How can I be sure the sample I provide will make the same mold?

The orinigal sample provided is very important. But what we do best is carefully select the right data and measurement which will match 100% as the sample. However, it is very commonly happened in our experience that the sample provided was deformed already. When the finished product comes out, it was not able to do assembly. 

To avoid this, you should always provide the best condition of sample and also to provide the other assembly parts along for engineer and designer to double check the measurement so that it can be ensured the assembly parts can fit.