Since a shear viscosity test was run and numbers and data have been generated the issue now becomes how to read or basically understand what is going on.
General assumption must be made in this part of the explanation;
1- shot size was held constant
2- pressure was set to max
3- barrel heats were held constant
4- machine speed was only variable adjusted
5- recorded data included
a. fill time
b. pressure at transfer
c. shot size
d. transfer position
6- other points were
a. allowable transfer time was increased to 10 seconds
b. A minimum of 10 points were taken with maximum as close to 10 seconds as could be found.
c. All parts were saved and labeled
Now than when plotting the curve there is a relative flat section as injection speed increases and also a relative steep section as the speed decreases. Between these two areas there is what may be called the knee of the curve or an area that connects the flat to the steep.
The question is where is the perfect spot? The answer is where good or acceptable parts are being made. That is it, as a spot can be chosen and if the parts are not acceptable at that spot than it is not a good spot. So it is the inspection of the parts that will determine where the process should be.
So why run the curve? Why waste the time? Because in the 10 to 30 minutes it takes to run this test you will have determined where injection speed should be set, a portion of whether or not there is an issue with gate size, what factors are important to processing, how big is the fill time window and finally a quality check, data point of the material that is being used.
Now the above illustration is showing a few things:
1- it is a resulting relative viscosity curve
2- The numbers 0 - 5 are shear rate numbers
a. The red line may show the relative flat portion of the curve
b. 1 second fill time is 1 on this line as 1/1=1, 2 on the line is a 0.5 second fill time as 1/.5 = 2
3- The number 0- 40000 show the relative viscosity numbers
a. The higher the number the stiffer the flow, more like concrete
b. The red line would represent the steep portion of the curve
4- The 2 green lines represent a point on the line that I state is critical in setup importance of operations and what is to be watched
a. To the right of line the process window is large as our relative viscosity does not vary that much.
b. To the left of the line the process window became very small as relative viscosity can change greatly with slight variations in fill time.
c. Note that the placement of the green line has yet to be determined exactly by modeling, and at this point is only reference and will vary in angle.
The fill time set point will relate to a viscosity number and shear number on the curve which gives a reference point. So now there is a number associated with the fill time that can be related back to material viscosity that can be checked against another lot of material, or process set up at a later date. There now is a QC check data point.
If the fill time selected, is to the right of knee in the flat section it can be observed that the viscosity does not change much, thus it is possible to adjust the fill time a bit and not have issues (not that you would want to), but more important, a change in temperature will not have a great effect in this area of the curve on the viscosity. While if in the steep section, fill time must be watched and keep to +/- 0.04 seconds and material temperature should also be watched very close, along with monitoring of the lot changes. This can be observed due to the steepness of the curve versus the fill time on the base line.
The portion that may refer to the gate size is that if we open the gate the viscosity of the material will go towards the left. (Opening of gate diameter reduces shear) Thus dependant of where you are on the curve will determine if you wish to even look at opening of the gate as to maintain that shear on material, and the resulting speed increase if the shear is to be maintained.
A point should be made here that fill time once established should remain the same and is a very important quality check point. It (fill time) should not fluctuate or if it does keep it to that small +/- 0.04 seconds. Understanding the concept and data generated will increase the knowledge base, and help to get to solutions a bit quicker. An example of this was a test (relative viscosity) a few years ago which resulted in every part having gate blush. What this showed is that the processor can not process out the blush with machine conditions, thus the issue was tool related.
Another example was a client whom knew and had done shear viscosity curves for the current material and than a new material, yet could not process the new material without bubbles. Just by reviewing the numbers on the curves the injection speed was slowed down to achieve the viscosity number of the old material and the bubbles disappeared. The newer material flowed easier.
Yes the shear viscosity and generation of data can be a pain, but knowing what is really going on and being able to show with data and parts those results is as they say priceless.
