The CRoCoMold project was initiated to develop an alternative production process for thin-walled plastic precision optical components. Innovative injection molding processes already allow the production of optical components with the highest precision. The demand and the requirements for optical plastic parts are constantly increasing. Optical fields within Lighting-, Automotive- and Medical industry are asking for different technologies. Highly efficient, load bearing production processes are required in order to be able to produce future high volume products. Production costs can be massively reduced by realizing manufacturing processes with the least possible use of plastic material while adhering to the highest quality.
polyoptics, as a leading optical components manufacturer, and the non-profit KIMW Forschungs GmbH decided to break the rules with the Continuous Compression Molding process (CCM).
The CCM machine of the Italian company SACMI that was selected for the project is named CCM 24, which means that it is equipped with up to 24 molds. SACMI is also known as a partner with high technological level and huge ability in new developments. The SACMI CCM machines can be equipped with up to 80 molds and have their benefits in the seamless and continuous high precision processing.
Due to the defined orientation of the extracted plastic parts an online quality control system is able to detect deviations in quality and assort, reject single molded parts and even disable several molds. Even other finishing processes like coating or other treatments can be easily included in one production line. The CCM technology is already an alternative to injection molding process for the high speed mass production of semicrystalline plastic parts. In the further development of existing plant engineering, not only the material-specific and component-specific requirements have to be taken into account, but also the highly technical machine conditions, such as the up to 80 molds, which are mounted on a rotating machine table.
Figure 1: left: PMMA plastic granulates; top: CCM 24 machine at polyoptics; right: planar PMMA lenses molded by CCM
One of the objectives is, to use injection molding materials for the CCM processing,to keep the number of different materials an injection molding company has to handle at current level. Moreover by using the same materials as for injection molding there is no approval of the customer for new materials necessary. That way a direct comparison of quality aspects can be made between the CCM process and the injection molding process.
Final state of the project
At the beginning of the project, product and quality requirements to machine and process have been defined. Product specimen in the optics industry for example are defect-free, without sink marks, without flow- and binding lines, part ejection scratches and delivered burr-free.
polyoptics and KIMW-F investigated several PC and PMMA materials that could possibly be used for CCM and choose the injection molding materials Plexiglas® 6N, 8N and 8H as well Altuglas® V825T. In close collaboration of polyoptics, Sacmi and KIMW-F first trials at the CCM machine were done, to determine different extruder setups regarding their suitability for the extrusion of PMMA materials.
Those practical trials showed the general possibility to use PMMA in the CCM process. Material analysis of extruded PMMA via gel permeation chromatography (GPC), which is a method to detect degradation of molecular chains, showed that there was no thermal degradation within the material. Another result of the CCM test run was the temperature control of the extruder, which had to be upgraded with several temperature controls for PMMA, an active degassing at the extruder was not necessary at that point. Even the transmission measurement, which is a quality attribute of injection molded lenses, showed exceedingly acceptable results.
With the improved temperature control all three PMMA materials were processed with variations of process parameters, to evaluate the best parameters for the existing mold. Further tests showed the result that the technical set up of nozzle, cutting and transporting device and even the mold had to be optimized.
In several steps the cutting and transport unit of the CCM machine was exchanged. It was accompanied by the need to program a controller for both components implementing the movements of the blade and the transport unit with the mold movements. This was necessary to cut, transport, mold and eject the plastics. Due to the technical improvement of the CCM machine the quality of PMMA molded parts rose, but still cutting marks were visible at the part surface.
In the next project phase lens molds for 5 mm thick planar lenses were constructed and built.
Figure 2: Compression molded planar plastic lenses
The sampling of the new mold was a success. It was possible for the first time to produce optical plastic lenses made of PMMA using the continuous compression molding.
In further sampling the blade quality was variated to find out best blade parameters. During those tests it came though, that the macro shape of the blade highly influences the surface quality of the plastic lenses. In order to further improve the lens quality the cooling concept of the CCM molds has been evaluated by KIMW-F. In order to validate the effects of different concepts on the surface quality, trials with various mold temperatures have been carried out. The mold was heated up with the success of producing a highly increased surface quality of the lenses. To evaluate the quality of the molded material and the process reliability the material was analysed via GPC. There was no degradation of PMMA material detected. The process control generally was in a condition that can be transferred serial production.
Further optimization of surface qualities can be reached by manipulation of contact temperatures between blade and mold with the melt. Thin thermal insulating CVD coatings were tested with the result that an optimization of the coating process for mold material and coating thickness has to be done to positively influence the lens quality.
The project team draws a positive conclusion from the project: The possibility of processing PMMA by means of CCM was developed within the project and by the production of optical lenses. The optimization of the quality of the parts as well as the constant checking of these by various analyses and measurements could prove the process capability.
The non-profit KIMW Forschungs GmbH supports plastics processing companies in their research and development tasks. This ensures the competitiveness of partnership, application and foundation-oriented R&D activities: by creating and securing innovation leadership, generating know-how and spreading knowledge. KIMW-F research topics are found in the fields of plastics processing and relevant tooling as well as coating technologies.
“This research and development project was funded by the German Federal Ministry of Education and Research (BMBF) within the “SME – Innovative: Research for Production” Funding Action (funding number 02P15K021/02P15K020) and implemented by the Project Management Agency Karlsruhe (PTKA). The author is responsible for the content of this publication.”