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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. Pressure limited: a condition which limits pressure.
In the previous installment the points of the graph were explained in a basic sense. This article is on the FILL portion of the graph, in explaining a condition referred to as Pressure Limited. Basically if one looks at the graph it is that portion of the curve prior to point B the point of transfer. In filling of the mold, this area shows the detail as to how the material is building pressure from the nozzle through the sprue runner gates and into the cavity.
Pressure limited, is a term that is used to describe a condition where the pressure is limited to what is necessary in the process. In the molding process this is typically on the filling side of things and is noticed in the graph by a flat area of the curve prior to transfer point. The other aspect of this condition or a result of it is that the fill time may actually change each shot.
Figure 1
Figure 1 this is the original curve with the points A, B, C, D shown, please review previous section for definition.
Figure 2
Figure 2 is an example of pressure limited. I is labeled as the original and the limited section is that area of the graph which is flat prior to B the transfer point.
In understanding pressure limiting is to understand the injection and what happens is that the machine uses pressure to force the screw forward; enough pressure has to be applied so as to be in control of this function. If however the machine pressure set point as compared to pressure used is similar or same (no Delta P), there is now limited pressure as to what is needed for the process. This condition may be that, we have set the pressure to use a lower amount; and or the machine has no more pressure available. Both of these conditions can be confirmed by viewing the graph and also fill time repeatability. Typically fill time well vary shot to shot when in a pressure limited situation and an adjustment to fill pressure availability (increase) results in a decrease in fill time and also an increase in pressure used. Only then can one for certain ascertain that yes the process is pressure limited. When maximum fill pressure is already set on the machine and that pressure used is at maximum, (same as machine set point) than that confirms that pressure is limited.
Why the other steps? Yes looking at the graph shows that pressure is limited, the question now is why? The author has observed and confirmed that should a gate, nozzle be small enough and the shot/cavity volume/ shape is large enough that one can observe this flattening of the curve which appears at first glance to be pressure limited when in fact it is not. It may appear like it is, but there is adequate pressure and if one fills enough of the cavity it does increase in pressure. Further the fill time is consistent, and the pressure used is greatly reduced with what is set / available for use.
So while one observes a graph, and may make observations, it is critical to have further data so as to confirm a condition and or disprove an initial observation.
While the graph Figure 2 does show that pressure used has plateaued out, the actual cause of this must be explored so as to apply the correct action, if necessary.
SL SILVEY
www.silveysplasticconsulting.com
Graphing 20052013.02 .03
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