I, Rajeswari, 36, 17th Road, Chembur, Mumbai 400071 (Tel- 25280767)
Though Nonwovens possess several distinct advantages over woven in terms of labour, space, manufacturing costs besides possessing unique properties, nonwoven industry is in a nascent stage till recently in India. One of the major problems is developing suitable and stable market that gives adequate returns for investment. But the advent of several automobile manufacturing units during the last few years in the country has changed the scenario markedly. Moulded floor carpets, luggage trunk liners, seat covers, roof liners and furnishings used in automobile are invariably made of nonwoven because of their special properties and cost advantage. Nonwovens have better adhesion property than woven and so binder application is more uniform. They also possess higher thickness for a given weight per unit length and so are more voluminous and comfortable . In addition they have a more uniform, smooth and random surface. As a result nonwovens are preferred for moulded carpets used in modern cars. Several nonwoven manufacturers in India have therefore added additional facilities like lamination, moulding and trimming to their plant to produce moulded carpets. Since automobile manufactories are highly quality conscious and are committed to QS 9000 quality management systems, nonwoven manufacturers were also required to upgrade their quality monitoring systems and procedures to establish a reliable name in the market, achieve a higher market share and keep down rejections. An attempt is made in this article to outline the critical factors that affect quality of nonwoven moulded carpets, from raw material selection to process parameter selection at different stages of manufacture and the precautionary measures to overcome them. Improvements in checking and inspection and packing of finished product are suggested. Some of the common causes for rejection together with remedial measures are enumerated.
Specifications and quality requirements vary from one automobile manufacturer to another. The specifications for moulded carpets for some of the major manufacturers are given in Table 1.
Table 1 : Specifications for moulded nonwoven carpets
Sr. No. Physical Property Maruti Zen Maruti Esteem GMI Opel Astra Tata Indica Hyundai Santro Fiat India Uno Toyota Plain Toyota Velour
Nonwoven felt Blend of 30%PES &70%PP
Blend of PES & PP of different shades Blend of 2 different shades of PP& 2 different shades of PES PES of single shade PES of single shade Blend of PES of 2 different shades Blend of
3 different shades of PP Blend of PP of 3 different shades
2 Binder back coating Acrylic Acrylic Thermo bondable Acrylic Acrylic Acrylic Acrylic Acrylic
3 Lamination LDPE Powder & Film LDPE Powder & Film Nil LDPE Powder LDPE Powder LDPE Powder LDPE Powder LDPE Powder
4 Namda Mixture of waste fibres Mixture of waste fibres
5 GSM (gms/sq.m)
Namda – 600
Namda – 1200
6 Tolerance GSM +/_ 10% +/_ 10% Min – 800 +/_ 10% +/_ 10% +/_ 10% +/_ 10% +/_ 10%
7 Surface of Non Woven Plain Rib Velour Velour Plain Plain Plain Velour
8 Thickness, mm 4 max 5 5-7 3 min 3.25 +/- 0. .75 3.2 3.2 3.5
9 Tensile Strength
(20 * 5 cm) kgf
10 Abrasion Resistance 10000
Cycles 3000 cycles Weight Loss after 1000 cycles – 20 g/sq m Weight Loss after 100 cycles - < 0.5% Weight Loss after 100 cycles - < 0.5% Weight Loss after 100 cycles – < 0.5%
Weight Loss after 100 cycles - < 0.5% Weight Loss after 100 cycles – < 0.5%
11 Burning Rate ISO 3795 Conform to FMVSS 302 Conform to FMVSS 302
12 Colour Fastness
- To Light
- To Washing
- To Rubbing
3 – 4
13 Temperature stability
The following important features emerge from Table I
1. PP and PES are the fibres mainly used in making needle punched fleece which forms the top surface of the carpet. In Maruti Zen, Esteem and GMI Opel Astra, blend of PP and PES is used.
