Inserts and core making (wing Test)

Inside the wing there are several inserts, cores, foam cores and a mandrel, which will be used to make the component. Only the inserts and core will remain in the finished part. After the cure, the silicone mandrel is extracted and the foam cores destroyed.

Since there are different materials, each one has a different manufacturing process. Here is the way to manufacture them:
Foam cores: Foam is usually machined because of the shapes it is designed for. Machining foam is not a big deal, but the fixing of it might be. Because the foam cores for the wing are machined all around, I designed a vacuum tool for one of them. The other two are machined into a big foam core and them cut with a staler knife.

Core trimming: The cores are made out of balsa wood for the test. I am planning to use structural foam such as Rohacell and honeycomb for the actual car. The thickness of the core is 1.5 mm so I used a skimmed 2mm balsa sheet. I trimmed it using paper templates and chamfer it with a knife and chisells.

I used an interesting techique  to curve the balsa wood which consists in wetting the side of the wood which will become the convex surface. I used a bottle with a nozel to get a uniform water layer. After 10 minutes, the balsa wood has absorbed the water and bends by itself. At this point I placed the cores on the mould in the right position and bagged them in vacuum. Then baked them for  90 minutes at 60ºC and cool down slowly under vacuum, with a release below 30ºC. With this process the curvature on the balsa becomes permanent. The alignment of the balsa natural fibre has to be in the direction of the bending axis. I did not tried to do it in the other direction so it might work as well but I guess it will be harder.

CFRP inserts:  To connect the wing with bolts and other fixing elements, CFRP are embedded inside the structure. CF is used because it is light and has good strengh to hold a metallic insert such as a dowel or keensert. The drilling direction for the metallic element, will be allways perpendicular to the CF layup plane. The layup is usually made with a high gsm fibre and with a QI (Quasi Isotropic) layer orientation. Since the longest insert of the wing is 16 mm, I produced a brick of CFRP of 19 mm (80 layers of 200gsm). I did not have higher gsm fibre so there you go...

The insert drilling is made after curing the component in the curing tool. The curing tool is used as well for the drilling.

Silicone mandrel: The silicone mandrel is used to apply internal pressure to the spar area during the cure cycle. The difference in CTE between the CFRP mould and the silicone means that the silicone will expand and squeze the laminate against the rigid mould.

The silicone I used is a ESSIL 20 from Axson. It is a bicomponent produce that can be poured into a cavity. It cures at RT in 7 days, but can be released after 24h.

The inserts in the tool cavity look like this: