This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
LEATHER GARMENT AND GOODS MANUFACTURING TECHNOLOGY DIRECTORATE
QUALITY CONTROL FOR LEATHER GARMENT TECHNOLOGY
Prepared by:
GETACHEW ADHENA (BSC TEXTILE ENGINEERING)
Thursday, March 25, 2010
Quality control for leather garment technology BY GETACHEW ADHENA page 1
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
ADISS ABEBA_ ETHIOPIA
Table of Contents
Chapter1Introduction to quality control...................................................................................................1
Chapter2 Quality control definition.........................................................................................................3
2.1. Basic concept of quality.......................................................................................................3
2.2. Dimension (parameter) of quality........................................................................................5
2.3. Basic concepts of quality control.........................................................................................7
2.3.1 The meaning of control.........................................................................................7
2.3.2. Stages of quality control......................................................................................7
2.4. Why quality is important.....................................................................................................8
There is today a noticeable increase of interest in quality control on the part both of
industries and society in general. There are several reasons for this greater interest:
Higher demands for quality from customers
Greater competition
Demand for better profitability. Often however there is a lack of knowledge of
ways of dealing with product quality.
It is not only for the users of industrial products that quality is meaningful. It is also an
important matter for the manufacturers. Poor quality means costs for manufacture in finding
and rectifying (repair or correct) defects. Poor quality leads to reduction in market share due
to a loss of confidence on the part of the customers.
It is apparent to most manufacturers that poor quality has a negative effect on profitability. It
is not so obvious to many manufacturers, however, that possibilities exist to improve profits
by systematic work on product quality even where the situation is considered to be
satisfactory.
Single results will not be achieved through work on single department in the enterprise, it is
necessary for all those functions which comes into contacts with the products during its
developments, manufactures and use to co-operate in this work. This means that quality must
be considered and controlled by all these functions.
Market research
Product development
Quality control for leather garment technology BY GETACHEW ADHENA page 4
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Manufacturing engineering
Purchasing
Production
Inspection
Marketing and after-sale service
Our lives are dependent in many ways of industrial product, shelter, nutrition,
communication, heath care work and national security. One basic aspect of products of this
type (regardless of whether they are goods or services ) if that must be fit for uses failure in
this respect can lead to death, injury, discomfort or economic loss.
Quality control for leather garment technology BY GETACHEW ADHENA page 5
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
CHAPTER TWO
QUALITY CONTROL DEFINITION
2.1. Basic concept of Quality
Meet specification, fitness for use, anything that can be improved, absence of
variation, conformance to requirements and bad quality is social loss.
Control
Preventing defects from happening
The subjective definition of element of quality relation to the design, style, color, and
\aesthetics. Objectively, quality is the ability to meet consistently the return and clearly
started specification. These aimed at producing a product suitable for end use and price.
Different scholars or authors define quality; however Professor David Garvin, from Harvard
University defines in to the following principal.
1. Product-based quality view
2. User (customer) based quality view
3. Manufacturing based quality view
4. Value-based quality view
1. Product based quality views
Quality control for leather garment technology BY GETACHEW ADHENA page 6
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
In this view, quality is determined as a precise and measurable variable and difference
in quality reflects differences in the quality of some ingredient or attitude seen to be
possessed by a product. In this view quality and quantity has direct relation
2. User (customer) based quality view
In this view, definitions are based on the premise that solely the user determined
quality. Individual customers are assumed to have different wants or needs and goods
that best satisfy the preference are the one they regarded, as having the highest
perceived quality. Quality is fitness for use that this view also reflects a highly
personalized and subjective view
3. Manufacturing based quality view
This view focuses on manufacturing and engineering practices. It emphasis
conformance to specified requirements. The higher the degree that meets specified
requirements the higher the quality. This view seeks to ensure that the deviations from
standards set design specifications are minimized.
To achieve the quality of conformance means improving in the design of the
• Equipment
• Materials
• Supervision
• Control
• Training
4. Value based quality view
The base for this view is physiological understanding of the meaning of value.
Consequently, customers have been conditioned to accept that the quality of product
is determined by the price. There for price and quality have a direct relation ship
5. Trader based quality view
Quality control for leather garment technology BY GETACHEW ADHENA page 7
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
This views deals about that we shall get the right product to the right place at the right
time while exceeding our customer’s expectation.
Quality has not specific or common definition or meaning. Some of general definition
of quality
Quality is a system which produces a product service, information or delivery on
target with manual variance which meets customers’ needs.
Quality is complete satisfaction (performance, appearance, longevity (long life) at
the lowest possible cost.
Quality is to reach customer’s needs at low rates (costs) to the company and
achieving employ satisfaction.
Quality is the extent to which products, services, products and relationships are free
from defects, constrains (limitation) and items which do not add value for
customers
The AMERICAN NATIONAL STANDARDS INSTITUTE define<< quality as the
totality of features and characteristics of a product or services that bear (accept or
allow) on its ability to satisfy given needs
Quality connotes different meaning to different peoples. Its concepts may be easy to grasp
but formulating a universal definition is difficult. Some definitions are given below.
Quality is fitness for purpose or use. Or quality is customer satisfaction. (Jurdan
1974G.C)
Quality should be aimed at the needs of the customer, present and feature. (Deming)
Quality is the degree of excellence (fineness). (Webster)
Quality means best for the certain condition
a. The actual use and
b. The selling price (Feugenbaum 1983 G.C)
Quality control for leather garment technology BY GETACHEW ADHENA page 8
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
2.2. Dimensions (parameters) of quality
Quality is an important factor which customers looks for in a product to give total
satisfaction. Some of the important parameters of quality are listed below:
1. Performance: (will the product do the intended (planed or proposed) job). Potential
costumer usually evaluate a product to determine if it will perform certain specific
functions and determine how well it perform them.
Example: producing document case leather bag, for what purpose does this bag
produced, does the bag has multi function pocket?
2. Reliability: (how often does the product fail?) different products may need repair
over their service life. The leather machineries should be also reliable so as to
increase productivity.
Example: when we produce the leather garment, it needs higher attention during
stitching. Because the nature of the needle is like cuter edge type and stitch the
component by cutting the part. So re stitching is not done, means no repaired and the
product is rejected.
3. Durability: (how long does the product last?). This is the effective service life of the
product customer obviously want products that performed satisfactorily over a long
period of time.
Example: producing leather garment coat for a big man and if the coat is not with his
physical structure. Does the garment have long life? No, because the person may loss
his comfort and reject the product in short period of time.
