why does the tapered exist in sprue in injection molding and what decide its lentgh
Answers
During the first 14 years of my career, when I was designing and building injection molds, gating and runners were not really a focus. I was concerned with building a robust tool and had no idea what was going on over at the molding side. In 2002 when I made a move from the mold shop to the molding floor, gating and runners jumped out at me right away as lacking standards.
I started studying gates and runners when I was developing in-mold degating with tab gates. To create a clean break, gate geometry and gate land were critical. I saw there was nothing consistent in gate geometry between material or part size from mold to mold. I saw large parts with gates half the size of the gates on a smaller part running the same material. I observed that the main focus in mold designs was on gate location on the part, and gate orifice geometry was basically shoot-from-the-hip.
For in-mold degating, I needed to have the gate as thin as possible and a minimal land to assist with a clean break. I started learning a lot about plastic pressures while I was welding and modifying gates on numerous tools. I remember the concerns people expressed about my making the gates thinner, but they gave no specific data to back up their concerns. It was very confusing when there was no standard for gates or runners, and sizes and geometries were all over the map.
GATE SIZE VS. SHAPE
Through the process of modifying gates on various tools I learned some very interesting things. Just because you decrease the gate size or thickness, it does not always mean that plastic pressure will increase. And it increasing a gate thickness or size does always not mean your plastic pressure will drop. I mention this because it goes against common assumptions.
Typically, when a gate is made smaller you can expect to see a change in plastic pressure because you have reduced the volume of the gate orifice. But a change in gate size or volume does not always produce a noticeable change in plastic pressure. I found out that when I decreased the gate thickness and had an increase of plastic pressure, I could widen the gate to compensate and reduce the plastic pressure. I also learned that the gate land can affect plastic pressure.