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Definition: Understanding: plural of understand, comprehension, knowing, Graph: a display of data in composite or visual display showing a range Injection Start: The start of movement on the injection screw, Injection Transfer: A point of transfer between filling and packing with the injection screw.
In the previous installment the points of the graph were explained in a basic sense. The concentration of this article is on the FILL portion of the graph. Basically if one looks at the graph it is that portion of the curve from point A the start to point B the point of transfer.
In filling of the mold, this area shows the detail as to how the material is flowing from the nozzle through the sprue runner gates and into the cavity. This is important to understand and if one has a machine which is capable and a shot size which is large enough one can in fact plot what is occurring and when on the injection stroke.
Some notes:
1. Pressure at transfer can vary
- This is due to the basic fact that pressure used is a result of resistance to flow and speed set to achieve the fill time desired.
- This is true when the machine is set to transfer by position
- Position transfer allows for volumetric change which is approximately exact.
- If one thinks of a syringe,(the dosage is calibrated in cubic measurement) by using the barrel of the machine and setting up transfer by position it is similar, and can be recorded as cubic measurement.
2. Time used to travel the distance should be the same each shot
- This results in Fill Time
- This results in a pressure used variability
- If time varies by more than 0.04 seconds +/- an issue with machine settings and or capability must be explored.
3. The horizontal axis of the graph is time.
The above example is the original curve presented (part 1) and the following curve shows a need for greater pressure at the transfer point. Since the second curve is smooth to the pressure transfer point it is consider just a need for more pressure to maintain speed.
Sometimes the nozzle or even a gate can interrupt the fill and this typically results in a spike prior to the transfer points.
In the above graph it can be noticed that upon injection (0, 0) the pressure moves immediately higher rapidly developing pressure and then declines and moves back into a gentle or controlled type of pressure increase. In this example it is the nozzle which is freezing off, and if the nozzle temperature were increased it would be expected that this first peak would disappear or at minimum decrease in magnitude.
Further one may see if one watches the screw moving forward a hesitation upon injection.
In the above example the peak is not at the nozzle as the material is flowing, than there is a sudden peak and decrease thus this area may be a gate. It could also be the use of an extremely large suck back on screw recovery that is causing this. As can be noted the screw is moving forward for a period of time prior to experiencing the spike and then once material passes this area it decreases and then gathers pressure as it fills the cavity further to the transfer point.
At issue in the previous examples is to understand filling of the cavity, and that anytime the pressure is decreasing, while the screw moves forward can be a possible concern of the process not controlling what is happening inside the cavity. Further on the graphing side it is sometimes helpful to expand out the time frame, thus in the above examples one is looking at the entire cycle for filling and packing, while the following example shows only the fill to transfer but on an expanded scale.
SL SILVEY
www.silveysplasticconsulting.com
Graphing 06052013.01
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