Open face casting
What you are doing with these moulds is open face casting. So you need to take the glass up to fusing temperature at the least. A little experimentation will help determine whether higher temperatures are required.
The moulds need to be filled with sufficient glass to form a piece at least 6mm thick if you want to avoid the needling. Find out the volume of the space in the mould in cubic centimeters, then multiply by 2.5 and that will give you the number of grams of glass needed for the mould. Normally, the glass frit will be heaped. The finer the frit, the higher the heap; also the higher the temperature required to get a clear result.