HML Test?

[iautry1973]

So I started at a MIM (Metal Injection Molding) place a month ago and the other process engineer here was showing me how they qualify a part. One of the tests they run is what they call an HML (High-Medium-Low Hold Pressure). It is sort of like an aesthetic window study, minus the temperature differences. They get the part filled to 98%+(the + part seems odd to me) in 1st stage and then add hold pressure until the part looks good. They then drop the switchover point .10" and start ramping the hold pressure up to at least 80% of peak pressure. I asked the other process engineer why they would drop the switchover point as they were just about completely full and by dropping the switchover would most likely be inducing flash just by doing that alone. It seemed to me like it would be limiting your process window by doing this. He didn't have an answer other than that is the way they always did it and it works. Makes no sense to me, but it is MIM and not thermoplastic so maybe I am missing something. Anyone got a good explanation?

[chrisprocess]

Only reason I could think of is to ensure transfer pressure and maximum injection pressure are the same..
Sounded like they pack 100% fill with cosmetic/peak pressure and ramp down to 80% pack pressure?
Some products and materials work well with first stage only

Is the end cushion smaller than transfer position or larger?
If the end cushion is larger than transfer position, than yeah I would think they're over-packing.