4. Serviceability: (how easy is it to repair the product?). There are many industries
where the customer’s view of quality is directly influenced by how quickly and
economically a repair or routine maintenance activity can be accomplished.
Example: planed maintenance of stitching machine.
Quality control for leather garment technology BY GETACHEW ADHENA page 9
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
5. Aesthetics :( what does the product look like?). this is the visual appeal of the
product, often taking into account factors such as style, color, shape, packaging
alternatives and other sensory features.
6. Features :( what does the product do? ). Usually customer associate high quality with
products that have added features: that is, those that has features beyond the basic
performance of the competition.
2.3. Basic concepts of quality control
Quality control is concerned with the operational techniques for detecting, recording and
taking action to eliminate quality problems. It refers to all activities (process) and
technologies that are used to achieve or maintain the quality of a product or service’
Quality control focuses on finding and eliminating of source of defects and monitoring the
manufacturing process. Quality control consists of developing, designing producing,
marketing, service products and services with optimum cost- effectiveness and usefulness,
which customers will purchase with satisfaction.
In leather industry, quality control should be concerned with the evaluation of test data and its
application to the control of the leather process, raw materials, intermediate products and
final products.
For effective quality control, it is necessary to exercises control over the input material,
process, and employment of skilled labor, management techniques and use of appropriate
tools, equipment and machines
Quality control in eliminating of material wastes and ensures in quality product and full
customers satisfaction.
2.3.1. The meaning of control
Any control system has certain essential features, as follows
Quality control for leather garment technology BY GETACHEW ADHENA page 10
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
1. There is a plan / in our case the quality standard/
2. We prepare to carry out that plan
3. We carry it out, and all the time we compare what is being achieved with our plan.
4. If we begin to deviate from the plan we ‘feed back’ instruction, so that we return to
the plan
2.3.2. Stages of quality control
There are five stages of quality control and which is written in quality control terms are
1. Set the quality standard or quality desidgn required by the customers
2. Plan to achieve the required quality. This will involve.
a. Planning methods
b. Planning equipment
c. Obtaining satisfactory materials
d. Selecting and training operators
e. Planning inspection and shop floor quality control
3. Manufacture right first time
4. Correct any quality deficiencies
5. Provide for long term quality control and planning.
2.4. Why quality is important?
According to Korathi (1999 G.C), various reports have indicated the following six benefits of
quality:
1. Greater market share
2. Higher growth rate
3. Higher earning
Quality control for leather garment technology BY GETACHEW ADHENA page 11
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
4. Premium price
5. Loyal customers
6. High motivated employs
CHAPTER THREE
QUALITY CONTROL
3.1 Quality determination
Quality can be determined through the following methods
A. By visualizing (inspecting)
This means that, if you feel better by observing on the product, we can say the product has
good quality. The reverse is true.
B. By touching
This means that, when we are touching the product and if it has shine and smooth surface we
feel better as compare to the product which has rough surface and we can say the product has
better quality.
C. By testing
This means that we takes sample of the product and taste some property in the laboratory then
compare the taste result with the given standard value. If the test result is approaches to the
standard given value, then we can say the product is good quality.
Quality control for leather garment technology BY GETACHEW ADHENA page 12
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
3.2. Main aspects to be considered in the quality control
A. set-up
If the set-up is correct, the whole lot will conform to the specification, such process includes
labeling, and cutting and checking is done by first inspection.
B. machine tools
During production time –to-time change can occur which leads to defects and necessary
checking production.
c. operators
The process is such that the results depend on the skill and attention of the operators.
It is important in manufacturing planning to decide on the operators working methods,
aptitude and experience must be considered when selecting operators.
D. Material components
Raw material and component influence results. A process of this type is assembling different
component. In this case it is very important to assure quality in earlier stage by in process
inspection
3.3. What to control
From the definition of quality, what we understand about preventing defect from happening,
so we should control anything that may made defects from the raw material up to finished
leather
Controlling in leather processing
Control the amount of chemical added in each production step
Control the duration for each production time
Quality control for leather garment technology BY GETACHEW ADHENA page 13
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Control the temperature
Control the ph value
Controlling in leather garment spreading and cutting
Controlling the size of cutting blade, sharpness of the blade
Cleanness of the cutting table
Pattern mixed
Controlling in sewing room
Needle damage
Feed damage
Thread broken
Inoperative zipper
3.4. When to control
In the garment industries quality control is practiced right from the initial stege of sourcing
the raw material to the stage of final finished garment, so quality control is carried out
throughout the whole system from starting to ending of the process.
To ensure at minimum practicable cost, that the requisite quality of product is being achieved
at every stage of manufacture from raw material to boxed stock.
3.5. Approaches to apply quality control
Itemize the variable that occurs in fabric and garment production in order to provide a
complete specification.
Develop a specification in a number of parts of sections to ensure that all design and
production staff has clear idea as what is needed
Establish acceptable working tolerances in relation to all values on the specification
Quality control for leather garment technology BY GETACHEW ADHENA page 14
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Establish fault rate recording systems
Improve technical understanding of the product including
• Fabric properties (skin and hide)
• Sewing problems
• Causes and prevention of seam break down
3.6. Who shall be involved in quality control?
Responsibility for the production of good quality lies with and in the hand of
The person teaching the job
Primarily by good communication
By making new comer aware of specifications and tolerances, faults themselves, their
effect and the appropriate action to take. Such communication or training if properly planned
and executed (implemented) will greatly assists in minimizing the cost of poor quality.
For controlling the quality, the following groups are involved:
• Individual operator
• Supervision
• Mechanics
• Quality control inspector
• Quality control head
• Trainer
• Instructor
• Chief executive
Quality control for leather garment technology BY GETACHEW ADHENA page 15
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
CHAPTER FOUR
PROCESS INSPECTION
4.1. Definition of inspection
What is garment inspection?
Inspection in reference to quality control in the apparel industry can be defined as the visual
examination or review of raw materials (such as fabric, buttons, zippers, sewing threads, etc),
partially finished components of the garments and completely finished garments in relation to
some standards, specifications, or requirements including the measurement of the garments to
ensure the satisfaction of the customers.
Principle involved in inspection
Quality control for leather garment technology BY GETACHEW ADHENA page 16
Inspection
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Advantage and importance
• Avoid wastage of time and money
• Avoid the unnecessarily use of resource
• Helps in timely delivery of goods
• Guarantees customers satisfaction
4.2. Objectives of inspection
1. Various objectives of inspection are:
2. Isolate good lots from bad lots
3. Isolate good items from bad items
4. Rate the capacity of the process
5. Measure the product quality
6. Rates the inspection accuracy
7. Measure the degree of precision of the measuring instrument used in inspection
8. Procure information about product design and
Quality control for leather garment technology BY GETACHEW ADHENA page 17
Detection of defects
Feed back of these defects to appropriate personnel
Determination of causes and defects
Correction of defects
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
9. To calibrate measuring standard and instrument
4.3. Function of inspection
There are different types of inspection function.
