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Materials Today: Proceedings 2 ( 2015 ) 3429 3437

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2214-7853 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization.doi: 10.1016/j.matpr.2015.07.318

4th International Conference on Materials Processing and Characterization

Process Improvement using Lean Principles on the Manufacturing of Wind Turbine Components a Case Study

Orville Sutari*

Faculty of Mechanical Engineering, St. Joseph Engineering College, Mangalore, 575028, India

Abstract

This research paper focuses on process improvement carried out in the nacelle unit of a wind turbine manufacturer. Lean Manufacturing (LM) is implemented to improve productivity in terms of manufacturing throughput time and to also reduce cost of rework/manufacturing costs. Tools like Root Cause Analysis and Kaizen were used in conjunction to create efficient results for the manufacturing processes. By assessing the current condition and establishing a baseline to work upon, improvement was achieved through the testing and implementation of solutions to the problems found. Process improvement was successful as seen in the acceleration of schedules and reduction in costs. 2014 The Authors. Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization.

Keywords: Kaizen; Lean Manufacturing; Process Improvement; Root Cause Analysis

1. Introduction

In todays competitive world, competitors can hire the same people, buy the same materials and, unless they build their own equipment (which could be a competitive advantage), buy the same machines. One interesting aspect of this equation is that the outcomes are dependent variables. One can vary the inputs and the methods but the outcomes will be the result of these, not something that one can independently manage and control. It is only the processes, procedures and policies that make up methods that are unique to a certain business throughout all its various departments. The key to differentiating any company is where process improvement projects are concentrated.

* Corresponding author. E-mail address: [email protected]

2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on MaterialsProcessing and Characterization.

3430 Orville Sutari et al. / Materials Today: Proceedings 2 ( 2015 ) 3429 3437

Process improvement is the systematic approach to closing of process or system performance gaps through streamlining and cycle time reduction, and identification and elimination of causes of below specifications quality, process variation, and non-value adding activities. Conducting process improvement will help give an advantage to any organization. When it comes to methodologies and techniques that are an integral part of this improvement ideology, LM, often simply, lean, usually comes to mind.

Lean is a set of tools that assist in the identification and elimination of waste that might improve quality as well as production time and cost [1]. Lean, when properly implemented, allows manufacturers to build quality products faster and more efficiently [2]. Many manufacturing companies have implemented LM in many different ways to suit their needs [3]. Lean is seen as a highly versatile tool and is adopted over a diverse range of work environments as seen in earlier works of research [4-7].

2. Problem Overview

This research work was carried out in a low volume manufacturing unit that produces components of wind turbines, namely the nacelle and the nose cone. These components comprise mainly of Glass Fiber Reinforced Plastic (GFRP) composite material that are manufactured using hand layup method, a technique that is labour intensive. As a result of budget cuts effective from 2011 there has been a substantial reduction in manpower. Therefore, proper utilization of the available manpower had become very essential.

The work standard for manufacturing one set of nacelle and nose cone is 1815 man-hours. The current man-hours consumption exceeds this standard and needs to be quantified and then improved upon.

The average cost of rework for one set of nacelle and nose cone was 50,000 INR (nacelle - 35,000 INR and nose cone - 15,000 INR). During the period of October 2012 to March 2013, it accounted for nearly 5% of the total manufacturing cost.

The main objectives were to:

x Improve the manufacturing throughput time. x Reduce rework/manufacturing costs. 3. Methodology

3.1. Time study for nacelle and nose cone

The actual total amount of work for manufacturing one set of nacelle and nose cone is calculated as 2260 man-hours. The man-hours for the individual processes in manufacturing are displayed in Table 1.

3.2. Data gathering

It is seen that there are many defects in the products due to which rework has to be carried out. It should also be noted that the Dry Finishing process which involves painting the outside surfaces of the nacelle and nose cone is a non-value adding one. It only gives a good surface appearance and does not add any functional purpose to the product. Dry Finishing is done due to the presence of many surface defects, which even after rework, looks displeasing and is not upto customer standards. This process can be altogether eliminated by tackling the problem of defects, thus improving manufacturing throughput time substantially.

To identify which defects need to be tackled first, pareto charts are drawn. Pareto analysis highlights the problems that require greater attention.

