I'd try to get the plastic fully melted and blended at around 340-35- degrees, then drop the temp to say 330-340 for injecting. Be sure the check for signs of yellowing or burning, being sure to keep the melted product well stirred often. I'd also do an incremental drop in temp to see what the lowest temp threshold is using the cooker you have.
This is a smart move on your part, using scrap plastic to get acquainted with everything. Don't hurry yourself and take time to use all of the safety things...gloves, long sleeves, ventilation, etc....so that you can enjoy doing this exploration. Injecting plastic has a broad learning curve and I think you are a wise one in your approach here. Have fun, good luck.