• Raw material inspection
• Process inspection
• Finished (final) inspection
• Purchased part inspection
• Tools inspection
4.3.1 raw material inspection
Fabric inspection: After fabric is received, it should be inspected to determine its
acceptability from a quality view point; otherwise, extra cost in garment
manufacturing may be incurred due to either the loss of the material or time, to say
nothing of customer returns and dissatisfaction due to poor quality. Some garment
manufacturers rely on their fabric suppliers to perform fabric inspection and mark
fabric defects.
Fabric inspection is usually done on fabric inspection machines, such machines are
designed so that rolls of fabric can be mounted behind the inspection table under adequate
light and rolled as they leave the table. Defects in a fabric can be seen readily with these
machines ,as the inspector has a very good view of the fabric and the fabric does not need
to be reversed to detect defects .These inspection machines are either power-driven or the
inspector pulls the fabric over the inspection form. Such machines are also equipped to
accurately measure the length of each roll of fabric as well as monitor the width of the
fabric.
There are various fabric inspection systems such as 4-point system, 10-point system and
Graniteville “78 “system.
Quality control for leather garment technology BY GETACHEW ADHENA page 18
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
A. 4-point system: The 4 –point system, also called American Apparel Manufacturers
Association (AAMA) point grading system for determining fabric quality, is widely
used by producers of apparel fabrics. Fabric flaws or defects are assigned point values
based on the following:
Points allotted
Length of defect in fabric, either length or width
Up to 3 inch 1
Over 3 inches and up to 6 inch 2
Over 6 inches and up to 9 inch 3
Over 9 inches 4
Holes and openings (Largest dimensions)
1 inch or less 2
Over 1 inch 4
Total defect points per 100 yd2 are calculated, and normally those fabric rolls containing
more than 40 points /100 yd2 are considered “seconds “However a garment manufacturer,
based on the price line and type of garments produced, may use more or less than 40 points /
100 yd2 as an acceptance criteria.
For example, a fabric roll 120 yd long and 48 inch wide contains the following defects:
2 defects up to 3 inch 2x1 = 2 points
5 defects over 3 inch but less than 6 inch 5x2 = 10 points
1 defect over 6 inch but less than 9 inch 1x3 = 3 points
1 defect over 9 inch 1x4 = 4 p0ints
Total defect points = 19 points
Quality control for leather garment technology BY GETACHEW ADHENA page 19
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Therefore,
Total points scored in the roll x 3600
Points /100 yd2 = Fabric width in inches x total yards inspected
= 19 x 3600
48 x 120
= 11.9 defect points /100 yd2
So if the acceptance criteria are 40 points /100 yd2, then this roll is acceptable. The maximum
number of defects to be counted against any one linear yard is 4 points. Overall, fabric quality
is assessed on the basis of the number of defect points per 100 yd2 of fabric.
B. 10-point system
Under this system, fabric defects are assigned point values based on the following
Length of defects Pointed allottedWarp defects
• Up to 1 in.
• 1 to 5 in.
• 5 to 10 in.
• 10 to 36 in.
1
3
5
10
Filling defects
• Up to 1 in.
• 1 to 5 in.
• 5 in to half width
• Larger than half width
1
3
5
10
No linear yard of fabric is assigned more than 10 points, no matter how bad or frequent the
defects are. Fabric is considered “first” quality if the number of defects point is less than the
number of yards of fabric inspected. In the case of fabrics wider than 50 in., the fabric is
Quality control for leather garment technology BY GETACHEW ADHENA page 20
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
considered first if the total defect points do not exceed the number of yards of fabric
inspected by 10 %.
Powderly speaking compared 4 point and 10 point systems and noted that a 4 point system
tends to classify more fabrics at first.
C. Graniteville “78” system
This inspection is used for the woven and knitted fabrics only, not concerned for non woven
fabric like leather.
Sewing threads: During the sewing process in a high speed lock stitch machine, the
thread is subjected to complex kinematics and dynamic conditions. The speed at
which it passes through the needle eye can reach 140 –165 km/hr and at the moment
at which the thread is caught by the sewing hook , the speed reaches 2000 m/sec
,while moving at such speeds ,the thread is subjected to friction from a number of
guides ,from the needle eye ,from the fabric being sewn , from bobbin case
assembly ,and from the bottom thread. At the same time, the thread is subjected to
many stresses, all of which take place very quickly and at high speed. Therefore
,sewing threads should be checked for the following characteristics :
Construction of yarn ( count ,number of twist, strength )
Sewability (ability to sew).
Imperfections (should be free from slubs, knots, etc).
Finish ( yarn should be smooth ,lubricate ,etc )
Colour (should match with the standard).
Package density.
Winding (winding should be uniform).
zippers : it should check for the following :
- Dimensions ( check for the correct width of tape )
- Top and bottom stops should be fastened securely.
- Zipper tape should be uniform in colour if that is important.
- Pull tab should be affixed firmly to the slider body.
Quality control for leather garment technology BY GETACHEW ADHENA page 21
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
- Slider should ride freely but must not be so free that it is loose on the chain.
- Check also to be sure the slider locks securely.
buttons, buckles, snap fasteners, and so on:
Buttons: should have large, clean sew holes that are free from flash and will not cut the
thread. Holes must be located properly in relation to the edge of the button. Buttons should be
of uniform thickness.
Buckles: should be checked for any visual defects such as sharp, burred edges. If a buckle is
cloth –or vinyl covered, there should not be an appreciable difference in the buckle and
garment materials.
Snap fasteners: The attaching machinery should locate the snap fasteners accurately and at
proper pressure .Component parts should be checked to close tolerances and free from dirt
and other foreign substances so that they will feed rapidly through the hopper and permit
uniform and trouble free assembly.
4.3.2. In –process inspection
In – process inspection means the inspection of parts before they are assembled into a
complete product. In apparel manufacturing, this means inspection at various points in the
entire manufacturing process from spreading fabric to pressing /finishing. In –process
inspection can be either quality control inspector or individual operators themselves after they
perform their respective operation (s).
Spreading:
Various factors that can affect spreading should be checked, such as the cleanness of the
spreading table, surface area of the table, the position of the table whether inclined or not,
spreading of the leather whether tight spread or spread with low tension.
