Tip of the Day 21: A Graphical Explanation

In tip #19 I discussed how cavity pressure curves determine the in cavity fill time summary value. The Pack time is similarly computed. To wit:

Process Time, Cavity Fill = time from stroke / volume zero crossing to a default of 1000 psi at the end of cavity sensor

Process Time, Cavity Pack = time from cavity fill until the post gate pressure reaches 98% of the post gate sensor peak

Pack Rate, Post Gate = Pressure rise on the post gate sensor from the time the cavity is full until 98% of post gate peak divided by the pack time.

The 98% value for pack is a way of getting around the use of time to peak for pack. Often the curves are so flat on top that the time at which the peak occurs wanders back and forth dramatically. Using 98% of peak tends to force it to occur at the same time each shot. If you have an oddly convex curve with a fast pack rate then this number may need to be adjusted downward.

The graph below describes fill and pack. Read on beyond for the cases in which more precision is required or you do not have one or the other of the sensors in the cavity. Note that as you change settings (sensor or levels) the values computed for Process Times and Pack Rates will change. Therefore, make any necessary changes before doing designed experiments or setting alarms.

The 1000 psi at end of cavity is an old rule of thumb that generally works acceptably. If you need more precision as to when the cavity is exactly full then use the pressure at the “kink” in the cavity pressure curve and add about 20% (for viscosity variation).

What if you do not have an End of Cavity Sensor? In that case open the Sequence Settings’ Fill tab and change the selected sensor to Post Gate (a “Mid Cavity” sensor can be similarly used). Then set the level for “fill” to a value read from the cursor at about the time the post gate reaches its greatest slope. This is a guess but it can be better than nothing. Showing here…

Finally, supposing you have an End of Cavity sensor and no Post Gate sensor. The cavity fill time can still be found by using a set pressure at the end of cavity sensor but there would be no pack computed. Open the Sequence Settings’ Pack tab and select the End of Cavity (or, if you have it, Mid Cavity) sensor. The 98% default can be used or you can adjust it if you have a strange shaped graph.

Here is the graphical explanation…