In addition spun bonding and meltblown nonwovens are nonwovens made directly from filament extrusion. In this article, we will discuss various factors that affect quality and performance of needle-punched fabrics and various developments in the needles and the machine.
Needle Punching
Needle punching is by far the most versatile and commonly used method of bonding accounting for 20 -25 % of the nonwovens. Further, needle punched fabric forms the base in making many composites. Needle punching is carried out passing a number of needles with barbs, mounted in a board, through the batt at a high reciprocating speed.. The machine consists essentially of a needle board, stripper plate and stitching plate, Needles are arranged in a number of rows (up to 23-26) in a needle board with about 1500 to 5000 needles per 1 m working width. Board size ranges from 200 to 320m working width. Stripper and stitching plates are perforated so that needles pass through them during up and down movement of each stroke. The stroke varies from 30 to 60 mm. The needles are usually triangular in cross-section with barbs at the three edges. As the needle penetrates through the batt, the barbs carry fibres with them thereby causing mechanical entanglement of fibres as shown in Fig 1 below.
Stitch density or punch density denotes the number of stitches or punches per square cm made by needles on the felt . Punch density is dependent upon the number of needles per board, number of boards, stroke frequency and delivery rate. Stroke frequency is indicated by the number of up and down strokes made by the needle board per min. This is usually 1000 in old machines and has gone up progressively upto 3000 in latest models.
Net Advance
Advance made by felt per stroke, mm =(Deliveryrate,m/min×100/(Stroke Frequency)
Needle punching machines also differ in regard to the direction of punching. In down stroke machines punching takes place from top to down while in up stroke machine punching takes place from bottom to top as shown in Fig 3
Fig 3 : Up and Down PunchingProcess Factors
The important process factors that affect quality of needle punched nonwovens are
- Type of batt preparation
- Punch density
- Depth of penetration
- Needle characteristics
- Feeding System
- Batt quality
- Fibre characteristics
Batt Preparation
The major methods of batt prepation and factors affecting batt quality have been discussed in an earlier article1.
Punch density
Punch density is determined by the number needles per metre width, production rate and stroke frequency.
Punch Density = (Number of needles per cm working width ×10)/(Advance per stroke,mm)
=(Number of needles per working width×10 ×stroke frequency)/(Delivery rate ×1000) Production rate,(m/min)= (net advance per stroke,mm ×stroke frequency (punches/min))/1000 Thus punch density can be increased by 1. increasing the needles per board 2. increasing stroke frequency at a given delivery rate and 3. reducing delivery rate at a given stroke frequency. Number of needles per metre width is determined by the make of the machine. Production rate can be increased by increasing net advance and or punching rate. There has been a substantial increase in needle density over the years contributing to marked increase in production rate. Number of needle boards per machine also varies with the model. With increase in number of boards, there will be more needles per metre width and consequently higher punch densities at a given production rate or higher production rate at a given punch density can be obtained. Needle punching is usually done in 2 stages viz; pre-needling and finish needling. This gives the best results in terms of uniformity as punching has been done in a gradual manner. However, later models have dispensed with pre needling machine.
Gradual reduction of thickness of batt, optimum control by means of adjustable gap between conveyors and close feeding of material to needle are required to minimize uneven draft and irregularity in product. Comprssive batt feeder by Dilo and RDF/FFS and DFS of Oerlikon Neomag, and SFD and DCIN batt feeder of Asselin, are equipped with such features.
Several authors have studied the effect of process parameters in needle punching on thickness, strength, elongation and other physical properties of Nonwovens,2,3,4,5,6,7,8,9. Apart from straightforward methods of varying process parameters, central composite rotatable experiment design and Taguchi methods have also been employed to study the effect of needling conditions on fabric properties9,12, Taguchi method with grey relational analysis is useful in getting multiple property optimization. Artificial neural network modeling has also been used to predict the effect of web gsm, punch density and depth of penetration on properties of nonwoven15 and the prediction shows good agreement with actual results.