Defects during spreading:
Pattern defects: during preparing the patterns of a garment, we should take care so that it is
possible to avoid the defects of garment. The following are the pattern defects.
1- Pattern parts missing: correct number of parts for all sizes not included by the marker
maker.
Quality control for leather garment technology BY GETACHEW ADHENA page 22
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
2- Mixed parts: parts not correctly labelled in marker, therefore a marriage of wrong –sized
parts.
3- Patterns not all facing in the same direction (either way).
4- Patterns not facing in the correct direction on napped fabrics.
5- Patterns not aligned with respect to the fabric grain.
6- Line definitions poor (e.g. chalk, too thick, etc) leading to inaccurate cutting.
7- Not enough knife clearance of freedom.
8- Mismatched checks and stripes.
9- Notches and drill marks omitted, indistinct, or misplaced.
cutting :
Cutting quality is a prerequisite for quality in a finished product. In addition, cut work quality
affects the ease and cost with which construction is accomplished. The quality of work
leaving the cutting room is determined by how true the cut fabric parts are to the pattern, how
smooth or rough the cut surface is material or fabric defects in the cut fabric parts, shade
differences between cut fabric pieces within a bundle.
Advantages of in-process inspection
1. Reduction of major supervision from the customers due to bad quality.
2. Decrease in labour cost due to a decrease in repair rates.
The operators and supervision are constantly reminded that the company has a specific
quality level to meet, just be the very presence of the inspectors in their section on daily
basic.
Because each worker will realize that his/her work is subject to being inspected at any time
throughout the day the quality of the work produced by workers will improve.
The data obtained can be effectively analyzed and utilized by the production supervision and
part manager in correcting problems or improving quality.
Sewing defects: defects of garment occur due to some problems of sewing .It is possible to
list some defects of sewing as follows.
Quality control for leather garment technology BY GETACHEW ADHENA page 23
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
1- Needle damage as evidenced by holes, picked threads, ruptured threads, or other damage
to the fabric, caused by wrong size or type of needle, blunt needle, needle heat, or machine
feeding difficulty.
2- Feed damage, particularly on thicker or sheer fabrics, or when machining over traverse
seams, arising from incorrect type of teeth, excessive pressure by foot, excessive machine
speed.
3- Skipped stitches, from the hook irregularity failing to pick up the loop of thread from a
needle’s eye owing to a number of causes.
4- Thread breaks, arising from too thick a thread for the needle, too thin a thread, needle
heat, too tight tension, etc.
5- Broken stitches ,arising from the wrong stitch type ,too tight tensions, a body formed
joint in the seam where the second line of stitch runs over the first and cracks it ,sharp feeds
,too great a pressure ,etc.
6- Seam grin arising from too loose a tension or too large a stitch, or the use of the wrong
stitch type.
7- Seam pucker, because of incorrect handling by the operator, misaligned notches, tight
thread tensions, etc.
8- Pleated seams, where operator failed to ease in fullness evenly.
9- Wrong stitch density: too many give rise to jamming and rupture of fabric threads, weak
seams.
10- Uneven stitch density: due to carelessness of the operator.
11- Improperly formed stitches, caused by bad tension, incorrectly adjusted timing.
12- Oil spots or stains.
Seaming Defects: It is the other type of defect of garment. Usually cased by errors arising from
the interaction of the operator and machine in the handling of the garment. Some of the seaming
defects are listed below.
1- Incorrect or uneven width of inlay, arising from bad handling by operator, incorrectly set
guide, incorrectly adjusted folder. In extreme cases, the seams burst open raw edges show,
slippage of weave threads occurs, or notches are exposed.
2- Irregular or incorrect shape of sewing line (sometimes called run-offs) in top stitching,
arising from lack of or badly set guide, not following a mark.
Quality control for leather garment technology BY GETACHEW ADHENA page 24
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
3- Insecure back stitching, because subsequent rows do not cover the first row of stitching.
4- Twisted seam leading to irregular puckering or the garment parts not hanging correctly
when worn, caused by improper alignment of fabric parts.
5- Mismatched checks or stripes.
6- Mismatched seam, where traverse seams do not match (eg inside leg seams at the fork of
trousers).
7- Extraneous part caught in seam, an unrelated piece showing through the seam.
8- Reversed garment part where part is sewn with face side opposite from specification,
perhaps when the part cut for one side of garment is sewn in the other, or when the whole
garment is assembled inside out.
9- Wrong seam or stitch type used.
10- Wrong shade of thread used.
Assembly defects : ( perhaps caused by errors arising in marking and cutting ,as well as sewing
operations in the sewing room, or a combination of these).The following are an examples of such
defects.
1- Finished garment not to size, arising from incorrect patterns, inaccurate marking or
cutting, shrinking or stretching fabric incorrect seam widths.
2- Finished components not correct to size or shape or not symmetrical.
3- Parts, components closures or features omitted, caused by bad work flow, and wrongly
printed work tickets, parts omitted in cutting.
4- Components or features wrongly positioned or misaligned arising from incorrect
marking ,or sewing not following the mark ( e.g. Pockets ,bar tracks, top stitching
buttonholes ,buttons, hooks and bars ,hooks and eyes, zips).
5- Interlining incorrectly positioned, twisted, too full, too tight cockling.
6- Lining too full, too tight, showing below the bottom of the garment, twisted, incorrectly
pleated, etc.
7- Garments parts cockling ,pleated, twisted, showing bubbles and fullness ,for example,
collar in relation to the under collar or the neck ,sleeve in relation to the arm hole ,pockets
,tapes zips, pads in relation to the shoulder.
8- Garment parts shaded owing to being mixed after cutting.
9- Parts in one way fabrics in wrong direction, usually only small parts, such as pockets.
Quality control for leather garment technology BY GETACHEW ADHENA page 25
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
10- Mismatched trimming.
Controlling of fusing operation
Adjustment of time, temperature and pressure for given types of material to be fused based on the
manufacture recommendation in order to attain certain peel strength.
Control screen printing operation
Placement of design
Ink coverage
Pressing /finishing
Appearance is the basic of most consumers’ judgment on whether or not to purchase a garment.
The quality of a pressing operation can be measured by evaluating the following:
1. Broken zipper or button
2. Creased not correctly formed
3. Fabric of finished garment not smooth, wrinkle free and showing its proper appearance
4. Pocket not smooth
5. Lining showing creases
6. Shrinkage due to heat and moisture
Methods of controlling of garment defects
Since manufacturers vary in organizational structure, products manufactured, technological
levels, and so on .there cannot be a standard text book answer on how to start a quality
control program. However, the following are some general guide lines that are used to plan
and start a quality control program for the manufacturers.
