Part One: the First Master Mold
While at least one individual artificial entity has taken the name of Master Mold, the term “Master Mold” itself was coined by the creator of the first Sentinels, Dr. Bolivar Trask, to denote a particular type of Sentinel. This model is specifically designed to complete the complicated task of manufacturing other Sentinels. No doubt the term of “Master Mold” itself was chosen to evoke the image of other types of manufactures processes, such as a plastics or metal mold, creating a fixed shape over and over again. However, unlike these simple devices, the Sentinel Master Mold would be tasked with the far more complex procedure of assembling servos, ablative armor, advanced weapons systems and even artificial intelligence circuits.
The first Master Mold, truly upon which all subsequent versions seem to have been based, was constructed in secret on a farm in rural South Dakota. It was built piece-by-piece in a barn over a period of years by Bolivar and his son, Larry. Nearing the completion of their work, the two Trasks intended to enlist the aid of government officials, such as a Special Agent Gyrich, to convince others of the need of their project for official support and funding. [Uncanny X-Men #minus1]
For years, Trask had worked tirelessly designing his first Sentinel, devoting every waking hour. No doubt the task was made that much harder by the fact that Trask was an anthropologist and not a cyberneticist. However, his primary vocation of anthropology had caused him to theorize and worry about the existence of mutants, a theory made startling fact when both his children, Larry and Tanya, turned out to be mutants themselves. Although he used a device embedded in a medallion to suppress his son’s abilities, Bolivar lost his daughter to the flow of the time stream when her ability to travel in time phased her out of synch with her family.
On the night before their meeting with Gyrich, she returned to her father to warn of the horror of the future that his project would bring. However, her task, as well as Bolivar’s memory of the incident, was stopped by the arrival of another time traveler, Rachel Summers, who did not want the time stream corrupted. Before leaving, however, Tanya left a program in the Master Mold’s memory core, dealing with a group of mutants enigmatically called The Twelve. This subroutine would remain in at least one future Sentinel design, suggesting that the Master Mold units would possess the same programming as the one being designed by Bolivar Trask in his remote South Dakota farm.
Though it is known that Bolivar Trask ultimately succeeded in his task of finishing his Master Mold and constructing a small army of Sentinels, it is not clear if this was with the help of – or in spite of the lack of aid of – people like Special Agent Gyrich. When the Sentinels first appeared, vast resources had clearly been employed, far beyond the means of a mere anthropologist. However, during the X-Men’s first encounter with the androids, it does not seem that any government office or department was involved. Indeed, at the time, the existence of mutants was deemed theoretical to the general populace, so it is unlikely that the government would have felt the need to expend funds without a clear and present danger. [X-Men (1st series) #14]
As Cassandra Nova, who years later would employ Bolivar’s technology, would refer to his family as “the Philadelphia Trasks,” it is probable that Bolivar was independently wealthy and intended to eventually obtain government funding once the need for his technology became apparent with the inevitable emergence of mutantkind. [New X-Men (1st series) #114]
Whether with help or not, Bolivar Trask eventually succeeded in construction of his Master Mold, complete with its manufacturing subroutines. The design of this ability was no doubt the result of mass production’s need to eliminate human error in the manufacturing of the androids, though it was still not completely automated. Rather than allowing machines to build other machines on their own, Bolivar Trask made his own presence necessary for the process. 
However, even with this safety feature in the procedure, Trask did not anticipate that the artificial intelligence he bestowed upon the Master Mold, and as a result the first Sentinels, would lead to the Master Mold deciding that its mental abilities made it superior to humans and thus no longer beholden to its creator.
As a result, the Master Mold rebelled and threatened Trask with the destruction of a nearby US city, should Trask not aid in the construction of an army of Sentinels. However, this Master Mold, as well as its Sentinels, was destroyed before the task could be accomplished. As these Sentinels had already captured two of their teammates, the remainder of the X-Men tracked the Sentinels to their base. Still, it was not the X-Men who defeated the androids but Trask himself, who at the last minute destroyed the Sentinels’ power source, which in turn destroyed the entire base and all Sentinels therein. [X-Men (1st series) #15-16]
However, this was not the last to see of the Sentinels. Months later, their menace arose once again 
from the progeny of Bolivar Trask – his son, Larry Trask. Larry had been incensed by the death of his father, going so far as to blame mutants directly for his death. Unfortunately, just like his father, the Sentinels refused to obey their creator and intended to enslave humanity to protect it from the mutants they were created to destroy. Also as before, the X-Men rose to stop the robots, ending their threat once again. [X-Men (1st series) #57-59]
During the conflict, it was never explicitly stated that Larry Trask had constructed the Sentinels using the same process, that of the Master Mold. Indeed, such unit was never seen nor referred to. However, as Trask only claimed to improve upon his father’s design, it is unlikely that he deviated too far from the process.
Part Two: Ecuador – the Forgotten Facility
Though Bolivar Trask’s Master Mold unit had betrayed its creator, threatening humanity in turn, others seemed to have decided the possible gains of a new version outweighed the risks. As it happened, they were wrong. It would be years later before the facility would be discovered by those other than who built it, but another 
Master Mold had been constructed by a remnant of a forgotten US government program and set in the highlands of Ecuador. The forgotten base was eventually located by Cassandra Nova, who described it as a basic Master Mold factory provided with a radical A.I. precision-engineered to adapt to its environment, using any and all technology within its test radius. Having been abandoned for years, it had since created a breed of unorthodox Sentinels out of makeshift scrap materials, which Cassandra called “Wild Sentinels.”
Cassandra tracked down Donald Trask, nephew of Bolivar Trask, to Albuquerque and had him brought to the Master Mold factory as the Wild Sentinels and the Master Mold itself would only take instruction from (or even allow entry to) a Trask. However, once Cassandra managed to use her abilities to copy Trask’s DNA and incorporate it into her own, she unceremoniously disposed of him, taking control of the facility herself. Now in charge, Cassandra Nova proceeded to use the facility to construct two immense Mega-Sentinels, which she then sent to the island nation of Genosha, home to over half the mutants on the planet. Within moments of the Sentinels’ arrival, the entire population, save a handful, perished. In her first scheme against mutants, Cassandra Nova had murdered sixteen million mutants, far more than the Trasks or Steven Lang had ever dreamed.
Arriving on the scene of the Master Mold factory too late to prevent the tragedy, Cyclops and Wolverine of the X-Men captured Cassandra Nova. Little did they know, however, was that Cassandra had used the Master Mold to create yet another breed of Sentinels. 
In contrast to the immense Mega-Sentinels, the Master Mold now constructed microscopic nano-Sentinels, which she used to infect her own body and thus, later, the X-Men themselves. [New X-Men (1st series) #114-116] While it was never stated plainly, it has to be assumed that, prior to taking her back to the Xavier Institute, the two X-Men destroyed the Master Mold facility, preventing it from ever being capable of manufacturing any more Sentinels.
It is still a mystery as to where in the timeline of the Trasks that the US government set up the factory. The inclusion of the necessity of the need of a Trask to run the facility would lead to the conclusion that either Bolivar or Larry were involved, but it never seemed to be the case that either of them was ever working for a government agency. Another possibility might be deduced via the combination of clues of the facility being abandoned and forgotten, in connection with the need for a Trask to be running the facility, a need very much unlike how a bureaucratic US agency would operate. It is possible that the US government used Trask’s technology and set up the facility, only to learn afterward that they unwittingly copied the subroutine requiring a Trask to be its main operator. With the inevitable rebellion that would have fallen, the agency could have written off the entire fiasco, the entire project eventually being forgotten.