Fibre Properties
For successful needle punching fibre length has to be beyond 40 mm to assist in entanglement. Strength and dimensional stability of nonwoven improves as length increases up to 80cm. Beyond that, strength improvements are marginal because of fibre breakages. With increase in fibre length, fibre entanglement during needle punching improves resulting in less slippage and higher strength (Fig 4). With reduction in fibre denier (tex), web becomes more compact and strength improves because of better entanglement due to increased specific surface area4,11,13. However, improvement is marginal beyond a point because very fine fibres are more susceptible to breakages. Further opening size, which is a critical property in geotextiles reduces11 with fineness because of compactness (Fig 5). So for filter and geotextile applications fibre denier has to be properly chosen to meet pore size requirements. Studies on mixture of fine and coarse fibre showed that compressibility, recovery and energy loss% increases with increase in fine fibre and later decreases16. Though not as important as in woven, fibre strength has also a significant influence on increasing strength of nonwovens. Contribution of fibre strength to nonwoven is, however, very low ranging between 5 – 15 %. With increase in ratio of kinetic to static fibre friction, nonwoven exhibits stick-slip tendency during load application. With increase in fibre friction, strength improves because of reduced fibre slippages and greater fibre carrying capacity of needles. Sometimes solutions are sprayed on the fibres to improve fibre friction With higher fibre crimp, consolidation improves resulting in better dimensional stability and strength of needle punched nonwovens. Rayon fibres consolidate more easily than courtelle or wool4. Fibre cross-section shape influences compression and recovery17. Initial thickness and thickness loss are highest with trilobal followed by round and hollow cross-section with polyester nonwovens. Hollow cross-section showed least % compression and minimum compression resilience. Nylon fibre has circular cross section while nomax has cocoon shape cross section. Nylon has therefore a greater packing factor when repeatedly pressed by hot press18 . Breaking strength and modulus increase with packing factor with nomax but decrease with nylon because of heat deterioration of latter upon subjecting it to repeated hot press action. Other properties like bending rigidity and antistatic properties have also some influence on properties of needle punched material. Jute has some inherent advantages as raw material because of its strength, rugged appearance, biodegradability and lower cost. However jute content should be kept within 30% in Jute/polypropylene blend, otherwise wear life and appearance of carpet will suffer19. By sandwiching jute nonwoven in between two layers of polypropylene nonwoven and needle punching, the loss in abrasion resistance and wear life can be minimized. This is because jute is hidden inside and does not come in contact with abrading material. In plain layered carpets made out of jute and polypropylene, needle punching in finish needling should be done from polypropylene side so that top surface is free from jute fibres. Resin bonded jute or waste cotton ‘Namda" is commonly used as an underpad to needle-punched carpet in automobiles as floor covering to reduce vibration and noise. Needle- punched floor covering from kenaf, jute and cotton blended with polypropylene and polyester with an underpad of polyurethane foam or soft cotton nonwoven reduces noise significantly in automobiles20 Kevlar, carbon, steel, teflon and glass fibres can also be needle punched to produce composites used in bullet proof fabrics, insulation,filtration, reinforcement and fire resistant products.
Thickness and gsm
With increase in punch density, weight per unit area(gsm) and thickness of fabric show a progressive reduction2,3 and dimensional stability of fabric improves5. The extent of reduction is initially more marked and progressively reduces afterwards. In needle punching, barbs in the needle push the fibres from horizontal to vertical direction as shown in Fig 1 . The fibre displacement leads to drafting of the felt. As a result gsm and thickness of felt decreases. The effect of these factors increases with punch density. Beyond a certain level of punch density, the fabric resists further compression and so rate of thickness and gsm reduction becomes less. The thickness of felt is lower with finer fibre than coarser fibre because of better cosolidation1 but the effect is more pronounced at lower punch density. Density of the fibre also influences thickness. With lower density, thickness will be higher because of increased fibre diameter. Thickness will also be higher with crimped fibres and hollow fibres. Effect of punch density on weight/unit area and thickness are shown in Figs 6 and7.
