The mechanical bonding of nonwoven fabrics with the help of needles has evolved into a popular method for manufacturing across a broad variety of products. These not only compete with conventional textile manufacturing processes but also feature new properties that were previously only achievable using other methods. As needling technology steadily evolves, new and decisive impulses are constantly opening up new and interesting sectors.
The December 2015 edition of the Magazine focused on the importance of pre-needling, and selection of the right needle for this manufacturing step. This article is now devoted to the process that follows: intermediate or finish needling. We present the Twisted needle, which has ideal characteristics for this part of the production process.
After pre-needling, a nonwoven is in most cases processed by one or several intermediate or finish needling machines. Finish needling gives the nonwoven maximum consolidation and compaction before it is ultimately finished. In this production step it is also possible to influence surface properties and other technical aspects.
The increasing densification of the nonwoven fabric can take place via different machine configurations. The following variants are possible:
In the design of today's production lines, a quattro punch machine is frequently used. This enables not only two-sided needling from top or bottom by means of two needle assemblies positioned side by side, but also two-sided needling as well as needle bars, which operate on just one side, either only from top or from bottom.
Every further development by machine-builders in regard to the performance of machines, terms of needle-bar stroke frequency and needle density per meter working width leads to re-selection of the right needle for the final product accordingly.
If this does not occur, or is done incorrectly, the surface structure of the final product often suffers during finish needling. Optical quality requirements also need to be met, especially with products that are visible in use – such as trunk linings in the automotive sector. Selection of the right needle also makes it possible to increase the efficiency of the machines.
Here, a needle type made by Groz-Beckert which has established itself in the industry is the Twisted needle.
The Twisted needle is a felting needle that has been specifically designed with a focus on the performance-optimized needling for the nonwovens industry. The Twisted needle can enable an increase in production and more intensive product densification – especially with manufacturers for whom the needling process represents a bottleneck concerning capacity. Moreover, the Twisted needle is the optimal solution wherever excellent surface quality and high strength are required. Thanks to the use of the Twisted needle, these product improvements are possible – especially in intermediate and finish needling.
The Twisted felting needle has an equilaterally-shaped triangular working part with a defined twist, thereby ensuring improved breakage/bending properties. This property is required because the twisting in the working part and the resulting increased efficiency mean that higher penetration forces are exerted on the needle during the needling process in comparison to standard felting needles.
A closer look at the Twisted needle compared to a standard felting needle shows that due to the twisted working part, there is a modified barb arrangement. This results in barbs 2 and 3 of the working part no longer being located in the "slipstream" of barb 1. For this reason, all the barbs can capture a larger share of fibers from the needle channel in the nonwoven, and transport them more strongly than with a standard felting needle. The resulting positive effect of improved fiber transport enables greater densification of the nonwoven and an accompanying increase in strength values.
Despite the twisted working part, all barb dimensions on the edges of the needle are identical, guaranteeing process stability during needling. To make efficient use of the twisted effect, at least two barbs should be available at each edge, and the distance from barb to barb should not be too low (preferably needles with regular barb spacing = R). Care should also be taken that during the needling process, all the barbs can work inside the product. With relatively thin products (3 mm or less) and lower penetration depths (less than 10 mm) the Twisted needle has fewer of the abovementioned advantages where efficiency and strength improvement are concerned.
• More efficient needling due to increased and more defined fiber transport.
In an in-house experiment with a sample nonwoven made of polypropylene with 6.7 dtex fibers, weighing 200 gsm, and needling from both sides with 80 p/cm² respectively, an improvement in strength values of about 10 % in machine direction (MD) and about 30 % in cross-machine direction (CD) was registered. This experiment was carried out in direct comparison with a standard felting needle.
The fields of application in which the Twisted needle can be used are wide-ranging. It is currently being used in the following sectors:
The Twisted needle has now firmly established itself in the geotextile and automotive application fields in particular, due to its many advantages. It is also used in the splitting of microfibers, and for needlle-punching spunbond material.
If you're interested in the Twisted needle and would like more information, the experts from Groz-Beckert will be happy to help you. The Groz-Beckert Technology and Development Center also offers the opportunity to carry out tests with Twisted needles: Needleboards there have been equipped with Twisted needles in different gauges.