2. While single shade of fibre is used in manufacturing non woven in Tata Indica and Hyundai Santro, a blend of 2 to 3 shades of fibres is used to get the shade requirements in Zen, Esteem, Opel Astra, Fiat Uno and Toyota. When a blend of fibres is used, more precautions are needed to ensure uniformity in blend proportion and colour of carpet.
3. GSM of the non woven fleece as well as final moulded carpet vary widely from car to car. GSM is lowest in Zen at 640 and highest in Esteem at 975.
4. Plain, Rib and Velour surfaces are found. For velour type, Dilour machine of Dilo would give the best appearance because of random and uniform lay of the piles. As most of the nonwoven manufacturers in India are not equipped with this machine, Diloop type machine is used with a suitable arrangement of fork needles in the board to get velour surface.
5. Though abrasion resistance is an important specification, different manufacturers prescribe different methods of estimating the same. For Zen and Esteem, the number of cycles to rupture or cause a hole in top surface is prescribed. For other manufacturers weight loss after 100 or 1000 cycles of abrasion are prescribed.
6. For assessing flammability, burning rate on a horizontal flammability tester as per ISO 3795 is prescribed by all. All manufacturers require conformance to FMVSS 302 specifications, which is 4 in/min.
Apart from those specified in Table I, specifications for temperature stability, static compressibility, peeling force, fogging resistance, dielectric weldability, and freedom from odour are prescribed by some manufacturers like GMI. Maruti prescribes specifications for static electricity generation and half decay time. Since nonwoven manufacturers do not have facilities for these tests, they are required to take the assistance of Research institutions like BTRA. Some of the test methods are peculiar to automobile industry and have to be developed and standardized even in Research institutes.
Raw material for needle punched top surface
Polyester and polypropylene are the commonly used fibres for manufacture of needle-punched fleece, which forms the top surface of carpet. Their relative merits are discussed below
1. The two fibres are nearly comparable in tensile strength though polyester fibre is available in higher tenacity grades.
2. Elongation is higher in polypropylene. This gives some advantages in terms of reduced tear during moulding.
3. Density of polypropylene (0.91g/cc) is much lower than that of polyester
(1.38 g/cc). As a result thicker, loftier and more comfortable carpets are made with the former for a given area density.
4. Polypropylene is dope dyed and is available in an extensive range of colours and shades. It is therefore much easier to achieve colour and shade matching by mixing a minimum number of shades of fibres. Dope dyed polyester, on the other hand, is available only in a limited number of colours and shades. The required shade has often to be developed through R&D work or fibre has to be dyed.
5. Melting point of polypropylene (165oC) is much lower than that of polyester (260oC). Heating time, temperature and pressing time are therefore more critical in moulding with polypropylene.
6. Flame retardancy by burning rate is inferior with polypropylene than with polyester. A flame retardant compound has to be added to the binder to meet the flammability requirements in Exports with polypropylene. This adds to the costs.
7. Resistance to UV light is inferior with polypropylene compared to polyester. UV stabiliser has to be added during manufacture of polypropylene to improve its resistance to UV light.
8. Polypropylene tends to form beads during carding. The beads get deeply loaded on the cylinder wire and also on the needles. There has to be a regular programme for removing the beads from the wire and needle. Achievable production rates are therefore lower with polypropylene than polyester because of card loading. Card processing problems are more acute with lower deniers and recycled fibres with polypropylene.
Moulded carpet manufacturing can be broadly divided into 4 sections 1. Needle punching 2. Back coating of needle-punched fleece with binder 3. Lamination, Blank cutting and Moulding 4. Trimming, checking and packing. A typical flow chart for manufacturing moulded carpets is given in Fig. 1.
Needle punching line is similar to that in a normal nonwoven plant. The following steps may be beneficial in getting good quality at lower manufacturing cost.
