Laminating a Bulkhead Penetration with a 3D-Printed Plug
Every hole through a structural bulkhead is a weak point unless it’s reinforced properly — and reinforcing it properly starts with a plug that comes back out again.
Matching laminate to the surrounding skin
The basic rule for reinforcing a penetration is straightforward: match the laminate going into the hole to the laminate already on the surrounding skin. A 600g double bias skin gets 600g of double bias reinforcement around the penetration; a 1200g skin gets 1200g. Lighter material works too, provided the total weight comes out equivalent — in this case, 300g double bias went in across multiple layers to reach the same effective strength as a single heavier layer would have provided, trading fewer, thicker layers for more, thinner ones that are easier to wrap tightly around a tricky radius.
Wrapping technique: continuous coverage, staggered joins
Getting continuous fibre coverage from the flat bulkhead face, around the curve, and onto the penetration flange itself is the part that actually matters. Each layer’s joint gets placed on the flat section rather than anywhere near the radius, and successive layers alternate which side that joint sits on — never stacking two joints in the same spot, and never overlapping them, since double bias fabric stretches enough that an overlap creates an inconsistent, lumpy build-up rather than a clean transition. Each new layer’s full length simply covers whatever joint sits underneath it, keeping the whole reinforcement smooth and consistent through the radius where it matters most.
Once every layer is wet out, a small amount of cabosil-filled resin gets worked into the hole itself, filling voids around the plug before it’s removed.
A plug built to come back out — and get reused
The plug at the centre of all this is 3D-printed, and its shape isn’t accidental. Every plug carries a deliberate taper — larger at one end than the other — rather than being built parallel-sided. That wedge shape, known as draw, is exactly what lets a fully cured, resin-covered plug tap out cleanly with a simple hammer strike, rather than hanging up on minor inconsistencies in thickness the way a parallel-sided plug inevitably would.
That taper is also what makes the plug reusable across multiple penetrations rather than a single-use consumable. Being a 3D-printed thermoplastic, a plug that’s slightly the wrong shape for a particular radius can be reheated with a heat gun and reshaped on the spot — pressed into the hole while warm and cooled into the exact curve needed, rather than requiring a whole new print for every slightly different application.
The video on the left shows marking out the new hole to cut in the bulkhead, the preparation of the bulkhead, cutting out the new hole and gluing in the plug with e-glass reinforcement.
Reading the finished part
Once removed, each finished penetration gets a light fairing pass on both the front and reverse sides before it’s ready to move on to the next stage. Multiple penetrations across a single bulkhead can be worked through with the same reused plug, resized and reshaped as needed between uses — genuinely efficient tooling for a job that would otherwise demand a new mould for every single hole.
In the Members Library
The 3D print file for this penetration plug is available as a free download for members inside the Members Hub, alongside the technical forum if you’ve got questions about adapting this for your own build.
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