Data was collected from the Non-Conformity Report (NCR) for 5 sets of nacelle and nose cone. It revealed the type of defects and the areas in which they persist. It also gave the frequency of these defects. The data revealed that nearly 75% of the defects are concentrated in the nacelle product.

Table 1. Time study for nacelle and nose cone

Product Process Manpower Man-hours


Nacelle

1. Material Preparation 8 96 2. Moulding 27 945 3. Wet Finishing 22 242 4. Dry Finishing 9 306 5. Assembly 4 64 6. Electrical and Dispatch 4 40 7. Child Parts 14 224 Total 88 1917

Nose- Cone

1. Material Preparation 8 16 2. Moulding 6 114 3. Wet Finishing 7 63 4. Dry Finishing 4 80 5. Assembly and Dispatch 5 70 Total 30 343

TOTAL (Nacelle + Nose Cone) 118 2260

3.3. Assessment

3.3.1 Pareto Charts

To identify which defects need to be tackled first, a pareto chart seen in Fig. 1. and Fig. 2., is drawn using the data collected from the non-conformity reports mentioned earlier. Pareto analysis highlights the problems that need greater attention and further analysis can be carried out on them.

Fig. 1. Inprocess NCR analysis for nacelle

Fig. 2. Inprocess NCR analysis for nose cone


Fig. 3. Ishikawa diagram for elephant skin defect

Fig. 4. Ishikawa diagram for air pocket defect

There are 3 defects that majorly contribute to both the nacelle and nose cone as a combined set:

x Air pocket in gel coat x Air pocket in laminate x Elephant skin

Eliminating these 3 defects will substantially reduce manufacturing time and are therefore focused on first.

3.3.2 Ishikawa diagrams

Ishikawa diagrams seen in Fig.3. and Fig. 4 are used to find out probable causes to the defects outlined in the pareto analysis.

The major causes for elephant skin were narrowed down to:


x Gel coat applied is too thin. x Insufficient catalyst in gel coat resulting in undercuring. x Back-up resin is applied too soon leading to styrene attack on gel coat.

The major causes for air pocket in gel coat and laminate were narrowed down to:

x Right angles at mould flanges, thus providing less area for contact. x Improper layering and rollering during hand lay-up. x Less thixotropy of resin causing it to flow out of the glass mat. 3.4 Corrective and improvement actions

3.4.1 Eliminating air pocket in gel coat

The air pocket in gel coat defect, shown in Fig. 5., is observed at corner (sharp angles) areas of nacelle and nose cone body. Upon performing a knock test on these areas, the gel coat breaks away if air pockets exist. This is caused due improper bonding between the gel coat and the first layer of glass mat.

A small amount of thixotropic resin paste (fumed silica filler) was applied along the corner areas of the nacelle mould over the gel coat layer; this creates more surface for contact between the gel coat and the subsequent layers of glass fiber mats. Figure. 6. shows the application of the resin paste in one of the mould corner areas.

To ensure proper bonding, the first glass layer is laminated onto the mould prior to the complete curing of the resin paste. Subsequent layers are then laminated as per normal work procedures. The de-moulded product showed no air pocket defects in the gel coat after performing a knock test. Figure. 7. shows the same corner area as in the previous figure, free of defects.

Fig. 5. Close-up photo of air pocket in gel coat Fig. 6. Application of thixotropic resin paste

Fig. 7. Defect-free gel coat on corner area of nacelle


Fig. 8. Close-up photo of elephant skin defect Fig. 9. Defect-free gel coat of nose cone

3.4.2 Eliminating elephant skin defect

In the existing work procedure, the amount of catalyst used in the gel coat was in the range of 0.8 1% by weight of gel coat and was found to be on the lower side. This increased the time for curing and caused any uncured areas of the gel coat to wrinkle due to chemical attack from the styrene in the back-up resin. This is illustrated in Fig. 8.

The amount of catalyst dosage was revised to 1.5 1.7% by weight after studying the materials technical data sheets to ensure in maintaining the integrity of the gel coat.

Gel coat application was made as per normal working procedures and proper wet-film thickness was checked to comply with customer specifications.

