From what material/s I need to create a mold so that I can pour in it safely any kind of molten plastic? I want to create a mold, so that when I want to make for example, planet Earth in plastic in 3D, then I can pour the molten plastic into mold. So that I can give a form. Not only this, but… when I buy Pepsi, or something with hard plastic, I can use that hard plastic to melt it and then create something with it. I want to be creative. 

Questions:


What do I need to make a mold so that I can pour any molten plastic in it? 
It is even possible to make a mold so that you can pour in it any molten plastic?
It is possible to make the mold in the you want? 


CLARIFICATION

What I’m trying to tell is, that I have found out that you can melt at home any plastic, and then you only need is a mold so that you can give a form to it. 

Problem

The only thing is that I don’t know what type of material I need to use so that I can create a special mold for this. From clay I can create mold so that I can pour a molten plastic? Don’t know.

NOTE: I have put “material/s” because, I’m don’t know what kind of materials I need so that I can put with plural or singular. Got me?
Injection molding can in fact be done at home. But judging from the OP this method would be far and above beyond the scope of his current knowledge on these types of things. 

The easiest way to cast plastic parts at home or in your workspace if that’s the case would be with the use of a commercially available product such as SmoothOn. 

Simple. Sold on amazon as well as most other online retailers. 

I suggest doing at the very least some cursory research into the process’ used when casting/molding with SmoothOn products, as they offer a variety of materials such as polyurethanes, rubbers, silicones etc. 
a quick google search will lead you in the right direction. YouTube aswell. 

Also, just as a quick tip to lead you in the right direction, try finding some videos of sculptors and character designers using this stuff for they’re own casting/molding needs and you’ll have a good point of reference to start from. 
Good luck :)