TEXTILE TESTING AND QUALITY CONTROL-II FABRIC DIMENSIONS

Transcription

1 TEXTILE TESTING AND QUALITY CONTROL-II FABRIC DIMENSIONS Fabric Length: During the manufacturing and finishing processes cloth is subjected to various strains. Some of these are recoverable if the fabric is allowed to various relax free from restrain. Recovery is often greater when immersed in water. Method-1: Measurement of dimensions should be made in a standard atmosphere. A table wide enough to accommodate the open cloth width is preferred and with one edge graduated. A trial measurement is made before it is allowed to condition. Marks are made on the selvedge at 5 yd intervals throughout the full pc, After 24 hours in the testing atmosphere, cloth is measured again. If any change in length is less than 0.25%, then the second measurement is taken on correct length. (If more than 0.25%, again conditioned for 24 hours). Method-2: Four pairs of marks across the width of the cloth are made, each pair is one yard apart. The distance between each pair is accurately measured. The samle cut out and conditioned for 24 hours. If change is more than 0.25%, further conditioning time is allowed Length Measurement in ware houses / Inspection rooms Measuring devices (Trumeter) : Measuring by accurate rollers is carried out on the cloth moves over an inspection table which provide information in the form of a printed ticket in addition to indication on the counter. Fabric Width: Some processes cause contraction in width (wet treatments) and some may stretch stentencing). The choice of cloth width is influenced by a no. of factors. Important amongst these is the end use of cloth. For example two gents handkerchief can be woven in the same reed space as required for three ladies hankerchief can be woven in the same reed space as required for three ladies hankerchiefs. The maker-up of fabrics bugs his cloth in widths which allow him to cut out his patterns with a minimum of waste. Measurement of width: a) Fabric conditions for 24 hours. b) Measurement of width taken before and after condition up.

2 Accuracy: 0.1 for fabric 18 or more in width Excess 4 but less than excess 4 or less. Continuous Measurement: Instrument is based upon a pair of photo electric cells, one at each selvedge, which scan the edges and detect changes in width. The signals are translated into cloth width and widicated on a meter or recorded on a chart. Fabric Thickness: Principal: The determination of the thickness of a compressible material such as a textile fabric consists of the precise measurement of the distance between two plane parallel plates when they are separated by the cloth, a known arbitrary pressure between the plates being applied and maintained. It is convenient to regard one of the plates on the pressure front and the other a anvil. Points for practical application of this principal: 1. Shape and size of pressure foot: A circular foot of common diameter of 3 is used. 8 Ratio of the foot diameter to the cloth thickness should not be less than 5:1 2. Shape and size of the anvil: When a circular anvil is used it should be atleast 2 greater in diameter of the pressure foot. 3. Applied Pressure: 0.1 lb/in 2 to 10 lb/ in 2 as veccomm ended. 4. Velocity of pressure foot: It should be lowered onto the sample slowly at about 2/ 1000 in/sec 5. Time: The thickness is read from the dial of the instrument when the visible movement of the pointer is stopped. 6. Indication of Thickness: A clock type dial gauge is used built into a thickness tester. It should be capable of measuring to an accuracy of 1 % for cloths of 5/1000 or more and for thinner fabrics. Use of Result of thickness tests: 1. For checking materials against specifications. 2. In the study of other fabric properties such as thermal insulation, resilience, dimensional stability, fabric stiffness, abrasion etc. 3. In the study of fabric geometry.

3 Fabric weight per unit area and per unit length: Weight/ unit Area: One has to merely weight a known area and divide the weight by area. Area measurement and cutting should be an accuracy of 1.L and for an area less than 100 in 2 a cutting die or template is recommended weighing should be accurate to 1 part in 500. The effect of moisture content can be accounted for either by conditioning the specimen in the standard atmosphere or by taking the specimen oven dry weight and adding the official regain. Weight/ unit Length: The points to watch are similar to as above. The minimum length should be 18 cm. Conversion of values: Let W=weight/ Sq. yard R= Weight/ Running yard w=fabric width in inches Then, W= 36R W and R=Ww 36 Thread per inch in woven fabric: Warp yarn= Ends Inch Weft yarn= Pide inch Fabric description: 20 5 X 20 5 / warp X weft, 56 E.P.I x 60 P.P.I Methods of Counting: 1. One inch counting glass- a simple microscope 2. Traversing thread counter- A travelling microscope fitted with a pointer for count. 3. Fabric dissection- A known width is unraveled and threads are counted (For difficult fabrics such as infelted threads, complex structure e.g. plied fabric 4. Parallel line grating- A rapid optical method 5. Taper line grating- A development of no. 4

