified by trainer (QE, IE)
- IPQC in line to audit correct operation of worker
- There is poke yoke and fixture for operator when assembling before inputting to welding machine
- Automation welding, Operator only input sub product Material - There is clear requirement (drawing, SER Sample) to suppliers
- Thickness and dimension are controlled and inspected by incoming quality of factory
After eliminating some causes, to analyze on the remaining causes, there is needed to get the consults from managers combined with using 5 why technique to look for potential root cause The application of 5 why technique to have deep root causes shown in Table 4.3 The fishbone diagram is updated with root causses highlighted in Figure 4.5
Table 4.3: 5 why technique for welding failure
Misalignment of machine and fixture when welding
Out of position after long time operation
There is not verification for alignment after PM
Welding time is random set in a large range given by machine supplier
There is not optimal welding time setting
Welding time is random set in a large range given by machine supplier
There is not optimal welding current setting
Not proper press force Press force is too high or low
There is setting range but no optimal press force setting
Welding head is not ground and clean effectively
Welding head is clean and grind only when
Welding head is ground manually and need scheduled down time
The Figure 4.5 showed high potential causes with following explanation
There is not optimal welding time setting, current setting, press force setting The setting of those parameters strongly affects to the welding process To have the welding finished, the welding tip will contact to the welding point by giving a press force to the point, then the machine will release a strong electric current which go through and heat up the material in some milliseconds, the materials (battery cell and strap) then will be melted and connected to each other Any changes on the parameters will affect to the welding failures Those parameters are not set up following any optimal setting
There is not verification of alignment after PM for welding machine The mechanical system is very important in the welding process If the mechanical parts are not aligned and verified after fixing or repairing, the process will not be stable The battery pack must be fixed and steady during welding Since the preventive maintenance is not showing any verification of alignment for the welding machine
The welding tips is serviced by manually and not on time The welding tip is also playing a very important role in welding process If the tips is dirty or worn out, then the electric current will be block when moving from the tip to the welding point and the current is not maintained stably during welding
Figure 4.5: Fishbone with highlighted causes
The same method that was explained in the previous section The team did the same thing to have the activities of brain storming, multi voting, eliminating causes, consulting, using 5 why technique
The fishbone diagram for pack voltage failure is shown in Figure 4.6 There are suspected causes described from 5 major aspect including people, machine, material, method, environment
- People: the physical contact to PCBA can cause the damage of components because of physical force or electrostatic discharge current Operators skip cleaning task which will let the solder balls exist on the PCBA, solder balls can cause the short circuit on components leading to voltage failure
- Machine: If the machine is not doing a good soldering process, then the soldering joint would be lack of solder or over soldering or there are many solder balls generated from the process The PCBA function would be affected Any misalignment, wire damage, current leakage can cause the failing of battery pack and affect to the voltage measurement
- Material: if the input material such as PCBA, strap, cell, solder wire are defective, the battery pack would not function as expected and voltage failure will occur
- Environment: dust and moisture can affect to measurement process, those also affect to the battery pack function including the voltage
- Method: the lack of cleaning instruction, calibration of temperature at soldering can cause of missing cleaning soldering balls and bad soldering quality Soldering balls as mentioned would affect to the function of PCBA of battery pack
There are many suspect root causes, the next step is to continue analyzing the causes and remove unnecessary causes Some causes are eliminated as explanation in Table 4.4 The environment, people, method are not necessary to be focused The remaining causes are machine and material
Table 4.4: Eliminate causes for pack voltage fail
Causes Explanation for narrowing potential cause
Environment Dust, humidity and particle are controlled in factory by ventilation and close area Machines also were covered
People - Operator is certified by trainer (QE, IE)
- IPQC in line to audit correct operation of worker
- There is poke yoke and fixture for operator when assembling before inputting to welding machine
- Automation welding, Operator only input sub product
Method Inspection and checking don’t directly cause the failure
Figure 4.6: Fishbone diagram for pack voltage fail
To continue analyze the remaining causes, The is application of 5 why technique to have deeper root causes as described in Table 4.5 The reason for having solder ball on PCBA because all solder balls created during soldering are not cleaned, the fixture doesn’t cover effectively and the machine generated so many solder balls
The fishbone diagram then was updated with highlighted potential root causes as shown in Figure 4.7
Table 4.5: 5 why technique for pack voltage failure
The Figure 4.7 showed the high potential causes for the pack voltage failure with following explanation
