I got the frame cemented together on the Kenworth W-925.

[Plumcrazy Preston]

I noticed the frame looks warped curving somewhat to the left while looking down the top of the frame rails from the rear. Plastic doesn't seem to have the precision of metal. The frame seems to lie flat while on a tabletop. 

What is the frame straightening trick? 

[Fat Brian]

The best you can do now is try to hold the rear end stable while gently forcing the front end to the right a bit. Just work slowly and keep checking the straightness and flatness in the other directions so you don't introduce any new misalignment. 

[Plumcrazy Preston]

It seems like I read somewhere that one could immerse the frame in hot water to soften the plastic then place the frame in a vise or under heavy weights like a book or blocks of wood to straighten out. I can't find the web source where I read that. The plastic should hold the proper shape once slowly cooled down again under some sort of jig. Die-cast metal might be ideal for vehicle model frames, at least the side rails. 

Edited December 15, 2020 by Plumcrazy Preston

[Oldmopars]

Boiling water works, but a heat gun can also be used. You have to be very careful, but it let you heat only one area, not everything. I just used it to fix an 1:8th scale Corvette hood. It also would have required a really big pot, the hood is huge. 

[Plumcrazy Preston]

I ran my frame under scalding hot sink water for about 20 seconds then gave the frame a bend to the right a couple of times. It now seems pretty dam straight. I monkey with it no more. 

[Bainford]

14 hours ago, Plumcrazy Preston said:

I ran my frame under scalding hot sink water for about 20 seconds then gave the frame a bend to the right a couple of times. It now seems pretty dam straight. I monkey with it no more. 

That method is one of the best. For stubborn distortion or bends, follow up the hot water with a dunk in cold water while holding the part/assembly in the corrected position, to lock in the correction.