If the screen is behind something like Gorilla Glass [1] like all the other tablets on the market, then that plastic will never scratch the screen. This is due to the way scratching and physics work. [2] A good reference is Mohs scale of mineral hardness [3]. Generally speaking, a softer material will not scratch a harder material. Plastics are really soft, glass and toughened glass are strong. There's a pretty good demonstration video on youtube. [4]
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[1] https://en.wikipedia.org/wiki/Gorilla_Glass
[2] https://en.wikipedia.org/wiki/Hardness
[3] https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
[4] https://www.youtube.com/watch?v=b7ANcWQEUI8