Hi,
One of the things that I noticed when looking at these cutters previously is that the Crikut is said to be able to do a heavier downforce than the Silhouette machines, at least as of a year or so ago. And as such some of the sites/reviews that I lloked at suggested that while the Crikut machines could cut thin styrene the Silhouette machines were more limited.
However, it should be noted that I was looking at the more low end models, and so I don't know if that necessarily applies to your machine.
As such, I would do so research on the internet, especially boards related to using those type machines, and see what others say there. It may be possible that your machine may not be recommended for cutting plastic do to an inability of the machine to provide enough downforce while cutting.
Regards
Pat