Had to touch basis on your outcome here.

As heat is one of the greatest factors, and looking at the molds venting also could be another issue.

starting with room temperature which should be around the 70 plus mark
In which dealing with it any colder, find you may need a little help with a Hot Plate or what I use is an old electric fry pan, where I preheat my molds and injectors, which is a trial and error for getting the proper flow with whatever type injector or pour you are using.

Another factor is your plastic again you may have to push the temp a little hotter of course without it burning or starting to smoke which is not good to do without using a respirator and Gloves

Now it appears you have no venting runners, if you don't have it inscribed in one mold halve one can experience an air gap resulting in what you have.

"Hope this helps"

"Remember Safety First"

billygee :rolleyes: