Laser-assisted thermoplastic tape laying

1- PROF. DR.-ING. CHRISTIAN BRECHER,

2- DIPL.-ING. ALEXANDER KERMER-MEYER,

3- DIPL.-ING. MARTIN STEYER,

4- DIPL.-ING. MARKUS DUBRATZ,

5- DIPL.-ING. MICHAEL EMONTS, FRAUNHOFER IPT AACHEN

(published on April 2010 – JEC Magazine #56)

Laser-assisted thermoplastic tape laying can be used for the automated, eco-friendly production of lightweight, highperformance structural components in a variety of industries such as aerospace and automotive.

Specific thermoplastics such as PEEK and PEKK can provide comparable or improved performance to epoxies [MOZ84], [BAI05]. Compared with thermoset resins, thermoplastic materials can be easily recycled and have superior fatigue and impact resistance, as well as better damage tolerance and vibration damping behaviour. These features are useful in the production of automotive underbody components and in offshore applications [AUT09], [MON00].

Laser-assisted thermoplastic tape laying is an out-of-autoclave process which provides high throughputs at comparatively low investment costs. By using an energy-efficient laser as the heating source, the required laminate bonding energy can be transferred with high accuracy to the nip point without wasting energy. The laser only heats a defined area with a minimum amount of energy. Thanks to the fast reaction time of the heating system, the temperature can also be controlled very precisely. These are outstanding advantages compared with the commonly-used hot-gas torch or open flame. As evaluated in [ESC01], higher throughputs can be achieved with a laser heat source than with other state-of-theart heating systems used for thermoplastic tape laying. According to Neitzel et al., lay-up rates up to 160 m/min are feasible with laserassisted thermoplastic winding processes, vs. only 60 m/min with open flame-assisted processes and only 18 m/min with hot gasassisted processes [NEI06]. These lay-up rates can be transferred to the tape laying process. The currently used fibre-coupled diode laser has an opto-electrical energy efficiency of 23%, whereas hot-gas and open-flame systems only achieve between 3% to 6% [NEI06]. Energy efficiencies of 40% can be achieved with direct and nonfibre- coupled laser systems. The advantages of processing thermoplastics with a laser heat source have already been proven for the laser-assisted tape winding process [DUB08], [KOE08].

As already introduced in [STE09], the Fraunhofer IPT develops modular tape laying units for multiple applications. One of its systems won the JEC Asia Award 2009 in the Automation category. The main components of a laser-assisted tape laying unit are a material storage system, a tape feeding system, a laser heat source and a consolidation system. The consolidation system is particularly important since it applies the pressure needed for in situ consolidation. Since the last publication, many improvements have been achieved in terms of system development, part production and process analysis.

Flat, contoured and tubular laminate structures made of carbonreinforced PA 12 and PEEK have been produced with the modular tape laying and tape winding units developed by the Fraunhofer IPT (Figure 1).

Combining tape laying and tape winding capabilities makes it possible to lay up precisely any angle between 0° and 90° independent from part size, lay-up rates and geodetical considerations on axially-symmetric parts. This makes the concept superior to common winding technologies. Moreover, product design can be based on load specifications and is less dependent on manufacturing requirements. This solution reduces material consumption while better utilizing the weight-saving potential of continuous fibre-reinforced thermoplastics.

After improving the thermoplastic winding process, the Fraunhofer IPT successfully produced flat and contoured laminate structures using the automated laser assisted thermoplastic tape laying method.

Investigations of unidirectional carbon-reinforced PA 12 and PEEK tape-laid structures clearly show the superiority of a laser heating system over open-flame and IR systems in the automated thermoplastic tape laying process.

The superiority of laser heating over IR systems can be shown clearly by comparing grinding surface patterns obtained with CF/PEEK laminate structures produced at Fraunhofer IPT (Figure 2). The laser-heated laminate shows a very homogenous pattern with a low void content and hardly visible interlaminar layers. In contrast, the laminate produced with an IR heat source has a very high void content and distinct interlaminar layers. The improved structure and low void content directly result from the precise energy transfer and temperature control associated with the laser heating system.

To evaluate the interlaminar shear strength of the laserassisted tape-laid components, wedge peel tests were performed on CF/PA12 and CF/PEEK specimens (Figure 3).

With laser-assisted tape laying, interlaminar shear strengths can be two to three times higher than those achieved with common hot gas-assisted tape laying processes. This is an impressive result considering that process optimisations for CF/PEEK prepregs are still ongoing. With the better investigated CF/PA12 prepregs, interlaminar shear strength values that are twice as high as the values for CF/PEEK have been achieved. The good performance of laser-assisted tapelaid laminates was also demonstrated by the three-point bending test performed by the Institute for Plastic Processing (IKV) in Aachen. No interlaminar fracture occurred during the examination.

In summary, the laser-assisted tape laying and winding process significantly improves production speed, energy efficiency and final product performance with respect to other state-of-the-art thermoplastic prepreg winding and laying systems.

Flat thermoplastic laminates were successfully produced with the novel Fraunhofer continuous form-adaptive consolidation system and a stiff roller. A form-adaptive consolidation system is obviously needed for the manufacture of three-dimensional components.

It was also demonstrated that such a consolidation system is essential for producing high-quality two-dimensional parts. In a future publication, stiff and conformable consolidation systems will be compared and their suitability for the production of flat and contoured thermoplastic laminates will be evaluated.

References

[AUTO09]http://www.autosieger.de/article7985.html, [20.07.2009] [BAI05] Bai; J.-M.; Leach, D.; Cease, S.; Pratte, J.: High Performance Thermoplastic Polymers and Composites, SAMPE 2005, Corina, CA, USA, S. 1391-1405

[DUB08] Dubratz, M.; Steyer, M. ; Brecher, C.: Thermoplastics speed up winding process, Composite Materials, September 2008, S. 36-38 [ESC01] Esche, R.v.d.: Herstellung langfaserverstärkter

[ESC01] Esche, R.v.d.: Herstellung langfaserverstärkter Thermoplastbauteile unter Zuhilfenahme von Hochleistungslasern als Wärmequelle. Diss. RWTH Aachen, 2001

[KOE08] Kölzer P.: Temperaturerfassungssystem und Prozessregelung des laserunterstützten Wickelns und Tapelegens von endlos faserverstärkten thermoplastischen Verbundkunststoffen. Diss. RWTH Aachen, ISBN 978-3-8322-7476-4, Shaker Verlag 2008

[LAT03] Latrille, M.: Prozessanalyse und -simulation von Verarbeitungsverfahren für faserverstärkte thermoplastische Bändchenhalbzeuge. Diss. Univ. Kaiserslautern, 2003

[MON00] Mondo, A.; Hauber, D.; Langone, R.;Quinn, L.: High Speed Processing of Thermoplastic Composites for Oilfield Pipe and Tubular Applications, In: Composite Materials for Offshore Operations, http://www.automateddynamics.com/tech_papers_final.php, PR 15, 2000

[MOZ84] Mozzy J. D.; Keys, A. O.: Thermoplastic vs. Thermosetting structural composites, Polymer Composites Vol. 5, No. 3, July 1984

[NEI06] Neitzel, M.; Mitschang, P: Handbuch Verbundwerkstoffe – Werkstoffe, Verarbeitung, Anwendung. Carl Hanser Verlag München, 2006. – ISBN 978-3-446-22041-6

[STE09] Steyer, M.; Dubratz, M.; Schütte, A.; Wenzel, C.; Brecher, C.: Laser-assisted thermoplastic tape laying system, JEC Composites Magazine, No. 47, March-April 2009