Fill Time; definitions

1-The time it takes to inject 90 to 99% of the plastic into a cavity, 2- time it takes to push the 1^st stage of material into a cavity minus the suck-back distance time, 3- a result of a distanced traveled as measured in time as it relates to injection molding process referred to as scientific molding, in regards to filling the cavity, 4- the time that is used on a boost timer to reach a set position during injection stage.

Yes all the above may be true and up for debate. Since injection molding is a process that needs to be repeatable, (why else would we use it except to make the millions of identical pieces) we need to monitor various factors within the process to see if we have that repeatability. Now our cost accountants may say cycle time is all we have to be aware of and from the time it takes to make a part on the machine they would be correct if we did not care for the quality of our part. Our cycle time is important in that it needs to always be the same, and at quoted or better if we wish to make money.

Now I should go into the generation of the relative viscosity curve (a later topic) but let's just go off the following statements.

1-     Our first stage injection (boost) will transfer by position

a.       We thus are pushing /injecting a set volume of material into the cavity/cavities

b.      Think of a syringe here, we will be filling via a set volume /cubic measurement

2-     By using 1 above we now have a measured distance we will travel (the screw/plunger)

3-     We now will measure the time it takes to travel this distance

4-     This is what we shall call fill time.

The fill time should be monitored and for present discussions take my word for it that the deviation we wish is a figure of plus/minus 0.04 seconds. So this gives us a range of 0.08 seconds. Again let me state this is for filling the part to 90-99% full, transferring by position. This variation will keep you in the ball park, ideally no variation but let's be real, (Yes Dan you can only fill to 75%, but what we want to monitor is the time to do that)

Please note that if you are transferring by pressure in the cavity, that the variation on fill time is greater, but you are now transferring based on inside cavity measurements which take precedence.

Now why should we monitor the fill time is that we wish repeatability in the process, we must think about what is happening in filling the part, flowing of material into the sprue, runner, gate and cavity. Are we not hoping to do it (filling of part) the same each and every cycle? If we do it the same do we eliminate a variable on our parts one cycle to the other? The answer to both is a YES.   Further if we establish a fill time with a prototype tool and keep the gating identical in the production tool our fill time would be the same, whether we had a 1 cavity, or 96 cavity tool.

Story 1,

Customer A produces a single cavity mold to run a prototype part. The production tool well be an 8 cavity stack tool. Upon hearing this I recommend that they look at the injection rate of the new press and run the prototype fill rate no faster than what an 80% rate for filling all 8 parts could be in the new press. Basically limiting the fill rate in the prototype tool, so that the result can be achieved in the new press given that it would have to fill an additional 7 cavities in the same time. What was found was that we needed a high fill rate to make a flat part, which than excluded the capabilities of the new press. What than happened was that upon completion of the 8 cavity tool we were back to square one in that a process had to be developed to use a slower fill rate.

Customer has a thin wall multicavity tool, with the results being that we do not make good parts on most of the cavities. Our computer simulation results show that fill time should be very quick, yet we are approximately 3 times the calculated fill time. We are able to block off half the cavities and than able to produce acceptable parts on the remaining half. Why? Because the machine now only had to fill half the parts and could now do so in about ˝ the previous time, still not what simulation data showed but good enough.

The point, a fill time is a data point. If we have an 8 cavity mold and block off 1 gate we have now changed the rate of fill to most of the cavities dependant of the feed design. We will not get the same quality of parts.

IF I have to move my mold to another press the fill time stays the same. It is independent of the machine; it is a data point that stays with the mold. 

The fill time is a result of function within the machine and mold and as such needs to be recorded and monitored for that mold and part quality.