I am in the process of rebuilding the wing of my crashed plane and now need to repair the wing tips which are made of formed plastic. I am thinking of taping them or something similar to hold the plastic pieces from falling apart. Is there something better I can do to repair them?

Do you have all the pieces and could they be put back together (like a jigsaw puzzle)? If so, see if lacquer thinner, acetone, dope thinner or MEK will "melt" the plastic (try a little on the inside of the plastric tip part and see if it softens it - don't use enough to damage the part). If you can find something that WILL soften it, use a tiny brush dipped in the liquid and, while holding each "piece of the puzzle" in place (one piece at a time, of course), wick the liquid into the joint where each piece joins the next. Hold it for a few seconds while the plastic softens, then "welds" together. Then go onto the next part until all the pieces are put back together. Let the joints harden (so they're no longer soft from the "glue"), then sand the tips lightly to smooth over the glue joints. Use a filler if necessary to get a smooth surface, then paint to match the rest of the model.

If the pieces are totally destroyed, you could remove any fragments still attached to the last wing ribs, glue on light balsa blocks, then carefully carve and sand them to match the general shape of the plastic tips that were damaged. Then finish them to match the wings (paint, plastic covering or whatever).

Glue a block of balsa on the end of the wing and carve and sand to shape.

Been there....done that.

I haven't done it yet, but I'm going to need to do the blocks on the wings of mine this winter. It's not even worth the effort to put the plastic ones on.

What type of plane is it? If it's a trainer, leave the caps off altogether and simply cover the wingtips.

yes the plane is a trainer. I already rebuilt both ends of the wing with balsa.