When you're working with cardboard, you may run into a narrow piece of cardboard that needs to be strengthened. On the plus side, it's easy to reinforce cardboard, because there are many materials that can be used, depending on your project. The biggest concern is whether or not the aesthetics of your project will be enhanced or ruined by certain methods of strengthening the cardboard.
Trace the piece of cardboard that needs reinforcing onto a piece of construction paper using a pencil. Cut out a slip of paper to use as a stencil.
Line a piece of tape along the piece of cardboard. Line the tape up lengthwise, or as close to lengthwise as it can be on your model. The piece of tape should stretch a little when the narrow piece of cardboard is bent in the wrong direction. Repeat this a couple of times to make an even stronger piece. If you need something stronger, or don't want the sheen of a piece of tape on the cardboard, try another strengthening method.
Wrap the cardboard in duct tape. Duct tape has a fabric weave on it, which makes it extremely strong. Wrap it along the length or width of the cardboard as works best for your needs. If you need something less flexible, or something that doesn't have the dull grey colour of duct tape, try another method.
Take a piece of wire (such as a straightened paper clip or a section from a wire coat hanger) and glue it against the back side of the cardboard piece. Depending on the width of the cardboard, and the strength needed, you may want to use a second piece of wire.
Trace the stencil created in Step 1 onto a piece of cardboard, card stock, or construction paper. If you need to copy a colour or design on your cardboard model, remove the stencil and copy that pattern onto the traced piece.
Cut out the traced piece and sandwich the piece or pieces of wire between it and the original cardboard. This not only further reinforces the cardboard, but also hides the wire pieces.
Adjust the reinforcement however you need to. You may need to tape the pieces of cardboard or paper together to hold them in place while the glue dries. Leave the piece to dry overnight, then remove the tape.
If you're using corrugated cardboard, you can slide a piece of wire between the corrugations. Place a healthy amount of glue inside the hole at one end, slide the wire in, and glue both ends. This creates an invisible wire inside the cardboard that makes it even stronger.