At the end of this year, is “Hurray, we’re still alive!” all we have to say? In the shadow of the Corona pandemic, 2020 was certainly a year that concentrated our attention on what is really urgent, what will bring companies through an economically difficult time and into a future that is still uncertain. The complete cancellation not only of important spring events, but of the plastics industry’s fall exhibition program doesn’t make the change of course any easier. But despite this, many companies are thinking about what they can still do thanks to digitalization and are moving their trade show presences into the Internet. You can find examples on page 23. In mid-November, Carl Hanser Verlag, too, launched a joint platform for this with its D-Expo Kunststoffe. Thanks to a background program supported by numerous partners, it still managed to tempt over 600 visitors in just two days.

The SKZ also achieved an impressive feat, drumming up 165 persons for an extremely compact format. In an approximately one-hour “coffee break,” they were not only able to hear talks by the organizer and sponsors via video, but, in a number of matchmaking rounds, also to make direct contact with other visitors who had entered compatible or complementary interests in their profiles. Not all virtual visitors coped with the technical hurdles of logging on to this kind of speed dating, and switching to the assigned partner in time – but who hasn’t lost their way in a real trade show? At any rate, the participants’ enthusiasm showed how strong the desire for interaction is in this time of contact starvation.

After five years in which I have followed developing upheavals in the plastics industry as editor in chief of Kunststoffe and Kunststoffe international, I want to wish you health and all the best for the coming year on behalf of the entire team, and also introduce my successor, Susanne Schröder, with whom, from January 2021, Kunststoffe and Kunststoffe international will continue their journey into a decade that will certainly continue to be challenging.

Dr. Karlhorst Klotz übergibt die Chefredaktion an Susanne Schröder (r.)

16:9

16:9(1)

Editorial Klotz

Klotz, Karlhorst, Dr.

Tips and Hints on the Campus Material Database

An important phenomenon that deserves attention in the use of plastics is the materials’ viscoelasticity. It has the consequence that plastics first deform spontaneously under constant load and then continue to deform over time. Spontaneous deformation can be predicted using the modulus of elasticity (single point value acc. to ISO 527-1/-2 in Campus). The subsequent creep requires more complex material data. For this, Campus provides the customary representation in the isochronous stress-strain diagram. The original experimental data, i. e. curves for strain over time, which are in each case measured for different loads, are not presented in Campus.

From these diagrams, the strains at constant time are picked off for all stresses, and then plotted as time-constant (isochronous) lines in a stress-strain diagram. The resulting curves are confusingly similar to the stress-strain diagram from the short-term test. In addition, Campus calculates the respective secant modulus for all curve points and then displays them as a creep modulus over time for different stresses. Both forms of presentation are used in design to calculate the creep of a material. It is important to note that both representations only apply at a single temperature. It is not possible to superimpose curves for different temperatures.

Users who prefer the original form of representation of strain against time are referred to the Material Data Center database, where this can be found back-calculated from the isochronous curves. This is particularly interesting for determining parameters for creep models, which usually fit the creep curves and not the isochronous.

As with all mechanical data in Campus, it should also be taken into account here that the tests are measured on type A standard test specimens per ISO 3167. The standardization ensures that the data from different manufacturers are comparable; however they cannot be readily transferred to the properties in any arbitrary parts. A designer must adjust the information by making appropriate corrections and reductions.

Campus

Standardization Guarantees High Data Quality for Future Applications

A consistent flow of data and information is essential for digital solutions and meaningful analytics. The introduction of potential solutions is therefore often associated with high costs for data preparation. Part 3 of the report on the Plastics Innovation Center 4.0 (PIC 4.0) at RWTH Aachen University describes information flow planning, which ensures a consistent database for future process optimizations as well as increased transparency.

A consistent flow of data and information is essential for digital solutions and meaningful analytics. The introduction of potential solutions is therefore often associated with high costs for data preparation.

Due to standardized interfaces and increasingly uniform communication protocols, data acquisition is still an important and necessary field of development but no longer a major challenge. Modern machines and peripherals as well as software systems are able to provide a large amount of data that has to be analyzed and archived. The disillusionment is all the greater when the collected data hardly offers any added value. The reasons for this are often found in a lack of consistency and missing data points, which prevent a semantic linking of data to information as well as an automated analysis of data [1, 2]. It is therefore not uncommon that data preparation takes up to 80 % of a data scientist’s working time before the actual analysis of the data begins.

Necessity of a Consistent  Information Architecture

For data to become information, it must be semantically linked to a use case or analysis focus and prepared in a way that can be interpreted by a decision-maker. If individual data packages are missing or have been labelled differently by another person, this makes interpretation more difficult. Examples of this are production data such as processing temperature or pressure that are not assigned to an equally continuously measured quality feature, or the sequence in which the cylinder temperature zones are labelled starting from the nozzle or material feed. In particular, the individual perspective of different operators and handwritten machine logs, which are subsequently digitized at great effort, increase the potential for interpreting individual data points in a database incorrectly or not at all.

