Does gravity affect injection moulding process?

In comparison to the shear stresses in the flowing polymer melt, the gravity force is very small. (You can do a simple order of magnitude comparison to confirm this). However, I have seen cases in Gas Injection Moulding where the melt sagged around the gas core during the gas pressure holding phase. This is reasonable. If all melt in the cavity is pressurized to approximately the same pressure, then the shear stress are zero (or near zero) and so the gravity force is the only significant directional force acting on the melt. (To put it another way, the buoyancy of the gas core causes it to rise up through the polymer melt. Of course it won't displace the frozen skin layer).

On the other hand, if the process applied is the short shot process gravity is very important. You can try to inject from top to bottom, at 100% the completed part will not have the same weight as you inject from bottom to top. In big diameter the melted polymer will fall down in the cavity only with gravity.

If you have a VERY big thickness into the cavity the pressure drop is dramatic and the gravity can influence the process. I have an example made with gas assistance process. Imagine a cylinder 30 mm of thickness. If you fill the resin (before the gas shot) with the cavity in horizontal way (from operator side to opposite operator side) you never made the part because the gravity drop the resin on the bottom side and, due the frozen skin, you don't fill the part and the gas doesn't help. If you rotate the part, from bottom side tool to top of tool, and fill the part from the bottom side part, you fill completely the part because the gravity drop the plastic and air is vacuum on the top of tool, the gas can push the plastic on the periphesical surface and help to complete the part.

Although have always thought in the contrary, there was one instance in which it seemed to have major impact. This was a center gated mould, with hinges at each end for arms to hold a regular paper towels standard roll. This was a long moulded single piece holder. When we got the mould the instinctive approach was to load it vertically into the platens. Technicians spend almost four hours trying to fill the klunker "upper" half complete. Since this was my own shop I got an serious and stood with him reviewing what we knew of SIM at that time, 1986. Then out of the blue another friend shouted out to tilt the mould 90 degrees. Guy, did not even finish HS, but was remarkably accurate with suggestion. We did and went back in almost all parameters never missing filling the part at shortest time and minimum amount of resin. Only explanation to this day is same: gravity.

Without gravity you can easily float away from the machine controls every time you push a button. Plus it is hard to keep your coffee in your cup which can lead to employee frustration and people floating off the job. In my experience and opinion, for a fully filled/packed standard injection moulded part, the answer is no. If it is not a fully filled part then you have other issues you need to address first.

I once had a Chinese injection moulder argue with me about this on a project where the weld lines where located in a bad location for the part's mechanical performance. He added gates (From one to four) and rotated the tool in the press. When the weld lines moved on the part he naturally said "See! Gravity does make a difference!"
If you are at the point in your process trouble shooting where you are considering gravity as a factor, it is a sure sign that you need to start the entire issue resolution process over again.