Not sure how reliable the site is, but Robocop Archive says:
The outer shell of the RoboSuit (the armour) is constructed of fiberglass. The interior segments (the black bits) are a combination of high-impact plastics and foam rubber.
... while also saying that there are multiple versions of the suit.
Altogether, seven complete robosuits were manufactured for the first RoboCop movie. Some had specific functions, such as one required for a scene in which RoboCop walks through an exploding gas station. Special fireproof fiberglass and other safeguards were incorporated into the suit worn by a stunt-man during the potentially hazardous shot. Two additional robosuits - termed 'third act' suits - were also manufactured. The third act suits reflects all the damage Robo had suffered in his fight with ED 209 and when the Detroit Police Department shoots him up. Basically they just pulled pieces off the molds for two fresh suits and then spent about a week using drills and all kinds of tools to make it look like bullets had burned their way through the armor. Of course, a lot of thought went into it and the bullet hits and gouges were very artistically placed. Finally, about thirty tygon tubes were laid inside the suits and hooked up to all of those bullet holes so they could have oil squirting out through the impact craters.