To obtain an overall picture of where you stand in terms of quality, perform 100 % final
inspection of all styles for at least two or three weeks to collect information. Then analyze
this information. The following are various defects to look for during quality control.
Broken button.
Broken snaps.
Broken stitching.
Defective snaps.
Different shades within the same garment.
Quality control for leather garment technology BY GETACHEW ADHENA page 26
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Dropped stitches.
Exposed notches.
Exposed raw edges.
Fabric defects.
Holes
Inoperative zipper.
Loose/hanging sewing threads
Misaligned buttons and buttonholes.
Missing buttons.
Needle cuts /chews.
Open seams.
Pulled /loose yarn.
Stains.
Unfinished button holes.
Zipper to short
4.3.3. Final inspection
Final inspection consists of inspecting finished garments from the customers’ point of view
,size measurement ,form fitting ( putting garments on the proper size mannikins to see if they
properly fit the labelled sizes ),and live modelling if necessary ( again to see if the garments
properly fit the labelled sizes ).
Final inspection may occur before or after garments are packed in poly bags and boxes. It is
done after garments are packed, then proper size and style markings on the package can also
be checked .In any case, there should be a list of points to be checked in a garment, including
a table of finished measurements.
Let us see for instance final quality inspection of men’s shirt.
Quality control for leather garment technology BY GETACHEW ADHENA page 27
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Serial
No Location Inspection For1 Collars Both points same length, uniformly stitched, No broken
stitches, should lay flat.2 Buttons and Button holes Properly spaced, No puckering or fullness, No cut
stitches, No broken buttons.3 Pocket Top of the pocket horizontal, uniformly stitched, corners
securely tacked, should lay flat. 4 Hems Uniformly stitched, No puckering, skip or broken stitch.5 Yoke or shoulder Pleats properly placed, Uniformly stitched, No
puckering, Skipped or broken stitches.6 Side seams Stripes, plaids, checks or patterns should match.7 Cuff Stripes, plaids, checks or patterns should match the
sleeve.8 Finished appearance Clean of all loose thread. No oil /dirt stains. Free of any
fabric defect.
4.4.4. Purchased parts inspection
The purchasing department will state in the purchased order that the vendor /seller/ is to
provide an initial sample before the first delivery. The aim of initial sample is to give the
customer an idea of how the vendor’s production will turn out in terms of quality. The sample
must be made by the same methods and in the same machines and tools which will be used in
future production. Both the size of the sample and the latest time for delivery must be given.
Before the initial sample is shipped, the vendor must inspect and test it to be sure that the
customer’s quality requirements are fulfilled. In order to avoid misunderstanding, only pone
department should have the contacts with venders on initial samples.
4.4.5. Tools inspection
Tools inspection helps to prevent lots too many defects getting through to production and
causing problem there.
Quality control for leather garment technology BY GETACHEW ADHENA page 28
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Tools inspection means identify and problem on the tools and justify whether the right tools
are used to the right position or place.
Tools inspection is done at each production department. For example, in case of leather
garment production, tools inspection is done in leather processing department, in cutting
department, in design and pattern making department, in sewing department and last finishing
department.
4.4 systems of inspection
4.4.1 floor inspection
It is a fast process as no material movement is involved in it.
It is a cheaper process too as a cost of transportation of the material from the floor to the
inspection room is nil.
It is highly efficient and most probably the only way of inspection for Buckley items.
The two main problem associated with floor inspection are
1. Service conditions at floor are not suitable for precision work.
2. There is a real possibility of the inspector being influenced by production personnel.
4.4.2 central inspection
It has certain advantages over floor inspection. So far as it can be carried our more precisely
and rapidly due to availability of all equipment at one place and the working condition are
better
Minimum possible interference is encountered in central room inspection.
It is well suited to incoming material inspection, finished production inspection and
inspection of high precision and delicate product; however, it is a time consuming inspection
as material movement is involved.
4.4.3 combined inspection
Quality control for leather garment technology BY GETACHEW ADHENA page 29
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Both floor inspection and central inspection are employed in a production unit, known as
combined inspection. On this case some component may be inspected at the floor level where
as other are inspected at the central room.
4.5. Types of inspection
There are different types of inspection:
A. 100%inspection
B. Sample inspection
C. First of inspection
D. Working inspection
E. Functional inspection
F. Testing complete product
4.5.1. 100% inspection
Every item in a lot is inspected for one or more characteristics.
It has its own advantages:
1. Inspection is done seriously and faults can be easily identify
2. Increase internal and external customer satisfaction
But the disadvantage is
1. Time consuming because of lot product for inspection each is difficult.
2. Cost for inspection is increased.
4.5.2. Sample inspection
Quality control for leather garment technology BY GETACHEW ADHENA page 30
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
A sample of the lot is inspected for one or more characteristics. Sampling inspection may be
divided into:
1. Spot check
2. Statistical sampling inspection
In spot check inspection
Just a few items in the lots are inspected; the items for inspection might not be selected at
random.
In statistical sampling inspecting
Procedures based on statically theory are used. The procedures provided information on the
sample size and criteria of acceptance or rejection. The sampling done at random. The
probability of acceptance is considered.
Every item in the batch has an equal chance of being selected from the sample
Taking sample from a batch
Every sample which we take must be representatives of the batch, because we are going to
judge the quality of the batch by the quality of the sample. Some care and experience are
necessary in doing taking of samples from a batch, the following point will help:
1. Never take the whole sample from one part of the batch
2. Make sure that the sample does not come entirely from the top layers of the batch.
The bottom must be represented as well as the top, and one side as much as other
3. If a consignment (batch) consists of more than one package, then from each package
we take a sample which is roughly proportional to its size.
Quality control for leather garment technology BY GETACHEW ADHENA page 31
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
4. We should never mix two small batches to gather to make one large for economically
in sampling, unless we know that both consignments are from the same production
runs.
5. We must beware of sampling bias if we have a batch in which some items are
obviously defective. Examples: suppose that finish is important and we received a
batch in which some items are obviously blemished (marked). It will be very difficult
to sample this fairly.
4.5.3. First off inspection / initial inspection
Initial/ first off/ inspection is a well known and valuable aid to quality control. Whenever and
wherever a production run is started, it will be prudent (careful) to check, the first piece,
assembly etc before the main run commence (began/start/).
