Manufacturing and Characterisation of High-strength Plastic-metal Hybrids
- 238 Downloads
KeywordsUndercut Adhesive Bond Torsional Test Metallic Component Torsional Rigidity
Metals and plastics have long since been rival materials when it comes to “plastic substituting metal”. Well-known exceptions to this rule include insert and outsert technology. This is primarily based on integrating locally arranged elements of the single material component into associated structures of the other material.
Plastic structures verifiably boost the performance of metallic designs by optimally transferring the forces exerted into the component and distributing them within the same. The high elasticity of plastic means that plastic-metal compounds also excel when it comes to absorbing impact stresses. For all these reasons, plastic-metal hybrid structures also offer clear cost and weight advantages when compared to similar purely metallic equivalents, and the greater the extent to which the additional functions are incorporated into the component, the more prominent the benefits mentioned.
Meanwhile, production applications are prioritised by the automotive industry (dash-board supports, front-ends etc.) and are set to be key components of large-scale serial production within this industry in a couple of decades’ time. One thing most applications currently in serial production have in common is that the bond between the metal and plastic is via a form closure, for example through- and circular injections at boreholes and beads.
Applying High-strength Plastic-metal Composites
Investigations by Zhao , which were conducted at the Chair for Plastics Technology of Erlangen-Nuremberg University, clearly show the potential offered by plastic-metal hybrid structures, provided an extensive adhesive bond can be established between the plastic and metal in question. Both rigidity and strength are significantly enhanced as a result of the adhesive bond. There was also scope to boost bending and torsional strength by around 45 % compared to conventional design (selective form closure over boreholes and beads). Torsional rigidity in particularly was even doubled. This large-scale adhesive bond prevents both any buckling of the thin-walled metal sheeting and any reciprocal movement between the plastic and metal components. The adhesive bond was established through the efforts of Zhao by heating up the sheet metal up to the melting temperature range of the plastic components. However, the adhesive bond failed when exposed to alternating temperature stresses.
Current developments in the hybrid technology field targeting a permanent adhesive bond are focused on developing primer layers to facilitate adhesion (Hybrid-Plus , HyLight ) or replacing the metallic components with so-called organic sheets, namely fibre-reinforced thermoplastics .
Inserts to boost adhesion by adding a microstructure to the metallic components were systematically investigated as part of the ExtraLight research project. The structuring of the metallic components was undertaken as part of efforts to generate an undercut topology. During processing, the plastic penetrates the undercut areas of the metallic components and then links up with the same via a microform closure.
Laser-structuring of the Metal Sheets
To generate the undercut structures required and implement the process in an energy-efficient manner, an approach combining melt and vapour ablation — a method also used for cutting or drilling — was selected. The melt ejection initiated by the vapour recoil pressure is harnessed here to shape the indentations.
Part of the viscous mass can become detached, in particular during lengthy laser pulses involving greater melt formation and higher intensities, and be ejected from the interaction zone in the form of droplets. High-speed imaging shows how the melt ejection is delayed during structuring with short laser pulses and actually takes place over a period equivalent to several times the actual pulse duration. The pulse durations during combined melt and vapour ablation ranged from a few ns up to a few ms. Intensities exceeding 100 W/cm2 were required to allow the material to be vaporised. The processing itself was conducted using high-performance nanosecond TruMicro 7050 and TruMicro 7240 lasers.
Characterisation of Plastic-metal Composites
▸ thickness of the sheeting, within the range 1 to 2 mm
▸ geometry of the rib (thickness of 2 or 4 mm)
▸ connecting geometries of the rib (base geometry).
The first step when manufacturing the rib test body is to fix the sheeting onto the fixed mould side using mechanical clamps until the movable mould side accommodates the fixing of the sheeting via a spring-loaded insert when closing. The spring-loaded inserts accommodate any desired sheeting thickness within the range of 1 to 2 mm. The mould has three cavities. The variable gating system allows simultaneous or serial filling of cavities, allowing the impact of a range of flow conditions to be assessed.
Both rigidity and strength are significantly enhanced as a result of the adhesive bond.
The process of characterising the front pull-off and shear strengths involves clamping sheeting and plastic rib in the relevant testing devices with a defined level of torque and initiating movement of the elements relative to each other at a speed of 2 mm/min until the point of failure. The tests were conducted at −40 °C, at room temperature and at 80 °C (PP) or 90/100 °C.
In addition, the failure stresses determined using this approach in the separately exerted loading directions were used as the basis for the complex geometries of the simulation.
“Berlin Test Beam” Demonstrator
In real components, complex multiaxial stress states act on the points where the metal and plastic elements are joined. To take this into account, a test component was developed, the so-called “Berlin test beam”, which exhibits complex geometry compared to the ribbed test body, but for which bending and torsional tests can be performed using comparatively simple means.
