lot of times, bare minimum prototypes can be made (for a faction of the cost of a full mold) that can be used to work out the bugs out of the part. Plastic can sometimes behave really strange and there are a lot of things that can impact the final part (shrink, cooling, packing/cycle time, gating....) If we need to change the part for any reason, then we will eather weld, or insert the part.
The mold shop that i work at charges only for the engeneering changes to the part. If we can prove that we can get a part out of that mold, that is in tolarence, and was approved by molders project menagement....then it's not our problem anymore. It's the molders responability to replicate the condisions we had to get the same resoult.