Monolithic Plastics Optics

  Plastics Optics Image: Thilo Vogel

The properties of plastic components are not only defined by their base material but also to a higher degree by the design of the surface. In many cases, surface structures with a size of a few micrometres are sufficient to drastically alter the surface properties, for example to avoid the adhesion of dust particles or water droplets or to create an optically functional surface. Caused by the progressing miniaturization in sensor systems, image and data processing, conventionally used macroscopic optical systems are increasingly replaced by micro-optical systems. Additionally, the general demands for compact designs in the consumer market as well as the general challenge in the adjustment of miniaturized optical systems contribute to the increasing integration of micro optical systems into monolithic components. Due to the size of typical optical components, they are well suited for mass production by injection and extrusion moulding. Such monolithic, multifunctional plastics optics are used in new imaging applications, such as light-field photography and microscopy, display and security technology, and miniaturized sensors.

 

Practical Issues

Structural Optics Cluster of Excellence Integrative Production Technology for High-Wage Countries

Drawbacks of established design and production methods are:

  • Insufficient flexibility relating to the design geometry of individual micro-optical elements
  • Limited compactness of optical components due to required functionality
  • Molding precision and deviations for optical elements

For a future increase in efficiency and functionality of products based on hybrid optics, it is necessary to improve the whole production chain by a holistic approach which improves not only design and production methods itself, but also views each step in respect to the other ones. An example for non-imaging micro-optical systems that is suitable for monolithic integration is the backlight of liquid crystal displays that in conventional designs consists of several components. Other areas of application are beam shaping (laser technology), security features (holograms) or warning labels (reflexion).

 

Approach

Machine Cluster of Excellence Integrative Production Technology for High-Wage Countries

In the test case “Optical components”, we are working on an optimized and holistic process chain for the design and production of optically functional plastics components. The process chain begins with the design of the hybrid micro-optical components. We are developing a design method that allows for the optimization of an optical surface with a high degree of freedom, optionally including deviations from the target geometry during production. In a second processing step the structures resulting from the design process are transferred into a molding tool with an ultra-short pulse (USP) laser. Through this so called “cold” ablation by USP laser structure sizes in the low μm regime can be produced, which largely occurs without any melting. For the production of complex functional 3D surfaces with foils and plates a variothermal extrusion or injection molding process is investigated. The utilized molding tool is therefore either a laser structured, coated embossing roller or a laser structured and coated injection tool. To improve the molding precision and release properties of tools, a coating is applied. These coatings are applied by Physical Vapour Desposition (PVD) and reduce the adhesion between the surface of the tool and the molded part to increase the precision and wear resistance of the tool. A coupling of the design and production
process is realized by an iterative process, which consists in measuring the deviation of the produced component in
relation to its simulated counterpart. These deviations are then used as an additional constraint in the design phase. The functional component or foil is a demonstrator application for test case “optical components”. The function of surface structures is checked and designed by the Chair of Optical Systems (TOS). The structure is generated by the Chair of Laser Technology (LLT) and afterwards coated by the Institute for Surface Engineering (IOT). The moulding process was carried out by the Institute of Plastics Processing (IKV).

 

Technical Challenges

Height profile of the negative structures Cluster of Excellence Integrative Production Technology for High-Wage Countries

The currently used design routines for hybrid micro-optical systems need to be replaced by a less computationally intensive approach that also allows the incorporation of restrictions and deviations arising from production. For the transfer of simulated structures into a moulding tool, new ablation processes that utilize USP laser radiation need to be established and validated, especially for the structuring of free-form surfaces. Depending on the size of the functional features, the structuring is applied to the surface of the molding tool or to the coating. Challenges in the coating process consist in the development of coating with low adhesion and high wear resistance, while also reaching a high uniformity of the coating layer on complex surface geometries. The variothermal moulding process is ideal for cost efficient reproduction of optical surfaces. Here, controlling the variothermal molding process through a choice of suitable process parameters is a major challenge. To this end, detailed knowledge of the various factors that influence the process is essential, while the effects of any changes to parameters should be considered in advance. In principle, variations in process control can be utilized to create a multitude of different molded designs from a single negative.