I have simple question, since my raspberry is new and i don't know if its ok when its 100 % of the time at 55 degrees and basically my question is does my raspberry will melt the plastic case at this temperature?
At 55°C, the raspberry pi cannot melt its case.
ABS plastic begins to soften at around 100°C.
Regarding the Pi being damaged, don't worry about it. Raspberry Pies throttle back their CPU speed if they reach 85°C.
*Actually there have been a few rare instances of a Pi melting its case or getting ridiculously hot. This happens when the SD card or CPU short out due to abuse. (like trying to run it on 12v)