AUTOMOTIVE | INNOVATION


Right:


Automotive lens Dura + hot runner system from Mold- Masters


to be introduced SeVG+ (Sequential Electric Valve Gate) system will also provide addition- al benefits to moulders of automotive lighting, says the company, which uses up to 16 servo motors to offer absolute control and precision over each individual valve pin. A collaborative research


project has been co-ordinated by IKV, the Institute of Plastics Process- ing at RWTH Aachen University in Germany, on injection-compression moulding of thick-walled plastic optics with demand-orientated mould temper- ing taking local cooling require- ments into account. Injection moulding of plastic optics requires a particularly precise and consistent replication with low internal stresses. Design needs result in requirements for the part geometry that contradict the typical design guidelines for conventionally thin-walled injection moulded parts. For example, thick-walled optics with sudden changes in wall thickness are often required which are susceptible to warpage due to inhomogeneous shrinkage. Injection-com- pression moulding has become established for the production of such components. Instead of the holding pressure, the volume shrinkage of the plastic melt is compensated by means of a pressur- ised compression stamp. As a consequence, a more homogeneous pressure distribution, and also homogeneous optical properties of the moulded part, can be achieved. According to IKV, due to the complex shrinkage behaviour of plastics, warpage-free plastic optics with high moulding accuracy and good optical properties can only be produced in injection-com- pression moulding with precise thermal mould design. Conventional approaches for thermal design are not able to meet the requirements and involve time-consuming and costly iteration loops. At present, independent of the wall thickness, uniform heat dissipation is achieved. As a conse-


IKV schematic diagram of the cooling of an injection compression moulded thick-walled lens as a function of the mould


tempering 54 INJECTION WORLD | September 2018 www.injectionworld.com


quence, thin-walled areas solidify earlier than


thick-walled areas. High stresses are induced locally into the already solidified areas by the compression, which significantly lowers the homogeneity of the optical


properties. This influence has not yet been taken into account either in the process control or in the thermal design.


In the first project phase of the Collabora- tive Research Centre SFB 1120 of the RWTH Aachen University a new method for the automatic, demand- oriented design of the tempering channels in injection moulds was developed in order to achieve uniform shrinkage behaviour


and minimised warpage. Based on a homogene- ous temperature and density distribution in the component, an inverse heat conduction problem is solved and the local cooling requirement is derived from this. The central component is a fitness function developed for thin-walled moulded parts with small differences in wall thickness, with which the homogeneity of cooling and density is evaluated. Following necessary further develop- ment, this offers a promising approach for the thermal design of injection-compression moulds for high-precision thick-walled optics in order to achieve homogeneous cooling. The overall goal of the current research project


at IKV, in collaboration with automotive lighting producer Hella and temperature controller company GWK, is therefore the transfer and use of this innovative method for thermal design. By determining the local cooling demand, demand orientated cooling of thick- and thin-walled areas of the lens during the compression phase should be achieved. This topic and other highlights from IKV research in the field of injection moulding will be presented at the International Injection Mould- ing Conference – IIMC 2019 – on 10-11 April 2019 in Aachen, Germany.


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