The mould includes a triple heat runner. The gating points can be used individually or in combination, allowing the impact of a range of filling scenarios on composite strength to be tested. Open nozzles are also used, to minimise any damage to the fibres for long fibre-reinforced plastics. As with the ribbed mould, there is also an option to heat up the inserted sheets via a mold-integrated induction heater to the desired temperature.
The “ExtraLight” research project focusing on the topic of “Multimaterial systems — future lightweight construction for resource-saving mobility” was required as part of the “Material innovations for industry and society — WING” framework program of the German Federal Ministry for Education and Research (BMBF) and supported by the VDI as project sponsor, funding code 03X3037. Companies involved included Albis Plastic GMBH, Allod Werkstoff GmbH & Co. KG, Audi AG, BASF SE, Daimler AG, inpro Innovationsgesellschaft für fortgeschrittene Produktionssysteme in der Fahrzeugindustrie mbH, Neue Materialien Fürth GmbH, SABIC Innovative Plastics, Technische Universität Berlin, thyssenkrupp Steel Europe AG, TRUMPF Laser- und Systemtechnik GmbH and Volkswagen AG.
Tests on the “Berlin Test Beam”
It is visible that the sheets, in themselves, have only minimal torsional rigidity and the polymer core is similarly torsion-flexible, albeit at a higher level. If both components are twisted at the same time, but without bond between them, then the resultant torsional stiffness is basically an addition of the single components stiffnesses.
If a bond between the polymer and metal is generated, it results in completely different load deflection curves with far higher torsional rigidity, and failure only occurs when a large torsion angle is reached. Also on video recordings, it proved impossible to ascertain whether this was due to the metal buckling or, the plastic becoming detached from the metal or cracking in the plastic. Even at large torsion angle, the remaining residual strength is higher than for the undamaged and unconnected individual components.
A four-point bending procedure was implemented for the bending tests on the Berlin test beam, whereby the punch was realised using a cuboid. Immediately after applying the initial load and with the initial deformation that followed, contact between the punch and the Berlin test beam only remains at both edges. Both the rotational support as well as the punch act on the polymer and prevent any peeling effects. During these tests, the free length between the supports was 250 mm and the total length of the supports was 350 mm.
The results allow, first and foremost, a comparison of various material pairs under a range of different processing conditions.
Applications in Trials, Transfer to Demonstrator Cockpit Crossbeam
The necessary condition for deploying ExtraLight technology for serial production was to ensure that the structuring process could be fully automated and incorporated into the injection moulding process and to ensure that the cycle time would not be extended as a result.
To investigate the strengths, crossbeams were structured from Trumpf at two fixed positions (GEO2, GEO3), Figure 10, using a laser and then incorporated into the serial production process. The structuring process involved two different forms of structuring being applied, which differed primarily in terms of their main orientation directions (longitudinal or transverse). Part of the crossbeam manufactured was also subject to alternating thermal and mechanical stresses.
The thermal ageing process comprised accelerated conditioning in accordance with DIN ISO1110 and ten temperature change cycles, each two hours in length at RT, −30 °C and 100 °C. The rate of change of temperature was 1 K/min. Crossbeams treated in this manner were labelled “OLD”. The connection surfaces of the GEO3 position were also additionally subjected to a mechanical fatigue test. This comprised torsional stresses alternating within the range −18 to 6 Nm over a total of 380,000 cycles and at a frequency of 2.8 Hz (labelled “with load”).
- Hoffmann, L; Zhao, G.; Ehrenstein, G.W.: Leichtbauteile in Kunststoff-Metall-Hybridtechnik. Symposium „Metalle und Kunststoffe — Verbindungen für die Zukunft, Fürth 2002Google Scholar
- Zhao, G.: Spritzgegossene, tragende Kunststoff-Metall-Hybridstrukturen, Konstruktion, Prozessanalyse und Charakterisierung. Dissertation, technisch-wissenschaftlicher Bericht, Erlangen 2002Google Scholar
- Michel, P.; Riepenhausen, H.: Neuartiger Metall-Kunststoffverbund als Struktur-Leichtbauelement am Beispiel Heckklappe. Kunststoffe im Automobilbau, Mannheim 2008, VDI KunststofftechnikGoogle Scholar
- Drummer, D.(Hrsg.): Handbuch Kunststoff-Metall-Hybridtechnik. Lehrstuhl für Kunststofftechnik, Erlangen 2015Google Scholar
- Hoffmann, L.; Linn, C.; Drummer, D; Gröschel, C: Hochleistungsfaserverbundbauteile aus der Spritzgießmaschine — Ein Verfahrensbaukasten für innovative Bauteilentwicklungen. Kunststoffe im Automobilbau, Mannheim 2014, VDI KunststofftechnikGoogle Scholar