The elephant skin was completely eliminated from the product as seen in Fig. 9. 3.4.3 Trial for using laminating resin as paint

The purpose of the trial was to verify the use of pigmented laminating resin instead of the existing PolyUrethane

(PU) paint coating system for the inside surfaces (non-gel coat side) of the nacelle and nose cone. In doing this there can be substantial reductions made in manufacturing costs as the PU paint is more expensive than the laminating resin, thus eliminating a waste of over processing.

The trials were carried out on scrap GFRP cut-outs. The pigment RAL 7035 (as per customer specifications) was mixed into the laminating resin at 10% by weight. Complete coverage in terms of opacity was achieved in 2 coats each having a thickness of 80-100 microns.

Fig. 10. Shade card matching of pigmented laminating resin


The result was a slightly glossy, defect-free coat that matched with the shade of colour required and fully met with customer requirements. This is illustrated in Fig. 10. 3.4.4 Work technique modification to reduce resin consumption

Resin and glass consumption was recorded and the resin to glass ratios was calculated for the moulding of 5 nacelles for the existing working conditions. Table 2. shows this data. In the current work method the glass mat is placed on the mould first and then the laminating resin is applied onto it.

Table 2. Recorded data for the existing work method

Resin weight (kg)

Glass weight (kg) Resin to glass ratio

1555.3 605.8 2.57:1 1542.1 608.6 2.53:1 1546.6 610.2 2.53:1 1537.8 604.2 2.55:1 1549.4 606.7 2.55:1

The modified technique required for the laminating resin to be applied first, generously over the gel coat. The first

layer of glass mat is smoothed into position. Efficient wetting of the glass mat takes place from the bottom to up and using excess resin is avoided. This is followed through for all subsequent layers.

Table 3. Recorded data for the modified work technique

Resin weight (kg)

Glass weight (kg) Resin to glass ratio

1352.4 607.9 2.22:1 1356.6 609.1 2.23:1 1309.1 609.2 2.15:1 1344.5 604.5 2.22:1 1317.5 607.1 2.17:1

The average resin to glass ratio for the existing work state was 2.55:1. After the workers were given instructions on how to correctly wet the glass mats from the bottom to up, the resin to glass ratio achieved was 2.2:1. Table 3. shows the data of the improved resin to glass ratios.

3.4.5 Adding filler to the laminating resin

A major concern during moulding is the draining out of resin from the glass mat layer against large vertical mould surfaces. The solution proposed was to add a desirable quantity of AEROSIL 200 fumed silica filler to the laminating resin. This would give it better thixotropy and thus the draining out of resin could be avoided. [8] shows the effect of filler material on the mechanical properties of GFRP.

Fig. 11. Effect of silica content on tensile strength Fig. 12. Effect of silica content on tensile modulus


To see if the idea was viable, the tensile properties of the GFRP laminate with the added fumed silica filler had to be checked. The desired amount of thixotropy could be achieved well within 2% by weight of resin, of the added filler.

The components of the GFRP system are isophthalic unsaturated polyester resin, cobalt octoate accelerator, methyl ethyl ketone peroxide catalyst and e-glass type chopped strand mat fibres.

For the purpose of testing, 3 different compositions of laminate were prepared. Fumed silica was added in the proportions of 0% (i.e. no added silica), 1% and 2% by weight of resin. The samples were fabricated using hand lay-up technique and tensile tests carried out using ASTM 3039 standards.

There is improvement in the tensile properties of the specimen by 6% and 21% for the addition of 1% and 2% by weight of fumed silica respectively as shown in Fig. 11 and Fig. 12. This positively verifies the use of fumed silica filler in the laminating resin.

3.4.6 Kaizen for console bracket area

For the main body of the nacelle, it is seen that there is substantial over processing in the assembly of a steel bracket onto a steel bed plate insert. The bed plate is an insert of the nacelle laminate which is incorporated during the moulding process, in other words, the bed plate is sandwiched between layers of composite material. Each nacelle has a total of 6 bed plate inserts.

The bracket has a flat base that is provided with slots through which the studs on the bed plate pass. Assembly is completed by tightening nuts over each stud to securely fasten the console bracket onto the laminate.

In the existing state there is excess work done to facilitate the assembly of the bracket after the moulding process. This involves clearing the area around the studs of excess accumulated composite material and levelling of the bed plate area for zero clearance fitting of the bracket, both of which were very time consuming operations. The Kaizen was implemented where both these problems could be taken care of during the moulding process itself.

