Hot runner vs. Cold runner (vs. hybrid)

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?

Hi ,
Another reason is the gate from back, where you have to use a banana sprue .... But you are correct and your question is good. Usualy design of the parts is crazy or requirements on the parts are too strong and toolmaker has to fulfill all. :-)

There are times when you can't get rid of gate blush, the gate can't be hidden in the part,or a new gate is added late in design due to late changes. Then it is what it is.
I've seen lots of class A interior parts that end up sequential valve gated with hybrids because there is no hidden area to gate into or you can't turn the part over and inject into the underside because you can't hide gate blush or use a reverse cold slug well!
Rick

I was an application engineer at a major hot runner company. Part geometry will be a factor - do you have the real estate to direct gate with the appropriate hot runner gating style?

For small parts hot-to-cold gating is often used. Large cold runners can extend cycle times, but direct gating can cause residence time concerns. Using hot-to-cold can reduce the cycle time but still move enough material through each cycle to reduce possible material degradation problems.

I agree with Team JAG and the rest. It is always trying to reduce the material waste and getting as close to the gate but sometimes one can get only so close. And that is when one would go hot to cold.
Suhas