The new Transformers film is better than the second one, says its director, but three is where he draws the line. For now.

In 2007, the film Transformers earned $710-million dollars. The second film, 2009's Transformers: Revenge of the Fallen earned $896-million and established Shia LaBeouf as a major Hollywood star. Given those profits, the idea of a third movie was a no-brainer for both director Michael Bay and Hasbro Toys.

Product placement is not a new thing in Hollywood. For decades, manufacturers have paid huge sums of money to have their products showcased in a major feature film. The greatest coup in the history of product placement in Hollywood movies is the deal done between Hasbro, the major toy producer that created the Transformers action toys, and Bay, who has used them in the series of blockbusting Transformers movies.

As he set about planning the third film, Bay had to make tough choices - the first of which was whether or not to make it in 3D. At a pre-release screening of the film at the Comic-Con movie showcase, Bay was having second thoughts about 3D, which has become the movie fad of the decade.

He told his fans: "This is so not me, and it's kinda scary, all right? I do real-world stuff, real actors in real locations, as opposed to the blue-screen stage, where the temptation is to manufacture too much.

"Nonetheless, producer Jeffery Katzenberg and director Jim Cameron were calling me all the time, saying: 'You've got to do it in 3D.' I visited Cameron when he was making Avatar, and after that I took about five months of investigation, literally breaking the 3D process down," said Bay.

"I'm a die-hard film guy who loves anamorphic, old-school lenses. Going digital seemed kind of crass."

Another issue for Bay was speed, because he likes to work fast.

"I've heard horror stories about other 3D movies, where they would do only 10 shots to 20 shots a day. I usually do 40-50 shots a day, and I was thinking that, you know, working on a $200-million movie, the cost will exponentially increase and it could a disaster.

"But they kept calling me, so I got into Cameron's space and we met his best guys, who made Avatar. I believe you must work with the best, or you're screwed, and we put the technology through a rigorous test.

"We were working on 3D systems that I had never used before. I spent several days testing ideas with film cameras and 3D cameras, and I was trying to keep my usual pace and timing, pretending this was a real shoot," said Bay, "and after I saw what we could do, I figured I could make a successful movie that way."

Bay was also candid about his difficult experiences with the second film.

"I had to think very carefully about a third film, because I remember the tough lessons we learnt with the second movie, because we got burnt in lots of ways. There was a writer's strike, and although we had agreed to a delivery date on the script, the unions just closed us down.

"It was a bad scenario, all the way around: it wasn't fair to the writer, it wasn't fair to me, it wasn't fair to anybody. It put huge pressure on us, until we finally got the finished script and started making the movie.

"We got it done, and Revenge of the Fallen was still an entertaining movie, but I think we failed on certain aspects," he said.

"With Dark of the Moon I think we have a much better script, and we got back to basics. There's really cool action, a really dark conspiracy and a lot of great robot stuff that was missing from the second film. This film shows a lot of great robot conflict, so I'm excited about it," said Bay.

"It's more serious. I got rid of all the dorky comedy. This is a dork-free Transformers movie. I think we've got our characters doing a lot more than in the previous films."

Once the technology had been sorted out, the production hit another problem. The sexy, dark-eyed, voluptuous diva, Megan Fox, left the production. That created a paparazzi frenzy, with accusations flying back and forth. It was a veritable "storm in a D-cup" that went on for months.

Fox was eventually replaced by an underwear model, Rosie Huntington-Whitely, who has never acted on stage or on screen. But she looks fabulous.

Most of the other characters, such as those played by Tyrese Gibson and Josh Duhamel, are back, but there's a new character, Bruce Brazos (John Malkovich), a political official who takes command of events, to the irritation of the hero, Witwicky (LaBeouf).

It was Witwicky who first exposed the evil Decepticons and he also saved the world. Why should he take orders from some random Pentagon egg-head?

The film was shot in Africa, China, Washington DC, Florida, Chicago and - where it all began - Los Angeles. Many fans believe that Dark of the Moon is the launch pad for a new set of adventures, but Bay is non-committal about the future.

"I've have to think - after three movies - how am I going to take it to the next level? What is the next level? What have we not done in this franchise? Has the whole conspiracy been unravelled?

"I think that Transformers may still have many stories to tell. It's just that I don't think I could do it again with Shia and everybody. I think it's time to move on."

So does the Transformers series end this year, along with the Harry Potter franchise and the Twilight saga? LaBeouf is 25 years old and Bay has told interviewers that he'd like to try his hand in a new genre.

The answers to that question are inevitably linked to the box-office returns of the latest movie. Who walks away from a multimillion-dollar success? George Lucas kept the Star Wars series going for decades, and James Cameron has announced his plans to make Avatar 2 and 3.

It all comes down to the box-office results, and we will only see how well Transformers 3: Dark of the Moon does now that the audience can give their vote.