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Crimp & Crimp Interchange

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Abdullah Al MahfujSeguir

Marketing Executive em Friendstex Fabrics Ltd.Anúncio

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- Introduction Crimp in a textile strand is defined as the undulations or succession of waves or curls in the strand, induced either naturally during fiber growth, mechanically, or chemically. Crimp in a fiber is thus considered as the degree of deviation from linearity of a non-straight fiber. Fiber crimp is the waviness of a fiber expressed as waves or crimps per unit length (figure 1) or as the difference between the lengths of the straightened and crimped fiber (expressed as a percentage of the straightened length)[1-3]. Fiber crimp characteristics have a big influence on the processing performance of the fibers. Crimp also contributes essentially to the properties of intermediate fiber assemblies, yarn and finished fabrics. Fiber crimp imparted to synthetic fibers, which are initially straight, makes it possible to process these fibers with existing machinery designed for natural fibers. Straight, slick synthetic fibers would not have sufficient cohesion for carding, combing, drawing, roving, and spinning. In nonwoven processes, crimp and crimp retention during processing are major contributors to processing efficiency, cohesion, fabric bulk and bulk stability. What is Crimp? Woven fabric made of two sets of yarns - warp yarns and weft yarns. Warp and weft yarns are interlaced with each another and form fabric sheet. When warp and weft yarn interlace in fabric they follow a wavy path. This waviness of yarn is called crimp. According to pierce “crimp”, geometrically considered is the percentage excess of length of the yarn axis over the cloth length. What is Crimp%? Percentage crimp is defined as the mean difference between the straightened thread length and the distance between the ends of the thread while in the cloth, expressed as a percentage. From the definition of crimp two values must be known, the cloth length from which the yarns is removed and the straightened length of the thread. In order to straighten the thread, tension must be applied, just sufficient to remove all the kinks without stretching the yarn. In practice it is seldom possible to remove all the crimp
- before the yarn itself begins to stretch. The standardized tensions recommended in the B.S. Handbook are given below: From those two values we can calculate the crimp percentage with the following formula: C% How to measure crimp% 1. At first we have to select the warp or weft way of the fabric. Then we should select the test length of the yarn. Here it is 10². 2. According to test length we will cut the flap of fabric. 3. Now a single yarn is to remove from the flap of fabric carefully as discussed in theory. 4. One end of the yarn is gripped in the fixed gripper of the m/c and the other end is gripped in the other setting the test length. 5. Now the tension for the sample is found out from its count and it is set in the m/c. 6. After that we will apply tension along the yarn length with hand by taking away the other end of yarn far from the first end. 7. As soon as the white marl on the tension bar is on the same line of its both sides white mark, we will stop far away the other end. 8. The length of the yarn after applying tension is taken from the scale. 9. Now from this two lengths crimp percentage is calculated from the given formula. 10. In this way at least 10 crimp percentage for warp and 10 for weft is taken and average crimp percentage is calculated from them. 100 p pl where, c = crimp, l = Straightened thread length p =The length of thread in fabric ,crimped length
- Then calculate crimp% by using following formula. Yarn Crimp% = 100 X (Straighten Yarn length - Yarn length in fabric)/Fabric Length Warp Crimp% = 100 X [(length of warps in straighten form - warp wise fabric sample length)/Warp wise fabric sample length] Weft Crimp% = 100 X [(length of weft in straighten form - weft wise fabric sample length)/Weft wise fabric sample length] Suppose p =The length of thread in fabric , crimped length=1” l = Straightened thread length =1.3” then, crimp% %30 30 100 1 13.1 Crimp amplitude: In cloth geometry, the term crimp amplitude is used. Crimp amplitude refers to the extent to which threads are defected from the central place of the cloth. Crimp ratio: Crimp ratio is the ratio of the yarn length to the fabric length produced from that yarn. Take up%: Take up % or crimp rigidity is a measure of the ability of a textured yarn to recover from stretch. It is related to the bulking properties of yarn.
