Now You Know Hydroentanglement Bonding Process for Production of Nonwoven Fabric (Part-3)
Friday, 8 March 2019
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Hydro-entanglement Bonding Process for Production of Nonwoven Fabric (Part-3)
Eng Mohamed Elsharkawy
Dept. of Textile Engineering
Alexandria University
Alexandria, Egypt
Email: m.elsharkawy.tex@gmail.com
Dept. of Textile Engineering
Alexandria University
Alexandria, Egypt
Email: m.elsharkawy.tex@gmail.com
2.6. Machine components
2.6.1 Web support system:
The web support system plays an important part in most nonwoven processes. Especially for the spun-lace process, it has a critical role in this process because the pattern of the final fabric is a direct function of the conveyor wire. By special design for the wire, we can have following varied products [11]:
- Ribbed and terry cloth-like products
- Aperture products
- Lace patterns or company logo can be entangled into fabrics
- Production of composites
- Formation
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| Figure (2.5): Support system for the web |
2.6.2. The hydro-entanglement unit:
Hydro-entanglement is an energy transfer process where the system provides high energy to water jets and then transfers the energy to the precursor. In other words, the energy is delivered to the web by the water needles produced by the injector. Therefore, we can calculate the energy from the combination of the water velocity (related to the water pressure) and the water flow rate (related to the diameter of the needles) [11].
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| Figure (2.6): Hydro-entanglement unit |
As we know, water is most critical part in spun-lace process. Therefore, there are some requirements for the water as follows [11]:
- Large amount of water – about 606 m3/hr/m/injector for 40 bar and 120m m
- Nearly neutral pH
- Low in metallic ions such as Ca No bacteria or other organic materials
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| Figure (2.7): Water system |
Due to the large amount of water consumed, the spun-lace process requires that it be recycled. Therefore, a high quality filtration system is necessary for the spun-lace process. Some of special filters are listed as following [11]:
- Cartridge filter
- Sand filter
- Bag filter
2.6.5 Web drying:
When the fabric leaves the entanglement zone the web, it is completely saturated with water. There are a few steps to remove water from the fabric. They include [11]:
2.6.6 Vacuum dewatering system
In general, the diameter of water needle ranges from 100 to 170 m. The highest number of needles is 1666 needles per meter of injector, corresponding to the smaller diameter. The water pressure ranges from 30 bars to 250 bars and it is increased stepwise from injector to injector.
The process variables are considered to have secondary effects on the performance of the finished product. The supporting substrate transport is an important variable influencing the fabric.
There are two systems of entanglement substrate systems: flat and rotary. For the most part [13], there is no difference in the mechanism used to achieve entanglement. The rotary concept uses a compact machine design with ease of sheet run that provides entanglement of both sides of the web. Entanglement is nearly achieved with as little as four meters (in the machine direction) of the material. Sometimes the fibers are driven through the substrate wire and, in the flat concept, it is seen that the wire (along with the fibers) is dragged over the suction box causing difficulty in the removal of the product. In the rotary concept, this problem is not encountered because the fibers are not pulled along the machine direction. Substrate texture seems to have important influence on the product. The size of perforations is usually measured in "mesh", which is the count of wires per inch of the substrate. It has been shown [12] that imposing the same energy into two webs with different substrate meshes; the finer substrate yielded a stronger product resulting from finer support. The coarser wire support (20 meshes) gave a bulkier product with more permeability, but with less strength. Water removal from the fabric was shown to be dependent on the mesh of the support belt. The lower the mesh, the more energy that was necessary to remove the remaining water. In addition to that, the surface of the fabric can be aperture (textured on the surface) with a specially structured substrate [13].
The amount of energy delivered in the web is a crucial parameter influencing the fabric structure and properties since
Water pressure is another parameter related to fabric energy intake. There are several water pressure levels used [7].
Another basic process parameter having influence on the fabric is the speed of the line. If a constant amount of energy is being delivered to a fabric, the speed of the fabric determines how much energy is going to be absorbed per fabric unit area. Logically, the higher the line speeds, the less the energy that is absorbed by the fabric and the lower the fabric strength that is achieved.
