am going to stay for 40 days or more to visit factories and between I need to go to visit Hong Kong

If you are invited by a Chinese company to inspect their factories they can provide you with the proper documents for a Business (F), i.e.: invitation letter, business license, etc.
You can apply for a 90 day F with one of the following options a) 1 entry (it is voided as soon as you leave Mainland China) b) 2 entries (you may re-enter China once) c) multiple entries (you may leave and re-enter as many times as you like)
multiple re-entry F might have some restrictions, so it's best to check the Chinese Consulate's homepage for your country.
Many nationalities are issued a Visa on arrival in Hong Kong, but it's best to check with the Authorities before you travel.
It is always a good idea to submit an itinerary with your application, as well as flight tickets, especially when applying for a multiple entry F. Whether you stick to your travel plans once you are in China is up to you. I have heard of no cases where foreigners were checked against their itineraries