Nonwovens
Feasibility study of spunbond plants considering the product width The pricing pressure on the production of spunbond products is constantly rising in spite of an increasing demand. One possibility for cost reduction is the widening of plants. Wider plant concepts require other technical solutions which, however, also involve various technical and commercial benefits. Dirk Laukien Oerlikon Neumag, Neumünster/Germany
Technical consideration For a plant design which is not limited in width, it is necessary to segment the plant components. It is of utmost importance that the segmenting is effected without influencing the product characteristics. Through the segmenting respectively the consideration of the effects between the segments, edge effects are also minimized, such as for example, drops or gradually decreasing basis weights towards the edge. Normally that part of the product, which has edge effects is separated and recycled. The components of a spunbond plant are: - raw material handling unit - extrusion - spinning equipment - filament cooling device - drawing device - forming table with vacuum box - thermobonding calender - winder - rewinder. Raw material handling unit and extrusion The components for the raw material handling and the extrusion can be obtained with practically all sizes and throughputs. Spinning equipment The spinning equipment of the Oerlikon Neumag technology is split up into segment widths of about 150 mm. Each segment is in this case fed with polymer from an individual outlet of a planet gear pump. For a uniform distribution, the polymer streams are separated up from the spinning pump to each segment and separately led to the spin pack. The streams meet again in the spin pack directly in front of the spinneret. Important in this case is, that all melt streams have exactly the same ‘history’, i.e. an identical temperature and retention time. E 62
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Slight differences make the polymer age differently and lead to changed melt viscosities. A pressure unification takes place directly in front of the capillary holes so that possible pressure differences can be hydraulically compensated. The identical temperature and retention time for the stream through all the capillary holes ensures an extremely uniform titer distribution from the first to the last spinneret hole and therefore equal cooling and drawing characteristics. Cooling device Beneath the spinneret, the filaments are cooled through quenching with cold air. The melt solidifies to filaments during this process. The cooling air must be led to the fiber bundle extremely uniform over the entire width. Smallest speed or temperature fluctuations result in process instabilities. The design of the quench box ensures a high homogenized air flow by use of fine screens before meeting the fiber bundle in order to compensate possible differences in the air stream. Drawing device The drawing device, the so-called draw slot, has the purpose to draw the filaments so that a defined ratio between the outlet speed of the capillaries and the filament speed is achieved. This ratio is responsible for the diameter of the single fibers. This is a system which has to provide an even air stream over the full width. The design of the slot must be designed in a way, that no separating devices obstruct the air stream and make a segment separation visible. This is attained with smallest segments of 20 mm length which homogenize again until they exit the slot. Forming table with vacuum box The forming table can be adapted to nearly any width by dimensioning the rolls accord-
ingly. Corresponding machines up to 12 m wide are known from the paper industry. The vacuum box ensures that the single filaments which hit the belt are fixed and in this way form the web. This requires a high pressure uniformity with regard to pressure and air flow uniformity because a non-uniform depositing pattern would result in a quality reduction. With the actual design of Oerlikon Neumag, a constant airflow is attained with a deviation in the machine direction before the exhausting is effected on the side. The particular advantage is that this design does not increase the construction height of the web former which means that the operator can look onto the web former to check the condition of production without having to use steps or platforms. Thermobonding calender, winder and rewinder These components have also long been available in widths of 12 m in the paper industry. The demand on the suppliers of these components was to simplify the technology in order to meet the strained price situation of the nonwovens industry. The supplier industry has successfully implemented this so that corresponding machines are available on the market. Commercial consideration The production costs of a spunbond plant can be ascertained with different methods. The diagram is based on the following types of costs: - polymer costs - investment costs - waste costs - energy costs - personnel costs - downtime costs. Polymer costs The polymer costs form the largest part (about 65%) of the manufacturing costs. There are two approaches for saving raw material costs. In both cases the Oerlikon Neumag technology offers more uniform production and processing of less refined polymer. Based on the segmented plant design, very uniform basis weights are attained over the entire width. In this way, a basis weight distribution of 3% or even less can be reached
Nonwovens
without any efforts. Less fluctuation in the product characteristics in which case the minimum must also lie within the specification, may result in savings in the production of 2-3%. An extremely uniform polymer distribution, temperature and cooling air velocity enable the process to be adjusted much finer than with non-segmented solutions. This in turn enables the application of materials with a wide molecular weight distribution, which do not have to be that strongly stabilized by the raw material producers. This results in a considerable commercial benefit of raw material supply. In this way, titers of dtex 0.7 can be properly economically produced without, the use of metallocene. One can revert to even less stabilized material for traditional qualities (dtex 1.7-2) which leads to savings up to 5%. Investment costs The second largest part of the production costs are the investment costs. In this case, one must of course not only consider the sheer purchasing costs of the plant, but also the building and utility costs. In the past, the necessary reduction of the investment costs has already led to a widening of plants. The procurement costs of the plants themselves decline up to a width of about 7 m. The building and utility costs are more significant. If, for example, a producer has to build 2 plants instead of one for the same capacity, the respective logistic paths must also be doubled. This results in a nearly 60% larger building area. The supply channels for energy, water and cooling also have to be doubled.
With about 5%, the energy costs cover only a smaller part of the entire production cost. Personnel costs Personnel costs considerably depend on the region. A European operator costs about € 40 per hour, whereas his Chinese colleague only costs about € 2 per hour. Experiences have proven that the number of workers for the ‘sheer’ production process (from the raw material storage up to the winder) does not significantly increase through the plant width. According to the plant capacity, a proportionally high number of personnel is required for the packaging. This can however be minimized with a corresponding degree of automation. For example, the same amount of personnel is necessary for the operation and maintenance of a 7 m wide plant as for a 5 m wide plant.
The costs caused by the product which directly occur during the conversion of the used materials hardly depend on the plant width. This is for example the melting capacity of the extruder, the energy consumption of the drawing device or drives of fans. With the energy costs caused by machines, can be saved up to 20%. This includes the heat radiation from the extruder or calender heating, but also the drive power of the web former edge which is not covered with the product.
Downtime costs The planned downtimes of a plant do not depend on the width. It is necessary to wipe the spinnerets after a few days (depends on the raw material quality). The spinnerets have to be changed at regular intervals in order to exchange the filter in the spin pack. Due to the weight, special equi pment for handling the spin pack already has to be used starting with a width of about 1 m so that no additional equipment is necessary ■ for 7 m wide plants.
Waste costs As mentioned above, the polymer costs form the largest part of the production costs. The reduction of waste costs is therefore of utmost significance for the economy of a production plant. The segmented technology of Oerlikon Neumag offers some advantages in this case. Based on the segmented melt pipeline, a uniform distribution also over wider widths is independent of the viscosity. Polymer which has been contaminated by recycling material, e.g. from SMS or bico (PP/PE) waste, can be tolerated to a great extent. Especially with bico plants, the higher product width is a further advantage. The higher width produces an edge cut with a small percentage so that eventually contained PE constituents can be fed back in a smaller percentage thus causing less ‘irritations’ in the spinning process. Energy costs The energy costs can be split up into energy costs caused by the product or the machine. Technical Textiles 2/2009
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