Precision manufacturing by controlling melt dynamics and solidification in production processes

Production processes where the material is turned into a molten phase as part of the process chain - which is the case with, e.g. injection moulding - frequently pose high requirement to the user, in terms of costs, processing and elaborate secondary finishing in order to meet increased demands of high component precision.

Volume shrinkage during solidification, inhomogeneous cooling due to restricted energy transport and accidental structure generation can lead to a variety of component inaccuracies and errors that significantly affect part precision.

To achieve high precision in melt-based processes, it is necessary to have a profound understanding of melt generation, inner dynamics as a result of external and internal driving forces, as well as of the processes taking place during solidification of the specific melt.

However, especially heat transitions, material states, energy transport through liquid phases, impact of solidification processes and crystallisation processes, with their effect on volumes, in addition to temperature profiles in terms of time and place, are hard or even impossible to predict and control. 

Targets of SFB1120 research

These issues are subject to research in the SFB1120 collaborative research project. The programme is aimed to find a description regardless of dimensions, for the processes taking place during melt-based production such as forming, joining, separation, generative manufacturing and coating. Precision is to be improved by at least one order of magnitude, related to geometric errors, internal part defects and surface accuracies.

To reach this aim, the scientist follow a fundamental approach that considers all partial processes, in terms of physics and material, that play a part in melt generation and its dynamics. The processes are submitted to temporal and local analysis using high-resolution methods. These analyses are the basis for the second stage, where an understanding is developed of the inter-relations between the partial processes which are based on nano-scale nucleation processes, up to macroscopic warpage in the component. Scientific approaches will be worked out eventually to control cooling conditions, energy transfer in the melts as well as solidification and structure generation. In addition to these primary objectives, another aim is to develop new approaches for process design and tools, so as to significantly reduce the work and costs involved in component and tool design, as well as manufacturing cost. 

The planned collaborative research centre is designed to enable multi-scale control of the melt, starting from melt generation and melt flow up to solidification, as a precondition to improve precision and prevent process errors in and on components generated by melt-based techniques. 

Sub-projects investigated at IKV

If manufacturing thermoplastic parts by injection moulding, inhomogeneous material shrinkage leads to warpage. Local shrinkage is obtained from cooling speed and dwell pressure while the plastic melt solidifies. During injection moulding, local differences in pressures and temperatures are significant, which is subject to geometrical and process engineering factors such as wall thickness. To reach homogeneous material shrinkage, these factors have to be compensated in a well-aimed process, i.e. by dissipation of heat oriented to the respective requirement. However, this is impossible to determine in an intuitive or manual way, due to the vast number of influencing factors and their inter-relations. This is why, in most cases, simplifications, for example homogeneous temperature distribution, are made during manual mould lay out, which fail to describe the mechanisms leading to warpage in a component, though. In a regular design process, this requires several iteration cycles to analyse various temperature control systems and their effects on the moulded part. After a system is found that provides for sufficient homogeneity, the mould can be manufactured and submitted to mould proving. Then again, this proving usually requires amendments until the necessary component precision is achieved. 

Sub-project B1 is aimed at reducing the warpage of injection moulded components, while, at the same time, minimising the number of proving steps. To reach this aim, the cooling required locally must be calculated by a computer and used as a target value during design. A suitable FE-based algorithm will be developed. In doing so, existing FE approaches and models are considered, whereas fundamental principles of present process simulation (e.g. forward simulation) are questioned and alternative approaches are developed according to the principle of inverted mould design.

Publications:

Bobzin, K.; Öte, M. ; Linke, T. F.; Alkhasli, I.; Hopmann, C.; Nikoleizig, P.: Development of Simulative Approaches for Precisely Designing the Properties of Plasma Sprayed Coatings for Application in Injection Moulding, Conference Proceedings of the YIC GACM 2015, Aachen, 2015

Hopmann, C.; Nikoleizig, P.: Minimisation of warpage for injection molded parts with reversed thermal mould design. Conference Proceedings of the YIC GACM 2015, Aachen, 2015

Hopmann, C.; Nikoleizig, P.; Schöngart, M.: Minimisation of warpage for injection molded parts with reversed thermal mould design. Conference Proceedings of the Polymer Processing Society, Graz, 2015

Hopmann, C.; Nikoleizig, P.: Präzision aus Schmelze – Ansätze zur automatisierten thermischen Spritzgießwerkzeugauslegung. IKV-Fachtagung Spritzgießwerkzeugtechnik – Im Spannungsfeld zwischen Klein- und Großserienproduktion von Kunststoffprodukten, Aachen, 2015

Hopmann, C.; Nikoleizig, P.: Inversed Cooling Channel Design for Injection Moulds Based on Local Coling Demand and Material Properties. Conderence Proceedings ANTEC, Indianapolis, 2016

Hopmann, C.; Nikoleizig, P.; Filz, P.; Schmitz, M.: Präzision aus Schmelze – Beherrschung der Erstarrung im Spritzgießen, Tagungsband zum 28. Internationalen Kunststofftechnischen Kolloquium: Integrative Kunststofftechnik, Aachen, 2016

There is a direct correlation between pressures and temperatures prevailing inside the cavity during injection moulding, and the material’s specific volume. Process control optimized on this basis is considered as the optimum process control for injection moulding in order to minimize warpage. At present, optimum process control is conducted according to the pvt concept, determining process data (pressure, temperature, etc.) for a local spot inside the injection mould. So as to optimize the moulded part in every spot along the flow path, it is thus necessary to compensate for pressure losses that occur along the flow path corresponding to flow resistances. For this purpose, temperature control over the flow path shall be included as another parameter to control process variables. This extension to process control is aimed to prevent residual stress caused by local differences in process control, and reduce warpage by locally homogeneous component shrinkage.

