If you are sweating copper water lines to a plastic outlet box at a laundry or fridge or any appliance, you have to be very careful not to melt the plastic box with your torch. At the same time, it is a delicate balance because you need to make sure it is HOT enough to give you a good connection. Otherwise, your pipes will leak, and that is BAD!
We wrap our pipes with wet rags around the connection to the outlet box while sweating the connections. This drops the temperature at the specific location of the plastic that needs to be protected and, meanwhile, allows the region of the pipe being soldered to get significantly hot enough to make the connection.
GL Barnhart Construction ...The Blog 