Achieving Zero Defect in Wedge Encapsulation used in Drinking Water Applications

Abstract

This project was studied in one of the leading MSME, manufacturers of the rubber encapsulation Wedge with an annual turnover of INR 3.2 million in Coimbatore. RGV Wedge parts is one of the product lines being manufactured using a special compound of EPDM Rubber, encapsulating the casting body.

This product contributes to 10% of the company’s total revenue an icon of brand image. This part gets predominantly rejected due to rusting in the side wall. In a highly competitive space, wherein high quality and lowcost had become the order of business, the goal to achieve zero defects in order to maintain the market share was the Challenge. 7.8% of Non-Conformance happened due to rust incurring a loss of lakhs of rupees during the period of January 18- to June -18 out of about 5000 pcs being produced.

The rolled yield was recorded at 89% where 100% could be achieved potentially.

3 covariates of Width, Depth, Weight of castings and 5 factors of molding process like Curing time, Curing temperature, Curing pressure, Loading and unloading Time along with rubber weight and 3 factors of preprocess preparation such as Time related to sand blasting, coating and preheating, were observed. The prediction equation ofprobability of acceptance was arrived using binary logistic regression, on the response ok , Not ok.

Accordingly, the significant vital factors were data mined and prompt corrective action were initiated, controlling the variation of Casting weight and Rubber weight between 1900-1950gms and 320-340gms respectively. This resulted in zero defects, eliminating the incidents of side wall rusting. The next incidental defects are under further research, and however this resulted in increased yield from 89% to 95%. This acclaimed recognized as best supplier from the OEM.

Key words: Zero Defect, EPDM- Ethylene Propylene Dyne Monomer, DMAIC- Define, Measure, Analysis. Improve, Control, RGV-Resilient gate valve and Binary Logistic Regression, Rolled yield.

1. Introduction

This project was studied in one of the leading companies in MSME sector, of a Polymers PVT group in Coimbatore. This is a well-known destination for rubber-moulded components serving a whole lot of industries requiring rubber products in their business line. When it comes to manufacturing rubber products, achieving the dimensions as per drawing, maintaining physical and chemical properties of the synthetic rubber compound, delivering defect free components to the customer are considered to be more challenging because of inherent elasticity or dimensional instability. Each and every product produced should justify itself and moreover it should comply the functional purpose when it is manufacturing critical rubber products. Rubber Encapsulation Wedges seem to be one of its kinds. With years of expertise in production of Resilient Gate Valve (RGV) Wedge Parts earned a space for itself in the company

2. What is Zero Defect?

Being a quality manufacturer of the Rubber Encapsulation Wedge with an annual turnover of around INR 4 million, RGV Wedge parts is considered to be a special product, and one of the product lines that is being manufactured using a special compounded EPDM Rubber encapsulating the casting body. Meeting the quality standards for these wedges have been a challenge, for drinking water application and required a very close attention in each and every aspects of manufacturing, towards zero defect. Zero defect as per Philips.C.Crosby, is doing right at the first time, and always”. Honourable Prime Minister of India, declared in Aug 2015 on the importance of “Zero Defect” and “Zero Impact” a Quality movement in India and the Simone Poisson principle for Zero Defect probabilities were worked out. 

2.1. Define Business case

Financially, this product contributes to approximately 10% of the company’s total revenue and established brand image. The rejection rates increased, for which an approach for practical problem through statistical modeling due to uncertainty in product output and providing statistical solution for practical implementation are suggested following DMAIC Approach (Define, Measure, Analyze, Improve, and Control. This part gets predominantly rejected due to rusting in the side wall as shown in Fig 1. In a highly competitive workspace, wherein high quality and low-cost had become the order of business, moving further was not fruitful unless the rejection rates were controlled. Therefore, the goal was to achieve zero defects in order to increase future orders. About, 7.8% of Non-Conformance happened due to rust during the period of six months out of about 5000 pcs, worth Rs.1.15 lakhs

2.2. Rolled yield

Productivity wise, the rolled yield was recorded at 89% where 100% could be achieved potentially. Various tests were carried out for testing the properties of rubber used, and all of which seem to be meeting the specified standards. Inspection process for this product was about one-and-half times more than the usual time taken. Several factors have to be considered to achieve rust free parts with less than 3% rejection on 90% of the production days throughout the year

2.3. Analysis

Statistically analyzing the following process approach, 3 covariates of Width, Depth, Weight of Castings and 5 factors of molding process like Curing Time, Curing Temperature, Curing Pressure, Loading and Unloading Time along with rubber weight and 3 factors of preprocess preparation such as time related to sand blasting, coating and preheating were observed. The prediction equation of probability of acceptance was arrived using binary logistic regression.

2.3.1 Equations and Findings: The data collected were shown in Table 1

Table 1-Final Production Results

Regression Equation

P(P) = exp(Y’)/ (1 + exp(Y’)) (1)
Y’ = -63.7 + 0.0202 casting-2056/2136 + 0.0539 pre heat – 0.1493 curing time + 0.082 Lug weight (2)

Variable Setting
Casting2056/2136 1990
Preheat30
Curing time30
Lug weight350
Fitted ProbabilitySE Fit 95% CI 0.994042 0.0230536 (0.0750091, 1.00000)

The probability of passing the product shown in Fig 2.

Figure 2-Fitted Probability on all 4 Factors

2.4 Improve

Accordingly, the significant vital factors were determined and prompt corrective actions were initiated, controlling the variation of Casting weight and Rubber weight between 1900-1950gms and 320-340gms respectively.

2.5 Control

This resulted in zero defects, eliminating the incidents of side wall rusting. The next incidental defects were taken for further research; however, this realized the yield from 89% to 95%. This acclaimed a recognition as best supplier from the OEM. A control plan was prepared to monitor the process at optimum levels.

3. Acknowledgements

The managing Director of this group is acknowledged for his thoughtful facilitation at his company. The Pedagogy of learning experience in Indian Statistical Institute, Coimbatore was a beginning of business excellence.

4. References

  • 01
    Over view of Rubber science- JK Tyres- Hasteri.
  • 02
    Review on Rubber -Metal Bondig-Dr. Samir Majumdar-Rubber Chem review
  • 03
    Introduction to Linear Regression Analysis – Douglas C.Montgomery. Elizabeth A.Peck, Geoffrey Vining – John Wiley & Sons, Inc
  • 04
    Multivariate Analysis and Its Application – K C Bhuyan – Central
  • 05
    Applied Statistics and Probability for Engineers – Douglas C.Montgomery, George C.Runger – John Wiley & Sons, Inc