The typical example of the Casimir effect is of two uncharged metallic plates in a vacuum, placed a few micrometers apart.
Why do the plates need to be metallic?
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According to the article linked bellow the effect works using nonconductive materials as well, but electrostatic force resulting from charge accumulation on nonconductive plates can make measurement impossible. Independent measurement of systematic error due to residual electrostatic forces is neccessary even with grounded metal plates. You can read more on this and Casimir effect in general in New Development in Casimir effect.