Always follow the recommended powder water ratio's and measure both the powder and water accurately.

Always add the powder to the water:

If too much water is added the mould will be weaker. This can create flashing on casting

If not enough water is added the slurry will be thicker.

This has 2 effects –

1) It will not be able to release the bubbles efficiently so bubbles may form on the surface of the casting

2) The slurry will set faster leading to flashing. This effect will be explained in more detail later.

Why do you need to vacuum the slurry?

When mixing the powder & water if it is not mixed and poured in a vacuum, air will become entrapped in the slurry.

If a vacuum mixer is not used, the mixing bowl must be vacuumed before pouring the flasks and the flasks must also be vacuumed after they have been poured.

If this process is not followed the castings will have bubbles on them as the air bubbles in the slurry will stick to the wax pattern and will be directly transferred to the casting.