Breaking tenacity=(Tensile strength,gm)/(Area density (g/m2)×Specimen width,mm)
Tenacity
Tenacity of the fabric is critically affected by punch density. In cross direction, tenacity initially increases with punch density, reaches a peak and thereafter falls2,21.. In machine direction tenacity increases continuously with punch density but the rate of increase is initially is steep and reduces progressively afterwards. The general nature of tenacity vs punch density relationship is shown in Fig 8
During needle punching drafting of batt takes place leading to orientation of fibres from cross to machine direction. With increase in punch density, draft increases and the reorientation of fibres increases. As a result CD/MD ratio reduces as shown in Fig 9
Elongation
Effect of punch density on elongation is shown in Fig 10 .Elongation is higher in machine direction than cross direction because fibre slippage during break is more in the former. Elongation reduces with punch density but the effect is more pronounced at low punch densities. With increase in punch density fibre entanglements become stronger, slippage of fibres is reduced and as a result elongation comes down. Beyond a certain level of punch density reduction of fibre slippage is reduced and so elongation is not much affected.
Tear Strength
Effect of punch density on tear strength follows the same pattern as tenacity2. Depth of penetration
With increase in depth of penetration, tenacity increases initially but afterwards levels off2,3(Fig 11). With increase in depth of penetration more fibres pass through the batt and to a greater depth. As a result interlocking of fibres is increased, leading to higher strength. But beyond a point, increase of depth of penetration does not improve interlocking much and moreover causes damage to fibres as more barbs pass through the batt. As a result strength improvement levls off beyond a certain level of depth of penetration. Strength is higher in cross direction than machine direction at all depths of penetration because of preferential orientation of fibres in cross direction.
Pore size or opening size is an important parameter in nonwoven geotextiles. This determines separation function of the material of different size of gravels and its ability to to retain soil particles without clogging. Dry sieving using well graded spherical glass particles is one of the methods used for determining pore size.Percentage of particle retained after sieving for a known time is determined for different particle sizes. A plot of particle retained % against particle size is drawn from which the opening size at which 90% of particle is retained 090 is determined. The effect of punch density on poresize is shown in Fig 12 for fibres of different denier11.
With increase in punch density, the fabric becomes more compact and as a result opening size reduces. The rate of reduction reduces with punch density. The fabric becomes more compact with finer fibre resulting in lower opening size.
Compression and recovery
With increase in punch density and depth of penetration, compressibility initially decreases sharply and afterwards reaches asymptotically a value. Recovery follows a reverse trend, increasing with punch density2. This is because the material becomes more compact and stiff with increase in punch density and depth of penetration. However in jute, compressional behavior decreases initially and later increases9. Recovery decreases with increase in punch density and depth of penetration9 because of greater compactness . Needle punched fabrics from staple fibre have a higher compressional energy loss than spun bonded22. This is because of the bulkiness of former. Needle punched nonwovens are more compressible with higher fatigue compression recovery than thermal bonded, wet laid and woven because of 3 dimensional fibre arrangement and higher bulkiness23. With hollow polyester fibres, cross laid batt has higher compressibility and lower recovery compared to parallel laid24. This means fibre orientation in the batt has influence on compressional behavior.
Sound transmission loss decreases initially and later increases10 with punch density with jute.
Air and water permeability
Air and water permeability are broadly unaffected by punch density2, 11. Though pore size is reduced, fabric becomes more compact one compensating for other. Water flow rate through nonwovens could be predicted by finite element analysis25 with good agreement with actual measurements.
Stiffness and Abrasion resistance
With increase and punch density and depth of penetration fabric becomes thinner and more compact and as a result more stiff14. However at higher levels of punch density and depth of penetration stiffness reduces because of fibre breakages. Abrasion resistance improves with punch density and depth of penetration because of compactness and improved fibre integration into the product. Sandwiching hollow polyester between two layers of normal polyester improves abrasion resistance without increasing stiffness15.
