The screw diameter is important for 2 reasons. The first reason is that it determines the maximum available injection pressure, the smaller the diameter the higher the available pressure. This is critical for parts that have thin walls and a long flow length and for plastic materials that are difficult to inject.
The second reason is the diameter determines the maximum shot size available. The smaller the diameter, the smaller the shot size.
It can be seen that there is a conflict between shot size and injection pressure when selecting a screw diameter. Initially it might seem advantageous to choose the largest diameter so that there is more flexibility in the types and size of parts that can be made in one machine but this is the wrong way to think about it.
The screw diameter should be chosen in line with the application otherwise quality and/or productivity rates will suffer. The injection unit must be capable of generating enough injection pressure (with some in reserve) to maintain consistent fill times and as a consequence, maintain the quality.