Pressing ahead - thermoforming advanced composites for transport applications
Work on thermoforming advanced composites and joining such parts form the core of a project at Queen’s University Belfast, UK, that investigates sustainable materials and manufacturing techniques in transport. Dr Saul Buchanan, from the School of Mechanical and Aerospace Engineering, explains.
Traditionally, continuously reinforced composites have been used for structural applications such as aerospace where costs are not a key design consideration. However, factors such as the demand for large-volume production, for example in the automotive and mass transit industries, continue to influence future material development strategies for transport structure applications.
The transport industry seeks ways to produce lightweight structures with high specific strength and stiffness but with a significant reduction in manufacturing costs. This can be achieved by improving manufacturing throughput, coupled with more energy-efficient processing technologies and strategies designed to recover and reuse valuable constituent materials at the end of the product’s life.
Recovery, reuse and sustainability in society’s approach to the materials, processes and disposal of the products we consume are receiving considerable attention because of the impact on the environment. This is now recognised by governments, which are legislating for a shift towards more sustainable manufacturing strategies. For instance, the EU’s End of Life Vehicles Directive, designed to encourage sustainable material and processing philosophies, is already in place and evolving.
Research scientists from Queen’s University Belfast (QUB) are working on the Sustainable Transport project whose principal aims are to develop future materials, lifecycle analysis and digital manufacture for the sector. A key function of the project, which is supported by the Northern Ireland Department for Employment and Learning, is to develop a rapid thermoforming press process and downstream fusion joining techniques for advanced thermoplastic composites.
The use of advanced composite materials for transport applications offers high performance and enhanced functionality, such as in situ health monitoring, self-healing, fire retardancy and improved wear characteristics. Structural composites employ reinforcement textiles made from carbon, aramid or glass fibres and are traditionally infused in a thermosetting polymer matrix system, such as epoxy. However, composites employing a thermoplastic matrix system, such as polyphenylene sulphide (PPS) or polyetheretherketone, are attracting considerable research interest because of their excellent mechanical performance coupled with rapid thermoforming technology, and the ability to recover and reuse them at the end of the component’s life.
A thermoforming station has been developed at QUB consisting of three key systems – transportation, heating and forming:
The pre-consolidated laminate blank employed consists of oriented layers of five harness satin woven carbon fibre in a PPS matrix fully consolidated to a 50% fibre volume fraction (FVF).
Thermoforming starts with loading the blank into the transportation system, which moves it into the heating system – an oven containing independently controlled banks of infrared elements – where it is held until the processing temperature is reached. The heated, pliable blank is then transported to the forming system, which consists of heated matched mould tools, one of which is attached to the actuator of a Promess electro-press. The blank is held between the moulds, triggering the actuator to close the moulds while controlling pressing speed to a specific thickness. Cooling is controlled by the moulds, affecting the level of crystallinity of the formed part. Finally, the consolidated part is ejected.
All together now
Complex parts can be produced with the rapid thermoforming process, but ultimately they need to be joined to construct the larger assembly. The team at QUB is assessing traditional adhesive bonding techniques with plasma surface treatments and thermoset adhesives. The performance of these joints is generally low because of the poor ability of the adhesive to wet the surface, as a result of the low surface energy and the presence of contaminants, such as mould release agents. Surface treatment therefore becomes necessary prior to joining to produce joints with satisfactory mechanical performance.
However, a major advantage of using a thermoplastic matrix-type composite is the ability to fusion-bond preformed parts together. Researchers at QUB are investigating joining parts via co-consolidation and ultrasonic welding.
Co-consolidation involves heating the preformed parts in a matched mould system to melting temperature and, then controlled cooling takes place, fusing them together. The key advantage is that there is no added weight or foreign matter in the joint and it requires minimal or no surface preparation.
Preliminary testing of co-consolidation indicates a good level of lap shear (LS) performance compared with adhesively bonded joints:
But, complex matched tooling is required to maintain the part’s thickness/fibre volume fraction when it is heated to the processing temperature, because of the tendency of laminates to deconsolidate. From a processing perspective, other techniques are, therefore, potentially more attractive as they do not require a heating/cooling cycle on the entirety of the joining parts, nor matched mould tools to maintain consolidation.
With the ultrasonic welding method, only the local joining area is subject to frictional heating, producing welds in less than three seconds without any prior surface treatment. At QUB, preliminary trials have investigated a welding time of two seconds followed by holding the joint under pressure for a further three seconds.
The chart above indicates that the LS performance of ultrasonic welding is superior to that of adhesively bonded joints, without the need for special surface treatment, and it is on a par with co-consolidated joints. Research on ultrasonic welding at QUB involves the design of energy directors. These are an essential component in the process as they provide the focus and initiate the induced vibrations to melt the desired area. The challenges in this respect relate to improved weld performance and energy director attachment.
The next level
Research will continue to explore rapid thermoforming of advanced thermoplastic parts and fusion joining techniques with a view to optimising the processes. Investigations will focus on the effects of thermoforming parameters on the formed part, for example the ‘spring-in’ of parts on cooling. Joint strength development as a consequence of the process parameters for each technique will also be explored. This will include experimental trials and process and mechanical performance models to simulate thermoforming and fusion joining.
Initial modelling simulations on the LS performance of adhesively bonded, co-consolidated and ultrasonically welded joints reveal reasonable agreement with the preliminary test results. These models will be used to enhance co-consolidation and ultrasonic welding.
With optimised joint performance at the coupon level evaluated, blade-stiffened panels will be assembled using fusion joining. Subsequently, structural detail tests will be conducted at QUB using seven-point bend (see main article image, top) and panel compression tests. Dr Ryan Wilson, research fellow at the School of Mechanical and Aerospace Engineering, comments, ‘The seven-point bending test allows replication of the joint failure modes observed in sub-component testing, but on a smaller scale. Using structural detail specimens in the verification of joints reduces the manufacturing time and cost and simplifies the test. This improves the ability to develop joining techniques over a shorter period.’
The lifecycle of the thermoforming and downstream joining processes is being studied and integrated into the digital manufacturing environment. The aim is to optimise the manufacturing process for minimum power consumption.
An experimental programme of power consumption monitoring, synchronised to the process parameters, has allowed the development of an inferential model, so that the entire manufacturing sequence can be optimised for selected pre-requisites.
Saul Buchanan, School of Mechanical and Aerospace Engineering, Queen’s University, Ashby Building, Stranmillis Road, Belfast, BT9 5AG, NI. Tel: +44 (0)28 9097 4147. Email: firstname.lastname@example.org Website: www.qub.ac.uk