Your electric radiator smelling like burning plastic could be dust accumulation that is burning off when the electric radiator is first turned on after it's been off for awhile.
In other cases the electric radiator smelling like burning plastic can be a result of faulty wiring, loose connections or even overheating and if the burning plastic smell persists or gets very strong you should turn the electric radiator heater off as it could potentially start a fire.
Also check the plug to the outside and ensure it's not getting too hot as it could be the plug causing the smell of plastic burning.
Slightly warm is okay but it should not be hot to touch.
A heater that smells like plastic or burning plastic could be dust and debris accumulating on the heating element or something plastic may have gotten lodged in the heater.
Other reasons for a heater smelling like plastic or burnt plastic are faulty wiring, loose wiring connections overheating components and if it's a new heater the burning plastic smell could be the protective coating burning off.
If your heater is clean and free of any dust including on the heating element if it's electric heater, it could be a loose or bad wiring or a bad connection or a wirenut is loose on the electric heater.
If the burning plastic smell goes away pretty quickly then the heater is likely safe but if the burning plastic smell persists with your heater you should shut it off and have it checked out to be safe.
It is normal for an electric heater and other heaters to smell burnt when you first turn it on after it has not been used for awhile due to dust.
When a heater sits unused for awhile it can accumulate dust and when you turn the heater on the heater can emit a burning smell due to the dust being burnt off.
This is normal and should go away within 10 minutes to 30 minutes or so but if not then it could be a sign that the heater is bad and needs repaired or replaced.
Even a brand new heater including electric heaters may smell burnt for a bit when first turned on due to protective oils and paint during the manufacturing process.
But the heater should not keep smelling burnt for too long or it's a sign that there's a problem which could result in a fire.
The burnt smell of the heater including electric heaters when just due to dust or the newness should fade away after a couple of minutes to a few hours and if not it could be a more serious issue.
A burning electrical smell is a more serious issue and can indicate overheating, damaged wiring or other electrical problems.
If you continue smelling burnt electrical smells it the heater should be turned off and unplugged or power cut off to it.
My electric baseboard heaters will emit a dust burning smell at the start of the season when I turn them on for heating as there can be dust that I cannot fully get out but it eventually burns off.