Although the PIC 4.0 only represents a research infrastructure for the development of intelligent production systems for plastics processing, the challenges of consistent data and information structures in science and industry are similar. In order to illustrate a consistent and expandable labelling of the gathered data as well as a consistent interaction of all linked systems and objects in a Smart Factory, the Institute for Plastic Processing (IKV) at the RWTH Aachen University implements an information architecture in the PIC 4.0 after the model of the Reference Architecture Model Industry 4.0 (RAMI 4.0) <a href="http://[3]">[3]</a>.

Cross-Domain Information Flow throughout the Value Chain

The information flow can and must be considered and modeled on different levels of abstraction (Fig. 1). When linking the individual departments of a smart manufacturing system, a distinction can be made between different types of data, which are exchanged via direct M2M communication along the value chain or via information exchange with a central server.

Master data refers to identification and general data for the production or test order as well as other data that are unchangeable over time or are considered as unchangeable, e. g. mold and target product dimensions or the characterization of the machine and peripheral equipment used. Material data are mandatory at many points along the value chain, such as for mold design in product and process development or for targeted process control in the technology center.

Simulation data is mainly generated in development and can then be used for data-driven process setup in the technology center [4, 5]. Process data of manufacturing processes and material handling, in turn, are gathered as digital shadows in order to implement a direct data-driven process optimization or to feed these data back into the development department for a targeted comparison of the simulated with the actual product quality.

On the next level, the generic data groups from the graphic (Fig. 1) can be more precisely considered along trial processing (analogous to order processing). Depending on the manufacturing process or separated process steps, quality and productivity relevant data are generated along the material flow during value creation of a product, which must be linked across all processes (Title figure).

According to the RAMI 4.0 hierarchy axis, the information flow is no longer considered on the company level, but on the level of the work centers e. g. across different manufacturing cells in production. Due to the more detailed representation of relevant information and data flows across several manufacturing levels, semantic correlations to a specific application or to quality-relevant conclusions from process data can be intuitively mapped.

Both the basic categorization and the specific assignment to the respective production steps and data sources support the creation of meaningful databases and process-specific synchronization across several manufacturing levels.

Modeling the Data Flow with the Help of Activity Diagrams

The detailed modeling of the data flows of individual process steps in activity diagrams specifies the information flow of the respective manufacturing processes. An example from blown film extrusion shows extracts of the individual process steps before the subsequent calibration of the blown film on the cooling section, independent of the machinery equipment (Fig. 2).

The respective setting values and measurable process parameters are directly assigned to the corresponding process steps and allow conclusions to be drawn about the relevant process section and the data source.

The activity diagrams of the PIC 4.0 processes are used to assign relevant setting and monitoring parameters in a process-specific manner and to consistently label the respective process parameters. Based on a uniform understanding of the variables in the process and the correlation between controllable setting parameters and relevant actual values, an information system can be set up that can be intuitively understood and expanded.

Through the object-oriented programming and implementation of an information architecture based on a reference architecture like the RAMI 4.0, data acquisition systems can be automatically configured in the future. Thus, it is ensured that gathered data can be interpreted for the employment of intelligent solutions in plastics processing continuously and with low effort.

Fig. 2. Extract from activity diagrams of blown film extrusion with assigned input and output data (Station-Layer)

Exemplary information flow across several steps of the value creation process in injection molding(Work-Center-Layer)

Fig. 1. General perspective of information flow planning in the PIC 4.0 with exemplary links between separated domains of a Smart Factory (Enterprise-Layer)

Name - Hauptartikel

The Reference Architecture Model Industry 4.0 (RAMI 4.0) was defined in 2016 in DIN SPEC 91345 and describes a reference architecture in the form of a cubic layer model for the representation of technical systems and objects in a smart production environment. Via its three axes (architecture axis; progression axis; hierarchy axis) it describes the interaction and the mandatory information exchange of interconnected objects across the value chain <a href="http://[3]">[3]</a>.

Univ.-Prof. Dr.-Ing. Christian Hopmann has held the Chair of Plastics Processing as Head of the Institute for Plastics Processing (IVK) in Industry and Craft at RWTH Aachen University, Germany, since 2011.

Pascal Bibow, <a href="http://M.Sc">M.Sc</a>. RWTH, has been working since September 2016 on intelligent production systems at IKV. Since January 2020, he has been Managing Director of the Plastics Innovation Center 4.0;  <a href="mailto:pascal.bibow@ikv.rwth-aachen.de">pascal.bibow@ikv.rwth-aachen.de</a>

The construction of the Plastics Innovation Center 4.0 is funded by the State of North Rhine-Westphalia and by the European Regional Development Fund (ERDF).