- The difference between length of yarn in the fabric after weaving, expressed as a percentage of the length of the yarn before weaving, is called take up %. Mathematically, Take up% (T) Crimp%,C ……………….(1) L-p Take up percentage,T = …….. X 100 ……………………… (2) L Where, T = Take up percentage L = Length of yarn before weaving p = Length of yarn in fabric after weaving Relation between Crimp% and Take up %: From equation (1) L-p C = …….. X 100 p => C/100 = L/p – 1 => L/p = C/100 + 1 100 + C => L/p = ………… …………………(3) 100 From equation (2) L-p T = ……….. X 100 100 p pl
- L = (1- p/L) X 100 = {1 – 100/(100+C)} X 100 ……………………….. [from equation 3] 100C So, T = ………… ……………………………(4) 100+C To express percentage crimp in terms of take up %, equation (4) is solved as follows: 100C T = …………… 100+C => 100C = 100T + CT => 100C – CT = 100T 100T => C = ……………. ……………………..(5) 100 – T Equation (4) and (5) are two relations between Crimp% and Take up% Crimp Rigidity of a textured yarn gives a measure of its ability to recover from stretch. It is determined by measuring the percentage reduction of length of a skein of yarn immersed in water when a minor load replaces a major load. For measuring crimp rigidity, a skein of known number of strands is prepared from the package of yarn under test. This skein is then immersed in water filled in a glass jar with a load of 0.1 g/denier/strand suspended on it. The length of the skein is measured on a scale. The load is then reduced to 0.002 g/denier/strand by replacing the major load by the minor load and the reading on the scale observed. Crimp rigidity is calculated from the readings obtained under the major and minor loads.
- Difference between crimp% and take up%: Crimp% Take up% 1. crimp percentage is defined as the mean difference between the straightened thread length and the distance between the ends of the thread while in cloth which is expressed as a percentage. 1. The difference between length of yarn in the fabric after weaving, expressed as a percentage of the length of the yarn before weaving, is called take up %. 2. It is not related to the bulking properties of yarn. 2. It is related to the bulking properties of yarn. 3. It is denoted by C. 3. It is denoted by T 4. Crimp % 4. Take up% (T) Shirley crimp tester: When yarn is removed from a fabric it is no longer straight but it is set into the path that it took in the fabric. This distortion is known as crimp and before the linear density of the yarn can be determined the crimp must be removed and the extended length measured. The crimp tester is a device for measuring the crimp-free length of a piece of yarn removed from a fabric. The length of the yarn is measured when it is under a standard tension whose value is given in Table below- Yarn tentions for crimp tester:
- Description: i. The instrument consists of two clamps, one of which can be slid along a scale and the other which is pivoted so as to apply tension to the yarn. ii. The sample of yarn removed from the fabric is placed in the clamps with each end a set distance into the clamp. This is because the length of yarn in the clamps has to be allowed for in the measurement. iii. The right hand clamp can be moved along the scale and it has an engraved line on it at which point the extended yarn length can be read. iv. The left hand clamp is balanced on a pivot with a pointer arm attached. On the pointer arm is a weight which can be moved along the arm to change the yarn tension, the set tension being indicated on a scale behind it. v. At zero tension the left hand clamp assembly is balanced and the pointer arm lines up against a fixed mark. As the weight is moved along the arm the clamp tries to rotate around the pivot, so applying a tension to the yarn. Procedure: i. The counts of the warp and weft yarns are first determined and the correct tension is calculated. ii. When a measurement is being made the movable clamp is slid along the scale until the pointer is brought opposite the fixed mark. iii. At this point the tension in the yarn is the value which was set on the scale.
- iv. The length of the yarn can then be read off against the engraved line. The crimp, which is the difference between the extended length and the length of the yarn in the fabric, is defined as: Crimp % = 100 p pl Let, the straightened thread length = l The length of thread in fabric, crimped length = p. EFFECT OF CRIMP OF YARN ON FABRIC PROPERTIES Influence of crimp on fabric properties: Warp and weft crimp percentage are two factors which have influence on the following fabric properties: Resistance to Abrasion: The abrasion resistance of a fabric will be more, if the crimp in the yarns is more. The yarns will high crimp take the brunt of abrasion action. This is because crowns, formed as the yarn bends round a transverse thread, will produce from the surface of the fabric and meet the destructive abrasion agent first. The other set of yarns lying in the center of the fabric will only play their part in resisting abrasion when the high crimped threads are nearly worn through. Shrinkage: When the yarns are wet, they swell and consequently say a warp thread as longer bending path to take a swollen weft thread. The warp thread must either increase in length or alternatively the weft threads must move closer together. An increase in length of warp requires the application of tension is absent, equilibrium conditions will be attained by the weft threads moving closer together.
