How is a nonwoven created?
Textile fabrics, such as wovens or knitted fabrics, are usually made of yarns or threads. In case of nonwovens, however, the interlocking results mainly from staple fibers adhering to other fibers surrounding them. Since numerous products require high strength, thermal, chemical and mechanical methods have been developed to bond these fiber layers together.
The production process of a needled nonwoven often comprises the following steps:
- Fiber opening and blending
- Web formation
- Web bonding
The fiber components are fed into the fiber opening line, consisting of various opening units and devices. The fiber tufts then are fed into a huge mixing chamber to achieve a homogeneous fiber blend. The fiber blend then is presented to the carding machine by the card feeder. All rollers of the carding machine are equipped with card clothing and are able to control fibers. The carding machine further opens the fiber tufts down to individual fibers and forms a fibrous web.
This web then is doubled to form a nonwovens web according to the required product weight. This web is then bonded to fulfil the requirements of the final nonwovens product. Finally the nonwovens product can be finished like coating or singeing depending on the subsequent requirements.
The influence of pre-needling during nonwoven bonding
Generally, the degree of bonding depends on fiber properties such as fiber geometry, fiber strength, fiber orientation in the web, as well as on the product weight and the bonding principle selected.
The process of bonding using needles begins with pre-needling. The feeding system of the machine enables dimensionally stable feeding of the web to the actual needling zone. Here, the gap between the stripper plate and bed plate has to be selected in accordance with the material thickness of the respective nonwoven. The nonwoven submitted is thus already compressed to a certain thickness.
By setting the needling parameters and selecting the appropriate needle, the nonwoven is pre-bonded and then transported further to one or more needling machines until the desired degree of bonding is obtained.
A major advantage of pre-needling in the case of two- and multi-layered products is the possibility of feeding the required layers to the preneedling machine either before or after.
Pre-needling frequently takes place on one side – with either one or two needling zones. Specific adjustment of the penetration depth and density, depending on the final product, results in an initial densification. Here, an average of 10 to 50 punches per square centimeter are made.
As a first step with pre-needling, therefore, the voluminous nonwoven is often brought to a certain density and a certain weight per unit area, as well as prepared for the material strength required for subsequent processes. In addition, the strength and elongation in machine and cross machine direction is already controlled to a certain extent, affecting the surface structure of the product.
The surface structure of the nonwoven fabric is obtained from varying penetration depth, feed per stroke, and needle type and gauge.
Selecting needles for pre-needling
With more than 2,000 needle types, including standard needles as well as special needle designs, Groz-Beckert offers the right needle for every requirement in nonwoven bonding.
During the first process stage of bonding in particular, fiber protection is especially important, and this can be influenced by appropriate needle selection. Especially in the pre-needling phase, nonwovens are still bulky and needle breakage occurs primarily in the first few rows of the needle board. The right needle selection during pre-needling already has an impact on the surface of the final product and on needle wear.
Most suited to pre-needling, therefore, are standard, conical and GEBECON® needles. With the parallel working part of the standard needle the best possible surface quality of the final product will be achieved. However, the standard needle is more susceptible to breakage due to strong deflection during pre-needling. The conical needle, in contrast, offers the best stability and is less prone to needle breakage and deflection. The GEBECON® needle offers the ideal combination of good surface quality of the final product and the best possible breaking and bending properties – the happy medium.
The GEBECON® needle is used mainly in pre-needling across all segments. Application fields include automotive, synthetic leather, geotextiles, filter felts, and many other technical nonwovens. Because of their staggered barb sizes, GEBECON® needles can also be used in the needling of fine, very fine, micro and special fibers, as well as natural fibers.
The conical working part and the continuous conical transition area all the way up to the shank lead to smaller penetration holes compared to a conventional conical standard needle, which in turn leads to improved surface quality of the final product.
Moreover, the shallow conical shape of the GEBECON® and the smaller barbs near the needle tip reduce machine load, thus also reducing penetration force.
Conical standard needle (above), and GEBECON®
The design of the GEBECON® needle makes it more flexible than either a standard felting needle or a standard conical needle. The improved needle guidance that results from this leads to less needle deflection and therefore less needle breakage. The service life of the needle can also be further extended by special coatings, such as GEBEDUR® treatment.
The GEBECON® needle is available in needle gauges from 25 to 43, and in needle lengths of 3 and 3.5 inches.