Fig I: Flow Chart for Nonwoven Moulded Carpets
The opening line consists of a Bale opener followed by 2 beating points. Use of high capacity mixing bin with automatic laying of material and simultaneous spraying of antistat such as found in Empto mix of Hergeth is helpful in ensuring intimacy of mixing and in minimising shade variations from place to place. Further performance in carding is improved because of uniform application of antistat. Feeding chute to card should have controls like photocells and vibrator or pressure switch to ensure uniform density of feed sheet. Incorporation of micro weighing system at the top of chute whereby small
pre weighed tufts are dropped into the chute will improve batt weight uniformity further. Card – Cross lap sequence is preferred to air laying because of long length of fibres used for making the felt. Since the fibre denier ranges from 6 to 15 d, card should be mounted with appropriate wire of coarser gauge. The card room should have a good humidification system with some control over temperature. This will minimise loading of cylinder, doffer and stripping rollers especially with finer denier. Loading of rollers not only causes production loss but also is a major source of variations in lenier density (gsm).
Needle punching machine of 2m width would be adequate, as the length and width of the carpets of automobiles is below 2m.The needle punching may be done in 2 stages
i.e., Pre needling line and a Finish needling line with a break in between. This has the advantage that if the gsm of the material is lower than the requirements, correction is possible in finish needling by the addition of a thin felt of required gsm. A continuous line with preneedling followed by finish needling does not provide such correction facility.
With Die pressed needles with regular barb spacing should be preferred as it minimises fibre damage and achieves uniform fibre interlocking. Further 3.5 inch needle should be used, as it allows wider gauge between stripper and stitching plate which in turn minimises distortion of the voluminous batt as it enters the needle loom. 32-34 gauge with a punch density of 90 – 100 punches /sq cm and depth of penetration of 11 – 13 mm depending on thickness of felt is normally used.
3 inch needle with close barb spacing has the merit that it allows all the barbs to enter the fleece with a relatively low depth of penetration. This improves consolidation and minimises fuzzy surface at the bottom. The lower needle length is also less susceptible to breakage. RB needle however gives more smooth surface. The penetration depth is between 10 – 12 mm with punch density of 150 – 250 punches/sq cm in case of plain carpets. In the case of ribs and velours, a lower punch density of 90 – 100 punches/ sq cm is used to enable easy penetration of fork needles at structure needle machine
In case carpets with rib or velour surface, a structure needle loom is used after finish needling. Diloop loom or its equivalent is used by majority of manufacturers though as pointed out earlier, Dilour machine by Dilo gives a more random and uniform pile surface. According to the position of fork in the needle board, rib or velour surface is obtained. For products with rib surface, penetration side of the fork needle will be reverse side of the finish-needled material, as it will give compact ribs. On the other hand, penetration side of fork needle will be the same side of finish needling in case of velour surface as this improves velour effect. Penetration depth is 7 – 8 mm in ribs and 8 – 10 mm in velours. The normally used fork needle is of 25 gauge.
Back coating with a binder in the form of foam, followed by curing in a stenter gives dimensional stability and strength to the carpet. An auto foaming unit will be helpful as it permits automatic adjustment of foam ratio so as to get uniform pick-up. Acrylic binder is normally used as it gives the required strength. The binder pick-up is kept low as the material is going to be subsequently laminated with LDPE. As seen from Table 1, binder pick-up varies from 10 – 20 % for carpet where subsequent lamination is done. Foam ratio is one of the important characteristics to be monitored and kept under control during the process to minimise variations in binder pick- up which ultimately leads to variations in linier density of carpet. Curing of binder is carried out in a stenter through heating by electrical heaters kept on both sides of the material. Temperature on the binder face is kept at 140oC and on the fibre side at 110oC. Material speed is adjusted as per the material and binder pick up and is usually between 2 – 3 m/min.
To cut down processing time and costs, thermo bondable binder is sometimes used. With such binder there is no need for lamination with LDPE powder/film. Thermo bondable binder is special styrene/ butadiene latex. The latex not only binds the fibres but also provides mouldability to the material. The binder film is thermoplastic with little softening in the region of 35 – 90oC. On further heating the softening increases very rapidly in the region of 120oC enabling moulding. The ratio of styrene to butadiene in the binder determines the stiffness and thermo plasticity of the binder.