Now, higher water pressure machines are mostly used since using high pressure, energy can be delivered into a web with less water needles and less water. This is economically more useful [7].
2.8. Water flow and nozzle characteristics
The underlying mechanism in hydro-entanglement is exposure of fibers to a non-uniform spatial pressure field created by a successive bank of high velocity water jets. The impact of these water jets with the fibers, while they are in contact with their neighbors, displaces and rotates them with respect to their neighbors. During these relative displacements, some of the fibers twist around others and/or interlock with them due to frictional forces. The final outcome is a highly compressed and uniform fabric sheet of entangled fibers. Since its infancy, hydro-entangling has shown promise for the textile industry.
The uniformity of the product and the repeatability of the hydro-entangling process require a continuous and locally uniform jet-fabric impact.
Water jets are known to break up somewhere downstream of the nozzle due to the interfacial forces between them and the surrounding air. A number of parameters, including nozzle internal flow patterns resulting from cavitation and/or wall friction, influence the behavior of the water jets.
Conventional hydro-entangling orifice nozzles have geometries that consist of a conical part and a capillary section.
The study of simulation and characterization of water flows inside hydro-entangling orifices employed a two-phase numerical simulation to make a comparison between the cone-up and the cone-down orifice nozzle geometries used in hydro-entangling.
The conclusion was that the cone-down configuration has a lower discharge coefficient than its cone-up counterpart. Therefore, a water jet discharged from a cone-down orifice has a slightly smaller diameter than that of a cone-up.
The cone- down configuration has a velocity coefficient slightly larger that of the cone-up configuration because the wall friction is higher in the cone-up geometry. The cone-down geometry forces the water to separate from the metallic walls once it enters the capillary.
This prevents cavitation, which is known to shorten the intact length of the water jet. The air gap between the water and the metal surface extends the lifetime of the orifice.
In the following Figure an orifice with the cone-up version is demonstrated to facilitate interpretation on the comments being made on this section.
The cavitation and hydraulic flip study (Vahedi Tafreshi and Pourdeyhimi 2004) reported that Hydro-entangling owes its success to the peculiar properties of coherent water jets.
For Hydro-entangling to be feasible at higher pressures, it is extremely important that water jets maintain their collimation (a straight line) for an appreciable distance downstream of the nozzle.
The discussions were regarding cavitation and its irregular and unsteady phenomena nature.
A realistic picture of cavitation can be described as an irregular, cyclic process of bubble formation, growth, filling (by water), and break-off.
In an actual water jet, a vapor cloud after formation and growth will be carried downstream, another cloud will form in its place, and the process will repeat. However, if the cavitation cloud reaches the nozzle outlet, the ambient air will be sucked into the nozzle and cavitation will stop (hydraulic flip).
When the fabric leaves the entanglement zone the web, it is completely saturated with water. There are a few steps to remove water from the fabric. They include [11]:
2.6.6 Vacuum dewatering system
In general, the diameter of water needle ranges from 100 to 170 m. The highest number of needles is 1666 needles per meter of injector, corresponding to the smaller diameter. The water pressure ranges from 30 bars to 250 bars and it is increased stepwise from injector to injector.
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| Figure (2.8): Drying unit |
2.7. Parameters affecting the product performance properties
Both the fiber and web properties have primary effects on the performance of the finished product. These parameters comprise of the web material and area basis-weight, and the way in which the web was manufactured. As mentioned in literature [12], spun-laced technology demands a high quality web, especially in its uniformity and isotropic orientation. The process variables are considered to have secondary effects on the performance of the finished product. The supporting substrate transport is an important variable influencing the fabric.