Bursting strength and puncture resistance
Bursting strength and cone puncture resistance decrease with punch density2 because of reduction in thickness. Liquid absorption rate of cotton needle punched nonwoven increases with punch density and net advance26. Water absorbency and thickness of jute/polypropylene needle punched nonwovens increase with jute content27
Thermal Conductivity
Thermal conductivity increases with reduction in punch density as material is less compact and more air is entrapped28. On compressing the material by putting a weight over it, thermal conductivity is reduced. Thermal insulation is highest with 9 barbed needle and higher ceramic content in a multilayered needle punched nonwoven made up of ceramic and glass fibres29. On the other hand higher glass content increases radiative transmission of heat30. Radiation thermal conductivity increases with reduction in area density, gsm and thickness. Increase in pore size increases radiation thermal conductivity31
Needle Design
Needle design and shape have considerable influence on the property of nonwoven. Needle consists of 3 parts as shown in Fig 13. 1.Crank 2. Crank shaft and 3 Working blade. In double reduced needle, there is an intermediate blade between crank shaft and working blade (Fig:13).
While the cross section of crank and crank shaft are circular, cross section of blade is triangular. The working blade has barbs on the the three apexes and the barbs are staggered. Essentially the barbs transport the fibres through the batt and carry out interlocking.
The chief properties of the needle that influence nonwoven quality are
- Gauge
- Type of barb
- Barb spacing
- Barb angle
- Kick up
- Length of working blade
- Cross section of working blade
Gauge
Gauge of the needle has to be chosen based on the denier(fineness)_ of the fibre. Finer barb has to be used with finer denier. Broad guidelines regard to the gauge of the barb to be used for different fineness is given in Table 1 below.
| Gauge | Diameter, mm | |
| Above 30d | 18 | 1.2 |
| 19 | 1.1 | |
| 20 | .95 | |
| 21 | .90 | |
| 30d | 23 | .85 |
| 25 | .80 | |
| 28 | .75 | |
| 18-20d | 30 | .7 |
| 32 | .65 | |
| 12- 18d | 34 | .6 |
| 3 - 12d | 36 | .55 |
| 38 | .5 | |
| 1.5d | 40 | .45 |
| 42 | .40 |
Type of Barb
Common type of barbs are conventional and diepressed types. Barbs are cut inside the blade with a chisel in conventional barb. As a result, the barb has a pronounced kick up and sharp edges. Fibres are therefore raked during needle punching causing disturbance and damage to fibres and sometimes fibre breakages. Further, the kick up wears away during use resulting in rapid change in needling efficiency and product quality.However, the products made from such needles are more bulky and lofty as a result of the kick up.Diepressed needle was developed to overcome these drawbacks (Fig 14). The barbs in the die pressed needle are formed on a die and so have rounded and smooth edges. Further a controlled amount of kick up can be had with these needles. As a result fibre breakages and damage are considerable reduced with die pressed needle. The products made are also more compact and have lower thickness. The strength is more and products are more smooth with these needles. Now a days, die pressed needles are invariably preferred for all products except for waddings and high lofts where conventional barb is used.
Needles of 4 barb spcings are commonly available, as discussed below and illustrated in Fig 15: Depth of penetration with different barb spacing is given in Table 2 . Regular Barb
This is the most common type of needle and is available from 13 to 46 gauge. Spacing between barbs is highest, 6.3 mm. Uniform packing distribution of fibres and smooth surface are achieved from top to bottom with this needles because of large spacing between barbs. RB is invariably preferred in pre needling and for high lofts and waddings as it results in higher thickness
Medium Barb (MB)
Medium barb improves needling efficiency and results in more compact products and has a barb spacing of 4.8mm. This needle creates more distinct fibre tufts in the felt and as a reult results in slightly inferior surface appearance. This needle is therefore preferred in waste fibre felts used as underlays in carpets and shoddy. If regular barb is used for such material, the fibres will be pushed out of the felt and rapid fibre accumulations will be found on stitiching plate. MB allows more fibres to be carried per stroke with less penetration depth as a result of the reduced spacing between barbs. At times MB is also used in synthetic leather or shoe felts in intermediate needling.
Close Barb
This needle is invariably preferred in finish needling. Spacing between barbs is 3.3mm Its main merit is that it allows all the barbs to pass through the felt with a relatively low penetration depth.The resuction in depth prevents the first few barbs from pushing fibres too far below the stitching plate and thus cause a fuzzy surface. When fibres are pushed out of the main body, they will not be contributing to the strength. Further, draft during needle punching will be lower with closed barb. So close barb needle results in a more compact and stronger nonwoven than regular barb. Needle breakages is also lower with close barb as needles travel less distance. However, close barb should not be used in preneedling as it creates distinct fibre bundlesat the bottom and variations in density from top to bottom.