We can detect many defects by checking the first off and so prevent the whole batch from
being wrong. The following are some of examples
1. We can check whether the machine or the equipment is correctly set up
2. We can discover whether the operator has fully understood his instructions
3. In many case we shall be able to detect the use of incorrect material.
A first off inspection is almost always worth doing, but some value of the work being
produced and partly on how far a correct first off ensures a correct run. The more expensive,
the work being made and the more it costs to put it right of it is incorrectly made, so the more
it will pay us to make sure that the run starts off correctly.
First off inspection ensures that the job starts correctly.
4.5.4. Working inspection
The objective of this type of inspection is to help the operators to make the whole run
correctly. From time to time the inspector will visit the machine or the operator concerned
and take a sample of the latest work made. If the quality is wrong on any point, then this must
Quality control for leather garment technology BY GETACHEW ADHENA page 32
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
be corrected as quickly as possible. If an operator goes wrong, it is much kinder to tell him
quickly, so that he can correct himself before he has made a lot of out of limit work.
The primarily task of a working inspector is to help keep running correctly and therefore,
whatever the sample size, it should always include the last or almost the last item made
because this gives the latest news of the machine’s condition. It will tell the inspector and
operator whether the machine is still running correctly.
4.5.5. Functional inspection/last-off/
At the end of the run, the last piece made should be checked to see whether the tools etc are
still satisfactory condition. If they need regrinding or any other attention before they will be
fit for another run, this must be arranged before they are returned to store.
The functional inspection/ last-off/ can usually be combined with the last working inspection.
It is usually desirable to do a working inspection at end of run, in order to check on work
made since the last routine inspection.
4.5.6. Final test and inspection
The term “final test and inspection” is usually used to denote any inspection done after
manufacture has been completed, with the object of making sure that the goods concerned are
satisfactory to send to the customers or may be to another department for the next operation.
This type of inspection should therefore, be regarded as a check that out production quality
control is working satisfactory, and not as the quality control system itself.
Quality control for leather garment technology BY GETACHEW ADHENA page 33
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
CHAPTER FIVE
PHYSICAL TESTING FOR LEATHER GARMENT
5.1. conditioning
Scope
This method describes the conditioning to be given to test specimens prior to physical testing.
Procedures
Quality control for leather garment technology BY GETACHEW ADHENA page 34
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
During the 48 hours immediately preceding its use in a test, keep each specimen for physical
testing in a standard atmosphere of temperature 20+ 20c and relative humidity 65+2%.
Support the specimen so that air has free access to its surfaces and keep the air in continuous
rapid motion by a suitably placed fan. Perform all physical tests in the same standard
atmosphere unless otherwise specified.
Note
The required relative humidity of 65 +2% at 20+ 20c can be maintained in a closed space
either by the use of saturated solutions of certain salts in water (in which the solid phase is
also present in excess) or by the use of a solution of given concentration of sulphuric acid in
water the selected solution should be placed in a large shallow dish inside the enclosure.
Saturated solutions of either pure ammonium nitrite or pure sodium nitrite are satisfactory.
A solution of sulpheric acid 35.6 % by weight’s=1.270 is satisfactory. It is necessary to check
the density from time to time and restore it to the value 1.270 as require by appropriate
additions of either water or more concentrated sulpheric acid. Continuous circulation of air is
essential.
Rapid conditioning procedure can follow for light leather (excluding heavy leather, e.g. sole,
belting) may be carried out by placing the leather in an oven for 2 h at 40 0c, followed by
standing overnight in the standard atmosphere: 200c/65%RH.
5.2. Measurement of the flexibility of light leather and their surface
finishes
Scope
This method can only be used for light leathers.
Outline of the method
Quality control for leather garment technology BY GETACHEW ADHENA page 35
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
The specimen is folded as described below and is clamped at each end to maintain it in a
folded position in machine designed to flex the specimen. One clamp is fixed and the other
moves backwards and fore wards causing the fold in the specimen to return a long it. The
specimen is examined periodically to assess what damage has been produced.
Specimens
The specimens are rectangles 70 mm by 45mm and well conditioned.
Apparatus
G 15 H
j
50 B c 22.50
25
E K F 25
A 55 D fig 1
The upper lamp consists of a pair of flat plates. One has the shape of a trapezium ABCD (fig
1), with sharp corner at D rounded to a radius of 2 mm. it has a ledge EF on which the folded
specimen rests. The other plate has the shape EGHCF. The two plates can be screwed
together, so as to hold one end of the specimens between them. The screw K which draws the
plates together acts as a stop, which prevents the end of the specimens from being thrust too
far towards the back of the clamp. Between the plates near the edge AB is a stop which
prevents them from coming together near AB, and so ensures that they clamp the leather
firmly near F. the upper clamp can be reciprocated by a motor about a horizontal axle J
(fig1). In the position shown in fig 1, the ledge EF is horizontal and the end F is at its height
point. The clamp descends through an angle of 22.50 and returns 100+5 time per minute. The
number of cycles is recorded by a counter.
The lower clamp is fixed and lies in the same vertical plane as the upper clamp. It consists of
a pair of plates which can be screwed together to hold the end of the specimen between them.
Quality control for leather garment technology BY GETACHEW ADHENA page 36
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
If the upper clamp has been turned to the position where the ledge EF is horizontal (fig1), the
upper edges of the plates of the lower clamp are 25mm below the ledge EF.
Insertion of the specimen in the clamps
• Term the motor until the ledge EF is horizontal.
• Fold the spacemen so that the two longer sides are brought together, turning inwards
that surface of the leather which is to be observed during the test. Clamp the folded
specimen in the upper clamp with one end of the specimen against the stop and the
folded edge against the ledge.
Test procedures
1. Clamp the specimens in the machine in the manner described above and switch on the
motor. After 100, 1000, 10,000 cycles, switch off the motor and example the leather
finish to see whether it has been damage. Record any damage observed its nature and
the number of cycles at which it was observed.
2. After 2, 4, 6, 8, 12, 16, 24 hours flexing, examine the leather itself to see whether it has
been damaged. Record any damage observed its nature and the number of cycles at
which it was observed.
Damage to finish may be of the following (and other kinds:
• Change of shade (graying) of the finish film without other damage.
• Crazing of the finish with smaller or greater surface cracks.
• Loss of adhesion of the finish to the leather with slight or considerable changes of
color in the folded area.
• Loss of adhesion of one finish layer to another, with slight or considerable changes of
color.
• Powdering or flaking off of finish, with slight or considerable changes of color.
Damage to the leather may be the following kinds (among others)
Quality control for leather garment technology BY GETACHEW ADHENA page 37
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Development of coarse grain folds
Loss of an embossed grain pattern.