In the following article, the authors report on the importance of the PIC 4.0 as a demonstration platform for industry-related developments in the Internet of Production. Presumably, it will be published in issue 03/2021. The previous parts have been published in issues 5/20 and 7/20.

[1] García, S.; Ramírez-Gallego, S.; Luengo, J.; Benítez, J. M.; Herrer<a href="http://www.kunststoffe-international.com/2020-10">www.kunststoffe-international.com/2020-10</a>d prospects. Big Data Analytics 1 (2016) 1

[2] Amin, R.; Arefin, T.: The Empirical Study on the Factors Affecting Data Warehousing Success. International Journal of Latest Trends in Computing 1 (2010) 2, S. 138–142

[3] DIN Deutsches Institut für Normung e. V.: Reference Architecture Model Industrie 4.0 (RAMI 4.0). Beuth Verlag GmbH, Berlin 2016

[4] Schmitz, M.; Hopmann, C.; Röbig, M.; Pelzer, L.; Topmöller, B.; Wurzbacher, S.: Jenseits menschlicher Fähigkeiten. Modellgestützte Prozesseinrichtung durch vollvernetzte Produktion im Spritzgießen. Kunststoffe 109 (2019) 9, S. 142–145

[5] Hopmann, C.; Bibow, P.; Kosthorst, T.; Lockner, Y.: Process-setup in injection moulding by Human-Machine-Interfaces and AI. In: Hopmann, C. (Hrsg.): 30th International Colloquium Plastics Technology. Aachen 2020, S. 157–196

Read the German version of the  article in our magazine Kunststoffe or at  <a href="http://www.kunststoffe.de">www.kunststoffe.de</a>

Spritzgießen

Kunststoffverarbeitung: Verfahren

Digitale Transformation

Wirtschaft

Innovationen und neue Technologie

Vernetzung / Cloudlösung für die Kunststoffverarbeitung

Industrie 4.0

Industrie 4.0 für die Kunststoffverarbeitung

IoT-Plattformen für die Kunststoffverarbeitung

Alternatives to Gluing and  Mechanical Joining

Thermal Direct Joining of Plastic-Metal Combinations for Industrial Applications

The joining of dissimilar materials is a special challenge for joining technology. Thermal direct joining together with suitable surface treatments enables stable joints between very dissimilar materials combinations, thus offering an alternative to classic procedures for joining hybrid parts.

Plastic/metal hybrid parts are being used increasingly in numerous branches, such as the automotive, aerospace, or electronics industries. The joining of dissimilar materials continues to present a special challenge to joining technology. In recent years, thermal direct joining of thermoplastics and metals has developed into a promising joining technology capable of replacing gluing or mechanical joining.

This joining process consists of two process steps: surface treatment of the metal joining partner and the thermal joining procedure. Depending on the materials combination and case of application, the various pretreatments and joining methods influence the properties of the hybrid joint. The required knowhow is still insufficient to transfer thermal direct joining to industrial series production. This applies to the effect on bond strength from the various surface pretreatments of each materials combination, as well as to the effect of ageing behavior, e. g., under temperature changes. Precisely these influences are being investigated by the AGeD, a joint project of the ILT, the Fraunhofer Institute for Laser Technology, and the ISF, Institute for Welding and Joining Technology of the RWTH Aachen, Germany, to investigate various pretreatment and joining methods in respect to ageing behavior and performance following thermal direct joining of metals and plastics.

Interplay between Surface Pretreatment and  Joining Process

Suitable pretreatment is a decisive factor for achieving long lasting connections between thermally joined partners. Bond strength is largely influenced by the energetic state, morphology, geometry, and chemical structure of the surface. Surface pretreatment aims at activating the surfaces, and/or increasing the bond between the joining partners through geometrical alterations to the surface. In addition to chemical surface pretreatments, emphasis has been placed on corundum blasting and laser microstructuring to create undercuts on the metal surface.

In corundum blasting, the metal surface is irradiated with a blasting material at high pressure in order to roughen the surface with irregular, unreproducible structures. Whereas undercuts result from corundum blasting only in rare instances, laser microstructuring modifies the metal surface by sublimation and remelting in such a way that the surface becomes larger, and defined undercuts occur that enable mechanical interlocking between the joining partners.

In the AGeD project, a single mode fiber laser is used to create linear cavities in the metal surface that exhibit an undercut. To do so, the laser beam is deflected linearly over the workpiece by a laser scanner. The material is partially evaporated and melted by the highly intense laser radiation. Steam pressure drives the melt out, and it partially solidifies at the structure neck. A teardrop-shaped undercut is formed by traversing the scan contour several times (Fig. 1, left). Targeted arrangement of the structures and increased structure density, e. g., in a cross-grid pattern (Fig. 1, right), can increase bond strength <a href="http://[1]">[1]</a>.