Carpets back coated with normal binder (acrylic) require lamination with LDPE powder or film or with both to enable moulding. The properties of LDPE powder have considerable influence on moulding. The average micron size of powder is 70 microns. For satisfactory moulding, the size of the granules should not be uniform throughout but should be variable from 30 to 100 with a peak value of 70. A distribution consisting of coarse and finer particles on either side of the average improves mouldability. The powder is dusted on the moving carpet on the back-coated side by an applicator, which is automatically controlled to achieve the required powder application. The material is then preheated in steps by electrical heaters to softening temperature and calendared by a pair of high-pressure calendars. Lamination of the powder on the carpet takes place as a result of softening and pressure application. With some products a pre-heated film is also introduced into the nip of calendar rollers along with the carpet, which causes lamination of film on the carpet.
Laminated carpets are cut to required length on a blank cutting machine. The length requirements vary with the car and is standardised after initial trials. Lengths are marked on the Table of bank cutting machine with different colours ( for identification) for the different cars.
The laminated and blank cut carpets are gripped on either side by grippers and moved to a pre-heating zone. The LDPE lamination is softened during pre heating. After preheating for a specified time the material is taken forward to a moulding press consisting of a male and female mould. The moulds are fabricated accurately as per the design of car body. Since car body is a patent of the car manufacturer, the moulds have to be prepared after a formal agreement with them to protect secrecy. Moulds are made either of aluminium or resin bonded fibreglass. The former though more expensive has a longer life and has a cooling arrangement, which reduces moulding time and increases productivity. Time for pre heating and pressing in moulding are critical factors affecting quality of moulding. They are determined by type of fibre, gsm of carpet, type of binder and lamination. Carpets made out of high proportion of polyester require in general more preheating than those with high proportion of polypropylene. Heavier carpets require more pressing time. Time for pre heating and pressing are standardised for each type of car after initial trials.
With some cars like Maruti, Namda, which is a bulky material made out of waste fibres, is attached to the underside of the carpet either during moulding itself or subsequently by an adhesive. Namda improves insulation and helps to absorb vibrations.
After moulding the carpets are trimmed to shape on a trimming unit. The extra material at the sides used for gripping the carpet during moulding is cut out and holes are punched at different place to accommodate car parts like seat legs, break, clutch and accelerator pedals. Trimming unit is made to match car body perfectly in minutes detail so that trimmed moulded carpets fit perfectly in the car.
The moulded and trimmed carpets are checked for fitment on car body or a template provided by car manufacturer. Minor defects in the carpets are mended. Carpets with major defects and improper fitment are rejected. As majority of problems come from improper fitment, a great care has to be exercised in trimming and inspection. Too reduce labour costs, some manufacturers get trimming and inspection done on contractual basis by an outside party.
Heel Pad Fixing
In some cases, heel pad is welded to carpet on a high frequency plastic welder. The object of heel pad is to prevent wear of carpet due to foot pressure from driver of car. Heel pad is usually of PVC material and car manufacturer gives specification.
Final checking for fitment is done on car body. As this is time consuming, it will not be possible to have 100% checking. So random checking of 15 – 25% of production is usually done. It would be advantageous to carry out checking some carpets on car body with seats in position.
Major Quality Problems
Some major quality problems in moulded carpets which lead to rejection of the material and precautionary measures to overcome them are discussed below.
Non matching of colour of the carpet with the master approved sample and variations in colour and shade from place to place and lot to lot form a major source of rejection.
The following precautionary measures will help to overcome this problem.
1. Raw material (fibre) should be matched in colour against master sample in a colour cabinet. It is advisable to have two master samples representing the extremes in tolerable colour variation.
2. Intimacy of mixing of components can be improved by increasing the number of layers in the sandwich and by having one more toppling before processing.
