Okay, I'll give the question first, then the background. Why would you want to make a mold with both a hot runner and a cold runner combined? It seems to me that if you are adding the cost & maintenance headaches of a hot runner to a mold, then why not make it a full hot runner so that there is NO sprue waste? What advantage is there to adding a hot runner but still leaving a partial cold runner?

I am a project engineer, not a molding expert. I am getting a new mold built in Germany, with program management assistance from our sister division in Germany. I understood at the beginning of the project that the mold was a cold runner type. I just found out that it is a hybrid.

The only reason I can think of for this, is that the gate is on the side of the part. Maybe the geometry of the gate forces them to make a small cold sprue area on the side of the part directly adjacent to the gate, but all the rest can be hot-runner type. Is this correct? Could there be any other reasons?