By Max Groenendijk

Ultrashort pulsed lasers have proven to be excellent tools to fabricate micro and nanotextured surfaces. These surfaces have a high potential for several applications where the tiny surface textures can improve or add a specific functional property. It was at ICALEO® 2006 that we first demonstrated a super hydrophobic surface obtained by texturing using a femtosecond pulsed laser. However, the capabilities were limited to small and flat surfaces. In order to be able to introduce this technology in industrial applications Lightmotif developed a 3D capable machine. At ICALEO 2012 results obtained on real 3D curved parts were presented, bringing this innovative technology closer to the market.

Functional surfaces are a hot topic. A lot of research has addressed the fabrication of super hydrophobic surfaces for example. Such surfaces exhibit textures on micro and nanoscale combined with a water repelling, low surface tension material. The textures “amplify” the water repelling property, leading to a partial contact of the liquid that only touches the texture’s peaks. The liquid is very mobile and rolls off the surface leaving it completely dry and clean.

A completely different example, are textures that can largely improve lubrication in a sliding motion by introducing dimples in one of the surfaces. The lubricant will start a circular motion inside the dimple that builds up a hydrodynamic pressure. The dimple serves as a lubricant reservoir and a trap of wear particles. By this combination, the friction coefficient and surface wear can be considerably reduced.

For dry contact, a different kind of surface texture is needed to reduce the friction coefficient. A very interesting application is the reduction of skin friction by a surface covered with tiny pillars. The contact area is largely reduced and a silky smooth touch is the result.

Optical effects that can be achieved range from diffraction gratings to surfaces with highly reduced glare. In biomedical applications surface textures can be used to improve cell adhesion, for example on implants.

Fig. 1: A selection of effects that can be generated by surface textures.

Fig. 1: A selection of effects that can be generated by surface textures.

However, the speed of the texturing process is a concern in most applications. Industry can only use this technology when an economic process can be realized. In most cases, direct texturing of surfaces by ultrashort pulsed lasers would add too much cost. A very attractive solution to this challenge is to texture a master surface that can be replicated in a cheap mass production process. The approach in our case is to use injection molds that have been laser textured. In the injection molding process, the textures are transferred to the polymer product surface. This technology is especially suited for producing surfaces with improved (skin) friction, super hydrophobic surfaces or products with anti-glare properties.

Notwithstanding, a big challenge is the fabrication of textures on 3D curved parts. Especially for texturing of injection molds this capability is needed, as the majority of potential polymer products have a curved surface. This challenge was targeted by Lightmotif, a company that started five years ago as spin-off from the University of Twente. A machine is now realized that uses a picosecond pulsed laser in combination with a five axis motion platform. The development of this machine was partly conducted in the collaborative project “Nanoclean,” financed by the European Commission. This project had the goal to upscale the technology of super-hydrophobic surfaces for the automotive market. This market is interested in using such surfaces for generating self-cleaning properties.

Fig. 2: The five axis manipulator, texturing an injection mold.  The machine can load up to 300 kg and has a travel of 1m x 1.5m x 0.4m.

Fig. 2: The five axis manipulator, texturing an injection mold. The machine can load up to 300 kg and has a travel of 1m x 1.5m x 0.4m.

The Lightmotif machine is constructed on a solid granite base with a weight of 9,000 kg and a travel of 1m x 1.5m x 0.4m. A kinematic model of this machine is fully integrated into the 3D capable software. Methods for generating the texture description on 3D curved surfaces have been developed in order to accurately place the functional textures on the surface. The surface is divided into tiles that are textured one after the other. The manipulator steps to a tile and a galvo-scanner applies the texture within the boundaries of this tile. Different methods can be used to arrange the textures inside a tile. This can have an effect on the functional performance and also determines the optical properties. When a regular texture distribution is chosen, the reflective properties of the individual tiles are different resulting in a kind of ‘carbon fiber’ look, as can be seen on the textured steel ball depicted in Figure 3.

Fig. 3: This steel ball was textured as demonstration object. The textures have been distributed into tiles, well visible in this picture. Determined by the texture arrangement the tiles can show a strong optical effect.

Fig. 3: This steel ball was textured as demonstration object. The textures have been distributed into tiles, well visible in this picture. Determined by the texture arrangement the tiles can show a strong optical effect.

In the Nanoclean project a demonstrator mold for a mirror cup was textured, as depicted in Figure 2 and Figure 4. Injection molding using this mold has resulted in super hydrophobic behavior.

The 3D texturing technology is an important milestone for using functional surfaces on a variety of products. Applications using injection molding are just one example. Other applications that can benefit from this development include texturing of implants to improve cell adhesion or biocompatibility. Forging tools or sheet metal forming tools could be equipped with surfaces reducing friction and wear.

Lightmotif concentrates on the development of the laser machine technology and is not the specialist in all the different application areas. The needed development towards applications is conducted in collaboration with industry and institutes. Lightmotif now offers the 3D texturing machine for application development and hopes that this can boost the development of new applications.

Fig. 4: This injection mold for a mirror cup was textured as the demonstrator in the Nanoclean project. The injecting molded products exhibit super hydrophobic properties.

Fig. 4: This injection mold for a mirror cup was textured as the demonstrator in the Nanoclean project. The injecting molded products exhibit super hydrophobic properties.

Max Groenendijk is Managing Director of Lightmotif.