At present, optical components manufactured through injection molding processes can already achieve maximum accuracy. However, the necessary number and additional requirements are also increasing. In the lighting and automotive industries as well as in medical engineering, a variety of production technologies are required. In the future, promising products will need to be produced in large quantities, so the underlying production process needs to be very efficient.
Polyoptics, Kleve and KIMW-F are working together to develop an alternative production process for thin-walled optical components, named "CRoCoMold Project". Continuous Compression Molding (CCM), a process consisting of extrusion and melt embossing, has so far been mainly used for simple packaging from free-flowing polymers and offers great development potential. This project is part of the federal government's "Innovative SMEs" project, which will further develop the process to enable it to be used in precision optical polymers such as lenses and optical accessories.
Compression lenses from the CroCoMold project (© polyoptics)
The purpose of the project is to develop the concept of the plant in terms of material properties and necessary process regulation and control. This means it also needs to address the heat input required for the polymer's melting and molding process. Newly designed embedded heating systems and molds are used to control tighter tolerances and more effective mold ventilation. On the machine side, design changes were made to integrate the concepts of heating and mold.
Background of the project
The first step is to determine the product and quality requirements of the machine and manufacture: Products designed for the optical industry must be free of defects, pores, flow and lines, shots, and flashes. A CCM machine (type: CCM 24) from the Italian company SacmiImola S.C. was eventually selected to participate in the project, which can fit 24 molds.
For new applications, the project partners decided to stick to the injection molding materials that have been used so far, so that long approvals and purchases can be avoided. Polyoptics and KIMW-F considered several candidate materials PC and PMMA used in the CCM process, and finally chose plexiglass 6N, 8N, 8 h and Altuglas V825T.
First attempt
Project partners worked closely with Sacmi in preliminary experiments to determine whether the design of different extruders is suitable for extruding PMMA materials on CCM machines. These actual tests confirm that PMMA can be processed by CCM.
With the help of an enhanced temperature control system, all three PMMA materials are processed under different process parameters in order to make the best materials for existing molds. Tests have shown that nozzles, cutting and transport units, and the mold itself must be optimized.
As a result, the CCM's cutting and shipping department was replaced in several stages. The controls of both components are set to match the operation of the blades and the molds of the transport unit. This coordination is critical for cutting, transporting, molding, and spraying plastic. Technical improvements to the CCM machine have greatly improved the quality of the molded PMMA parts, but cutting marks can still be seen on their surface.
Continuous compression molding
The next phase of this project is the design and manufacture of molds for 5 mm thick flat lenses. Project partners used continuous compression molding (CCM) to produce optical lenses made by PMMA, and tests showed that the newly designed mold was suitable.
Further testing includes changing the composition of the cutting blades to determine the optimal parameters. These tests show that the macroscopic shape of the blades has a great effect on the surface treatment of the lens.
In order to further improve the lens quality, KIMW-F also studied the cooling concept behind the CCM mold. To achieve this, different tests were performed at different mold temperatures to determine the effect of different concepts on surface quality. The results show that heating the mold greatly improves the surface finish of the lens. By manipulating the contact temperature between the blade and the mold containing the melt, the paint finish can be further increased.
The quality and process reliability of the parts were analyzed by GPC, and no deterioration of PMMA was found. This means that the process control system is ready for expansion into series production.
The experiments have studied thin, thermally insulating CVD coatings, and pointed out that increasing coating thickness and optimizing coating processes are necessary conditions for improving lens quality.
In conclusion
The project and the successful production of optical lenses confirm the possibility of processing PMMA through CCM. By optimizing the quality of the part and continuously monitoring it through various analyses and measurements, it has proven its production capacity.