3. Dope dyed fibre should be preferred to dyed fibre as better consistency in colour and higher fastness to rubbing and light is obtained with the former.
4. Scope for improvement in fibre opening in the line should be explored by addition of Kirschner beater.
Contamination with foreign material and fibres from previous lot arises because of inadequate cleaning of card wires and needle board and stripper and stitching plate. Molten plastic beads and nep like clusters in needle board also result in contamination. Card wires should be washed with petrol once a shift and during lot change. Needle board should be removed and cleaned with compressed air. Fibre clusters firmly adhered to the barbs should be removed with a wire hook. Needle board cleaning should be done once every 4 hours and at the time of lot change.
High variations in weight (gsm) and thickness from place to place cause rejections. The variations in weight can be at needling stage or during back coating. Stoppages of card due to loading of material on cylinder, workers, strippers and transfer rollers is one of the causes of variations in gsm. This problem is more often encountered with fine deniers and with polypropylene. Optimisation of antistatic spray applied on the fibre and maintenance of proper humidity and temperature in card room will help minimise loading. The card wire should be washed with petrol and damages in wire rectified. Lower doffer speed with higher web density will also be helpful. Control of foam ratio in the binder during back coating will also minimise variations in binder pick-up.
Lines in the Fabric
Two types of defects are common.
1. Longitudinal lines running along machine direction of needle punching line, arise from broken or worn out needles. A proper schedule for replacement of needle should be drawn up and adhered to. Life of needle depends on type of needles, barb size, type of product, punch density, gsm etc. and it would be difficult to give a schedule for needle replacement that will hold good at all times. Phased replacement of needles in the needle board as suggested by Foster Needle Co., will help minimise lines caused by needle wear and breakage. Needle board should be divided into 3 parts as shown in Fig. 2. All needles occupying one third of the width of the board (section A) is replaced at a time after about 10-15 million strokes. Broken needles in other 2 halves of board ( B & C) are replaced by good needles removed from the board at this time. After another 10-15 million strokes, all needles in section B are replaced and broken needles in section A & C replaced by good needles retrieved from B The same process is continued after another 10-15 million strokes in Section C.
Fig 2: Needle Replacement procedure
. A A A
B B B
C C C
After 10-15 ms After 20- 30 ms After 30-45 ms
2. Horizontal lines in cross direction of fabric at regular intervals arise because needles are penetrating in the same hole as fabric advances. This arises because net advance of fabric during a stroke matches with the distance between two adjacent rows of needles. The defect can be overcome by altering the slowly the stroke frequency while keeping fabric delivery speed constant.
Rejection due to deformation or non retention of the shape of moulded carpet is another source of rejection. Moulded carpets laminated with LDPE film in general retain he shape much better than those laminated with LDPE powder. But lamination with film alone is more difficult and wrinkles are encountered if the speed of film is not properly controlled or if the film is not properly rolled. So a good compromise is to laminate with both powder and film. Improper packing and transportation of carpets also leads to loss of shape. If a large number of carpets are stacked one over the other and transported, those at the bottom get deformed. Carpets should be laid on specially prepared rack with number of trays conforming to the shape of carpet. The carpets in different layers should be separated from another.
Improper fitment is one of the major complaints. This is mostly due to improper trimming arising from wrong methods or improper tools. Use of pre heated knife helps in getting sharp cuts. The tools used for cutting holes should be checked for wear and tear and sharpness. The car body used for checking carpets in moulding plant must match accurately in shape with the cars in automobile manufacturing unit.
Holes in Carpet
Excessive pre heating is one of the causes of holes particularly at corners. Excessive tensioning of material during moulding may also contribute to holes. Mould should be free from burrs or scratches or extraneous matter.
Wrinkles in Carpet
Wrinkles are caused because of inadequate or excessive pre heating or inadequate tensioning of material during moulding. Pre heating time has to optimised for different types of fibres and area density of carpet.