In the field of mold making and casting, it is generally assumed that you make one mold of the original and are done with it. The same mold can be used to produce multiple castings as required.
However, experienced mold makers will be quick to point out that there are times when it is prudent to make secondary molds. Such as:
Multiple life casts: Alginate is generally used to capture molds from the live human body as it is skin safe. However, the only drawback is that alginate tends to shrink quickly as the water evaporates. Therefore, the same alginate mold will not hold up for making life casts over and over. Now instead of putting the model (especially babies and infants) through the trouble of posing again and again for the cast, it makes sense to make secondary from the original alginate mold or even one of the casts.
Correcting mistakes: A mold often does not come out perfect at the first attempt itself. Air bubbles may appear on it as air tends to get entrapped around undercuts despite the utmost care. Some imperfections are also likely to creep in to the details and necessitate minor repairs. The maker can smoothen the mold and even trim off the excess or add more material to the cast to gently correct the flaws. Now a secondary mold from the repaired one will turn out perfect. And the perfect secondary mold will surely result in the most flawless castings!
Varying casting materials: At times an artist may be called on to make the same cast with different materials. But the material is not always compatible with all kinds of casting materials. Making a new secondary mold from the original mold (or subsequent cast) but with a different material will allow the liberty to use more types of casting materials.
The procedure for making a secondary mold is just like when making the original mold. However, the artist gets the option to change the material or even the mold making technique when making the secondary ones. Keep in mind that a secondary mold should be made only after the original mold or cast has cured properly.
In sum, secondary molds come with a lot of utility. They can save mold makers and casting artists a lot of time that would otherwise be wasted in capturing molds from the original model once again. They even allow the artists to aim for perfection in their works!
It is little wonder then that professional mold makers often continue to make secondary molds till they are completely satisfied with the results. This option proves especially useful when making life castings or even a latex mask. The imperfections or other problems will quickly disappear and you will be left with the most perfect life cast or latex mask!