SQC

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Statistical Quality Control in Textiles
Module 1Introduction to Quality and Quality
Control
Dr. Dipayan Das Assistant Professor Dept. of
Textile Technology Indian Institute of Technology
Delhi Phone [phone] E-mail
Dipayan_at_textile.iitd.ac.in
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Introduction to Quality
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Concept of Quality
Simply, quality refers to one or more desirable
characteristics that a product should
possess. Quality is inversely proportional to
(unwanted) variability.
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Quality Characteristics
Every product possesses a number of properties
that jointly describe what the user or consumer
thinks of as quality. These properties are known
as quality characteristics. For example, fiber
length is known to be one of the important
quality characteristics of a fiber.
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Quality Cost

• Preventing, detecting and dealing with defects
  cause costs that are called quality costs or
  costs of quality.
• Quality costs can be broken down into four broad
  groups.
• (1). Prevention Costs
• product/process design.
• Process control.
• Burn-in.
• Training.
• Quality data acquisition and analysis.

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Quality Cost (Continued)

• (2). Appraisal Costs
• Inspection and test of incoming material.
• Product inspection and test.
• Material and services consumed.
• Maintaining accuracy of test equipment.
• (3). Internal failure Costs
• Scrap
• Rework
• Retest

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Quality Cost (Continued)

• Failure analysis
• Downtime
• Yield losses
• Downgrading/ off-spacing
• (4). External failure costs
• Complaint adjustment
• Returned product/material
• Liability costs
• External costs

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Quality history 1
Year Event
1700-1900 Quality was largely determined by the efforts of an individual craftsman.
1915-1919 WWI- British government began a supplier certification program.
1919 Technical Inspection Association was formed in England, this later becomes the Institute of Quality Assurance.
1924 W A Shewhart introduced the concept of control charts in Bell Laboratories technical memorandum.
1928 Accepatance sampling techniques were developed and refined by H. F. Dodge and H. G. Romig at Bell Labs.
1931 W. A. Shewhart published Economic control of quality of manufactured product outlining statistical methods for use in production and control chart methods.
1932-1933 British textile industry began use of statistical techniques for product/process development.
1944 Industrial Quality Control began publication.
1954 E. S. Page introduced CUSUM control chart.
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Quality history (Continued)
Year Event
1960 The concept of quality control circle was introduced in japan by K. Ishikawa.
1960 The zero defects (ZD) programs are introduced in US induatries.
1975-1978 The concept of total quality management (TQM) was developed in the US.
1989 The journal Quality Engineering was appeared.
1989 Motorolas six sigma initiative began
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Introduction to Quality Control
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Concept of quality control

• Quality control describes the directed use of
  testing to measure the achievement of a specified
  standard.
• Quality control is a superset of testing,
  although it often used synonymously with testing.
• The concept of quality, namely that the object,
  the service, the performance whatever it is
  should be fit for purpose.
• Quality does not mean being at the top of the
  range, but it does mean being efficient (so
  things happen on time and on cost), reliable
  (whatever the weather, every day of the week) and
  giving good value for money.
• The goal of quality control should be to
  identify process steps that best predict
  outcomes - A. Blanton Godfrey

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Link Among Quality, Productivity, and Cost
Take an example. A ring frame machine produces
1000 cops per day. The production cost per cop
is Rs. 15. It is seen that 75 of the cops
conform to specifications and 60 of the
non-conforming cops can be reworked at an
additional expense of Rs. 5 per cop, the rest 40
of the nonconforming cops is scrapped. The cost
per good cop is then After implementation of
process control program, it is seen that 80 of
the cops conform to specifications and 60 of the
non-conforming cops can be reworked at additional
expense of Rs. 5 per cop, the rest is required to
be scrapped. The cost per good cop is then
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Quality Control Methodology

• Statistical process control
• Statistical product control
• Six sigma
• Total quality management (TQM)
• Total quality assurance (TQA)
• Note TQM and TQA are beyond the scope of this
  course.

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Statistical Process Control

• Statistical process control involves measurement
  and analysis of process variation by using
  statistical techniques namely control charts.
• This is most often used for manufacturing
  processes, in the intent of statistical process
  control is to monitor product quality and
  maintain processes to fixed targets.

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Statistical Product Control

• Statistical Product Control involves in making
  dispositions on accepting or rejecting a lot (or
  batch) of product that has already been produced.
• It is most often used to evaluate products that
  are received from outside sources and where it is
  not possible to implement statistical process
  control.

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Six Sigma

• Technical products with many complex components
  typically have many opportunities for failure or
  defects to occur. Motorola developed the
  six-sigma program as a response to the demand for
  such products.
• The focus of six sigma lies in reducing
  variability in key product quality
  characteristics to the level at which failure or
  defects are extremely unlikely.

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Frequently Asked Questions Answers
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Frequently Asked Questions Answers
Q1 A ring-spun yarn shows more strength
variability that a rotor-spun yarn. Can we say
that the ring-spun yarn offers better strength
quality than the rotor-spun yarn? A1 Yes Q2 Is
it so that the application of statistical
techniques for quality control in textiles has
started recently? A2 No, the British textile
industry began use of statistical techniques for
product/process development in 1932-1933. Q3 Why
quality control is at all required? A3 Effective
quality improvement can result in increase in
productivity and reduction in cost.
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References

• Montgomery, D. C., Introduction to Statistical
  Quality Control, John Wiley Sons, Inc.,
  Singapore, 2001.

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Sources of Further Information

1. Montgomery, D. C., Introduction to Statistical
   Quality Control, John Wiley Sons, Inc.,
   Singapore, 2001.
2. Grant, E. L. and Leavenworth, R. s., Statistical
   Quality Control, Tata McGraw Hill Education
   Private Limited, New Delhi, 2000.
3. Leaf, G. A. V., Practical Statistics for the
   Textile Industry Part I, The Textile Institute,
   UK, 1984.
4. Leaf, G. A. V., Practical Statistics for the
   Textile Industry Part II, The Textile Institute,
   UK, 1984.