A very stable connection is created when the connection surfaces are first welded deeper and then at the edge.
For the deeper welding, a thin soldering tip is suitable and for the edge seam a larger one.
Do some temperature tests.
I use around 250 ° C for HDPE and 270 ° C for PP.
Please use a gas mask for this work!
See also: https://community.preciousplastic.com/academy/plastic/safety
Position the parts and fix / clamp them.
Weld the parts first with the thin soldering tip and then with the thick soldering tip. Not too fast!
Check the connection and change the parameters: temperature, welding depth and speed if necessary.
The recycling guy in this video shows how he combines 12 plates to one hanging lamp ;)
In addition to HDPE, you can also use this welding technique with PP. LDPE or PS.
After welding, you can also improve the edges. I rub with additional hard plastic over the edges, but you can also polish.
For more information about the lamps visit https://johannplasto.de/.de