Cracking of the grain layer
Continuation of the breakdown of fibers to such an extent that a hole develops through
the entire thickness of the leather
5.3. Measurement of tearing load (tear strength )
a. Double edge tear
Scope
This method is intended for use with any type of leather.
Preparation of specimens
The specimen is rectangle 50mm and 25mm wide in which a slot, having the shape and
dimensions shows in fig 1, has been cut, preferably by use of a pres knife which cuts out the
specimen and slot in one operation. Cut the specimens by pressing the press knife through
from the grain to the flesh side.
25
15 20 15
Quality control for leather garment technology BY GETACHEW ADHENA page 38
5 A B
R=5
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Fig 1 specimen (dimension in millimeter)
Apparatus
Use a tensile strength machine having a uniform speed of separation of the jaws of 100+
20mm per minute, and such that readings of lad fall in that part of the scale which has been
shown by calibration to be correct within 1 %
Attach to the jaws of the machine a pair of specimen holders of the type, each consists of a
strip of steel 10mm wide and 2mm thick, bent through a right angle at one end and welded to
a bar which makes the strip rigid, and which fits one pair of jaws of the tensile strength
machine or replaces them.
Procedure
1. Condition the specimen and measure the thickness.
2. Adjust the machine so that the turned up ends of the specimen holders are in contacts
with one another. Slip the specimen over the turned up ends, so that they protrude
through the slot with the widths of the turned up ends parallel to the straight edges of
the slot. Press the specimen down firmly on to the holders.
3. Run the machine until the specimens torn apart and record the highest lad reached
during tearing as the tearing load.
4. Report the tear load in kgf and the thickness in millimeter.
b. Single edge tear
Scope
To determine the resistance of all types of leather using single edge tear
Sampling
Quality control for leather garment technology BY GETACHEW ADHENA page 39
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
The specimen is rectangle 70mm and 40mm wide in which a slot, having the shape and
dimensions shows in fig 2, has been cut, preferably by use of a pres knife which cuts out the
specimen and slot in one operation. Cut the specimens by pressing the press knife through
from the grain to the flesh side.
70mm
50mm
20mm
40mm
Fig. 2
Procedure
1. Determine the average thickness
2. Set the separation rate of the jaws of the tensile testing machine to 100 + 20mm/min
and the distance of the jaws to 50+ 2 mm.
3. Clamp the ends of the test piece in the jaws for a length of 20mm, ensuring that the
slot is aligned to the traction axis.
4. Start the test and record the load- extension plot
5. Divided the plot in 4 equal parts from the first peak to the last one.
6. In the second and third sections record the load value of the two higher and two lower
peaks.
7. Determine the single edge tear load making the arithmetic mean of the tested sample.
Quality control for leather garment technology BY GETACHEW ADHENA page 40
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
5.4. Measurement of tensile strength
Scope
These tests are intended to used will all kinds of leather.
Preparation of specimens
Fig 1 shows the shape and dimensions of the internal surfaces of the press knife which must
be used to cut the specimen. When specimens are used for tensile strength measurement the
leather must be conditioned before the specimens are cut.
30 R=5
25 A E C 10
B 50mm D
110
Machine to be used
• Use a tensile strength machine having a uniform speed of separation of the jaws of
100 + 20mm/minute.
• The jaw shall measure at least 40 mm in the direction of the applied load. They shall
be designed to apply, by pneumatic or mechanical means, a clamping force which
remains constants as the end pieces of the specimen compress and whose center of
action shall lie as close to the center of the clamped end pieces as possible and in any
case never outside it. The texture and design of the inside faces of the jaws should be
such that at the maximum load attained in the test the specimen should not slip at
either jaw by an amount exceeding 1% of the original jaw separation.
Quality control for leather garment technology BY GETACHEW ADHENA page 41
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Procedures
A. Tensile strength
1. Measure the width each specimen to the nearest tenth of a mm at the positions on the
grain and at three positions on the flesh side: in each group of three , make one
measurement at the mid-point E of the waist of the specimens (fig 1) and the other
two at position approximately midway between E and linen AB,CD.
2. Measure the thickness of the specimen and make three measurements at appoint
equally spaced, between AB and CD. Take the mean of the three thickness
measurements as the thickness of the specimen.
3. Calculate the area of the cross section of each specimen by multiplying its width by
its thickness.
4. Set the jaws of the tensile machine 50mm apart. Clamp the specimen in the jaws so
that the edge of the jaws lies along the lines AB, CD. When it is clamped the grain
surface of the specimen shal lies in one plane
5. Run the machine until the specimen breaks and take the highest load reached as the
breaking load.
6. Calculate the tensile strength by dividing the breaking load by the area cross section
of the specimen. Express the results in Mpa.
Tensile strength Tn = F/w*t [N/mm2]
B. Percentage elongation caused by a specific load.
Quality control for leather garment technology BY GETACHEW ADHENA page 42
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
1. Clamp the specimen between the jaws of the machine and measure the distance
between the jaws to the nearest half mm and take this distance as the initial length of
the specimen for the purpose of the test.
2. Start the machine. Unless the machine automatically draws a load extension graph
with the necessary accuracy, follow the distance between the pairs of the load
increases with the aid of dividers.
3. Note the distance between the pair of the jaws at the instant when the load first
reaches the specified value. Take this as the length of the specimen at this load.
4. Calculate the elongation at the specified load by taking the difference between the
initial length and the length at the specified load. Express this difference as the
percentage of the initial length (L1).
Percentage elongation E1 = L1-L0 [%]
L0
c. Percentage of elongation at break
1. Take the same principle with percentage of elongation.
2. Note the distance between the pairs of jaws at the instant when rupture of the
specimen occurs. Take this as the length of the specimen at break.
3. Calculate the elongation at the break by taking the difference between the initial
length and the length at break. Express this difference as a percentage of the initial
length.
Eb = L1-L0 [%]
L0
Quality control for leather garment technology BY GETACHEW ADHENA page 43
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
5.5. Test for colo0r fastness
Color fastness to TO – AND FOR RUBBING.
Scope
To determining the behavior of the surface of a specimen to rubbing.
Condition of test pieces.
For tests with wet test pieces:
• Immerse the test piece in a vessel containing distilled water.
• Dispose the vessel in the desiccators and produce a 5kpa.Vacuum for 2 minutes.
• Restore the atmospheric pressure and repeat the procedure twice again.
• Dry the test pieces using filter paper.
• Test pieces must not be immersed more than 1 hour.
