JEC COMPOSITES MAGAZINE - Issue #73 - June 2012 - 72

micro-pultrusion

fibre volume of more than 80% using micro-pultrusion. Figure 1 shows the cross-section of a glass-fibre-reinforced profile manufactured with the micro-pultrusion technique. Reducing the diameter of pultruded profiles requires essential new process technologies that have no equivalent in the state of the art. As the aspect ratio of the profiles increases with decreasing diameters, the influence of friction forces applied to the single filaments grows substantially. Therefore, the need to reduce process forces and optimize process parameters is even more important for micro-pultrusion compared to standard pultrusion.

Fig. 4: Principle of a pullwound profile

Micro-pullwinding technology
The pullwinding process is a combination of the pultrusion and filament winding techniques. Within a continuous and automated process, fibres are pulled from a roving unit through a resin bath and a die comparable to the pultrusion process. This pultruded profile with a fibre orientation parallel to the pulling direction (zero degree) is wrapped and consolidated with additional layers of fibres in several winding units. The fibre orientation angle of the additional layers can be set from zero to ninety degrees. These additional layers can improve the properties of the profiles significantly,

as not only tensile strains, but also bending and torsional stresses can be absorbed. Profiles made with stateof-the-art pullwinding technology are stiff tubes with diameters starting from 10 millimetres. These tubes are used, for instance, for ski poles or feed pipes where high torsional stiffness or high pressure resistance are required. In connection with its research and development work in the field of micro-pultrusion, the Fraunhofer IPT is also miniaturizing the pullwinding process to diameters under 500 micrometres. Figure 3 shows the developed micro-pullwinding system. The micro-pullwinding process can be used for many applications such as medical devices like guidewires or catheters, where several aspects like high torsional stiffness, moderate bending stiffness and high kinking resistance have to be combined. Micro-pullwinding offers the possibility to adjust these mechanical properties independently from each other by

changing the fibre orientation angles of the wound layers. Not only can the general properties of the manufactured profiles be adjusted, but they can also be varied over the length of the profiles. By changing the different pulling and winding movements, the fibre orientation angle can be changed over a certain distance without interrupting the process. Profiles with variable bending or torsional stiffness can be manufactured in a single automated and repeatable process. This can be crucial, especially in applications with small dimensions and high requirements in terms of complexity and precision.

Applications
In many applications, miniaturized fibre-reinforced profiles are superior to other materials such as metal or plastic. In addition to their outstanding mechanical properties, other adjustable characteristics like conductivity or thermal behaviour make such profiles valuable for many applications. Micro-pultruded rods can be used for high-precision measurement systems, where a thermally-stable framework is necessary to achieve the required high measurement resolution. With the help of the miniaturized pultrusion process, the Fraunhofer IPT has developed profiles made of carbon-fibrereinforced plastics that have nearly no thermal expansion over a wide range of temperatures. Micro-pultruded or pullwound profiles can also be used as reinforcing elements in a wide range of applications to improve their mechanical properties. Due to their high fibre volume content, micro-pultruded profiles can be used as reinforcing material replacing bare fibre strands or fabrics to produce a selective reinforcement. Compared to fabrics or filaments, pultruded rods with very small diameters can be handled much more easily and could be used to reinforce structures in very specific areas. It is also pos-

Fig. 3: Micro-pullwinding system at Fraunhofer IPT

72 jec composites magazine / No73 June 2012

JEC COMPOSITES MAGAZINE - Issue #73 - June 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #73 - June 2012