Surface Flattening and Nanostructuring of Steel by Picosecond Pulsed Laser Irradiation
- 307 Downloads
Steel is used as a mold material for press/injection molding of plastic products. High accuracy and releasing ability are required for a steel mold surface. This paper proposes a surface finishing method for steel molds by using picosecond pulsed laser irradiation. The process involves two steps: one is surface flattening by removing the surface asperity through laser ablation, and the other is forming nanoscale laser-induced periodic surface structures (LIPSS) on the flattened surface. The two steps are realized by using the same laser at controlled laser fluence and focus position. Experimental results showed that LIPSS was successfully formed after surface flattening under specific ranges of laser fluence and defocus length. Furthermore, plastic forming experiments demonstrated that a steel surface with LIPSS significantly decreased the effective contact area and, in turn, reduced the mold releasing force. These findings provide the possibility of fabricating high-performance steel molds by picosecond pulsed laser irradiation.
KeywordsSurface flattening Nanostructure Steel material Picosecond pulsed laser Plastic molding Mold releasing ability
In modern industry, various kinds of plastic products are produced by press molding and injection molding processes. In high-precision molding technologies, it is important that the surface of a mold has high accuracy because the accuracy of the mold directly affects the accuracy of the resulting plastic products. At the same time, high mold releasing ability is also necessary to prevent adhesion of plastic onto the mold.
Conventionally, mechanical polishing has been used to improve the surface quality and modify the form error of a mold. However, polishing becomes difficult when the mold surface feature is smaller than millimeter size and its shape is complicated. To solve this problem, laser polishing of a mold has been attempted . In laser polishing, a layer of mold material is melted by laser irradiation and the surface becomes flat due to the surface tension effect. Laser polishing can improve surface roughness without deforming the surface shape . However, the heat-affected zone in the laser-polished mold is very thick (~ tens of microns) because a thick layer of material is melted [3, 4]. The heat-affected zone might reduce the strength and, in turn, the service life of a mold.
On the other hand, during plastic molding, a releasing agent is necessary to prevent plastic from sticking to the mold surface. However, it is difficult to apply the releasing agent uniformly on a small mold with complicated surface geometry. The releasing agent, even uniformly applied, may be damaged and even peeled off after molding. As a result, it is necessary to apply the releasing agent repeatedly, which significantly reduces the production efficiency.
In this study, the authors propose a surface finishing method for a steel mold by picosecond pulsed laser irradiation. The method involves two steps: the first is surface flattening by removing the surface asperities through laser ablation, and the second is forming nanoscale laser-induced periodic surface structures (LIPSS) on the flattened surface. In recent years, the ultrashort pulse laser has been extensively investigated, and it has been demonstrated that irradiating ultrashort pulse laser can significantly decrease the thickness of heat-affected zones . Previous studies have focused on ultrashort pulse laser irradiation to form fine patterns or clean holes [6, 7, 8]. However, there is very few research on surface flattening using ultrashort pulse laser ablation. LIPSS formation has also been focused for various materials at femtosecond pulses [9, 10, 11, 12]. It is known that the period of LIPSS depends on the laser wavelength [13, 14], and its orientation and shape are defined by the polarization of incident laser [10, 15]. A lot of researches have been carried out to investigate the physics underlying the formation of LIPSS for different materials [13, 16, 17, 18]. Although the formation mechanism of LIPSS is still under investigation at the moment, LIPSS has been widely utilized to improve surface properties, such as wettability [19, 20, 21], optical performance [22, 23], and tribology .
In this study, picosecond pulsed laser irradiation was performed on steel to realize surface flattening and to form LIPSS on the flattened surface. It is expected that the LIPSS can improve the mold releasing ability of a steel mold and reduce the necessity for applying releasing agent. Surface flattening and LIPSS formation will be combined into a single process by using a single laser beam. This study can improve both surface accuracy and releasing ability of steel molds, thus contributing to the high-precision micro-/nanomanufacturing industry.
2 Mechanism of Surface Flattening and LIPSS Generation for Steel
3 Experimental Methods
3.1 Ablation Threshold/Surface Flattening/Nanostructuring
The resulting surface was observed by a scanning electron microscope (SEM), INSPECT S50 produced by FEI Company. The depth of the irradiated area was measured by a laser-probe profilometer, NH-3SP produced by Mitaka Kohki Co., Ltd. The depth of spot and the surface roughness of irradiated area were measured by a laser microscope, VK-9700 produced by KEYENCE CORPORATION.
3.2 Releasing Ability Evaluation
After laser processing of steel mold surfaces, the samples were used to evaluate the releasing force by press molding experiment. The pressed area of the sample was 8 mm × 8 mm. Polybutylene terephthalate (PBT) was pressed on the steel sample surface by a high-precision molding machine GMP211 produced by Toshiba Machine Co., Ltd. Molding temperature was 230 °C, near the melting point of the PBT plastic (224 °C). Pressing time was 20 s, and pressing force was 0.2 kN. Releasing force was calculated from the load which was used to separate the PBT sample from the stainless tool steel.
4 Results and Discussion
4.1 Ablation Threshold
4.2 Effect of Laser Fluence on Surface Morphology
4.3 Effect of Defocus Length on Surface Morphology
4.4 Effect of Defocusing on Material Removal Amount and Surface Roughness
4.5 Surface Releasing Ability
Picosecond pulsed laser was irradiated on steel mold surfaces at controlled laser fluence and focus position, and laser-induced surface flattening and formation of LIPSS were investigated. After surface flattening by ablation at a higher laser fluence, LIPSS was successfully formed on the surface with a low surface roughness (0.56 μm Ra) at a specific defocus length. Forming LIPSS on a steel mold surface decreased the effective contact area during plastic molding, and the mold releasing force was decreased by a factor of three. The findings from this study demonstrated the possibility of improving surface flatness and releasing ability of steel molds by irradiating a picosecond pulsed laser.