To obtain a long-lasting joint between plastic and metal in the actual joining process, the thermoplastic has to be locally heated to melt temperature, and external joining pressure applied in order to wet the metal surface and fill the surface structures. The metal surface is often heated for this reason, and the plastic melted indirectly via thermal conduction. Various heat sources can be used, such as induction, heating elements, or laser irradiation.

Each joining process has its particular pros and cons, so that processing requirements, part geometry, and materials properties are very important when selecting the process. For example, laser joining enables flexible reworking at short processing times, but the laser beam must have access to the joining zone. In induction joining, even hard to access parts can be joined.

On the other hand, the processing times are longer, and the inductors may have to be adapted and configured according to the geometry of the joining zone. No significant differences can be observed among the various joining procedures, as long as the procedure ensures that the metal surface and structures are completely wetted with plastic melt.

Materials Selection Affects Bonding Strength

The requirements for surface pretreatment and the prevailing bonding mechanisms differ according to the materials combination. Within the framework of the AGeD project, samples were joined and subsequently tested that combined plastics (PA6, PP, PBT and POM) with various materials (aluminum and steel) after different surface pretreatments (corundum blasting and laser microstructuring) by diode laser radiation. It turns out that roughing the surface by corundum blasting suffices to create a bond between many materials combinations (Fig. 2) This is in part due to the polarity of the plastic and the formation of specific adhesion forces between the joining partners.

However, clearly higher bond strengths can generally be achieved by introducing reproducible undercuts by means of laser microstructuring. Moreover, all materials combinations be combined with each other regardless of the polarity of the plastic. This is helpful if the process is intended to open up the broadest possible field of application. In addition, this promises to provide good results in investigations of ageing, since the mechanical interlocking of the plastic in the structures suggests that interfaces are less susceptible to external influences.

Other surface pretreatments, such as irradiation by atmospheric pressure plasma, or the use of adhesion promoters can only be used for a few materials combinations. Here the tensile shear strengths of the hybrid connection lie clearly lower than the results from laser microstructuring.

Climatic Change Resistance of the  Hybrid Connection

Besides the selection of suitable surface pretreatment procedures and the joining process for a particular materials combination, the conditions of application also play important role in subsequent industrial implementation. Due to the different thermal expansion coefficients of plastic and metal, thermally changing loads can have great influence on bond strength.

In initial investigations, climatic change tests based on an automotive standard (VW P1200) were performed on hybrid connections of PP/GF30 joined with laser structured steel or aluminum. To this end, the samples were stored in a climatic exposure test cabinet at changing temperatures between 80 °C and 95 % relative humidity and –40 °C and tested for bonding strength following 2, 10, and 30 cycles. The results (Fig. 3) show that the bonding strength of the hybrid connection is not significantly affected by temperature change. Even if the results cannot be transferred to all other material pairings, they provide a promising future for the use of thermal direct joining over a wide field of applications.

When suitable surface pretreatment methods and joining processes are selected, thermal direct joining offers a suitable alternative to conventional joining processes, such as gluing and riveting or screwing. High bond strengths can be achieved by laser microstructuring to create undercuts, almost regardless of the material combination.

However, for certain combinations of materials, strong connections can also be achieved by simple methods. Initial climatic change tests indicate promising results with respect to long-term join stability. These investigations should be expanded in the future to include further materials combinations and different surface pretreatments and joining processes.

For applications in the automotive field, the corrosion resistance of the join plays an important role in addition to thermal shock endurance. Media tightness is important above all for housing applications, such as for electronic components. In the near future, further investigations are planned within the framework of the project, e. g., corrosion tests according to  DIN EN ISO 11997. The goal is to transfer the project results to a recommended action catalog in order to give users assistance when selecting the procedure for a particular industrial application. Since these results show high bond strengths and good long-term stability, short processing times, as well as high flexibility in the selection of materials, they are a good starting situation for rapid implementation in industrial series production.

This electronics housing composed of an aluminum die casting with PA6 covers is a potential application case for thermal direct joining

Fig. 1. Cross-section of a laser microstructured DC06-galvanized metal sample (left) and a reflected light image of the structure pattern (right)

Fig. 2. Influence of surface pretreatment: tensile shear strength of various materials combinations subsequent to laser microstructuring (left) and corundum blasting (right)

Source: Fraunhofer ILT; graphic: © Hanser

Fig. 3. Tensile shear strength of hybrid connections between glass fiber-reinforced polypropylene and steel (green) and aluminum  (orange) prior to climatic change tests and subsequent to varying  long temperature cycles from 80 °C to –4 0°C 

Fig. 4. Corroded tensile shear samples subsequent to ageing investigations

Kira van der Straeten, <a href="http://M.Sc">M.Sc</a>., is the team leader for polymer processing at the ILT Fraunhofer Institute for Laser Technology in Aachen, Germany;  <a href="mailto:kira.van.der.straeten@ilt.fraunhofer.de">kira.van.der.straeten@ilt.fraunhofer.de</a>

The IGF project AGeD (IGF-No. 20326N) is funded by the Federal Ministry for Economic Affairs and Energy based on a resolution of the German Bundestag.