A novel type of mould concept is designed to enable these investigations of process control. The concept uses a large number of small cooling circuits to provide for different cooling temperatures within the mould (so-called grid tempering). In addition, this complex system made up of many system elements will be implemented as a self-optimizing system. For this purpose, the behaviour of the overall system is adjusted by adapting the target values of the individual system elements (elements to control temperature as well as mould internal pressure) to the global target value of warpage. 

Sub-project B3 will investigate overall process control of pressure and temperature during injection moulding. This will be done by local investigations over the flow path, to minimize residual stress as a result of local differences in process control, in order to achieve reliable and high precision of the moulding.  

Veröffentlichungen:

Bobzin, K.; Öte, M. ; Linke, T. F.; Alkhasli, I.; Hopmann, C.; Nikoleizig, P.: Development of Simulative Approaches for Precisely Designing the Properties of Plasma Sprayed Coatings for Application in Injection Moulding, Conference Proceedings of the YIC GACM 2015, Aachen, 2015

Hopmann, C.; Nikoleizig, P.; Filz, P.; Schmitz, M.: Präzision aus Schmelze – Beherrschung der Erstarrung im Spritzgießen, Tagungsband zum 28. Internationalen Kunststofftechnischen Kolloquium: Integrative Kunststofftechnik, Aachen, 2016

Hopmann, C.; Nikoleizig, P.; Theunissen, M.; Schmitz, M.: Development of a highly segmented temperature control in injection moulding for reduced warpage and increased process stability. Proceedings of the 32nd International Conference of the Polymer Processing Society, Lion, 2016

What is common to plastics and metal processing, considering primary shaping techniques, is that a melt is transferred into the cavity of a moulding tool and solidifies there. During this process, melt, mould and the structure generated seek to compensate temperature differences, thus determining the states of order both at molecular and atomic level. These are the crucial factors to determine morphology as well as the generation of residual stress. When producing plastics parts, these internal properties frequently lead to severe problems such as shrinkage and warpage. Due to insufficient models and lack of material data, the underlying mechanisms have not been considered in an overall investigation, as yet.

Sub-project B4 is due to investigate and, in a following step, model temperature compensation processes aimed to improve the predictability of warpage in plastics components. Integrative simulations that use approximate approaches to determine thermal interaction are available but cannot prevent entirely the expensive step of subsequent adaptation of the injection mould. With thermal interaction considered only insufficiently today, it is therefore essential to analyse, understand and control it much better than at present so as to improve simulation. No method is currently available to measure the temperature field in the melt and in the solidifying structure. This is why the central target of the sub-project is to develop, manufacture and implement a new measuring device for the locally resolved determination of melt temperatures. As a measuring approach, the technique of ultrasonic tomography is suitable. Using this method, first the ultrasonic velocity field is determined, which can then be transferred into a temperature field. 

Veröffentlichungen:

Hopmann, C.; Spekowius, M.; Wipperfürth, J.; Schöngart, M.: A concept for Non-Invasive Temperature Measurement During Injection Moulding Process. Proceedings of the 31st International Conference of the Polymer Processing Society, Jeju, 2015

Hopmann, C.; Spekowius, M.; Laschet, G.; Spina, R.: Effective thermoplastic and thermal properties in injection moulded parts. Proceedings of the 3rd ECCOMAS Young Investigators Conference, Aachen, 2015

Hopmann, C.; Spekowius, M.; Laschet, G.; Spina, R.: Towards a precise simulation of effective material properties in injection moulded parts taking into account inhomogeneous microstructures. Proceedings of the 2nd International Injection Moulding Conference, Aachen, 2015

Hopmann, C.; Spekowius, M.; Laschet, G.; Spina, R.; Poppe, E.; Wipperfürth, J.: Morphologiebasierte Vorhersage inhomogener Werkstoffeigenschaften. Tagungsband zum 28. Internationalen Kunststofftechnischen Kolloquium: Integrative Kunststofftechnik, Aachen, 2016

Hopmann, C.; Wipperfürth, J.; Schöngart, M.: A concept for Non-Invasive Temperature Measurement During Injection Moulding Process. AIP Conference Proceedings 1713 (2016) 40009

Hopmann, C.; Wipperfürth, J.; Schöngart, M.: Spatially Resolved Temperature Measurements in Injection Moulding Using Ultrasound Tomography. Proceedings of the 32nd International Conference of the Polymer Processing Society, Lion, 2016

Dr.-Ing. Matthias Theunissen

Head of department Injection Moulding +49 241 80-93827 matthias.theunissen@ikv.rwth-aachen.de