How to Choose Carbon Fiber Plate Thickness

Stiffness Scales With the Cube

Bending stiffness rises with the cube of thickness. Going from 2.0 mm to 3.0 mm is not a 50 per cent improvement - it is roughly a threefold one. This is why flat composite parts are almost always sized by deflection rather than by strength.

Stock thicknesses are 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 mm, all in 500 x 400 mm sheets, so the step you need is usually available without a custom lay-up.

The Lay-Up Is Half the Answer

Two plates of the same thickness can behave very differently. Stock plates are fabricated with multiple layers of high strength carbon fiber prepreg using a high pressure moulding process, with a specific construction: the outer material is 2 x 2 twill carbon fiber prepreg and the inner material is 0/90 balanced unidirectional carbon fiber prepreg.

That balanced core is what makes the plate predictable. A part cut from it does not have a weak axis the designer did not expect, which is the usual failure mode with plate bought on thickness alone.

Where the Material Should Be

In bending, the fibres furthest from the neutral axis do most of the work. That is the logic behind a twill skin over a unidirectional core: the outer layers carry the load and give the surface, while the core keeps the two faces apart.

If a part is deflection-limited and weight-limited at once, adding thickness is often more efficient than adding layers of the same thickness, because thickness is what moves fibre away from the neutral axis.

Machining Changes the Answer

Plates can be cut to size or machined to print via CNC milling or waterjet cutting. Both expose the core at the cut edge. On a visible part, decide which edges are cosmetic before the profile is programmed.

Large pockets and lightening holes remove material from exactly where bending stiffness comes from. If a plate needs both cut-outs and stiffness, it is often better to go one step thicker and keep the cut-outs small.

A Simple Selection Order

Start with the span and the allowable deflection. Work out the required section. Pick the nearest stock thickness at or above it. Then check the weight, and only then consider whether a custom lay-up is worth the cost over stock plate.