Definition: understanding: comprehending, becoming clear Relative Viscosity curve: a graphic visualization of calculated pressure plotted over reciprocal seconds (time) for a specified mold, material, nozzle at set temperatures for both mold and material Data: numbers / information generated by specific test.
The shear viscosity curve... or as properly stated" the relative shear viscosity curve", as it is relative to the performance of the machine, the mold and material used.
This tool / test is great, as it allows one to establish a base line for incoming resin, the gate, establish fill time, and evaluate a machine.
Understanding the graph and data that is generated by a shear viscosity curve is important if one elects to use the tool. There are a few programs out there that well take the data and generate the curves, these are off the shelf and some are exceptionally well done, in all that they provide.
Once the curve and or data are generated, it now depends on how one uses it for their betterment and or profit. In reviewing the graph provided, the horizontal axis is the shear axis meaning that at the 0 point there is no speed of injection and at the far right is the fastest speed for the machine.
On the vertical axis is the relative viscosity. At the 0 point the resin being extremely thin, (like water in a manner of speaking) and at the top of axis it is thick (concrete)
On this example there is another line drawn that bisects the curve at a point. This is drawn by the author at a point of supposed transition for explanation sake, only.
If one is to the right of this line it can be seen that the RV of the resin varies from approximately 15000 to 1523, though in section A there is little change in the viscosity. If one were to find this is the area where they were set up to mold, than changes in the temperature, and lot of material, would have a minimal effect on the filling of the part, with fill time remaining constant, and fill time may change with the result being that the viscosity still does not change that much. Basically one has reached the area of where shear is affecting the resin flow.
Now to the left of the line it has been found that lots, temperature, fill time become even more critical as when one looks at the spread for B one can see the variation in RV from 20,000 to 60,000. If molding in this area than yes, changing the temperature definitely has an effect on the viscosity, changing lots of materials which might have a different melt flow well show up. Further it now becomes critical that the fill time be constant as a minor change in the fill time may in fact have great changes in viscosity.
The part cosmetics and functions control where one is on the curve, and most shall find themselves around the knee of the curve whether they perform the test or not, the point here is to know where one is.
Establishing a base line for the resin is a result of the testing. What is meant here is that instead of a melt flow number one has now created a RV number for the resin being used. For example let us pick a point of say 8000, with a fill time of approximately 0.6 seconds. All is good and the parts are being produced but suddenly the parts are short. After checking everything is correct to the process sheet, the material may be suspect. Finding that the lot was changed, (or possibly not) it is possible to record the RV for what is going on. Now if someone were to quickly run a RV curve they could generate the entire range, does it match the original curve, is it entirely different. A quick check might be to calculate out that point for RV, how does it compare to the original testing. One may find that it is completely different, giving data that can be shared with your material supplier that something change, not that you think the material is different but you now have data to prove it is different, when having been processed under the same settings. Note that minor changes are always there, and a robust process well take care of them, but sometimes this data comes in handy, especially when we don't have that other lot of material around to drop in or a new lot.
The gate is one of those areas that many may change to provide for filling of the part. It is common for toolmakers or molders to say "open the gate," make it bigger, and in fact many times it helps, but sometimes it does not. If in fact one knows where one is on the curve than they can decide what will happen when the gate is opened. Since the gate size can be related to shear, if one opens the gate than shear is reduced. A reduction in shear moves the viscosity higher, for the same speed settings. There shall be a whole movement of the curve upward on the viscosity. In many cases one can say the movement shall be to the left on the original data graph. If one is at the knee than there is not much of a problem as the injection speed may be increased to maintain the RV number previously established if it is necessary. But if one were for example to the right of area A on the graph than opening the gate moves the viscosity(RV) higher or to the left (original data), the question than is there enough speed left in the press to create the RV number that was established, if that is necessary.
Machine linearity is another benefit of the RV testing and using some of the software that is available out there. In performing the RV test one has established a stroke (distance/ shot size) and is measuring a time for the travel of that distance. Also one records what the machine setting are for speed. In looking at the comparison of what the results are as compared to what is set it now shows how the machine is preforming to standard. Understand that dependent on stroke the speed may not be reached, because the stoke is not long enough to attain that speed. But what can be seen is how are we comparing to the set point, is the valve (hydraulic) oversize such that flow above a set point does not change. Is the curve similar to the set point or are there areas that they cross over and then back indicating a possible issue with machine, this in calibration or sizing of fittings, dependent on type of machine.
From just a knowledge base of knowing the machine it is a benefit, and also allows the data to be shared with maintenance and or possibly a machine supplier.
An example of a machine issue:
The owner tried to convince his processors that a 1% change meant something in the speed setting. (Percentage setting on the machine should always have a reference as to what is 100% equal to)Upon preforming the RV test the following was found, 100% to 50% setting on speed control no change in fill time. From 49% to 40% there was a change of approximately 1.5 seconds but from 39% to 30% there were approximately 6 seconds of change. By examining the area where they had been molding a setting of 35% to 37% on speed setting a 1% change did mean a second or so in fill time. Yes they should have noticed this on their own but did not, but also the use of any speed setting above a 50% setting had no effect.
An example of material issue:
The processor, a highly qualified and knowledgeable shop was testing out a new material grade in an existing production tool. The new material was easier flow, and they had performed the RV test on the new material and provided the RV for the current production grade of material. They were very different in that the newer one was well below the production grade, and uniformly along each set point, showing that yes it was easier flow. The issues were voids in the part, and after review of the curves it was suggested to mold at the same RV number (as in production) and see if the issue went away, which it did. Yes the fill time changed but in this case one is looking at the flow into the tool and being able to pack it out which when one thinks about it is based on viscosity.
The above curve is an example of the generated data for relative viscosity.
S L Silvey
Silveys Plastic Consulting
silveysplastics@hotmail.com
http://silveysplasticconsulting.com
