Problem

So, I need to decide which core to use in the sides of the tub, where I don’t really need huge stiffness like I do with the base, but I still need to do a trade-off between thickness, ease of working, stiffness and weight.

Weight wasn’t going to be my only trade-off here. I have some others to think about

• Ease of preparation started to become important when I started calculating the time required to prepare a standard foam such as the Airex. It needs to be scored every 20mm as a grid, and then at each intersection, needs to have a hole pit in (only 2mm or so) to allow the resin to flow through to the other side of the core. I did a square metre of it, and it took over two hours. Very tedious to do accurately.
• Ease of layup is also a significant factor. Some cores are very bendy, some are very rigid, and some will thermoform. I don’t want to choose a core of marginal better physical properties if it takes me days of frustration to get it into the stack. This isn’t just a simple flat sheet, but a large female mould with complex curves in multiple planes.
• Physical strength

Method

• Prepare three cores of the three materials I had readily to hand 3mm Soric (by Lantor), 5mm Airex T90 (from Trident Foams) and 3mm 3D Core (supplied by EC Fibreglass)
• Choose a very simple stack that demonstrates the properties of the core rather than the stack (so I chose 2 layers of 2o00gsm e-glass either side of the core)
• Infuse
• Trim all cores to the same size
• Weigh

• do a flex-test to see how rigid it is (clamp one end to a flat surface – hang a standard weight off the other end). You can see how the steel rule is horizontal, and the part has deflected.. 24g is the weight.

Results

• Weight – Soric (24g), 3D Core (23g), airex (29.5g). So, the airex is 2/5 thicker than the other two, and is 26% heavier.
• Deflection (therefore infer stiffness) – Soric 30.9mm, 3D Core 31.6mm and airex – 20.5mm

Ease of layup

1. Soric first – it’s more or less like a thick cloth, and will bend into most angles without being damaged
2. 3D Core second. It’s a bit like hexagons of foam which are stitched together. It will easily follow simple curves, and can be thermoformed into the tighter ones. It’s not great in tight curves and will separate at the meeting points between the hexagons
3. Airex last. It has some flex but does thermoform very well. However, getting the hot-air gun in there and pushing and shoving to thermoform it is going to be an arse.

Conclusions

• I was surprised a bit between the 3D Core and the Soric. Deflections were very similar for very similar weight, which is what one would hope to see. This means the cores were performing as a function of the distance they separate the cloth. I thought the Soric would hold a lot more resin than the 3D core mind, and the results were so close.
• The airex outperformed for stiffness, which is a symptom of the thickness it separated the cores. Interestingly, i could hear the fibers snap when i weighted this core, and it deformed at the end. Not sure why – further thought is needed.
• I will use Soric for the side pieces. The ease of putting it into the stack and having it follow tight curves makes it far better for me than the foam. The time and effort to score, hole, form and shape these pieces isn’t worth the extra 600g of weight the chassis will carry as a result.

This is for the side. It’s a complex shape so multiple pieces of card cut-and-shut. Once I’ve transferred this to a 5mm core I will cut the core and heat shape it to the side. 

I’m using solid core rather than flexible in pursuit of minimum weight. 

So, I think my tub is going to be an interesting thing to demould – basically more than 4m² of cloth and it’s nearly all sloped edges. I’ve heard of people using air to demould and I have an air wedge, but that only gets in from the top. I’ve even optimistically bought a couple. The other way to do this is to build a function into the mould or part to inject air.

So, the way is to somehow get a thread into the part for an air-line connector to make a seal good enough to take some pressure. One needs something that gets through to the mould/part interface, without anything so ungainly as drilling and tapping a thread for said airline connector.

Ideally, one builds it in to the part.

The technique is to stick the face of a nut to the mould, and then build the layers of cloth around it. In order to stop the resin infiltrating the nut, one should take a bolt, coat the threads in wax and put that in the nut. If you just put the bolt in, there’s a strong chance infusion resin will get down the threadsl. Then, when you demould, you turn the bolt, crack the thread and you have a way to get the air in.

In order to get the air in, you need to take something that matches the thread, drill it out and then weld it to a air-line fitting. So, you can connect it to your airline and force air in. The other thing you can do, rather than do this, is just turn a bolt in to apply physical force to push the part off the mould.

OR, you can make one feature do two parts.

So, my plan is to weld the nut to a plate, cross drill the plate with a bunch of holes, and embedd the plate in the stack – like a hard-point. I will replace the core with the actual plate, so I am putting in a 5mm thick plate (ouch for weight).

I will then use this as a harness anchor point. So, rather than adding a couple or air-release points that are convenient for the mould, I’ll be adding four either side. Two below for the crotch straps, and one either side of the thighs for the leg straps. needless to say I need to be quite accurate for my positioning, and I’ll probably have to use 7/16 UNF rather than M10 to stay with the standard larger size anchor points (rather than have to do lots of explaining to scrutinisers that may not understand what I’ve done). The other smart bit of advice Vic gave me was to double up the cloth over the anchor point, which makes a lot of sense.

Conclusion: reuse is best. Anchor points can become release points. Viva El Presedente.