Why are pieces of plastic melted and mixed together to become a whole?

I don't know if you can understand the question stated in this way.

For example:

Regardless of whether it is virgin plastic or recycled plastic, most of them are plastic pellets when they leave the factory. Put these small plastic pellets into a molding machine to heat and melt them, and they will become a large piece of plastic after cooling. What is the principle?

Surface tension (surface free energy) tends to minimize the outer surface area of ​​the fluid.