Proofing means making a sample.
To open a mold is to make a mold and start drying it.
These terms are often used in the general printing industry. For example, if you want to print a batch of color pages, it will first proof, that is, print out one copy. If the customer is satisfied, it will open the mold, that is, make a mold, and start printing. Because it is expensive to make a mold (copperplate printing, etc.), generally the more copies you print, the cheaper the average price per copy will be.