The simplest solution is to design the component better. Avoid large ares of unsupported plastic (or anything else). See if you can put in cross beams underneath (think about the flooring in a house, it's the same principle).

Tor: High flex modulus is probably not the answer. Plastic with little or no internal stress is what you need. Don't use stuff cut
from a commercial product, use sheet purchased from a supply house. High impact
styrene is readily available inthickness from
.010 to .250 with digestible prices.

Pat http://www.aquasite.com/skonkworks

Thanks for your responses!

I will check the things you have said.

But, another thing, what if the piece has to me made using polypropylene?

Any suggestions?

Thanks!

You can also achieve stiffness by using a core. Basically have the high density polypropylene on the external surfaces and a low density material (eg expanded polystyrene) bonded in between. Thus your sheet material is actually 3 sheets bonded together.

What are you building, so we can come up with suitable suggestions? What are your restrictions or constraints.

TOR,

When asking for help it's a good idea to explain what you are designing, what environment and loads it will see, and the manufacturing process your plan to use. These questions must be asked before you'll get meaningful help.

If you plan to use polypropylene in an injected molding process, Insideman offers one of the best suggestions to reduce warpage. That is to reduce the stress of the part during manufacturing. Injection molded parts tend to warp when internal stresses are too high or reinforcements such as glass fibers align in one direction. You'll need low stress filling and an isotropic shrinkage pattern. PP is notorious for warping due to high stress and anisotropic shrinkage.
Best of luck.