You are best of trying to weld a polymer to itself. Failing that, the
polymers should be compatible with each other - a very rare situation.
And both polymers need to have similar melting temperatures, a further
Ultrasonic might work. But run from the LDPE. It will be a nightmare.
Aspen Research, -
"Turning Questions into Answers"
Opinions expressed herein are my own and may not represent those of my