Dynamic tooling, dynamic blades!

Norco GRP and its 5-axis CNC machining sister company, MouldCam were approached jointly by leading wind energy technology company, Blade Dynamics to produce a flagship tool for its next generation modular blade development.
The carbon fibre used in the blades is necessary for achieving the desired stiffness, and being able to join the two halves of the blade using the curing tool is essential for the rapid turnaround of blades. Hence there was the requirement for a carbon tool.

One of the biggest failings and indeed false economies of any composite development programme is not to provide tooling that is matched to the part and expected production rate. Keeping the correct shape and stability during cure is widely understood, but just as important is matching the size of the part and tool after cure, where in-mould assembly operations take place. A part that doesn’t fit in the mould after it has come out of the oven poses a significant problem if any in-mould assembly operations are required.

This is not a problem for applications such as wet layup and infusion which utilise ambient curing resins, however when venturing into prepreg processing, an understanding of tooling that matches the thermal behaviour of the part becomes vitally important.

The main driver which steers moulders towards using prepreg is the improved structural benefits and weight reduction which justifies the extra material cost. Prepreg therefore also lends itself to the use of carbon fibre where the fixed cost of producing prepreg is a smaller percentage on the cost of carbon fabric than glass.

Carbon fibre is a unique material with a near zero coefficient of thermal expansion (CTE) i.e. it remains pretty static through both heating and cooling. Whilst in prepreg format and under the pressures of a vacuum bag the carbon in its uncured state will conform to the geometry of the mould or pattern which is expanding with heat during a prepreg cycle, it will continue to conform to this shape until the resin begins to gel, this gelation period occurs approximately 20-25% into the total cure time. If the CTE of the pattern or mould material is greater than that of carbon you can expect the part to come out larger than the design.

After this gelation the carbon part is going to remain that size. If moulding in a female pattern or mould constructed of a material with a greater CTE than that of carbon, there is going to be a battle between part and mould once the oven starts to cool back down to room temperature. The forces involved are huge, with the implications being either an inability to remove the part from the mould, the mould forcing the part out or some sort of stress damage to the part or mould.

The aerospace industry has widely adopted the use of metallic tooling using an alloy called Invar 36, a 36% nickel/iron alloy, which uniquely for a metal has a CTE which is almost identical to Carbon. The inventor received the Noble prize in physics for such a find. Needless to say this also comes with a significant cost premium, whilst other disadvantages are high thermal mass and overall weight requiring heavy lifting equipment to move the tooling around factories.

Tooling dynamics

The required tool for Blade Dynamics was an 18m matched carbon tool for 120°C processing. The epoxy tooling paste is applied in a thin layer onto a glass skin which encapsulates a polystyrene base. It is this volume of low cost polystyrene which makes large patterns very cost effective and more easily recyclable. The finished pattern utilising high temperature epoxy paste is supplied at a cost of approximately £1,300 per square metre. For the finished pattern, this is about the same as sourcing the equivalent volume of tooling block on a pallet before blocking up and machining. Significantly compared to a solid mass of tooling block, this also reduces the CTE by about one quarter due to the fact that the thin layer of paste which has the highest CTE of the materials constituting the final pattern is restrained by the glass and steel which overrule the expansion of the paste.

The cure and post cure were carried out in Norco’s 6m oven, with an extension of 12m and the assistance of an additional three-phase heat source. Before receiving the patterns the oven was assembled and the air flow levels inside were checked. Air flow is key in any high temperature environment to avoid hot and cold spots. For the post cure we conducted a quick thermal study on the available power input, thermal mass of the tool and the U-Values of the PU insulation to check we could reach the desired 140°C target post cure temperature. The 140°C post cure would provide a tg of around 145°C enabling the customer to confidently cycle the tool at 120°C without any stability issues.

The post cure went extremely well, there was a small hiccup six hours in when one of the ducting panels gave way due to heat so there was a glitch in the graph while a repair took place. Tooling suppliers always suggest a very slow ramp rate when post curing tools so as not to overshoot the tg of the resin system. Embedded thermocouples in the laminate show exactly why this is necessary with long thermal lag between the air temperature and the part temperature.

Detailed thermal analysis

The use of carbon fibre as a tooling material presented other challenges when designing and integrating the hinging steel backing support structure. A detailed thermal and structural analysis had to be carried out to ensure that the support frame was able to support the tool throughout the rotational cycle of opening and closing yet not distort the mould during the high temperature bake cycles. The entire frame rests on silicone pads to eliminate the distortion. In addition, FEA analysis on the frame checked that the sag at the tips was to an acceptable level. The hinging system had to allow the tools to separate and fold away in production then come together and close within 10 minutes from a single point lift. This was achieved in under six minutes. Male and female locators on the tool surface ensures the tool comes together in the exact location for repeatable joining processes.

A multi-zone integrated heating system with an external control unit was another challenge, so Norco teamed up with Lamina heat, which provided its power film technology, a conductive homogenous film which is encapsulated in the tool skin isolated from the conductive carbon tool providing an almost instant controllable heat source exactly where it is needed. Norco has since used this heating film technology on a number of other applications warranting high cycle rates of predominantly prepreg parts.

www.norco.co.uk