I have already experimented with melting plastic bottles. But experiment of plastic bottle cap has not done. So, I experimented with melting plastic bottle cap.　It came from the extruder but it looked like a stick.

And, unfortunately the viscosity was high, the hot end was clogged up, plastic melted from the gap leaked out. This hot end has not been used anymore.
From the perspective of project management, I decided not to use plastic bottle cap at final project.
I have to make hot end again.....(T_T)