Innovate UK NATEP Collaborative R&D Project

6 June 2024
Author: Guy Brown

ZETTAS - Zero Expansion Thermoplastic Tools at Scale

Large Format Additive Manufacturing is becoming widely used for composite tooling applications across many industry verticals. It offers huge benefits in terms of both cost and lead time, thanks to the inherent benefits of additive manufacturing (automated digital manufacturing process and simplified supply chain). 


Aerospace, however, has been one vertical where the use of LFAM for composite tooling has been extremely slow in uptake. The primary reason for this stems from the fact that composite structures in aerospace are typically cured in a heated oven or autoclave up to 210℃, which is needed to impart superior mechanical properties to the end parts. Any mould tool that is used in one of these ovens must remain as dimensionally stable as possible across the temperature range, and any deviation that will occur needs to be modelled in with an expansion factor to ensure the end part is dimensionally correct when the cure temperature is reached. This requirement has proved to be a blocker for LFAM in this segment primarily because the coefficient of thermal expansion (CTE) of printed thermoplastic materials varies wildly depending on the orientation of deposition, making modelling in an expansion factor to a tool near impossible. For fibre-filled printed parts, CTE in the X-X direction can be as low as 3 µm/(m⋅K), while CTE in the X-Y and X-Z directions can be as high as 70 µm/(m⋅K), and modelling the 3-dimensional interactions of these CTE values is a highly complex problem that is yet to be solved.


Our NATEP project ZETTAS looked to solve this problem by simplifying it. Instead of attempting to model the complex 3-dimensional interactions, we sought to ensure the CTE is equal (isotropic) in the relevant directions on the critical moulding surface of an aerospace composite mould tool, meaning a simple, known, expansion factor can be applied to the printed mould tool, as with conventional mould tools.

Conventional (Invar) mould tool

Our approach to achieve this comprised of including a feature in Aibuild software to enable a double-edged printing strategy:

  • Printing the critical mould surface non-planar, meaning the layer orientation is always kept parallel to the moulding surface.
  • Crosshatched infill and skin to build up the mould surface, meaning the orientation of deposition is rotated by a defined amount layer-on-layer. 


These critical software features enable moulds to be printed that have a single, isotropic, CTE value along the moulding surface. It also brings down the CTE value much closer to an X-X CTE value (4 µm/(m⋅K) on the material we tested with).


This approach does not control CTE in the X-Z, but the impact of expansion in this direction on the dimensions of the moulding surface is limited due to the layers being kept conformal to the surface across all regions, allowing a single expansion factor to be applied to the mould tool. This approach means that LFAM printed mould tools can be treated the same way as traditional mould tools, allowing their use for high-spec autoclave parts, but with all the cost and lead time advantages of additive manufacturing.


This project was enabled by the National Aerospace Technology Exploitation Programme (NATEP) through Innovate UK. As well as the critical R&D funding provided, the programme helped to bring on our technical partners – the National Composites Centre (NCC), as well as supporting end users Boeing, Hamble Aerostructures and Pentaxia.


The NCC were critical to the success of the project, providing essential composite moulding know-how, as well as producing real composite parts from the demonstration printed tool and carrying out all the requisite metrology to prove out the concept.

The National Composites Centre

The project was ultimately successful in proving the feasibility of the approach through small scale lab tests followed by a full scale demonstration mould tool for an aircraft wing structural component.

Demonstration mould tool during annealing at the NCC

One outstanding technical problem was uncovered at the end of the project – when the full scale demonstration mould tool was being annealed, due to the difference in CTE between the crosshatched layers, delamination occurred, rendering the tool unusable for production. Other than the difference in CTEs, the primary driver for this delamination was due to the poor inter-layer bonding between the layers, due to the size of the mould (the layer time was very long, meaning interpass temperatures were very low).


There are many possible solutions to solve this problem (e.g. localised layer re-heating), and now the approach has been proved feasible, and Aibuild software has the capability to enable it, the next step for this work is alongside our end user partners to iron out the wrinkles and bring the technology to market.