Procedure
1. Cut a rectangular test piece with length not less than 120mm and width not less than
20mm.
2. Dispose the test piece in the holders.
3. Put the test piece in traction corresponding to a 10% elongation.
4. Dispose the 1 k.g weight on the piston.
5. Set the number of cycles and star the test.
6. After reconditioning the wet component, assess the change in color of leather
Quality control for leather garment technology BY GETACHEW ADHENA page 44
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
CHAPTER SIX
STATISTICAL QUALITY CONTROL
Statistica1 quality control (SQC) is the term used to describe the set of statistical
tools used by quality professionals. Statistical quality control can be divided into three
broad categories:
1. Descriptive statistics are used to describe quality characteristics and relationships.
Included are statistics such as the mean, standard deviation, the range,
and a measure of the distribution of data.
2. Statistical process control (SPC) involves inspecting a random sample of the output from
a process and deciding whether the process is producing products with characteristics that fall
within a predetermined range. SPC answers the question of whether the process is
functioning properly or not.
3. Acceptance sampling is the process of randomly inspecting a sample of goods and
deciding whether to accept the entire lot based on the results. Acceptance sampling
determines whether a batch of goods should be accepted or rejected.
The tools in each of these categories provide different types of information for use in
analyzing quality.
Descriptive statistics are used to describe certain quality characteristics,such as the central
tendency and variability of observed data. Although descriptions of certain characteristics are
helpful, they are not enough to help us evaluate whether there is a problem with quality.
Acceptance sampling can help us do this.
Acceptancesampling helps us decide whether desirable quality has been achieved for a batch
ofproducts, and whether to accept or reject the items produced. Although this information
is helpful in making the quality acceptance decision after the product has been produced,
it does not help us identify and catch a quality problem during the production process. For
this we need tools in the statistical process control (SPC) category.
Quality control for leather garment technology BY GETACHEW ADHENA page 45
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
All three of these statistical quality control categories are helpful in measuring and evaluating
the quality of products or services. However, statistical process control (SPC) tools are used
most frequently because they identify quality problems during the production process. For
this reason, we will devote most of the chapter to this category of tools. The quality control
tools we will be learning about do not only measure the value of a quality characteristic. They
also help us identify a change or variation in some quality characteristic of the product or
process. We will first see what types of variation we can observe when measuring quality.
Then we will be able to identify specific tools used for measuring this variation.
Variation in the production process leads to quality defects and lack of product consistency.
The Intel Corporation, the world’s largest and most profitable manufacturer of
microprocessors, understands this. Therefore, Intel has implemented a program it calls “copy-
exactly” at all its manufacturing facilities. The idea is that regardless of whether the chips are
made in Arizona, New Mexico, Ireland, or any of its other plants, they are made in exactly
the same way. This means using the same equipment, the same exact materials, and workers
performing the same tasks in the exact same order.
sources of variation: common and assignable causes
If you look at bottles of a soft drink in a grocery store, you will notice that no two bottles are
filled to exactly the same level. Some are filled slightly higher and some slightly lower.
These types of differences are completely normal. No two products are exactly alike because
of slight differences in materials, workers, machines, tools, and other factors. These are called
common, or random, causes of variation. Common causes of variation are based on
random causes that we cannot identify. These types of variation are unavoidable and are due
to slight differences in processing.
The second type of variation that can be observed involves variations where the causes can be
precisely identified and eliminated. These are called assignable causes of variation.
Examples of this type of variation are poor quality in raw materials, an employee who needs
more training, or a machine in need of repair. The machine may need to be readjusted. This
would be an assignable cause of variation. We can assign the variation to a particular cause
(machine needs to be readjusted) and we can correct the problem (readjust the machine).
Quality control for leather garment technology BY GETACHEW ADHENA page 46
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
1. Descriptive statistics
Descriptive statistics can be helpful in describing certain characteristics of a product and a
process. The most important descriptive statistics are measures of central tendency such as
the mean, measures of variability such as the standard deviation and range, and measures of
the distribution of data. We first review these descriptive statistics and then see how we can
measure their changes.
A. The Mean
In the mass of leather’s box, the average mass of the box is 190 K.g. The arithmetic average,
or the mean, is a statistic that measuresthe central tendency of a set of data. Knowing the
central point of a set of data is highlyimportant. Just think how important that number is
when you receive test scores!To compute the mean we simply sum all the observations and
divide by the totalnumber of observations. The equation for computing the mean is
n
where x _ (x bar)the mean
xi _ observation i, i _ 1, . . . , n
n _ number of observations
B. The Range and Standard Deviation
In the leather box, we can state that the amount of natural variation in the box is between 100
& 200 K.g.This information provides us withthe amount of variability of the data. It tells us
how spread out the data is around themean. There are two measures that can be used to
determine the amount of variationin the data. The first measure is the range, which is the
difference between the largestand smallest observations. In our example, the range for natural
variation is 180K.g.
Another measure of variation is the standard deviation. The equation for computing
Quality control for leather garment technology BY GETACHEW ADHENA page 47
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
the standard deviation is
n-1
where _ standard deviation of a sample
X- (x bar)_ the mean
xi _ observation i, i _ 1, . . . , n
n _ the number of observations in the sample
Small values of the range and standard deviation mean that the observations are closely
clustered around the mean. Large values of the range and standard deviation mean that the
observations are spread out around the mean.
The given below figure illustrates the differences between a small and a large standard
deviation for our leather box. You can see that the figure shows two distributions, both with a
mean of19K.g. However, in the first distribution the standard deviation is large and the data
are spread out far around the mean. In the second distribution the standard deviation is small
and the data are clustered close to the mean.
Small standard deviation
Quality control for leather garment technology BY GETACHEW ADHENA page 48
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
Large standard deviation
100 120 140 160 180 200 220 240 260 280 K.G
Mean
Symmetric distribution
Skewed distribution
100 120 140 160 180 200 220 240 260 280 K.G
Example; the production of ELICO leather industries is given below and also the production item is men’s leather jacket.
Week 1_ 35, 35, 30, 30, 35, 25, 30
Week 2_40, 35, 40, 35, 30, 30, 30
Calculate the mean, the range and standard deviation?
A. mean = x1 + x2 + x3.....+xn
n
Quality control for leather garment technology BY GETACHEW ADHENA page 49
LEATHER & LEATHER PRODUCT TECHNOLOGY INSTITUTE 2002 E.C
=35+35+30+30+35+25+30+40+35+40+35+30+30+30
14
=32.857 33
B. range = maximum -minimum
= 40-25
=15
C. standard deviation = sum of (individual-mean) 2