So many solder balls generated during soldering, not cleaning all solder balls, fixture doesn’t cover the PCBA effectively Those solder balls would affect and cause the short circuit of PCBA and affect to the voltage of battery pack
The testing probe is not aligned during preventive maintenance activity, there are a lot of needles pressing to PCBA if those needles are not aligned properly every period, the probe can damage the PCBA components and cause the voltage failure
Cell supplier issue, the cell play an important role to the total voltage of battery pack If the voltage of each cell is not maintained by a good physical quality, the battery pack voltage would be affected
- Not cleaning all solder balls
- Fixture doesn’t cover PCBA effectively when soldering
- So many solder balls created during soldering
Testing probe misalignment and oxidation
- Testing probe is not in the check list when
PM Defective cells - Supplier issue
4.2.3 Gap between demand and actual output
The fishbone diagram to understand appearing gap between the demand and actual output is shown in the Figure 4.8, some explanations for not addressing causes are shown in the Table 4.6
Table 4.6: Explanations for eliminating causes of gap
Causes Explanation for narrowing potential cause
Machine Unscheduled downtime for machine < 3%
People - Operator is certified and given certificate by trainer (QE, IE)
- IPQC in line to audit correct operation of worker
Material - Unscheduled downtime for material waiting is not high
Figure 4.7: Highlighted causes for pack voltage fail
Improve
By having high potential root causes from analysis section The improvement plan for those failures was given as shown in Table 4.7 to address to potential root causes The improvement will set up to solve each potential cause and to be assigned to related department The improvement actions also organized by each failure
Figure 4.8: Fishbone diagram for gap between demand and actual output
Y Why - Why Answer Improvement plan Person In charge
- Not cleaning all solder balls
- So many solder balls created during soldering
- Standardize the cleaning process after soldering, update to work instruction
- Improve the fixture cover PCBA
- Grove the solder wire to reduce the solder ball created
- Soldering tip is replaced daily
- Testing probe is not in the check list when PM
Alignment task is added to daily check list
Check and replace the test probe when pm time PIE Engineer
Cell Supplier issue Analyze the CPK of supplier PIE Engineer, Quality
There is not verification for alignment after PM
Gemba and define critical point of alignment, update to PM check list
There is not optimal welding time setting DOE study, update setting form PIE engineer, Quality
There is not optimal welding current setting DOE study, update setting form PIE engineer, Quality
There is setting range but no optimal press force setting DOE study, update setting form PIE engineer, Quality
Engineer Welding head is ground manually and need scheduled down time
Have the grinding by automation set the frequency of grinding PIE engineer,
Gap between demand and actual output
Current bottle neck is not meet the takt time
Do line balancing and reduce bottle neck by finding improvement opportunity
PIE engineer Low line balancing rate
There are many activities to be proceeded so there is a need of organizing the work by sorting by priority Hence, the priority table is built to calculate the priority of each activity base on the easiness and cost of actions Priority table are shown in Table 4.8
Table 4.8: Priority for improvement actions
# Defect type Improvement plan Cost Easiness Cost x
Gemba and define critical point of alignment, update to PM check list 4 4 16 3
Failure DOE study, update setting form 3 2 6 10
Set up the grinding by automation set the frequency of grinding 4 2 8 9
Failure Improve the fixture cover PCBA 3 3 9 8
Standardize the cleaning process after soldering, update to work instruction 5 5 25 1
Grove the solder wire to reduce the solder ball created 3 4 12 5
Failure Soldering tip is replaced daily 5 4 20 2
Alignment task is added to daily check list
Check and replace the test probe when pm time
Failure CPK to analyze the cell supplier 5 3 15 4
Gap between demand and actual output
Do line balancing and reduce bottle neck by finding improvement opportunity
The Table 4.10 and Table 4.9 show how the value is given for the Table 4.8 by evaluating the easiness and the cost of activity The improvement action would be ranked base on easiness and cost of action then the value is used for the Table 4.8
After having the priority table, all the activities are organized to proceed from the highest priority number to lowest priority number The next section would show how the improvement actions are in detail
The Gemba activity to find some weak points which affect to the alignment of the welding machine The mechanical alignment is one of important factor which would assure all the cell pack is in right place and all the points are welded in correct positions, otherwise the welding position is incorrect and leading to welding failure Those finding and actions to resolve the finding are shown in Figure 4.9
4 Cost between 1 and 5 mil VND
3 Cost between 5 and 20 mil VND
2 Cost between 20 and 50 mil VND
The welding tip also play an important role for welding process, the current situation is that the welding tips are cleaned and serviced not frequently, which make the bad quality of tip and weaken the welding process One of the solutions to enhance the quality of welding tips is to set up the automatic grinding described in Figure 4.10
- The welding head is often observed having bavia
- The tip is taken out from machine manually by maintenance technician every shift 12hrs to be serviced (clean and grind)
- The automatically grinding is set up by having sandpaper and brush in machine Machine automatically move the tip to grind every 3 hrs.
