Aerospace seals embrace direct ink writing technology

Materials World magazine
,
7 Aug 2019

A new layering method is helping create terrific seals.

Gaskets have been used to fill and seal gaps between mating solid materials for many years. Allowances for engineering tolerances, differential thermal properties, and load transferring are some of the desirable attributes, either singularly or in combination, that a gasket needs to exhibit.

Gaskets and seals are found in almost every assembled device that has dissimilar materials mating with one another either functionally or by form. These can vary from high-temperature metal seals, low-temperature polymer seals, and seals that need to cover a wide temperature range. They can be solid materials, tubes, formed or foamed.

Although gaskets are designed to allow for forgiveness, they must also be made and perform within their own tolerances, in order to fulfil their purpose. The space shuttle Challenger disaster of 28 January 1986, was traced to a failed seal at the aft field joint of the right solid rocket booster. Due to the cold temperatures at the time of launch, the O-rings were too slow to return to their correct shape. It is believed the O-ring in question could not be pressure actuated enough to achieve complete sealing of the gap it was supposed to protect. Rogers Commission Report member Richard Feynman summarised this in incredibly simple terms – because of the cold, the O-ring was too stiff to do its job. It is important to be very careful in challenging situations to understand the static and dynamic demands sealing gaskets and manage material inventory to fully meet the design requirements needed.

Rubbers and foams

In aerospace applications, gasket materials are often used to fill small gaps in assemblies due to engineering tolerances and to allow thermal/shock movement, often absorbing and transmitting loads. These gaskets may be larger flat or curved pads, as well as the usual round ring seals. Materials must be carefully selected for compatibility with other nearby materials, atmospheres and temperatures, engineering requirements based on expected and likely abnormal loading modes, and ageing performance over extended times.

Currently, some of the most common materials used are polyurethane, rubber, acrylonitrile butadiene (NBR) rubber, ethylene propylene diene monomer (EPDM) rubber, polytetrafluoroethylene (PTFE), Viton, fluorinated ethylene propylene (FEP) and silicone. The type of material is determined by the operating conditions or limits due to pressure, chemical compatibility or temperature. The material may also be filled with particles, fibres to modify its properties pre or post-set, or foamed to reduce density and increase compliance. Aerospace seals may often be under a pre-load, so the response can be better understood. However, maintaining a seal underload for extended periods of time requires an understanding of creep and stress relaxation.

Modern pad design

Defence supplies company AWE makes tailored, material-specific, gaskets in several shapes and forms that act as both engineering gap fillers and load transfer materials between, at times, dissimilar materials experiencing rapid temperature fluctuations.

Current gasket components meet the requirements but come at great cost, especially as size and shape complexity increases. To make a large 3D-shaped pad, there is a significant investment in quality tooling required, which is expensive to make, certify, and change when needed. There is also the difficulty in producing reproducibly accurate components. Experience has shown that with tightly specified aerospace foamed gaskets even a change in operator can severely affect the attrition rate in manufacturing. Understanding the effect of variability in density and pore distribution on the gasket performance is made more difficult by the lack of consistent production.

As a result of such challenges, industry is constantly striving to improve production methodologies. For example, direct ink writing (DIW) offers a way of addressing the requirements of gap filling and load transferring gaskets for aerospace. AWE is working across functions to fully understand the materials, the manufacturing, and the behaviour under service conditions of flat and curved gaskets made from DIW polydimethylsiloxane (PDMS), commonly referred to as silicone. PDMS is inert, non-toxic, non-flammable and optically clear. Used in many applications, it also has a wide temperature use range.

The team is taking silicone two-part formulations and modifying the processing to use in the DIW process, initially for flat multilayer sheets. The objectives are:

  • A controlled structure, examining design variation versus mechanical property measurements
  • Tailored requirements to tight aerospace specifications
  • Low part attrition – a major issue with traditional foamed materials
  • A less labour-intensive production method and parts that are less sensitive to labour variations
  • A more automated and streamlined manufacturing process to maximise part consistency, and
  • Printing final parts with little or no post-processing required afterwards.

