Max Velocity Settings for Thin Wall Molding

A blog reader submitted an entry this morning…

Jean Francois
We are using a decoupled process. During fill, the pressure is set to a max pressure and should not to be reached. However, when molding thin wall parts with long flow paths, it is common to reach the maximum machine capacity pressure before reaching the expected velocity.
Question: In this case what is the best recommended process: 
1) Lower the programmed velocity to a point where requested pressure in lower than the machine capacity (with 10 to 15% margin)? 
2) Keep high programmed velocity with the maximum pressure set at the maximum of the machine capability.
My guess is solution 1) should be the best because the process will be more stable. The drawback of this solution is that we are not using the max velocity that the machine can really provide.
If I choose solution 2) any variation of melt viscosity will change the process…
So what is the best?
My Response
Your choice is correct… but I should clarify the reasoning.
In establishing a robust injection molding process, the purpose is not to use the fastest velocity possible, but to use a high velocity which provides a shear rate which is above the point of shear thinning. You should perform an In-Mold Rheology Worksheet to determine where shear thinning 
occurs in your process.
If necessary, you can try processing at a velocity consuming approximately 90% of max pressure if it provides the best appearance or performance. Just ensure that you closely monitor the fill time consistency, possibly placing a lower limit alarm on your process.
Additional Thoughts
You should always avoid a pressure limited process. If necessary, you can avoid pressure limiting the process by reducing the injection speed through the use of profiling.
  1. MJ left a comment on 2009/10/08 at 11:24 am

    Hello Jean Francois.
    I’m recomending to do In-mold rheology test, but also Injection speed linearity test.If you compare these test you will see, how your machine is able to control injection speed.

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