For this improvement, an acrylic sheet having the same dimensions as that of the base plate and drilled holes at the locations of the studs was placed onto the console bed plate laminate area while still in the wet (uncured) state. In doing so the studs were free from composite material accumulating around them, by pushing the wet laminate down into place. The console bracket is then placed on the acrylic sheet to serve as a weight, this would also ensure zero clearance between the bracket and laminate later on. After the laminate is fully cured the bracket and acrylic sheet are removed. After this step no post moulding work is required to ensure proper assembly of the console bracket. Before and after photos of the kaizen is shown in Fig. 13. and Fig. 14.

Fig. 13. Before Kaizen Fig. 14. After Kaizen


4. Conclusions

4.1. Manufacturing throughput time reduction

The benefits of lean implementation with regard to manufacturing time are:

x The amount of rework saved by eliminating all surface defects like air pockets in gel coat and elephant skin, for 1 set of nacelle and nose cone is 15 man-hours.

x Eliminating all surface defects helped us in eliminating the entire non-value adding process of Dry Finishing, which saves 386 man-hours for 1 set of nacelle and nose cone.

x Kaizen implementation reduced amount of work in nacelle by 16 man-hours. 4.2. Manufacturing costs reduction

The benefits of lean implementation with regard to the manufacturing costs are:

x Manufacturing costs saved through avoiding rework and repairs is 50,000 INR for 1 set of nacelle and nose cone. x Material cost saving through using pigmented laminating resin in place of PU paint for 1 set of nacelle and nose

cone is 25,000 INR. x After implementing new work technique in moulding there is a saving of 450 kg of laminating resin translating to

material cost saving of 67,500 INR for 1 set of nacelle and nose cone.

4.3. Overall summary of lean implementation

To summarize, the implementation of lean principles in this manufacturing company proved to be highly beneficial in terms of maximizing the plants productivity. Lean methodology suited the needs of the plant and its manufacturing processes, through a simple and low cost approach that proved to be very effective in this type of environment due to manufacturing budget costs. There is a reduction of 18.45% in manufacturing throughput time and a total cost saving of 1,42,500 INR for every set of nacelle and nose cone produced.

References

[1] Nor Azian, Abdul Rahman, Sariwati Mohd Sharif, Mashitah Mohamed Esa, Lean Manufacturing Case Study with Kanban System Implementation, Procedia Economics and Finance 7, 2013, pp. 174-180.

[2] Lucas Simmons, Robbie Holt, Glen Dennis, Clay Walden, Lean Implementation in a Low Volume Manufacturing Environment: a Case Study, Proceedings of the 2010 Industrial Engineering Research Conference.

[3] Rasli Muslimen, Shari Mohd Yusof, Ana Sakura Zainal Abidin, Lean Manufacturing Implementation in Malaysian Automotive Components Manufacturer: a Case Study, Proceedings of the World Congress on Engineering, London, 2011, Vol. 1.

[4] Biman Das, Uday Venkatadri, Pankajkumar Pandey, Applying lean manufacturing system to improving productivity of airconditioning coil manufacturing, International Journal of Advanced Manufacturing Technology, 2014, pp.307-323.

[5] L. N. Pattanaik, B.P. Sharma, Implementing lean manufacturing with cellular layout: a case study, International Journal of Advanced Manufacturing Technology, 2009, pp. 772-779.

[6] Remigiusz Horbal, Robert Kagan, Tomasz Koch, Implenting Lean Manufactruring in High-Mix Production Environment, in IFIP International Federation for Information Processing, Lean Business Systems and Beyond, 2008, Vol. 257, pp. 257267.

[7] Roberto lvarez, Roque Calvo, Marta M. Pea,Rosario Domingo, Redesigning an assembly line through lean manufacturing tools, International Journal of Advanced Manufacturing Technology, 2009, pp.949-958.

[8] K Sadashivappa, M.C.Murugesh, Effect of Notch Orientation, Temperature and Filler Material on Impact Toughness of GFRP Composites, Advanced Materials Manufacturing & Characterization, 2013, Vol. 3, pp. 345-348

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