Now that you know how to ensure that the eDART computes good cavity fill times, what do you do with them? The simple answer is that for any type of molding Process Time / Cavity Fill can be used for at least the following:

• Containment: Add Process Time / Cavity Fill to the Alarm Settiings tool to sort for part quality including molecular orientation, stress, warp, texture splay, flow lines, knit line location and some specific issues relating to crystalinity. Process Time / Cavity Fill is the closest we can get to the plastics variable called “Flow Rate”.

• Analysis: Detecting check ring leakage (looking for variability), cavity blocking, balance, valve gate actuation problems and hot runner problems.

• Control: Ensuring consistent process setup by setting machine speeds so that cavity fill times come as close as possible to the template. This matches the in-cavity flow instead of matching machine flow.

Multi-Cavity Applications

In a multi-cavity mold one of the one of the first questions is: Is it balanced? Is it balanced when new? Shot to shot? At each time the process is started up fresh? Over the long haul?

The eDART computes fill balance as a number called Balance / Cavity Fill Time. This is a percentage number. A value of 100% means that all cavities are filling in exactly the same time. Anything less than that means that the cavities are slightly out of balance. You can see the imbalance on the cycle graph

Medium to High Cavitation: 8 or more cavities

The cycle graph generally gets very busy with many cavities. This makes it difficult to know which cavity is causing the imbalance. If you want to observe the whole process you can remove all of the individual cavity curves and just put up End of Cavity #High and End of Cavity #Low (or Post Gate or whatever). These are the “phantom” curves in which the value at each point is the value of the highest or lowest cavity respectively.

So the question immediately arises: How can I know which cavity is causing my mold to get out of balance and the Balance / Cavity Fill Time number to fall if I can’t see all of the curves? Here is a simple technique:

1. Use “Add Value” on the Cycle Values window to add all of the Process Time / Cavity Fill numbers (except for the phantom curves, #High, #Low, #Avg and #Rng). After selecting Process Time on the left you can choose one cavity on the right and then hold down the shift key to select a range with the next click or the control key to select individuals.

2. Click the header word (button) labeled “Value” on the Cycle Values window. This will sort all of the cavity fill times by, either ascending or descending. Click the “Value” button a second time to reverse the order. Note that the Cycle Values window only sorts when you click. It does not sort continuously.

Now you can discover which cavity is filling first and which is filling last and by how much. You can compare each to the average by putting Average Value / Cavity Fill Time on the Cycle Values window. This can be much less confusing than looking at a cycle graph with say 32 cavities and tying to pick out an offending cavity with color.

To look for the most variable cavity add Coefficient of Variation / Process Time / Cavity Fill to the Statistics window and sort there as well.

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…

After sending Tip #15: “What are Fill and Pack?” I got some questions that showed confusion between machine fill and pack v.s. cavity fill and pack.

The short explanation is this: Cavity fill is when the material reaches the end of the cavity so that it can begin to pressurize. Machine fill (in a decoupled process) is when the machine slows down (D3) or transfers to hold (D2) BEFORE the cavity is full. The melt then decompresses and the machine slowly packs more material into the subsequently full cavity.

Machine Fill:

In the eDART we want the user to set the fill volume for a particular process. This is either done with the cursor on the cycle graph, or a button on the Sequence Settings fill tab (with a short shot) or by typing in a number. If you don’t set fill volume then the eDART uses the time at injection pressure peak. This tells the eDART when the fill portion of the cycle ends from the machine’s point of view.

Cavity Fill:

The cavity fill settings in “Sequence Settings” tell the eDART when to interpret the cavity as full. Experience has shown that using 1000 psi at the end of cavity is a good estimate of a repeatable fill time.

The Sequence Settings tab lets you change the meaning of when a cavity is full. These are saved with the processs, i.e. the mold / material /cavitation combination.

This tip is focused on fill. The pack settings follow the same idea. Machine pack ends at transfer. Cavity pack starts at cavity fill and ends at 98% of the Post Gate sensor peak. If there is no post gate sensor you must choose another that exists. If someone needs more elaboration on machine v.s. cavity pack, let me know. This is probably a separate tip.

More Details for Those Interested: Values Computed

Machine Fill:

Knowing the end of machine fill allows the eDART to compute viscosity during fill, machine fill time and speed (for repeatability and process setup). It does not tell you how fast the cavity filled to the end. The sequence trace Machine Sequence/ Fill comes from this computation. The summary values are:

Cavity Fill:

The eDART uses the Sequence Settings, Cavity Fill setup to decide when the cavity is full. It can then compute a variety of summary values that represent part quality in-cavity far better than the machine variables.

Many of the eDART’s computations of the in-cavity values depend on defining when the cavity is filled and when it is packed. This affects such things as cavity fill time (“Process Time”, “Cavity Fill”), pack rate, fill and pack time and so on. So the eDART must decide when the cavity is full and when it is packed. If you do not have cavity pressure sensors (and perhaps someday temperature) then this is irrelevant.

By default it assumes that the cavity is full when the pressure at an End of Cavity sensor reaches 1000 psi. It assumes the cavity is fully packed when the pressure at the Post Gate Sensor reaches 98% of its peak. The 1000 psi at end of cavity (for fill) is an old rule of thumb. The 98% of peak (for pack) prevents variation in pack times caused by curves with very flat tops.

These settings can be changed in the “Sequence Settings” tool on the “Cavity Fill” and “Cavity Pack” tabs.

All of these settings are saved with the “process”: The combination of Mold, Material and Cavities in the Job Setup. If you change the “process” (e.g. change materials on the Job Setup) then you will need to make the changes for that new material.

• What if you don’t have a Post Gate sensor?
  
  
  
  On the “Sequence Settings”, “Cavity Pack” tab select the sensor that you do have (e.g. End of Cavity or Mid Cavity).

• What if you don’t have an End of Cavity sensor?
  
  
  
  On the “Sequence Settings”, “Cavity Fill” tab select the sensor that you do have (e.g. Post Gate or Mid Cavity).

• When should you adjust the level for “Cavity Fill”?
  
  
  
  – If you do have an End of Cavity sensor you can sometimes get more precise measurements of fill time by lowering the 1000 psi to just above the “kink” in the End of Cavity curve where the pressure begins to rise rapidly.
  
  
  
  – If you do not have an End of Cavity sensor and must change to Post Gate then you need to estimate from the cycle graph how much pressure appears at the gate when the cavity is full. You must pick a number higher than the “dynamic” (filling) pressure that is caused during flow. Otherwise the eDART will assume that the cavity is full when it is only filling.
  
  
  
  – Some very low pressure materials (liquid silicon, TPE etc) do not even reach pressures much above 1000 psi when they are fully packed. So you may need to set the fill level quite low to pick up the actual fill point. As above, refer to the cycle graph to find a good pressure.

• When should you adjust the level for “Cavity Pack” (i.e. % of peak)?
  
  
  
  We have had some odd pressure curves with early discharge and late packing during hold. This kind of process can cause two peaks and the eDART may oscillate as the level of each peak varies relative to the other. Of course, this begs the question: When is the cavity actually packed? A the first or the second peak?
  
  
  
  If there is a lot of mold deflection then deciding for when the cavity is packed may be open to discussion. The first peak during pack speed or hold is, of course, a packed part. But if the mold is deflected then sometimes when it springs back at the end of hold it can compress the material even further. Yet if the material is sufficiently cooled it may not actually pack the molecules together, even though the pressure my rise dramatically.