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I have, very simply, modelled the production of various plastic polymers, using the ELCD (v3.2 for openLCA version 1.7) database and used the process "Polyethylene terephthalate (PET) granulate, production mix, at plant, amorphous" for PET, "Polyethylene low density granulate (PE-LD), production mix, at plant" for LDPE and so on..

This works for PET and HDPE (for example looking at GWP), however I get strange results for PP where there is an unexpected big negative GWP associated to PET production. Similar strange results for LDPE but, in addition, the LDPE production itself generates negative GWP!

Can someone please help?

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I think it was me creating a product system with the wrong options. When going from "Prefer default providers" to "Only link default providers" in the Provider Linking options, all those odd results disappeared.
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Your information are a bit scarce but if you open the tab "impact analysis" after calculating the analysis results you can open the impact category climate change under which you will see what processes exactly contribute the negative emission. I hope this helps.