From a material modelling point of view, the team is examining the reaction chemistry of the silicone-based polymeric components of DIW material using ab initio density functional theory (DFT) and transition state theory (TST) to calculate reaction energetics followed by an Arrhenius kinetics assessment. Atomistic modelling techniques are used to link materials properties to structural properties to predict bulk polymer ageing as well as to calculate transport properties of reactive species. This includes continuum-scale modelling with the above to enable a material/component level assessment of the effects of ageing. It is important to understand the role of impurities and minor chemical constituents of the DIW material, which the team is investigating the practicality and potential benefits of, including course-graining techniques in the model.

The DIW process involves extrusion of a thixotropic ink – a two-part silicone – through a narrow orifice to deposit continuous beads of self-supporting material in a layer-by-layer approach. Tailorable mechanical properties and thickness can be achieved by varying the internal printed architecture to reach the desired properties. AWE uses an Aerotech motion stage with a Nordson air pressure dispensing system to deposit 250µm beads of Dow Corning SE1700 silicone. The architecture of the beads deposited is controlled by the Aerotech A3200 software, which defines a series of x, y, z coordinates to move the stages while integrating with the Nordson dispenser controller, defining the pressure at which the material is deposited. Once the build has finished printing, the build plate is transferred to an oven to cure. Parts are then removed from the plate, measured and tested.

Currently, several gasket sheets have been manufactured in several different architects. These samples have been mechanically tested and modelled at different length scales including macro finite volume. Initial results show significant improvement in repeatability and control in manufacturing but also show a surprising sensitivity to variations in architecture. DIW requires a tight control of both the starting material and the manufacturing environment as both can introduce production errors rendering parts unusable. 

Taking an integrated computational materials engineering (ICME) approach in designing a product, the materials they are made of, and their processing methods by linking materials models at multiple length scales. The focus is on the materials, i.e. understanding how processes produce material structures, how those structures give rise to material properties, and ultimately deliver the required performance characteristics which therefore enables the select of materials suitable for a given application.

ICME arose out of the Defense Advanced Research Projects Agency, USA – sponsored Accelerated Insertion of Materials programme and aims to integrate computational tools to accelerate the development process, hence reducing the overall project time and costs. The first stage in applying ICME to a process-driven materials development task is to define a set of performance characteristics. Once these requirements have been established, a system chart is developed for the proposed material/processing route.

In this example the focus is on the DIW processing of PDMS and how this affects the product’s performance. An example system chart developed in collaboration with the Center for Hierarchal Materials Design (CHiMaD), USA, was made.

‘Designing novel materials of specific properties for a particular application requires simultaneously utilising physical theory, advanced computational methods and models, materials properties databases and complex calculations. This approach stands in contrast to the traditional trial-and-error method of materials discovery. CHiMaD aims to focus this approach on the creation of novel hierarchical materials, which exploit distinct structural details at various scales, from the atomic on up, to obtain enhanced properties,’ the CHiMaD website reads.

The system chart shows material flow from left to right and the associated relationships between processing, structure and properties and hence performance (PSPP). These relationships need to be described through materials models, or in cases where such models do not exist, explored via experimental techniques. Once this framework has been established, it is possible to quickly check how the material’s performance characteristics are affected by changes in items including processing, and then enable the production process to be optimised.

AWE has looked at the initial stages of machine qualification and focused on the deposited materials and specifically the influence of additive manufacturing, in particular DIW, nozzle diameter, pressure, speed, as well as how this influences load retention via the cell architecture. The team hypothesised that more layers and face centred tetragonal (fct) versus simple cubic (sc) structure would have an impact on the load retention properties at a simple level and initial data from printing trials supports this.

The printed pads load retention results showed a positive correlation with these properties and hence have confirmed the draft PSPP and the team’s understanding of the DIW process. Now that the sensitivity test and machine qualification have been completed, AWE will move onto more challenging aspects of the PSPP, such as molecular weight and degree of crosslinking.

A way forward

Gaskets have a set of clear performance requirements depending upon application. PDMS has a long history of use, hence has considerable experimental characterisation to baseline any predictive capability. ICME enables a rigorous exploration of a material’s performance and the best practices for manufacture, and in turn, helps AWE maximise its potential to deliver new components with previously difficult to achieve performance characteristics.