Near-unscratchable screens that are expected to be one of the biggest selling points of the iPhone 6 when Apple unveils the latest model in September are to go into large-scale production this month.

To create industrial quantities of man-made sapphire, the material already used to cover the fingerprint-sensing home button and camera lens on its phones, Apple has a $578m (£350m) deal with manufacturer GT Advanced Technologies, which has built a plant powered by renewable energy in Mesa, Arizona.

The iPhone 6 will make its first public appearance on 9 September, when Apple has scheduled a big media event. "You should be good without a screen protector on your next iPhone," predicted technology blogger Marques Brownlee, who has subjected what he claimed was a leaked prototype of the new screen to his own durability tests on video: stabbing it with a hunting knife and trying to snap it while standing on it.

Until now Apple has relied on toughened glass, which can however easily be shattered and marked. But screens already demonstrated by GT can withstand scratches from concrete. Its thin sapphire layers, which could also find their way onto Apple's rumoured smart watch, are flexible, potentially improving resistance to knocks and falls. In March 2012, Apple patented a concept for laminating thin layers of sapphire to each other and to glass, producing surfaces that are strong and resist chipping.

Apple is said to be preparing two new iPhone models, both of which will have larger screens than their predecessors. The biggest will measure 5.5 inches corner to corner, while the smaller iPhone will have a 4.7-inch screen, according to reports.

The Wall Street Journal claims Apple has ordered an initial batch of between 70m and 80m handsets, its biggest first run production ever, to be sent out from factory gates in time for Christmas and New Year. Last year's initial order for the iPhone 5S, which introduced Apple's first fingerprint sensor and had a 4-inch screen like its predecessor, the iPhone 5, was for between 50m and 60m devices. But Apple is preparing for a ramp in sales from China, where its recent distribution partnership with China Mobile should boost demand.

GT chief executive Thomas Gutierrez told investors on a call this month: "The build-out of our Arizona facility, which has involved taking a 1.4 million square foot facility from a shell to a functional structure and the installation of over 1 million square feet of sapphire growth and fabrication equipment, is nearly complete and we are commencing the transition to volume production."

GT's new plant will reach full operational capacity in early 2015, but the company is expecting to collect a final $139m payment for its construction from Apple in October.