- The largest amount of shrinkage is that represented by increase of crimp. Yarn shrinkage takes a second place and generally it is much less than increase in crimp. Since shrinkage is mainly due to yarn swelling and the resulting crimp increase, mechanically means of controlled pre-shrinkage have been developed such as- Sulfurizing and Ragmen processes. Fabric behavior during strength testing: When a stripe of fabric is extended in one direction, crimp is removed and the threads are straightened. This causes the threads at right angles to the loading direction to be crimped further, i.e. when the load is applied along the warp threads, crimp in the warp is removed and that in the weft threads is increased. This is known as crimp interchange. The sample loses its original rectangular shape the middle position to the stripe contracts. This is known as wasting. Due to the removal of crimp, the load- elongation curve will show relatively high extension per unit increase in load in the early stages of strength testing of a stripe of fabric. Faults of fabric: Variation in crimp can give rise to faults in fabric, eg, reduction in strength, bright picks, diamond bars in rayon’s, stripes in yarn dyed cloth etc. The crimp variation is mainly due to the improper tension on the yarn during yarn preparation and weaving. Fabric design: Control of crimp percentage is necessary when a fabric is design to give a desired degree of extensibility. Some fabrics require control of crimp in the finishing process to give the correct crimp balance between warp and weft so that the finished appearance is satisfactory. Therefore, the tension applied must be carefully controlled. Fabric costing: Since crimp is related to length, it follows that the quantity of yarn required to produce a given length of fabric is affected by the warp weft crimp percentage. Therefore in calculating the cost and the yarn requirements, the value of crimp play an important role. Measurement crimp percentage: From the definition of crimp, two values must be known, the cloth length from which the yarn is removed and the straightened length of the thread. In order to straighten thread, tension must be applied, just sufficient to remove all the crimp without stretching the yarn.
- The principle of yarn crimp determination is very simple. With a fine pen and rule, lines are drawn on a piece of cloth at a known distance. Some of the threads are raveled out, the yarns are straightened without stretching and the stretched length is noted and form that the crimp is calculated. The difficulty lies in the straightening of the yarn without stretching it. To do this, the following three methods are available: 1. Straighten by hand: This is inaccurate since we do not know force applied. 2. Straighten by a standard weight: This is satisfactory it we know what weight to use. Determine the straightened length from the load-elongation curve: This most accurate method. Factors that affect crimp%: i. Physical properties like elasticity, rigidity, bending behavior etc. of fibres and yarns. ii. Count of warp and weft iii. threads/inch or cm iv. tension on threads during weaving v. yarn and fabric structure vi. Physical and chemical treatment of the fabric after weaving. Crimp interchange Warp and weft yarns lie to a certain extent in a wavy form in a fabric. There is, therefore, a difference in length between straightened yarns and the yarns in the wavy form in the fabric, both in warp and weft direction. In order to be able to produce the fabric required, information is needed for predicting the length, width and mass per unit area of the fabric. This difference in length between the yarn and the fabric is usually referred to as crimp, a parameter which is significant for the structure of the fabric and also for the pore shape and size. When the fabric is loaded there is often a change in the ratio of the crimp of the warp and weft system. This change is referred to as crimp interchange.
- Wave Length, Crimp Frequency and Crimp Length The wave length λ of fiber crimp is twice the distance between two crossings of the fiber with the zero axis. It cannot be measured directly, since fiber crimp is by far too irregular and the small measuring quantities- wavelengths in the order of 1-3 mm - cause problems. Therefore, the crimp length lc, as the average length of fiber in one crimp, is sometimes used to describe crimp which can be measured as equation (1); (1) More commonly used is the crimp frequency Cf may be calculated from the equation (2) of a fiber, defined as twice the average of the inverse of the wavelength. It is also called crimp number or crimp count, and characterizes the number of crimp bows or waves Cn per unit length of straightened fiber L0. The unit length L0is taken as 1 inch in the US, whereas in Europe, 100 mm or 1 cm is used[2-6]. (2) Crimp Angle The angle α between the leg of a crimp wave and the zero line may be used to characterize crimp geometry. The crimp angle φ is the angle between the two legs of a crimp bow, as shown in Figure 2; Φ indicates the sharpness of a crimp[2].