4 Crimp of yarn in fabric: When warp and weft yarn interface in the fabric they follow a wavy path. Crimp percentage is a measure of this weaviness. Percentage crimp is defined as the mean difference between the ends of the thread while in cloth, expressed as a percentage. l d 2 t h1 2 h2 2 d1 p Crimp %age = l p p X 100 Warp Crimp amplitude= h1 Weft crimp amplitude= h2 Crimp and Fabric Properties (Importance of Crimp): 1. Resistance to abrasion: Yarns with high crimp take the brunt of abrasive action because crown formed as the yarn bends round a traverse thread will protrude from the fabric surface and meet the destructive abrasive agent first. 2. Shrinkage: When the yarn are wet they swell, and consequently a thread ( say a warp thread) has a longer path to take round a swollen weft thread, the weft threads move closer.( Collin states that the largest amount of shrinkage is that represented by increase of crimp; yarn shrinkage takes a second place, being generally much less than increase in crimp, while fabric shrinkage is almost negligible). Since shrinkage is mostly due to yarn swelling and resulting crimp increase mechanical means of controlled pre-shrinkage have been developed e.g. Sen-forizing and Rigmel processes. 3. Fabric Behaviou during tensile testing: When a test strip of fabric is extended in one direction crimp is removed and the threads straighten out. This causes the thread at right angles to the loading direction to be crimped further. Crimp Interchange is said to take place. The specimen loses its original rectangular shape and waisting occurs. i.e the middle region of the strip contracts. The load

5 extension curve will show relatively high extension percent increase in load in the early stages of test. 4. Faults in fabric: Variation in crimp can give rise to faults in fabrics, e.g. reduction in strength, bright picks and diamond barring in rayons, strips in yarn dyed cloths, and so on. The cause of crimp variation is often loss of control over the tensions employed during yarn preparation and weaving. 5. Fabric design: Control of crimp %age is necessary when a fabric is designed to give a desired degree of extensibility and satisfactory finished appearance. 6. Fabric costing: The amount of yarn required to produce a given length of cloth is affected by the crimp percentages of warp and weft. Knowledge of values in calculating the cost and yarn requirement is useful. Measurement of Crimp %age: 1. Simple method by a pair of scissor and a scale-accuracy is not great. 2. W.I.R.A Crimpmeter: Test length upto 16 specimen is gripped and recommended tension is put on the sample by means of a sliding weight, when the beam becomes horizontal, touch bulb lights up and movement of sliding grip is noted. This is increase in length from which crimp %age is calculated (20 warp reading + 30 weft reading are noted) Yarn count diameter and cloth cover: The yarn diameter of a cotton yarn count N is: D= 1 28 N where d is the diameter in inches. 1 n d s s= 1 n n= no of threads/ inch Fig. represents the cross-section of threads in a fabric. If the no. of threads/inch is n, then distance s, from one thread to the next will be 1 inches. The ratio d, will represent the fraction of n s the spacing, S, covered by the projection from a thread. This fraction may be expressed in terms of yarn count and ends/inch. Thus, d s = 1 28 N X n 1

6 Multiply by 28, we get a value known as cover factor K= n Cover factor we calculated for warp and weft therefore pairs of values can be quoted in descriptions of fabric in much the same way as pairs of counts or pairs of threads/inch. Cover factors do not necessarily indicates textilement because differences in count, twist factor, fibre etc. all play their part. The fraction of the area covered by both sets of threads is given by k1+k2 28 k1 k2 28 X 28 Where, k1= warp cover factor k2= weft cover factor Again multiply by 28 for convenience, we obtain cloth cover kc=k1 + k2 - Cover factors and cloth wt. of unit area: An appropriate relationship between the sum of the warp and weft cover factors and the cloth weight per unit area is: K1+k2= w(n1+n2) Where, w= cloth weight per square yard in ounces n1= no of warp ends/inch n2= no of picks/inch An estimate of the closeness of the weave can be made only when the cloth setting and the weight per unit area are known. Crimp, count and cloth setting: The above dimensions are related by the formula C1% n2 + c2% n1 = 0.28 e ( 1 N1 + 1 N2 ) Where, c1= warp crimp %age C2= weft crimp %age n1= warp setting N k1 k2 28 n2=weft setting e= factors to accommodate yarn flattering N1= warp count (cotton) N2= weft count (cotton)

7 % Crimp, crimp amplitude, cloth setting and cloth thickness: Crimp amplitude is defined as the extent to which threads are deflected from the central plane of the fabric. The values in thousandths of an inch, are given by: h1= 136 c1% n2 and h2= 136 c2 % n1 Since, yarn diameter on a fraction of an inch is equal to 1/28 N, the diameter in 1000 th of an inch will be e or 28 N N Cloth thickness t, will thus be higher of the two values which follow: T=h1+ 36e 36e or h2+ N1 N2 FABRIC PROPERTIES AIR PERMEABILITY Air Permeability: A.P of a fabric is the volume of air measured in cubic centimeters passed per second through / cm 2 of the fabric at a pressure head of 1 cm of water. Air Resistance: The air resistance of a fabric is the time in seconds for 1 cm 3 of air to pass through 1 cm 2 of fabric under a pressure head of 1 cm of water. Air Porosity: 1. Same as air permeability alternative def 2. The porosity of a fabric is the ratio of air space to the total volume of the fabric expressed as a %age. This is a calculated value based on an estimation of the volume of the component fibres and the estimation of the volume of the fabric from measurement of length, width and thickness. Note: a. Air resistance is the reciprocal of air permeability. b. The advantage in using air resistance values in preference to air permeability values lies in the fact that when a no of fabrics are superimposed to form a multy layer assembly, the total air resistance is merely a sum of the individual values. Type of finish gicven to a fabric have a considerable effect on the permeability even though the porosity may remain the same.

8 Air Permeability(Air passing ability through fabric) :