Sandwich panels with honeycomb core

ThermHex Waben GmbH and the Fraunhofer Institute for Microstructure of Materials and Systems IMWS in Halle are working together in a joint research on the development of production and processing methods suitable for series production of organic sheet sandwich semi-finished products.

Wabenstruktur Firma Leichtbau
© ThermHex Waben GmbH

Continuous production of thermoplastic honeycomb cores

The patented ThermHex process enables continuous in-line production of thermoplastic honeycomb cores. In other processes, each layer of the honeycomb must be cut individually from a block and then laminated. This makes the conventional production methods for honeycomb cores time-consuming and expensive – with ThermHex, all production steps take place within one production line.

After the extrusion the material sheet is deep-drawn by rotary-vacuum forming, folded, laminated and cut to the length required by the customer. The production plant in Halle can be used to produce honeycomb cores from 3 to 30 mm thick and with an individual length of up to 6 metres; the production rate reaches up to 10 metres per minute.

The honeycomb cores are used in particular by the fiber composite industry to make sandwich panels and components which, for example, are used in truck bodies, car interiors or bathroom pos and in swimming pools.

The process is more resource efficient and significantly more cost effective than the conventional processes for the production of honeycomb cores. With this process, ThermHex Waben GmbH can produce large quantities considerably more cost effectively and since the start of production in 2010 has repeatedly increased its production volume up to the present day 500,000 m².

Most recently, in December 2015, the production capacity for honeycomb cores was doubled again with the installation of a new extruder with higher throughput rate. In this way, on the one hand the delivery periods for customer orders reduces significantly. On the other hand, scope results for forward-looking development projects and innovations, for example, for a joint research project with the Fraunhofer Institute for Microstructure of Materials and Systems IMWS. This objective of this project, supported by the EU through the EFRE programme, is to research into the production and processing of organic sheet sandwich semi-finished products suitable for series production.

Flat organic sheet sandwich semi-finished products

Organic sheet sandwich semi-finished products are made up of very thin layers of thermoplastic fiber composite materials (organic sheets), which are held apart by a thermoplastic honeycomb core, so that a high level of stiffness is made possible with minimum weigh, and without additional stiffening ribs. These semi-finished products are particularly cost efficient when they are attached to the honeycomb in-line in a continuous process and are bonded with the core.

In further production steps they are then processed further to make components. To this end, for example they are formed and functionalized through injection moulding. Only the continuous ThermHex process of honeycomb production in one production line enables the production of cost-efficient flat organic sandwich semi-finished products in continuous production, which integrates the continuous lamination of the organic sheet cover layers. With automated in-line quality control, critical quality characteristics can also be monitored efficiently during the process.

 

Herstellung Fertigung Prozess Schema Darstellung
© ThermHex Waben GmbH

Automated in-line production of organic sandwich semi-finished products.

Complex sandwich components from the injection moulding machine

Further processing of the organic sheet sandwich semi-finished products can take place in the fully-automated hybrid injection casting process for large production series applications. This process, with semi-continuous working with four process steps enables functionalized sandwich components to be produced, ready for use, within short cycle times.

© Fraunhofer IMWS

Process chain for the production of thermoplastic-based lightweight structures in hybrid injection casting.

The flat sandwich semi-finished products are first heated contact-free with the help of medium-wave infrared (IR) heaters. These enable precise control of the heat input and thus a defined temperature profile to be set across the cross-section of the semi-finished products. The matrix material of the cover layers is heated above its melting temperature, while the thermoplastic honeycomb core is significantly below its melting temperature and thus retains its elastic-plastic behavior and its stability.

After reaching the optimum temperature profile in the semi-finished product for further processing the cut-to-size sandwich is inserted into the cavity of the injection moulding machine. This process step is performed fully-automated, quickly and with high precision via an articulated arm robot. This ensures that the temperature profile for the subsequent forming is maintained in the sandwich during the insertion process.

The thermoforming of the flat sandwich semi-finished product into shell-shaped structures takes place during the closing movement of the injection mould. During this, sliding processes between the fibers, within the cover layers and in the molten matrix material and the plastic forming of the honeycomb overlap. A special test rig was developed for process-orientated examination of the thermoforming behavior in the vertical installed position. This allows differentiated investigation of the shear forces in the individual laminate layers and in the sandwich composite.

Following complete closing of the injection moulding cavity and on reaching a predefined closing force the formed sandwich components are functionalized in the conventional injection moulding cycle. Current investigations are focussed on the development of new methods for economical production of selective point and full face joints by means of injection moulding technology for thermoplastic sandwich construction. Such functionalized sandwich components can be joined with other technical components or systems without additional rework, for example, by means of screwed or riveted joint and thus increase the potential for integration in existing lightweight solutions, for example, in the automotive industry. The development steps described are supported by systematic experimental studies and numerical analyses during the test, of the forming and material behavior. This approach enables highly efficient development of the necessary basic knowledge for the component and process design at a later stage. The results of the project are due to be demonstrated on a generic test structure at the end of the project period in August 2017.

Nutzfahrzeuge Leichtbau Einsatzmöglichkeit
© ThermHex Waben GmbH

Sandwich panels with honeycomb core are used, for example for truck bodies.