High density Barb
These needles have an extremely close barb spacing (1.3 mm). This combined with the short distance between first barb and needle tip enables all barbs to enter the felt with only 8.3mm penetration depth. This needle is preferred in finish needling with thin products. HDB barbs with extremely low kick up or no kpick up are commonly used to get very smooth surface combined with high punch density. These needles are preferred for manufacture of synthetic leather, automotive package tray, spun bonds, automotive trunk liner, shoe and hat felts. Because of of low depth of penetration, needle deflection and breakages are lower.
| Type of Needle | Penetration depth |
|---|---|
| RB | 23.3 mm |
| MB | 19.1 mm |
| CB | 14.8 mm |
| HDB | 8.3 |
Fig 16 below shows the barb angle and kick up in a needle
Kickup
Higher kickup rakes up the fibres without contributing to interlocking.Fibre damages are therefore more and further fabric surface is rough and strength is lower.However, thickness is more and hence higher kickup is preferred in high lofts and waddings.
Working blade length
Lower blade length reduces needle deflection and breakages and result in higher strength.Shorter blade lengths are therefore preferred in finish needling.
Working blade length varies from 11 to 30mm length. Longer working blade lengths are preferred for thicker products but they are more prone to deflection and breakage.
Needle Crosssection
Needle crosssection is usually triangular. However, newer designs have been developed for certain applications like geotextiles for getting higher strength.
Pinch blade
Pinch blade has 2 barbed edges with a diamond shaped crosssection (Fig 17).Needle apexes have an angle of 30 0 as against 600 in normal needle.Consequent to this narrow radius, fibres tend to wrap round the barb face firmly with reduced fibre slippage. Fibre carrying capacity of barb is increased and strength is therefore hogher. Moreover, each barb carries separate fibre fringes and the two barbs work independently. It is futher possible to orient the needle in such a way to reduce damage in machine direction or cross direction scrim cloth threads.Further damage to both warp and weft threads can be minimised by orienting the barb at 45 0. Pinch blade is eminently suitable for geotextiles and filters Pinch blades are preferred in geotextiles and filter fabrics. Pinch blade is however more aggressive than triangular and is therefore not preferred for smooth fabrics.
Star Blade
Star blade has either 3 or 4 barbs.(Fig 17) The 3 edged star blade by Gros-Beckart has a slightly concave working area and a more acute edge angle which results in better grip of fibres and intensive bonding. The cross sectional area is lower by 8% compared to conventional needle. The blade is available primarily in 32 to 38 gauge. The additional barb in the 4 edged star blade in Gros-Beckart's 4star and Foster, increases the fibre carrying capacity and interlocking by about 14% The angle between the outer edges of barb is 600 which increases carrying capacity of barb. Tensile strength and abrasion resistance of fabric is improved by 20 to 30% as a result with this barb. In addition, productivity is increased by 20%.Star blade is preferred with geottextiles particularly from spunbonds and automotive interiors and heavy duty materials.
Normal needle has 9 bars 3 on each apex. However, to have a smother fabric, needles with 3 or 6 barbs are preferred with each apex having 2 or 1 barb instead of 3. For extremely smooth fabrics 2 barb needle is recommended.
Blade Style
Flexing strength is an important characteristic that determines the needle's ability to withstand flexing forces.Flexing loads are high with waste and regenerated fibres and cotton and with rigid materials like aramid and at high punch densities. With normal needles of cylindrical crossection, needle breakages will be high and quality of product will be inferior under such conditions. Needle with conical shape has been developed to increase flexing strength. With conical shape, needle with a lower crosssection first enters the batt when resistance is low and the larger diameter enters afterwards when the resistance is higher Fig 18). So bending and breakage of needle is reduced. Gros-Beckert has developed an improved version of conical needles with a lower diameter, termed as Gebecon. It is claimed to have even higher flexing strength and breakage resistance. The working part of the needle is thinner, more flexible and has extended wear life.
