Velocity Study

Hi all,

I just finished reading the Optimization of the Injection Phase - Rheology Study and found myself confused

The procedure is to establish a 95-98% filled shot at close to the machines maximum. Record the data and reduce the injection velocity by 10% until producing parts at the slowest injection velocity possible.


I completed my study and found that the viscosity stays fairly constant at about 50% of the machines maximum velocity.

Now here is where I get confused...The procedure doesn't exactly spell out what I am to do next...unless I overlooked it in the text.

I set the machine to around 60% of the machines maximum to ensure that the process will stay consistent with slight viscosity variations. At 60% of the injection velocity, the parts appear to be 70% filled.

I was inclined to adjust the transfer position such that the parts were filled 95-98% at this speed. But then questioned myself because for process development activities, I was asked to get a High and Low value for injection speeds to see the effect injection speed has on part dimensions. If I were to choose 60-70% injection velocity for the test, would I have to adjust the transfer position for both during the study?

Here is an example of a rheology study and the parts made at each speed.

Attachment 407

If you want to make full and packed parts to determine the effect of injection speed on part dimensions I would adjust the transfer to 95-98% filled parts for each injection speed tested.
For the general rheology study the example you posted is how I've always done it.

Thanks for the input.

I just looked at several process sheets for our current machines running and we have fill time as a critical to quality validated parameter.

Plenty of times I have talked to technicians about troubleshooting issues and they throw around changing the Injection fill time...I ask if that is all they changed, and they said yes.

If they had a 95-98% filled part at say 0.50 seconds fill time and now are filling slower at 0.75 seconds, shouldn’t they have changed the transfer position to achieve 95-98%?

I'd agree with your statement. For parts with tight tolerances I will get my fill only shot back to the same weight as the qualified process if I make a change to injection speed.