I'm attempting to make some PVC films in a hot press using DOP and PVC powder (itself mixed with powder additives) as ingredients.

I tried mixing DOP and PVC at a 50:50 ratio (w/w) by adding all of the powder to the liquid DOP all at once. The DOP was being mixed using a magnetic stirrer and was kept mixing after the addition of the powder

After 15 minutes, I stopped the mixing and immediately hot pressed the mixture at 170 °C. Apart from thermal degradation, which I fixed by hot pressing at a lower temperature), I saw clumps of PVC in the film (see picture).

My question is, what should I do differently to prevent these clumps from forming?