- Crimp Amplitude and Crimp Index The crimp amplitude A is the maximum distance of a crimp bow from the zero axes. Since the measurement of the amplitude of single crimp bows is practically impossible, average crimp amplitude of the fiber is derived geometrically with Pythagoras from length measurements of the crimped and the uncrimped fiber. The crimp index Ci is an indirect measure of the crimp amplitude. It is also called crimp ratio, crimp percentage, crimp contraction or crimp retraction and is the ratio of the difference of extended length L0 and crimped length Lc of a fiber, in percent of the extended length of the fiber L0 as shown in by the equation (3). Ci describes the crimp potential of a textile fiber as its ability to contract under tension. A crimp index of zero (Ci = 0) indicates that the fiber is straight with no crimp. A crimp index of one (Ci = 1) indicates that in the relaxed state, the fiber is in collapsed loop form in the case of helical crimp, or ideally plied together at zero length in the case of planar crimp[3-4]. For a given crimp frequency Cf, the crimp index Ci is a measure for the crimp amplitude. Sometimes, the crimp index is denoted as percentage of the crimped length as the equation (4). (4) (3)
- Crimp Width and Crimp Depth These parameters were defined for the use in numerical image analysis of crimped fibers. The crimp width Cw is the distance between the midpoints of successive valleys of a crimp[4], as shown in Figure 3. The crimp depth Cd is the perpendicular distance between a peak of a crimp and a line joining the valleys of the adjacent crimp waves. Cw , Cd or Cw /Cd characterize the size of individual crimps (Figure 3). The mid points are determined by numerical regression of fitting curves. For an idealized triangular crimp bow, Cw corresponds to λ/2 and Cd corresponds to 2A[5,7]. Crimp Degree The crimp degree Kg is defined as L0 / Lc, where L0 is the length of the straightened fiber and Lc is the length of the crimped fiber[9,12] .It is related to the crimp index by the equation (5); (5) Three-Dimensional Character of Crimp Crimp form of textile fibers is essentially three-dimensional in character because the two parameters, geometric curvature and torsion, accurately describe the space path of fiber crimp. Measurements needed for determination of these parameters are tedious and impractical to obtain by manual methods. An instrument built by DuPont makes these measurements automatically [11,14]. It differs from published instrumental techniques by providing data which allow definition of torsion and curvature of crimp as well as the more usual the fiber crimp frequency and extensibility parameters[8, 12-13]. A measurement of a series of fibers shows that fiber crimp, expected to be planar by virtue of mechanical crimping process, and has a three-dimensional character. A curved line in space can be defined by curvature and torsion at any point along the line. These two quantities as a function of arc length of are basis for development of crimp parameters. Statistics of curvature and torsion distribution along a fiber and among fibers are also important descriptive parameters of crimp. Geometric torsion should not be confused with mechanical or fiber torsion, commonly called twist. Curvature is by definition a mathematical expression, which is more easily visualized in a physical sense than is geometric torsion. The mathematical statement of these parameters can be shown as by the equation (6).
- (6) The following diagram (figure 4) shows a curve in three-dimensional space (x, y, z Cartesian co-ordinates) for a crimp forming geometry over a textile surface. We generally used the torsion (T) and curvature (K) at any point on this curve can be defined in terms of three orthogonal vectors. The normal vector which is in the direction of the radius of curvature, the tangent vector which is tangent to the curve at the point and orthogonal to the normal vectors and the binomial vectors which is orthogonal to the other two has been shown by this diagram. Curvature, K and torsion, T can be expressed in the x, y, z-co-ordinates system, so that data obtained by the automatic crimp-measuring instrument are used to calculate these parameters for any crimped fiber[13-15]. Crimp Distribution The purpose of the study of fiber crimp distribution in nonwoven fabric is to quantify fiber mechanical behavior during de crimping and re-crimping, and relate it to fundamental fiber properties, nonwoven fabric properties, and process ability in nonwoven equipment by the study of crimp stability of fiber crimp in nonwoven fabrics. PET fibers of three different crimp levels have been carded and webs have been produced under various processing settings. Samples from different settings and processing stages are being tested for their single fiber crimp characteristics. An empirical function fitting the stress- strain curves of several single fibers in the crimp region has been found and may deliver parameters that characterize the crimp removal behavior of these fibers and their processing parameters. Synthetic fibers must be crimped to process on conventional carding equipment[16]. The initial crimp level and crimp retention during processing influence nonwoven fabric performance. Effect of Crimp on Filtration Performance Use of crimped rather than un crimped fibers improves both efficiency and drag characteristics. The reason for the reduced drag and higher efficiency with crimped
