Large-Scale Additive for Composite Tooling Part 1: Understanding the Advantage
An Introduction to Composite Tooling
Over the last 50 years, carbon and glass fibre composites have allowed us to make products that are lighter, faster and stronger than ever before, revolutionizing the way we make aircraft, race cars, wind turbines, and a myriad of other high-end technologies.
These performance advantages are all thanks to the exceptional mechanical properties that composite materials provide.
Why is it, then, that these vastly superior materials have so far been confined to extremely high performance products — would your everyday Volkswagen Beetle not benefit from the exceptional fuel savings that lightweight composites could provide?
The sad reality is that, today, composite materials are currently prohibitively expensive to use in the majority of products.
The cost of commercial grade carbon fibre is between $25 and $30 per kg, compared to the much lower cost of steel and aluminium alloys which are priced between $1 and $4 per kg. A 10x increase in part costs is a hard proposition for anyone to argue, even with the massive mechanical advantages that carbon fibre brings.
Does this mean that composite materials will be forever limited only to very high-performance-requirement products? What are the costs of manufacturing and how can we reduce them?
According to a recent white paper by Technology Consultancy Infosys, for a medium-size run, the variable costs of manufacturing carbon fibre parts can be broken down as follows:
Labour, material and overheads are what we expect to see on a graph like this and dozens of innovative companies are working on decreasing these costs — but what is tooling and why does it make up a staggering 25% of the cost of a composite part?
Composite tooling refers to the moulds, patterns and jigs that enable shape to be given to a carbon fibre part. Carbon fibre weaves are laid on to a mould before the part is infused with a resin (e.g. epoxy), and cured to solidify the form.
Today, there are several ways that composite tools are manufactured. ‘Hard’ tools are those that are manufactured from specialist metal alloys, typically by taking a large block of the metal alloy and machining material away to achieve the final mould geometry. Hard tools bring high durability, allowing many thousands of parts to be manufactured before a change of tools is needed.
They are, however, relatively heavy and expensive and are mismatched in coefficient of thermal expansion (CTE) with the composite material they are forming. Steel and aluminum alloys are common choices for metal tools because they are less expensive and usually involve shorter lead times than high-performance alloys.
During heated cure, the CTE mismatch between the less-expensive metal tool materials and the composite often is too extreme for use in molding close-tolerance composite parts. Only the higher-priced metal alloys offer closer CTE matches. For example, Invar — alone among tooling metals — offers a CTE very near to that of carbon fiber composites. For that reason, it has been the perennial choice for parts that must be manufactured to extremely tight tolerance. But Invar also is the most costly tooling material and, especially when it is used to large parts, the sheer size and weight of the tools makes them difficult to handle.
‘Soft’ tools are manufactured from polymer and composite materials. Pure polymer moulds are often machined from large blocks of epoxy or high-density polyurethane. The machining process is much easier here than for hard tools, but it is still very costly and generates a large amount of material waste. Pure polymer moulds also have large CTE mismatch and are not typically suitable for heated cures. A common practice is to use a pure polymer tool as a ‘pattern’ to then manufacture a room-temperature-cure carbon or glass-fibre production mould from. These composite tools typically have a CTE very close to the final composite part but often end up being more expensive to manufacture than hard tools.
Weighing up the pros and cons of the various types of composites tools is a science in itself, with different types being utilised to serve different durability, tolerance, CTE and curing temperature requirements. However there is one common theme amongst all existing tooling types — they are expensive and complex to manufacture.
Additive Tooling and the Square-cube Law
Additive manufacturing is already being used at desktop scale to attempt to solve the composite tooling problem, spearheaded by the likes of stratasys. These small-scale tools are printed with high-temperature, fibre-filled thermoplastics, allowing them to meet the high heat and CTE requirements of autoclave tooling.
Desktop additive manufacture has begun to solidify its niche in the world of composite tooling, but has not been entirely disruptive. Why, with all the ease of manufacture and material efficiency of 3D printing, has desktop 3D printing not taken over the world of small-scale composite tooling?
Look no further than high-school physics’ Square-cube law. First formulated by Galileo, the Square-cube law states that as a shape grows in size, its volume grows faster than its surface area.
In composite tooling terms, this means that if you double the size of a wind turbine blade, the volume of material that must be machined away to form the tool increases eight fold. Additive manufacturing uses only the amount of material required for the final shape, so the material savings become vastly more important as part sizes scale.
This fundamental law of physics is why Ai Build is so excited about the possibilites of large-scale additive tooling. The manufacturing cost and ease advantages that make additive tooling sometimes the best option at small scale, will dominate the market at large scale.
Realising the Potential
Ai Build has been working hard over the last five years to perfect the science and technology of large scale additive manufacturing. The challenges faced have been primarily in toolpath generation (‘slicing’) and process control. The response to these challenges have culminated in the development of AiSync, the worlds first user-friendly toolpathing and process control platform for large scale additive manufacture. AiSync makes it easy to go from CAD to part and has been extensively developed for six-axis deposition, allowing the elimination of support structures for a majority of part geometries.
Ongoing integrations of toolpathing support for surface finishing of additively manufactured parts using subtractive milling has enabled Ai Build’s customers to manufacture composite tools that meet the tight dimensional and surface finish specifications of composite tooling.
So we can make large-scale additive tools. That’s great — but how do they stack up against their traditional counterparts in terms of performance and cost?
Part 2 of this blog series — Your Additive Materials Field Guide — will take you on a deep dive in to the material properties that are important for a composite tool and explore if additive materials are good-enough, as well as a direct cost comparison against existing soft and hard tooling options.
We hope this blog has been a helpful introduction in to the world of large scale additive manufacture for composite tooling. If you are interested in seeing how this could be integrated in to your supply chain, please contact info@ai-build.com.