1 Engelmann, C., Eckstaedt, J., Olowinsky, A., Aden, M., Mamuschkin, V.: Experimental and Simulative Investigations of Laser Assisted Plastic-metal-joints Considering Different Load Directions. Phys. Proced. 83, 1118–1129 (2016)

Aluminium

Metall Werkstoffe

NE-Metalle

Laserstrukturieren

Oberflächentechnik

Laserbearbeitungen

Polypropylen

Kunststoff Werkstoffe

Thermoplaste

Verbindungstechnik, hybride

Verbindungstechnik Verfahren

3D Printing with Pellet Extruded Plastics on Metal

Plastic-Metal Hybrid Cuts Costs and Time in Additive Manufacturing

Additive manufacturing processes count as future-oriented manufacturing techniques. Numerous studies and award-winning demonstrators underline the interest of the industry. Nevertheless, up to now there are only a few plastic-based applications with significant quantities. Today, screw-based additive manufacturing is a solution for the economic and technical challenges and has the potential for series production.

Additive manufacturing processes count as future-oriented manufacturing techniques. Numerous studies and award-winning demonstrators underline the interest of the industry.

The debate surrounding the conditions for cost-effective and scalable additive manufacturing processes is currently dominated by a certain amount of wishful thinking. In many cases, the feasibility of a specific realization can be called into question as from the very beginning of the project due to the lack of an economic basis and the high requirements on the mechanical properties of the components. Certainly, additive manufacturing offers advantages in terms of tool maintenance, flexibility, freedom of design and functional integration. However, the simple fact remains that the transfer to mass production is never carried out if the resulting production costs per part exceed the current possibilities of a mold-based process. We also need to break free from the mindset of manufacturing current injection molding designs with a 3D printer. Like any production process, additive manufacturing processes require specific designs to reach their full potential.

In this situation, a team of scientists at the Institute for Plastics Processing (IKV) at RWTH Aachen University, Germany, developed a screw- and robot-based additive manufacturing process and introduced it for the first time during the Plastics Technology Colloquium 2016 in Aachen [1]. This concept was applied by Yizumi Germany GmbH, Alsdorf, Germany, involving the same research team, and was industrialized into a scalable production system.

The technical key element is a lightweight extruder weighing only 7.5 kg. It can still be used to process all common materials from PMMA and PC to PP and PA – with and without fiber reinforcement. At the same time, the lightweight construction approach also allows for small robot sizes to be used. Their range is usually more than sufficient.

Using Strand Shapes to Systematically Influence Anisotropy

Considering the required mechanical properties, the use of fiber-reinforced plastics is particularly beneficial for structural 3D-printed components. The use of pellets enables a higher content of reinforcing materials compared to other additive manufacturing processes. All manufacturing processes influence the fiber orientation and therefore the mechanics of a plastic part. Due to the excellent mechanical and rheological properties as well as the high thermal conductivity, which contributes to the dissipation of heat during layer composition, carbon fiber-reinforced materials are highly suitable for a fast additive manufacturing process. A carbon fiber-reinforced polyamide 6 (grade: Akromid B3 ICF 30 9 AM black (7451); manufacturer: Akro-Plastic GmbH, Niederzissen, Germany) is used in the following investigations.

By adapting the processing, the anisotropy of the mechanical properties can be significantly influenced. An anisotropic behavior is influenced by the parameters of the selected nozzle diameter as well as the ratio of feed rate and mass flow relative to the nozzle diameter.

Three different types of strand shaping can be defined. This means that we are dealing with either a preceding melt, a drawn melt or an intermediate state. Based on these boundary conditions, the fiber and molecule orientation of a drawn melt is stronger than that of an preceding melt.

Examining the flexural modulus and flexural strength, a significant increase in the mechanical properties can be seen with increased nozzle diameter at identical strand width (Fig. 1). Due to the increased anisotropy, the 3D-printed test specimens reach the property range of injection molded parts.

Why Does 3D Printing Plastics  Need Metal?

The integration of metal components in 3D-printing is advantageous for several reasons. Firstly, hybrid structures offer the opportunity to use materials according to requirements. Secondly, flat structures, which can only be printed inadequately by hatching operations, can be easily integrated. The flat, aerial pattern is simply added to the process as a semi-finished product.

This also replaces the necessary building platform management. The “building platform” remains in the part as a functional component. Thirdly, both aspects save time and money. Apart from additive manufacturing on metal sheets, the hybrid process also opens up the option of direct manufacturing on three-dimensional structures (metallic or polymer).

