A study in cores for my tub


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






  • 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.


  • 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.


  • 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.

Feed the attention-whore