- fibers may be found that straight fibers seem to form groups of two or more where the fibers run close together for a considerable length[17]. The space between them becomes clogged with filtered particles, and the group then acts as a single, wide, flat fiber with a higher resistance to air flow. Efficiency decreases because of the larger spaces between these groups. None of these groups is visible in the case of crimped fibers, which maintain an open structure. Crimp Frequency Experiments with fabrics made of crimped and uncrimped fibers showed that the crimped fabrics gave higher efficient lower pressure drop. This information is of limited value, since in practice uncrimped fibers are difficult to process and are therefore rarely used[15,18]. The extended study of the effects of crimp was therefore aimed at detecting any dependence on crimp level. It is done with two sets of fabrics. The first was made from three rayon fibers samples having crimp levels designated zero, normal (5.8 crimps/cm) and high (7.6crimps/cm). Micrographs of the three kinds of fiber are shown in Figure 5; Figure : Micrographs of rayon fibers (7.8X); A = no crimp, B = normal (5.8 crimps/cm), C = high crimp (7.6 crimps cm)[15] Efficiencies of the three fabrics (weighing 0.17 kg ml) were measured using atmospheric dust and no cake, and the results are shown in Figure. In agreement with earlier results for a variety of fabrics when no dust cake forms, no differences in capture efficiencies are, seen. When the fabrics were used to filter a fly ash aerosol with accumulation of a dust cake, however, the results in figure were obtained. These show that in the presence of a cake, crimped fibers give slightly higher efficiencies
- than straight fibers. There is no significant advantage of high over normal crimp level, however. The advantage of crimped fibers over uncrimped extends into the diffusion region, showing again that the differences are probably due to different cake formations[12,19], since in the absence of a cake no differences in penetration were detected in this region. Table 1. Pressure-drop characteristics on crimp level[19-20] Crimps, cm Density, g/cm3 K (1),104 N.s/ Kg.m K (10),104 N.s/ Kg.m Air permeability,m. s (ft. min) 7.6 0.112 0.99 2.17 1.52 (300) 5.8 0.112 1.05 2.67 1.43 (282) 0 0.112 1.29 3.35 1.28 (242) With regard to pressure-drop characteristics, however, additional differences can be discerned. Table 1 gives results for two sets of experiments comparing the three rayon fabrics, and it can be seen that a higher crimp level brings higher air permeability and low specific cake resistance. The effect of crimp level on pressure- drop characteristics was also measured and the table 2 shows that at the tenth cycle the final pressure drops and specific cake resistance of both high and regular crimp sa Fiber orientation The experiment were simulation and tracking scheme done by R. Dent to produce image for characterizing fiber orientation varying different parameters in nonwoven structure[22]. Incorporation of crimp Most real nonwovens have fibers or filaments with crimp or curl. For simplicity, we assume a sine wave to be sufficient for describing crimp. A sine wave is also convenient because it can be easily represented by its wavelength and amplitude. In defining crimp, we allow crimp percentage and period (wavelength) to be specified. The height (amplitude) is computed from these parameters[29-31].
- Orientation Distribution Function (ODF) The orientation distribution function (ODF) f(α) is a function of the angle α. The integral of the function f (α) from an angle a α1 to a α2 is equal to the probability that a fiber will have an orientation between the angles α1, and a2. The function f(α) must additionally satisfy the following conditions (7) and (8); (7) The simulation ODF is computed while the simulation is in progress depending on the segment size for crimped fibers[27,31]. Simulation Results Some sample results are presented in Tables 4 show the structural features of the simulated images. Table 4. (i) displays image series I where the orientation anisotropy varies for a set composed of continuous lines with no crimp. They were generated using the μ-randomness procedure. All other parameters were kept constant for this series. Table 4 (ii) shows the image series II also vary in their orientation anisotropy. These are composed of discontinuous lines and incorporate 10% crimp[32]. They were generated using the I-randomness procedure. The third series was also generated using the I- randomness procedure and they vary only in their crimp percentage, shown in Table 4.
- Conclusion: Crimp is an important property that determines processing behavior in carding, drafting and fault incidences in yarn. Crimp frequency, amplitude, crimp stability, crimp elongation, crimping point are some of the important properties that determine crimp Fiber crimp characteristics have a big influence on the processing performance of the fibers. Crimp also contributes essentially to the properties of intermediate fiber assemblies, yarn and finished fabrics. Fiber crimp imparted to synthetic fibers, which are initially straight, makes it possible to process these fibers with existing machinery designed for natural fibers. In this assignment I know about various thinks about crimp. Its help me in my working life.

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