In a first step the required metal sheets are coated with a storage-stable plasma-polymerized adhesion modifying layer using the PST process (Plasma-SealTight) developed by Plasmatreat GmbH, Steinhagen, Germany, and Akro-Plastic GmbH. The process is successfully used in injection molding to produce high-strength plastic-metal bonds [2, 3]. The metal sheet can then be printed on directly after the coating.

At this point, it is important to ensure a sufficient contact temperature between the sheet metal and the melt. A variothermal fixture is required for this. In contrast to variothermal injection molding, it can be very compact and feature heating elements close to the surface, since the forces on the mold are much lower than in injection molding.

The processes can be combined with different metals [2]. In an initial process evaluation, an aluminum alloy and stainless steel were used to produce beam structures and the test specimens used in the following investigations [4]. However, to evaluate the mechanical potential of the plastic-metal hybrid bond, it is necessary to investigate the influencing parameters.

Approach to Analyze the  Mechanical Properties

In order to investigate the influences of the combination of processes and materials on the mechanical properties of 3D-printed plastic-metal hybrids, test specimens for three-point bending tests were developed in accordance to DIN EN ISO 178. The plastic component of the test specimens consists of a closed contour of four parallel strands over the entire test area. The height of the test specimen is set at 10 mm.  To reduce the influence of shear stress on the layered test specimens, the length to height ratio was defined to 25 [5]. Plastic test specimens and hybrid test specimens fitted with 1mm thick stainless steel inserts were used in testing (Fig. 2).

To assess the process influences of 3D-printing, the nozzle diameter is varied at constant strand width during the production of the plastic specimen. As part of the experimental design, the peak temperature of the variothermal fixture is also varied. The surface temperature of the metallic insert was set to 175 °C, 190 °C and 220 °C. During the production of the plastic test specimens, the variothermal heating with a peak temperature of 175 °C is maintained on the surface of the fixture.

The plastic and hybrid test specimens are tested in a three-point bending with a span of 200 mm and scaled test speed. The force is applied both on the substrate side and on the side facing away from the substrate, so that the metal inserts are loaded in tension or compression.

Mechanical Properties at  an Optimal Level

The temperature influences of the metal insert on the mechanical properties of plastic-metal hybrids are shown in the results (Fig. 3) and exemplary force/displacement curves (Fig. 4). The data shows a high consistency with minimal scattering of the maximum force. Due to the negligibly small standard deviation the processing is regarded to be robust.

The achieved maximum force, which is almost twice that of a plastic part, illustrates the mechanical potential of 3D-printed plastic-metal hybrids. Moreover, this is also reflected in the increased stiffness of plastic-metal-hybrids (Fig. 4). On the other hand, it can be seen that the flexural strength does not increase significantly above an insert temperature of 175 °C, irrespective of the type of load on the metal component. Thus, excellent bonding behavior is achieved over a wide temperature range.

However, the fracture behavior varies considerably throughout the temperature range, as the various temperature and load-dependent strain at break illustrates. At the same temperature (175 °C), when load is applied to the metal component, the plastic component fails in the extreme outer fiber under tensile load resulting in a migrating crack through the plastic at the slightest deflection. The bond between metal and plastic remains intact.

When load is applied to the plastic component at the same temperature, the applied force leads to plastic deformation of the metal component. The failure in turn occurs in the plastic under tension and thus in the interface between metal and plastic. This results in a complete separation of the plastic component (Fig. 5).

The fracture patterns indicate a well-developed bond between plastic and metal. The residues on the metallic surface after test show that the failure occurs in the plastic component.

With an increased insert temperature of 220 °C, the failure mode changes. It is remarkable here that the deflection nearly triples until the total failure of the test specimen (Fig. 6).

With regard to stiffness and the progression before reaching the force maximum, the test characteristics of specimens at higher temperatures largely match the lower temperature levels (Fig. 4).

The fracture mechanics are also impressive at the macroscopic level. Here, too, plastic deformation of the metal component occurs. The fracture also starts at the interface between plastic and metal. However, the bond remains intact. This means that initial failure does not occur between the layers of the 3D-printed plastic, but perpendicular to the direction of the buildup up to the neutral fiber. Further and complete failure is caused by compressive stresses.

As shown, the process for producing a plastic-metal hybrid component can be easily automated (Fig. 7).

According to the investigations, the bonding strength between metal and plastic via the plasma-polymerized adhesion layer is very high and the resulting parts exhibit improved mechanical properties compared to solely plastic parts.

In addition, the hybrid approach to additive manufacturing also offers economic advantages in comparison to conventional approaches in injection molding. A reduction in component costs of 10 % for the demonstrator (Title figure) shown clearly demonstrates the economic potential of additively manufactured plastic-metal hybrids.

