First, return the ink cart of the digital printing machine to its original position, then pull out the waste ink pipe from the waste ink bottle, insert the 50ml injection needle into the waste ink pipe mouth, and pull the needle cylinder backward with force until the waste ink is extracted by the needle cylinder. After the extraction, insert the waste ink pipe back into the waste ink bottle. Here we need to pay attention to the return force of waste ink. When there is no return force, you can release your hand. After drawing, you can't pull out the syringe immediately.