Some HDPE plastic bottle caps have a plastic inner lining that is not HDPE and causes problems when attempting to melt the caps.
The inner lining is kept in place by an inner lip in the cap.
To remove the lining, you have to open the lip by bending the edges back
To remove the inner lining, use a pair of gardening shears and make multiple cuts along the edge of the cap from the edge of the cap to the lip where the lining is held.
Do this all the way around, then flatten out the entire cap. It should look like a flower.
You should be able to bend back the "petals" and grab the inner lining with a pliers and pull it out slowly without tearing it.