The bicycle frame (600 g, produced in 80 min) is printed directly onto a metal sheet. This combination unites the advantages of two worlds

Fig. 1. Increasing nozzle diameters have a positive effect on the anisotropy of mechanical properties in the part

Fig. 2. Test set-up: variothermal fixture in production area of a SpaceA manufacturing unit, temperature load cycle and test specimen geometry

Fig. 3. Maximum load at different insert temperatures. This demonstrates the strong bond between plastic and metal over a wide temperature range

Fig. 4. Force-deformation chart of the tested specimens. The hybrid structure increases the stiffness and strength compared to plastic-only test specimens

Fig. 5. Testing of the hybrid part: initial position (force applied to the metal component on the left, force applied to the plastic component on the right), maximum deflection of the test specimen and failure with fracture mode (insert temperature 175 °C)

Fig. 6. Test in starting position (force application to the plastic component), maximum deflection of the test specimen and failure with fracture mode (insert temperature 220 °C) 

Fig. 7. A modular plant strategy allows scaling (here the largest model SpaceA 2000–500-S) and integration of all known automation concepts

Dipl.-Ing. Thilo Stier is Global Sales Director & Innovation Manager at Akro-Plastic GmbH, Niederzissen, Germany

Michael Rieck, <a href="http://M.Sc">M.Sc</a>. RWTH, is Business Development Manager at Akro-Plastic; <a href="mailto:Michael.Rieck@akro-plastic.com">Michael.Rieck@akro-plastic.com</a>

Dr.-Ing. Nicolai Lammert is Head of Business Unit Additive Manufacturing at Yizumi Germany GmbH, Alsdorf, Germany; <a href="mailto:N.Lammert@yizumi-germany.de">N.Lammert@yizumi-germany.de</a>

1 Hopmann, C.: Additive Fertigung in der Kunststoffverarbeitung. In: Hopmann, C. (Hrsg.), Integrative Kunststofftechnik 2016. Umdruck zum 28. Internationalen Kolloquium Kunststofftechnik. Aachen, Februar 2016, 978–3–8440–4033–3

2 N. N.: PST Der besonders haftungsstarke Hybridverbund. Firmenschrift der Akro-Plastic GmbH, URL:https://akro-plastic.com/de/prozess-und-technologie/plasma-sealtight/

3 Melamis, A.: Ultra-Strong and Media-Tight. Kunststoffe international 106 (2016) 6–7, pp. 8–13

4 Heinz, C.; Lammert, N.; Stier, T.; Unger, N.: Additive manufacturing based on screw extrusion – Applications, machines and materials in the automotive industry. PIAE, Mannheim 2020

5 Grellmann, W.; Seidler, S.: Kunststoffprüfung. 3. Aufl. Hanser, München 2015

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A Research Project Highlights the Recycling Potential of Lightweight Composites

Composite materials are often the key to lightweight design for many applications. However, these hybrid materials face a major challenge where recycling is concerned. There is still no solution to the recovery of long and continuous fiber-reinforced materials. Where it is possible at all, only downcycling has been feasible until now. Four research partners have therefore set themselves the goal of developing a new press forming technology as part of a zero-waste project.

Composite materials are often the key to lightweight design for many applications. However, these hybrid materials face a major challenge where recycling is concerned.

Whether for automotive, aeronautical, teletronics, construction or technical applications – diverse industries require ever more lightweight designs together with ever better part properties. The obvious advantage of using composite materials often fails at the hurdle of recycling. Three companies – Engel Austria GmbH, St. Valentin, Alpex Technologies GmbH, Mils, both in Austria, and the Hexcel Corporation, based in Stamford, CT/USA – are working on a solution together with Johannes Kepler University in Linz, Austria.

The research work focused on manufacturing carbon fiber-reinforced sheet molding compounds (SMC) by hot press forming, abbreviated to carbon-SMC, or C-SMC technology. The one-step process is regarded as especially cost efficient, in both its fully and partly automated versions. Critical regions are reinforced locally, in line with the load paths, by means of fiber-reinforced prepregs.

The project, which was sponsored by the Austrian research funding association (FFG), operates at various levels. It concerns the use of recycling materials, reduced quantities of carbon fiber waste, cost-optimized manufacturing and greater material efficiency. The goal is to use material only where it is required by the loads and safety requirements.

From the Idea to the  Automotive Component

The processing concept for manufacturing fiber composite solutions with a focus on zero waste was first validated at laboratory level, with both recycled carbon fibers and SMC offcut wastes. To permit implementation in large series, fully automated press forming was preferred from the start. The case study, an automotive transmission crossmember, effectively illustrates the feasibility and market potential of the innovation (Fig. 1).

