Consolidating composites - polymer materials for aerospace and electronics
Processing improvements have hastened the development of polymer materials for the aerospace and electronics sectors. Dr Alan Wood from Victrex outlines the route to faster production of polymer matrix composites.
Aluminium and titanium alloys have been the aerospace industry’s materials of choice for years. However, in recent times polymer-matrix composites have become more widely accepted due to better fatigue resistance, better thermal and acoustic insulation and increased noise and vibration damping.
The composite business is historically based on thermosetting polymer matrices, but the developing trend is towards thermoplastic polymer matrices, as these offer many advantages over thermosetting materials, including infinite shelf life and fast out-of-autoclave processing.
Move to thermoplastics
To date, most polymer-matrix composites in aerospace have been based on thermosetting polymers, but the current trend is towards the introduction of thermoplastic-matrix composites. This change is driven by a number of factors including better impact resistance, fatigue and, in the case of semi-crystalline thermoplastics, mechanical performance. There is better chemical resistance in these thermoplastics, as well as the potential for shorter processing times because no chemical reaction occurs, in contrast to thermosets, where the material crosslinks during manufacture and thus the properties tend not to be a function of processing time and temperature. Pre-pregs based on thermoplastic matrices effectively have an infinite shelf life and do not require storage in freezers and thermoplastic matrices also offer a wide range of assembly and connection techniques.
In practice, shorter processing times are not often achieved due to the use of thermoset polymer processing techniques being applied inappropriately. In the case of thermoplastics, significant increases in output, with the consequential savings in processing costs, can only be achieved by using the most apposite manufacturing technique.
The manufacturing process comprised the consolidation of 10 plies of carbon fibre Victrex PEEK unidirectional (UD) tape. PEEK is a thermoplastic semi-crystalline polyaryletherketone polymer. The lay-up was spot welded into a pre-form using an ultrasonic hand-held pistol welder, four spot welds being used to keep the individual plies in position. The bagging films used were Aptiv PEEK film with a thickness of 50μm.
The production to functional specifications (PtFS) RS system uses layered tool architecture to minimise thermal inertia. This construction methodology is optimised for transient rather than steady state conditions, and supports high tool ramp rates with simultaneous close tolerance control. The tooling used in this work was a flat 200mm square stainless plate with 16 active heating and cooling zones, controlled using a Surface Generation M18 PtFS master control system. Ceramic insulation inlays were positioned around the perimeter of the heated/cooled tooling surface to reduce the temperature on the exterior surfaces of the unit. This allows low-temperature adhesive systems to be used to seal the film against the tool surface.
A vacuum was applied throughout the process cycle, drawn through small holes in the hot-plate area and through the channels containing the ceramic inserts. VACsealG(20) was used to seal the edges of the Aptiv fi lm.
In PEEK condition
The thermal cycle can be followed by the visual changes to the panel during manufacture. The initial bagging film is crystalline and has the typical light-brown appearance of PEEK. As the film melts during stage three, it becomes transparent due to the destruction of the crystal structures within the material and the panel becomes black in appearance. As the panel cools, the surface, which is rich in PEEK, solidifies and crystallises and so the colour returns to that of the original film. Micrographs of the crosssections of the consolidated panels, produced using a scanning electron microscope show the panels are well consolidated with a very low level of porosity. The crystallisation processes occurring during manufacture are very important. The properties, in particular chemical resistance and fatigue, of a semi-crystalline polymer depend heavily on an adequate level of crystallinity being developed. PEEK crystallises very quickly and so the rate at which it is cooled through the crystallisation temperature zone depends more on the thermal characteristics of the tooling and the required uniformity of cooling than on the polymer itself. If the cooling is not uniform, then different levels of crystallinity are produced in the panel. This leads to differing levels of shrinkage associated with the crystallisation processes and can result in residual stress, which may cause the panel to distort. Thus control of the temperature changes, and hence the rates of crystallisation, is important.
In this work, the rate of cooling from 300ºC to 250ºC, is 85ºC per minute. Differential scanning calorimetry was used to determine the level of crystallinity in the panels produced. The degree of crystallinity at various points across the surface was around 31.2%, showing that the cooling rate was well balanced across the complete area of the panel. A crystallinity of greater than 25% is usually required to produce good chemical resistance and fatigue properties – typical levels in injection-moulded components are 25–35%.
The work has shown that it is possible to produce fully consolidated panels made from a CETEX AS4/ PEEK pre-preg in less than 20 minutes using an out-of-autoclave method. Work is focused on reducing the cycle time even further by reducing dwell times and increasing ramp rates. Previous work has indicated that cycle times of eight minutes can be achieved and the objective of the ongoing work is to achieve controlled cycle times of less than 10 minutes.
The heating/cooling cycle used consists of seven stages:
1. Initial rapid pre-heat to a temperature below that at which PEEK melts (343°C).
2. Short dwell time. It is advisable for the film to melt uniformly as the material consolidates to minimise any degradation effects due to the molten fi lm being exposed to atmosphere for long periods.
3. Control temperature ramp up to around 400°C to melt the polymer and bring about consolidation of the panel.
4. Short dwell time at the consolidation temperature. This is to ensure all the material has thoroughly melted.
5. Rapid cooling to 300°C. The temperature of solidifi cation for a semicrystalline material is related to the formation of crystal structures within the polymer matrix. In the case of PEEK, the crystallisation process occurs around 285°C.
6. Ramped cooling from 300°C to 250°C, this being the temperature range where crystallisation occurs. The cooling is controlled in order to control the rate and level of crystallisation.
7. Rapid cool to room temperature. Once the transition from melt to solid has occurred, the crystalline structure of the matrix is defined and so cooling can be carried out very quickly.
It should be noted that a similarly structured cycle would be used with an amorphous thermoplastic such as polyetherimide. In the case of an amorphous thermoplastic, no crystallisation processes occur, the ramped cool is used to take the polymer through its glass transition temperature.
Dr Alan Wood: firstname.lastname@example.org