Factorio Forums

Every fluid recipe has internal fluid buffers for both inputs and outputs.

For example, for molten copper from lava, the internal input buffer for lava is 2x the recipe (500 lava per craft, so 2x500=1000), and also 2x the recipe for the molten copper internal output buffer.
A problem that arises especially in the end-game with legendary quality with high machine crafting speeds, and especially for recipes where the craft output value is in the hundreds/thousands (such as molten iron/copper, advanced thruster fuel/oxidizer, acid neutralization/steam condensation), is that the expected production values are quite off from what you actually get.

While to some degree this is due to the 6000/sec (or 100/tick) max fluid per input/output limit, there is one more thing limiting it:
If the output fluid buffer is 2x the recipe instead of minimum 3x the recipe, then whenever the normal craft and the bonus productivity craft line up, they guarantee the machine will get stuck for at least 1 tick if the speed is high enough, as with high enough speed the buffer won't be completely empty.
An example is molten copper/iron:
You'd expect up to ~9.9k molten copper per second, or ~594k per minute, but the max possible is ~474k.

Here's an example one tick before both crafts finish:
The foundry then gets stuck for 3-4 ticks before it can output the 2nd finished craft