How can I create a perfect 90 degrees angle using my styrene sheet? I've used Model Master Liquid Cement and super glue but does not do the trick. It bends at some point after drying.

Hello Red I'm not getting what you mean, by one piece, or two pieces, or what are you doing?

Let's say for example I want to make a box.Now how can you do a perfect 90 degree angle on a side of the box using two pieces of same styrene sheets? how about a single piece?

Here's what I would try. First, find a metal box with a 90 degree corner you can use. After you glue both pieces together, place the part in the corner of the box and then tape the edges of the plastic to the metal so it stays there. I think there is also somthing from Micro-Mark (?) for this purpose. Hope this helps.
Kyle

That would be a good idea. I'll try that out.

I do this. I set the two pieces together and use a fast drying cement on them like Ambroid's Pro-Weld or Tenax 7R on the pieces. Then I take a small metal drafting triangle with a right angle (90 deg) and line the sides up. I let it sit for a couple minutes then I cement in a couple small blocks of styrene on either side of the triangle. I continue to hold it in place for a minute or two until the cement is mostly set. Then I remove the triangle (be careful, if any of the wet plastic got on it, it will stick a little). Then I will cement another side on in a like fashion. If you are doing a box, make sure the last three sides are a little long and just trim them down once the box is completely dry. Trying to cut them in advance and then hoping they line up is a royal pain.

Rob