Whilst metal alloy tooling materials like Invar offer a closer coefficient of thermal expansion match very near to that of carbon fibre composites, their desirable properties come at a price. As the perennial choice for parts needing to be manufactured to extremely tight tolerances, Invar is the most expensive metallic tooling material and, particularly for large parts, the sheer size and weight of the tools makes them difficult to handle.
Aerospace manufacturers have expressed a need for carbon fibre tooling materials that can withstand thousands of autoclave cure cycles, like Invar does, so to increase the durability of composite tooling, several suppliers offer hybrid tool designs that combine a thin Invar facesheet with a composite backup structure.
Ascent Aerospace, a provider of layup tooling for composite aerostructures, was exhibiting at the recent JEC World exhibition to champion its new HyVarC, patent-pending hybrid Invar and composite mould. HyVarC layup tooling is said to offer a cost-effective, lightweight, short lead-time solution for prototype and development applications.
HyVarC combines a thin Invar backup structure and facesheet with a bonded, high-temperature composite working surface. The resulting tool is 50% lighter and with a 20% shorter lead-time than a traditional Invar layup mould, while maintaining the same superior vacuum integrity and dimensional precision. The hybrid mould capitalises on the desirable performance characteristics of Invar and composites to offer an ideal layup mould for prototype aerospace parts.
The weight is over
At half the thickness of a traditional Invar mould, the thin Invar backup structure takes less time to weld and manufacture. It serves as both the master mould and the deliverable mould, eliminating the time and cost of creating a second composite backup structure. Lead-times are reduced by at least 20%, compared to an all-Invar or all-composite tool.
Thin Invar, and the resulting lighter weight, is easier to handle when transporting the tool within the facility. It has thermal mass benefits, for faster autoclave cycles each time the customer is curing a part. And as Invar is a commodity by weight, raw material savings can be passed along to the customer.
The machined composite working surface offers better dimensional accuracy than net-mould composite tooling, while the Invar structure provides vacuum integrity and durability. A part fabricated on a HyVarC mould is bagged to the Invar facesheet, which provides vacuum reliability that is independent of the composite surface and does not degrade with age or thermal cycling.
The composite working surface is easily re-machined to support modifications for rapid prototyping and R&D applications. A damaged surface can be repaired with material add (layup) and subtract (re-machine) operations. The base mould can be reused by burning off the composite working surface and laying up an entirely new surface.
“HyVarC is a reconfigurable, patent-pending ‘masterless’ tool system that is positioned between a standard composite tool, which has a limited life of 100-200 cycles and a full-on Invar tool,” begins Ascent Aerospace’s director of technical engineering, Colin Birtles. “It’s roughly 20% cheaper to manufacture because we’ve removed a lot of the cost of the Invar in the first place.
“We were looking to manufacture some high temperature tooling masters, so we took our Group’s metallics knowledge and used a 5mm gauge of Invar instead of 10mm plate for the backup structure. Instead of using 20mm plate for the facesheet, we’re using the same 5mm plate, thereby cutting the weight of the tool in half. And it reduces the cost of the raw material too. Fabrication of the tooling becomes easier because we’re now working with something much lighter. We’re using an approximate Invar shape with a pad of carbon fibre on top, so that the carbon fibre becomes the shape and the Invar secures the vacuum.”
How to meet the heat
Birtles says that one of the problems with using carbon fibre tooling is that it eventually breaks down over time. Durability depends many factors, including the number of cycles, temperature, material, shape, handling, etc. whereas Invar doesn’t deteriorate and exceeds aircraft part lifecycles. The 5mm thick HyVarC lightweight structure is covered with a modified carbon fibre epoxy system, constructed from Ascent Aerospace’s own secret recipe.
“We have a proprietary surface preparation on the Invar, which helps the epoxy ‘key’ to the Invar base. HyVarC is suitable for low runs of up to 20 parts and is ideal for development work, i.e. for engineers that want to check potential ‘spring back’ problems. Our solution has the flexibility where it can be modified in response to design changes, still maintain the basic structure of the tool and the vacuum tightness portion of it, and just modify or replace carbon fibre pad on it. It lends itself to being modified very easily, and if you don’t like the shape you’ve got, all you do is heat the tooling to an elevated temperature and the pad will burn off whilst maintaining the original structure.”
Birtles adds that the company is continuing to develop HyVarC by going down to thinner gauges at 3mm. However, this will mean the introduction of some preformed tooling shapes to create some form of inherent stiffness in the tool itself.
“Part of our ongoing development work with HyVarC will be to introduce features that you would expect to see on a normal layup tool, such as edge bars, drilling features and integral vacuum systems for example,” Birtles concludes. “Ultimately, we will be moving towards a production tool system.”
Sample HyVarC tools were on display at Ascent’s stand at JEC World, at AeroDef Manufacturing, held in Long Beach, California from March 27-28 and also during SAMPE 2018 also held in Long Beach, California, May 21-24.