I design injection molded parts and injection molds (tooling) for a living. I also establish molding machine injection / temperature parameters when I prove out (do the initial test run) of a new mold. I'm an M.E. by training, but have specialized training within the injection molding industry.
Its not clear from your post if you are asking a question or if you are offering advice.
If I assume you are asking a question, here is my reply:
There are multiple considerations to creating a successful production run of injection molded parts. I would not say any particular consider is "key", as all are important.
1) Which material is the part intended to be made of? Different materials require different machine parameters (settings) to run, and to some extent, influence tool design (mold design). When I am asked to design a part and mold around a material with high viscosity, I will design larger runners, sprues, and gates. If the material has a tendency to form voids in thicker areas, I might add overflow cavities to push these defects into. These become sacrificial molded "pieces" that are picked off the part after its molded.
2) Part design along with Molding simulations to optimize part design for material flow within the cavity, as well as gate location, injection speed, gate location, etc. I typically use Autodesk Moldflow for simulations. Part design must consider how the tool will be designed to mold the part. For example, are there undercuts on the part design. Will a side action (slide) be needed for these undercuts? Are there any inserts that need to be placed in the mold before the mold is closed that will become part of the molded part?
3) Machine limitations: Available clamp force (tonnage of the platen clamp), injection speed, and pressure limits. Generally, injecting faster is better for part quality, but the machine can only inject so fast and has a limit on clamp tonnage. If molding simulations show that you cannot get good molded parts at the available injection speeds, the part must be re-designed (for example, for 2 step molding) or the machine must be upgraded with a second injection unit to double the cavity filling speed. Pressure = Force x Area. If the cavity pressure x the "silhouette" area of the part = force greater than available clamp (platen) force, the mold will get "blown" open. note: The silhouette is the area of the molded part projected onto the parting line of the mold and must include any other molded parts that are at parting line such as runners.
Regarding materials molds are made from:
I typically use 7075-T6 for prototype "soft" tooling meant for very short production (ie prototype) runs.
For production molds, P20 or A2 tool steel is typically what I spec. Sometimes I spec a type of Stainless (I don't recall which one off the top of my head), but this is very rare and is very unusual.
P20 can also be heat treated to harden it for even higher production run volumes.
-------------
Chris, M.E. Low Pressure Molding Expert, Molded Part and Mold tooling design
Also: Machine design, CNC metal part design, Sheet metal design