What material(s) does a dentist use to make the plastic trays for tooth whitening?

I understand the dentist takes a negative cast and then from this is able to produce a positive cast. But what happens next?
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He places a sheet of soft plastic like vinyl material over the model and with a vacuum suction machine heats and draws the material over the teeth. The excess is then trimmed away. Now you have a custom bleaching tray. The same is done for mouth guards except a different type of material is used.
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