If you want to get a China Business Visa in Hk, you need an invitation letter from the factory in Shenzhen and copy of its business license, then you can apply for Business Visa to China, if you need a multiple entry visa, the invitation letter should prescribe that you need to enter and exit China for many times. It's advisable for the Shenzhen factory to apply for a visa notification letter for you from an authorized Chinese government office, such as Office of economy and economy.