Why Not 2-Part Mold Making?

2-Part Mold Making is hard to scale, and requires some tending.

Although 2-Part mold making is fairly easy to use, one must develop some ingenuity when it comes to making good molds. This can be tricky. Suspending originals is tricky to do properly.

2-Part mold making is also a fairly rapid process, but it is not advised that leave the molds unattended for too long. The negative molds especially can split and ruin the mold if they aren't maintained.

The hardest part of 2-Part mold making, though, can be finding a positive original to cast. One popular way is to mold an existing object. If you are trying to make a plastic original, however an original item must first be sculpted before it can be cast. For the artistically inclined, this can be easy. 

2-Part mold making could also be combined with 3D printing to easily and rapidly duplicate complex plastic parts.