Introduction

Most nonwoven products are in 3D form. They have to be made from flat fabrics through costly conversion processes. The joints in the product can also be a problem due to their deviation from the normal fabric.

Research in nonwovens is mainly related to the manufacture and use of the fabric in 2D sheet form, although there have been a few reports on the production of 3D nonwovens. We present in this paper a new technology for the production of 3D nonwovens from staple fibres and discuss the design of the fibre flow system using CFD techniques.


Process Design

The process is based on the air-laying principle for web formation and thermal through-air bonding for web consolidation (Figure 1). The fibre-opening unit is based on a roller card. The fibres are dispersed in airflow and carried to perforated 3D moulds. The moulds move across the machine width during web formation and into a separate bonding section for consolidation.



The airflow velocity and pressure distributions in the web forming area are critical for achieving the desired fibre distribution over the 3D mould surface. These can be studied using CFD technology. We used the Fluent CFD package in our study. Figure 2 shows an example of air velocity distribution in various cross sections of the air duct. A variety of 3D shapes can be formed with a controlled fibre distribution within the 3D shell structure by optimising the design of the air duct and the mould chamber, the airflow velocity and also the design of the appropriate airflow control devices. Airflow control is essential because the angle between the airflow and the 3D mould surface varies. It is also important to avoid vortex in the airflow because it will cause fibre entanglement.


Once the 3D web is formed, it is moved into a bonding chamber for consolidation. After evaluating numerous bonding techniques we found that the thermal through-air method was the most appropriate. Similar to the web forming area, the design of the boding chamber is simulated using CFD. However, the simulation of the bonding chamber is more complex because it involves temperature distribution and heat exchange in addition to the velocity and pressure distributions in the web forming area. Because of the heat exchange, time-dependent simulation must be carried out to examine dynamic hot air flow distribution. Figure 3 shows such an example.



The four pictures in Figure 3 show the temperature distribution in the bonding chamber at intervals of 0.2 seconds. The top left picture is at 0.2 seconds after bonding starts. In this particular case, a uniform temperature over the 3D web is achieved after 1 second. Obviously, this is dependent on the airflow velocity and pressure distributions as well as the shape and size of the 3D web.


Experimental Results

Based on CFD simulation study, we built an experimental machine capable of making 3D products directly from staple fibres. We measured the flow parameters experimentally in order to validate the simulation results.


Figure 4 compares the theoretical air velocity distribution (top) around a semi-sphere mould to the experimental data (bottom). The agreement between the two is very satisfactory. Figure 5 compares the theoretical temperature distribution in the bonding chamber (dotted line) to the experimental data (large dots). There is good agreement between the two results. In order to achieve uniform web property, the temperature distribution must be controlled within a very tight range, typically 1 (K). This should also be achieved quickly to reduce the cycle time which determines the production speed of the process. Figure 6 shows an example of a 3D nonwovens structure that was produced using the new technology. Products of various other shapes and web area densities have been successfully produced.



Conclusion

Seamless 3D nonwoven shell products can be produced directly from staple fibres using air-laying and thermal through-air bonding. This can significantly reduce the cost of many products and eliminate the property variation caused by joints.