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…

Blog Reader
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.

Additional Thoughts
Basically, this is why it is critical to document your process based on process outputs such as fill time, part weight at the end of fill, and plastic pressure at transfer. Such an approach will eliminate the need to make such conversions.
-Andy
1. Steve Cavalier left a comment on 2013/11/26 at 3:21 pm

Question. Say i have a process with .94sec fill time and 36mm screw. When i change to a 50mm screw, what is the formula to match the same fill/shear rate/?

2. Andy Routsis left a comment on 2013/11/28 at 12:06 pm

Good Question,
I will put the basic formula on my next post for the speed. This is a good starting point, but the most important factor is to know the 1st Stage Fill Time and 1st Stage Short Shot Weight. If these are matched, then the outputs match which is critical.
Andy