## Calculating Injection Speed When Changing Machines

I received a series of follow-up questions from yesterday’s blog entry. I will do my best to paraphrase and answer…

One machine has a 30mm screw (3100 bar max pressure), and one has a 40mm screw (1800 bar max pressure). Both have the same size injection cylinders. When I transfer from the 30mm to the 40mm machine, do I need a higher pressure pump to move the screw the same distance.
My Response
In reality, the larger diameter screw changes all the aspects of pressure, velocity, and distance.
Since the surface area of the 30mm screw is 56% of the 40mm screw, you need to approach the process with the following assumptions:
1. To maintain the same injection volume, the 40mm screw will need to travel approximately 56% of the distance.
2. To maintain the same injection rate, the 40mm screw will need to travel at a speed approximately 56% of the 30mm screw.

In most cases, you will need more pressure only if the machine is maxed out… and you must run a pressure limited process to make a good part.