Quadro needle by Singer has a working blade in rhombic form. (Fig 19 ). The barbs are in two opposite edges that are most distant from one another. The barb arrangement with a coarse shank results in more intensive transportation of fibres with a smooth surface. It is possible to increase strength in machine or cross direction depending upon the orientation of fibres in the batt.Damage to scrim threads is also less.Quadro needles are preferred in geotextiles, filters, blankets and needle felts with support material.
Gebecon needles by Grosbeckert have been developed for fine and micro fine fibres
Orientation of Barbs
By changing the orientation of barb in relation to orientation of fibres in felt needling efficiency, strength and surface appearance of product can be altered. We will first consider the case with a needle with one barb. This is often used to minimize damage to scrim cloth.
We will now consider the more common triangular needle which has barbs on all the three edges. Fig 21 shows two types of arrangement of needles in relation fibre flow.
With arrangement in Fig 21 B two barbs are favourably disposed to the fibre flow and will have high fibre carrying capacity and so this arrangement is normally preferred. But if needle marks are to be minimized arrangement in Fig 21 A may be used at the cost of slight reduction in strength.
Scrim Cloth
Scrim cloth is sandwiched between 2 layers of nonwoven and needle punched in making certain products like filter fabrics and blankets. Use of scrim improves dimensional stability, strength and life. Cotton, polyester, blends and HDPE woven tapes are used as scrim The cloth should be of sufficiently open construction, otherwise, threads will be pushed out of the fabric in the form loops and show as defects. Stringent specifications are prescribed in export for nonwoven filters. Calendering and heat setting of needle punched material have been found helpful in meeting export specifications with polyester filters32.
Orientation of barb is even more important in products involving a scrim cloth. During needling such products, the needle barb cause damage to the warp and weft threads and lower the strength. Further, the barbs push the threads out of the felt and cause rolled up nep like defects. The most efficient way to minimize damage to threads is to have needle with barb only on one edge and orient it so that the barb is in most unfavourable position as shown in Fig 22A . Damage to scrim is minimum with such an arrangement but interlocking will be poor as there is only one barb. For improving interlocking without damage to scrim, barbs on 2 edges is used (Fig 22 B)
Alternately to increase interlocking arrangement, pinch needle with 2 barbs ( Fig 23) is used. The barbed edges are perpendicular to fibre flow and parallel to warp threads. Damage to warp threads are minimized while fibre carrying capacity of barbs are increased. By turning the barbs by 900 damage to weft threads can be minimized. The interlocking will be lower as the barbs are less favourably placed to incoming fibre. Pinch blade is more suitable for avoiding damage to scrim cloth as it has only 2 barb edges and at the same time a higher fibre carrying capacity. By arranging the barbs parallel to warp threads as shown in Fig 23 A, damage to warp threads is minimized and by arranging the barbs parallel to weft threads as shown in Fig 23 B, damage to weft threads is minimized. By keeping the barbs at 450 to warp and weft, damage to both is minimized.
Teardrop shaped needle with a single barb is offered by Grosbeckert to minimize damage to scrim cloth (Fig 24).
Different type of needle points available are given in Fig 25 . Needle point is never totally sharp and there is slight rounding of the edges. Extent of rounding or its radius is based on the product. Nonwovens made from monofilaments get damages with needle with a sharp point. Rounded ballpoints are used with such products. Rounded points and polished points are preferred with papermakers felts, filters and blankets. Chisel point needle has been developed by Foster for needle punching into a support base/backing consisting of foam. Needle point has a chisel shape instead of conical. Buckling up foam is avoided by this needle.
Some amount of fibre damage and breakages takes place during needle punching. Damage can be minimized by using diepressed needles and needles with low kick up, fewer number of barbs and regular barb (RB). Fibre breakages and loss in strength with wool fibres increase with needle punch density, depth of penetration and kick up of the barb33 Needle punching of cotton
Needle punching of cotton is difficult because of lower fibre length and higher frictional and penetration resistance. Special needles have been developed by various manufacturers for needle punching of cotton (Fig 27).
Fig 28 : Phased replacement of Needle
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