We used to test plastics additives where I worked, so we had an injection moulding machine. After the melted plastic is injected into the mould the mould opens up and little pins push out the hot part. If the part is too hot when it is ejected it can cool too quickly and may not only warp but it will also have stresses that will cause warping later on. If you have to heat up a chassis you should try to cool it slowly to avoid stresses.