Corrugated Plastic

Corrugated Plastic is a simple material: two layers of thin plastic with parallel"flutes" that run between them. The plastic itself is polypropylene, and the most common brand is Coroplast. It comes in various thicknesses, most notably 4mm and 10mm. Being a plastic, and in a form that is meant to live outside in the elements, it is fairly resilient; even signs that are over a decade old and left outside can often be in good enough shape that they can still be used once the fragile and broken bits on the edges are removed.

It is well suited to making shield cores, and can be laminated together, with each layer's flutes perpendicular to adjacent layers, to create a a stiff, yet lightweight, panel. Smaller shields can get away with just having 2 layers of 4mm Coroplast for a core, while larger shields would need an additional layer for added strength. Shields could be made with as little as a single 1/2" layer of foam on either side of a core made from 2 layers of Coroplast, and you can get away without covering the entirety of the rear face with foam, depending on the design of the shield. This can see you end up with a shield that is 1-1/4" to 1-3/8" thick.

Shield cores made from Coroplast are mostly air, even moreso than many foams, and are extremely lightweight. They are not as strong as plywood, but are so much lighter, while being considerably stiffer than just laminating foam sheets, that they make an excellent replacement and firmly claim the middleground between coreless and wood-cored. These layers can be held together with double sided indoor-outdoor carpet tape or a contact adhesive to great effect. The latter is more efficient for both cost and time, and also goes better if the surface has been cleaned and roughened with a coarse-grit sandpaper. To make panels that are larger than the signs you get would allow with a single sign, doing overlapping layers (with no aligned seams) is a viable path. Any seams that are exposed on the outer surface of the finished panel, as well as the exterior perimeter, should be covered in either duct tape or strapping tape before adhering foam to the core, as the backing on the tape will help protect the foam from the sharp edges of the Coroplast.

See the Shield (stub) page in the Build Techniques/Complete Implements section for more information about making and working with Coroplast Shield cores.

Yard Signs

4 mm Coroplast is most commonly seen as 16x18 and 18x24 yard signs. You will see some that are larger, generally up to 36"x60" at most, unless intended to be mounted to another supporting surface. It is not strong enough for being a core without at least 2 perpendicular layers.

Realty Signs

Larger and stiffer, but also bulkier to layer together. Can also be harder to get, unless you have a friend at the company that would be offloading them regularly as they wear out (visually) and need replaced.

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