You probably realize toothbrush handles are injection-molded pieces of plastic. But how do they get the nylon bristles into the head? And without using adhesive, how do they get the bristles to stay put?
Here's how. This is a shot of the machine that loads the bristles into the heads, at actual speed:
Here's the process from a different angle, slowed down:
The close-up in slow motion, below, provides the answer. The tool that grabs the bristles also folds them in half (off camera) and binds them into a bunch by wrapping them with wire near the fold. In this shot below, you can see the wire binding that's holding the butt end of the bristles.
The wire binding provides a friction fit in the molded holes, both keeping the bristles together and preventing them from falling out.