Yes, waiting for molten metal cubes to cool down is boring. But there is no boring thing without a clever solution.
In order to cool down your molten metal cubes faster, you can use some water.
Fortunately, you can find some quantities of water near the central flat area. There is only one big problem : you have nothing to take them.
To take some water, you'll need a pump. So let's make one.
There is several pumps in Blackvoxel. But pumps are expensive, except for one model, the T0 Slow Pump.
As it's name said, this model of pump is (very) slow and you'll take advantage to make better models as soon as you'll get more resources, machines and automation. But at this time, this pump will fit our needs for getting some water.
Now, make the needed resources and follow the manufacturing instructions for the T0 pump at this page in the manual.
Once done, this mission is over, go to the next in order to learn how to use your pump.