Plastic type to make mobile covers

Hi, I and some friends have bought a moulding machine and a iphone 8 mould, but we have a problem… The mobile cover become very hard. Is it possible to get them soft er with some special liquid or something. We have used PP plastic, is there any other sort of plastic that is beter for making soft mobile cases? I need to know how to get soft recycled mobile covers 😊Please help me💛

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I have not made phone cases, but both HDPE and LDPE should be softer