- Check the welding head in the beginning of shift
1 The locating pin in the plate which hold the mold is losing.
Add a lock screw to fix the locating pin
2 The locating hole in mold seem to be worn out
Add to weekly PM check list
3 Mold and plate are not tight connection
Add to weekly PM check list
Figure 4.9: Doing Gemba to improve alignment
Figure 4.10: Set up automatic grinding for welding tip c Full Factorial DOE for welding machines
The parameters of machines are not optimum value, current situation is that the setting in the machine is not following any suggested value as long as the setting is in the limit Since the setting limit is a big range and there is a need to find the optimum set of parameters which lead to stable welding process The DOE is a good tool to address the improvement Full factorial DOE is used to determine the main effects, interaction on the response and find the optimal setting
The DOE carry out 3 factors of the experiment and number of levels corresponding to each factor That information is described in the Table 4.11 The welding process is controlled by time, current and force Those setting are very critical to the process
The limit spec for welding time is from 3 to 5 milliseconds, the welding current is from 3.1 to 3.4 KA, the press force is from 5.5 to 6.5 kgf Those specifications are suggested from the machine supplier
Y Cause Level 1 Level 2 Level 3 Level 4
From the inputs as above, the treatment of DOE is generated as shown in the Table 4.12 the treatment is created by the Minitab software It is seen that there are
36 run orders need to be executed for 36 set of parameters This table is going to be referenced to set up the parameters every 6 hours to get the failure percentage from the welding machine
Run Order Welding Time Welding
After having the treatment, there were a run order which execute each set of value for machine setting The execution of each set was done with duration of 6 hours The results of welding failure percentage are recorded from the production line, then the data was input to the Table 4.13 in the column “Welding Failure Percentage” The next step is going to use this data table for the regression analysis
The data is inputted to Response Surface Regression Model in Minitab The analyzing on the regression model would be described next The first regression model is generated from the Minitab is shown in the Figure 4.11 Based on the p- value of each term, we would continue to remove one at a time for terms which p- value < 0.05 in this case the terms “Welding Time * Press force “ and “ Welding Current * Press force” would be removed out of the model
The task is to remove unsignificant terms and rerun the regression analysis in Minitab The best regression model is showed in the Figure 4.12 The welding time, welding current, press force all has a strong significance on the voltage failure It is
Figure 4.12: Terms after removing insignificantly terms
53 reasonable since any changes on time, current, press force would create over welding or under welding
The best regression equation is shown in Figure 4.13 the regression model not only described by welding time, welding current, press force individually but also reflected by the interaction of the welding current and welding time The affect of the welding time to the pack voltage based on the value of welding current the model also showed that the square value of welding time, welding current, press force affect to the failure
CONCLUSION AND SUGGESTION
Conclusion and discussion
The project focus on application of DMAIC methodology in the battery assembly line to improve quality and productivity In around 6 months, the project is carrying out The thesis researched knowledge of Six Sigma methodology and defined the status of productivity and quality in the battery assembly production line, DMAIC methodology is applied in the battery assembly line to improve quality and production in the battery assembly line The results are recorded and assessed Some results are listed as below:
- Sigma level of overall production line improved from 3.95σ to 4.12σ
- DPMO reduced from 7120 in July to 3982 in Nov 2022
- Production output capacity increased from 76 pcs/h to 81 pcs/h
- Production output attainment of plan increased from 88% in Aug to 96% in Nov 2022
Suggestion
The project is successful in battery assembly line Based on this experience and reference, the project can be applied to other product line in the company Increasing the scope of the project to other area can increase more value of project by improving quality and productivity in other production area of the company
The sigma level reached 4.12 but it can be improved by continuing the DMAIC cycle and focus on the remaining defects Since the thesis focused only on 2 top defects which are welding failure and pack voltage failure
The control activity needs to be maintained as mentioned in the control phase
The schedule attainment is 96% even reached the target of the project but this number is not perfect which needs to improve more to meet the demand in the company The project still needs to focus more on closing the gap between demand and the actual output To do so, the suggestion is to perform more deeply measurement and investigation on the gap between demand and the actual output
[1] "Battery: Global Strategic Business Report." Internet: https://www.researchandmarkets.com/reports/5303140/battery-global- strategic-business-report, Nov 25, 2022
[2] D Pohekar and R S Reosekar, "Six Sigma methodology: a structured review," International Journal of Lean Six Sigma, vol 5, iss 4, pp 392 -
[3] J Antony et al., "Six Sigma in service organisations: benefits, , challenges and difficulties, common myths, empirical observations and success factors," International Journal of Quality and Reliability Management, vol
[4] M J Harry, The Vision of Six Sigma: A Roadmap for Breakthrough, Sigma Publishing Company, 1998
[5] S Nataraj and M Ismail, "Quality enhancement through first pass yield using statistical process control," International Journal of Productivity and
[6] D A Desai et al., "Reducing failure rate at high voltage (HV) testing of insulator using Six Sigma methodology," International Journal of Productivity and Performance Management, vol 67, no 5, pp 791-808, 2018
[7] P A D A Marques and R Matthé, "Six Sigma DMAIC project to improve the performance of an aluminum die casting operation in Portugal,"
International Journal of Quality & Reliability Management, vol 34, no 2, p 307–330, 2017
[8] L D Tran, "Cải tiến chất lượng chuyền lắp ráp thiết bị âm thanh theo quy trình DMAIC," Master thesis, Ho Chi Minh University of Technology,
[9] N Q Thai, "Nghiên cứu và ứng dụng Lean Six Sigma trong hế thống dịch vụ tại công ty TNHH MAERSK Việt Nam," Master thesis, Ho Chi Minh University of Technology, 2011
[10] F W Breyfogle, Implementing Six Sigma, John Wiley & Sons, 2003.