HIPS (High Impact Polystyrene)

Polystyrene is a very durable plastic, used for some early bricks (PRIMA, waffle plates) and modern baseplates. Despite its impact resistance, it can be brittle, and has a tendency to turn white along the edges as it wears (see the factory-cut edge of the baseplate pictured, and play-worn Minitalia bricks). Older polystyrene bricks have a very high-pitched, glassy sound when dropped. I believe LEGO now adds a rubber additive to HIPS to increase its strength and flexibility, as well as decrease the whitening effect.

[Original post and comments on Flickr.]

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