There are two systems of entanglement substrate systems: flat and rotary. For the most part [13], there is no difference in the mechanism used to achieve entanglement. The rotary concept uses a compact machine design with ease of sheet run that provides entanglement of both sides of the web. Entanglement is nearly achieved with as little as four meters (in the machine direction) of the material. Sometimes the fibers are driven through the substrate wire and, in the flat concept, it is seen that the wire (along with the fibers) is dragged over the suction box causing difficulty in the removal of the product. In the rotary concept, this problem is not encountered because the fibers are not pulled along the machine direction. Substrate texture seems to have important influence on the product. The size of perforations is usually measured in "mesh", which is the count of wires per inch of the substrate. It has been shown [12] that imposing the same energy into two webs with different substrate meshes; the finer substrate yielded a stronger product resulting from finer support. The coarser wire support (20 meshes) gave a bulkier product with more permeability, but with less strength. Water removal from the fabric was shown to be dependent on the mesh of the support belt. The lower the mesh, the more energy that was necessary to remove the remaining water. In addition to that, the surface of the fabric can be aperture (textured on the surface) with a specially structured substrate [13].
The amount of energy delivered in the web is a crucial parameter influencing the fabric structure and properties since
Water pressure is another parameter related to fabric energy intake. There are several water pressure levels used [7].
Another basic process parameter having influence on the fabric is the speed of the line. If a constant amount of energy is being delivered to a fabric, the speed of the fabric determines how much energy is going to be absorbed per fabric unit area. Logically, the higher the line speeds, the less the energy that is absorbed by the fabric and the lower the fabric strength that is achieved.
Now, higher water pressure machines are mostly used since using high pressure, energy can be delivered into a web with less water needles and less water. This is economically more useful [7].
2.8. Water flow and nozzle characteristics
The underlying mechanism in hydro-entanglement is exposure of fibers to a non-uniform spatial pressure field created by a successive bank of high velocity water jets. The impact of these water jets with the fibers, while they are in contact with their neighbors, displaces and rotates them with respect to their neighbors. During these relative displacements, some of the fibers twist around others and/or interlock with them due to frictional forces. The final outcome is a highly compressed and uniform fabric sheet of entangled fibers. Since its infancy, hydro-entangling has shown promise for the textile industry.
The uniformity of the product and the repeatability of the hydro-entangling process require a continuous and locally uniform jet-fabric impact.
Water jets are known to break up somewhere downstream of the nozzle due to the interfacial forces between them and the surrounding air. A number of parameters, including nozzle internal flow patterns resulting from cavitation and/or wall friction, influence the behavior of the water jets.
Conventional hydro-entangling orifice nozzles have geometries that consist of a conical part and a capillary section.
The study of simulation and characterization of water flows inside hydro-entangling orifices employed a two-phase numerical simulation to make a comparison between the cone-up and the cone-down orifice nozzle geometries used in hydro-entangling.
The conclusion was that the cone-down configuration has a lower discharge coefficient than its cone-up counterpart. Therefore, a water jet discharged from a cone-down orifice has a slightly smaller diameter than that of a cone-up.
The cone- down configuration has a velocity coefficient slightly larger that of the cone-up configuration because the wall friction is higher in the cone-up geometry. The cone-down geometry forces the water to separate from the metallic walls once it enters the capillary.
This prevents cavitation, which is known to shorten the intact length of the water jet. The air gap between the water and the metal surface extends the lifetime of the orifice.
In the following Figure an orifice with the cone-up version is demonstrated to facilitate interpretation on the comments being made on this section.
 |
| Figure (2.9): Example of a jet strip and its orifice diagram Kasen Nozzle website. |
For Hydro-entangling to be feasible at higher pressures, it is extremely important that water jets maintain their collimation (a straight line) for an appreciable distance downstream of the nozzle.
The discussions were regarding cavitation and its irregular and unsteady phenomena nature.
A realistic picture of cavitation can be described as an irregular, cyclic process of bubble formation, growth, filling (by water), and break-off.
In an actual water jet, a vapor cloud after formation and growth will be carried downstream, another cloud will form in its place, and the process will repeat. However, if the cavitation cloud reaches the nozzle outlet, the ambient air will be sucked into the nozzle and cavitation will stop (hydraulic flip).