The structural component, which is traditionally made of aluminum, was developed from carbon SMC with the premise of zero waste. The part specifications were provided by a leading OEM and the component design was optimized for the material. During the product creation, the development partners developed approaches to process and structural simulation. They were able to determine essential process data in advance by means of trial components. This reduces the development work and increases process reliability.

As the component topology was optimized at the beginning of the project, the main priority was to adapt the new process to the particular characteristics of the material. Different SMC semi-finished materials from Hexcel were used, some of which are still at the development stage. Various sandwich architectures were also used. The reference material was HexMC-i 2000, a commercially available carbon-SMC type. The feasibility as a whole was evaluated with four different materials (Fig. 2):

<ul><li>HexMC-i 2000 is given the designation primary C-SMC. It consists of rectangular chips (50 x 8 mm) cut from continuous fiber-reinforced, unidirectional (UD) carbon-fiber prepregs. The chips have a random orientation in the sheet molding compound.</li><li>rC-SMC is designated secondary C-SMC and contains recycled carbon fibers from the US supplier Carbon Conversions, which are impregnated with epoxy resin in a prepreg line at Hexcel. The carbon fibers have an irregular structure, similar to conventional GF-SMC.</li><li>UD tapes with an epoxy resin matrix were obtained from primary unidirectional carbon fiber prepregs and used for local reinforcement of HexMC-i 2000, which is made from the same material.</li><li>C-SMC Prepreg Byproduct is obtained from the UD prepreg HexPly M77 (offcut waste from the industrial mass production of automotive components). As delivered, it is in the form of tapes, which are cut at distances of 50 mm. This material allows defined fiber orientations of the chips to be integrated into the component architecture.</li></ul>

All the materials were pre-impregnated with the same epoxy resin (type: Hexcel M77). This resin system cures at 150 °C in about 2 min and is therefore especially suitable for automotive applications.

Fully Automated Process in the SMC Manufacturing Cell

The experimental components (Fig. 3) were manufactured in a fully automated SMC manufacturing cell, developed by Engel, an injection-molding machine with compression function (type: Engel v-duo 700) and an articulated robot (type: Engel easix KR35) (Title figure). The blanks were prepared on an NC-controlled cutting table, gravimetrically adjusted and introduced into the mold together with the aluminum inserts for the fastening points (Fig. 4). The mold is from Alpex, a supplier of tooling systems for manufacturing composite parts.

The carbon-SMC was pressed with a clamping force of 5000 kN. The resulting cavity pressure was in the range from 200 to 300 bar. With a component thickness of 8 mm and maximum dimensions of 520 x 340 x 100 mm, part weights of 1270 up to 1280 g were measured. The cycle time was 180 s. The actual press forming of the material was performed after a 15 s compression phase at 8 bar cavity pressure. The UD tapes were positioned before the stacks were inserted into the mold.

To validate the zero-waste concept, the structural components were mechanically tested in the laboratory on a servohydraulic testing machine (type: MTS Damper Test System; manufacturer: MTS Systems Corporation, Eden Prairie, MN/USA) according to the OEM’s specifications. The component test rig modeled the connecting points to the actual body. The force was applied via the piston of a dummy transmission. The displacement and reaction force were determined using a load cell.

The component behavior under monotone, quasistatic loading was investigated by means of a critical load path in the Z-direction with a test velocity of 2 mm/min. To estimate the expected lifetime under service conditions, cyclic fatigue tests were performed in the critical Z-direction (Fig. 5). These tests were performed under sinusoidal loading (5 Hz), both displacement and force controlled.

The first investigations already helped to optimize the process sequence with the new SMC processing technology. The fact that the fibers are bound in the form of chips instead of taking the form of bundles or cut rovings significantly changes the processing behavior compared to the conventional carbon-SMC process. Only after a defined heating time does the viscosity of the matrix fall to a level at which the chips can slide with respect to one another. In the optimized process, therefore, before the actual press forming, the system is subject to a low pressure of approx. 8 bar for about 15 s until the stack reaches a core temperature of at least 60 °C. In addition, the pressing force is increased to cause the material to flow during subsequent pressing.

A graph illustrates the profile of the cavity pressure during the curing time. The reference material is shown in green and the recycled material in orange (Fig. 6).

During the heating time, the cavity pressure is still low. In parallelism-controlled press forming, a pressure maximum occurs which, despite precise control of a constant clamping force of 5000 kN, rapidly decreases again. Here, the conversion of the liquid resin to the stiff and solid component at the pressure sensor influences the result of the measurement. In the continuation of the curing reaction, the pressure rises again, which can be primarily attributed to the thermal expansion of the C-SMC. Toward the cycle end, the pressure curves converge because of the identical matrix material.

In another series of experiments, the four investigated material configurations were compared with one another. The component tests were performed on a transmission dummy test rig. The readings for the comparison obtained from the quasistatic component test were the linear stiffness, the secant stiffness at the force maximum, and the fracture load (Fig. 7).