The 18 minute process explained m'halisha (from kneading) or m'hashulchan (from table)
Jewish law states that Matzah dough worked and kneaded constantly, and not stopping even for a moment, can last all day and be kosher for Matzah. If the dough is left without working for more than the determined time, the time it takes to walk the talmudic distance of a ”mil”, then it is chametz. There is disagreement among the Poskim whether the time to walk a mil is 18 minutes or 24 minutes. In practice, we follow the more stringent position of 18 minutes.
Therefore, by the strict letter of the law, as long as the dough is being worked on, it can last much longer than 18 minutes. The concern is that during the preparation, a tiny piece did not get worked on for 18 minutes, became chametz and got stuck to the dough. The bakery, therefore, thoroughly cleans all the tools and surfaces every 18 minutes and begins another cycle of matzah production.
There is a difference between what is called 18 minutes m’halisha (from kneading) or m’hashulchan (from the table), which are different levels of stringency, beyond the letter of the law. The matzah baking process is constant: mixing and kneading the flour and water, transferring to the one who works the dough, moving to the table for cutting and rolling until it becomes a thin matzah, perforating, laying on the wooden rod, and baking it in the oven. The process itself takes a few minutes from start to finish. This stringency is about cleaning all of the tools and surfaces every 18 minutes.
The kneading tools are typically changed every few minutes with every new dough, so, the main concern is not at that point. The main concern is once the dough reaches the cutting and rolling table. The concern is that there will be some minute piece of chametz crumb that is stuck to the table. Therefore, the table in every 18-minute cycle is thoroughly cleaned, so even if it has some crumb, it does not sit 18 minutes. This is the stringency called 18 m’hashulchan.
Some are even more stringent that the entirety of matzah manufacturing cycle, from the initial moment when the flour was touched with water, will be no more than 18 minutes. This is the stringency called 18 m’halisha.
