Definition: the state or quality of being viscous, 2. physics: a: the property of fluid that resist the force tending to cause the fluid to flow, b. the measure of the extent to which a fluid posses this property. 3. How one puts measures to the flowabilty of a fluid or material example, concrete versus water one is easy flow the other stiff or hard to flow.
In a previous article presented the topic was shear and how to calculate and or at least understand shear for what it is. Basically that as the force/volume of a fluid is increase through an orifice the result is it shears and that the fluid can possibly reach an area where degradation will occur.
When evaluating materials a lot of time people will discuss the melt flow of the material or melt index of the material ( a calculated interaction) When they discuss melt flow it has been common knowledge that a higher number is an easier flow from the processing side of things, this due to the fact of shorter molecular chains. Other things are given up for this easier flow, but when looking at it from a viscosity point the easier flow is less viscous and the harder flow is more viscous. This could be the difference between a 60 melt and 1 melt material of the same family. It is also common for manufactures to list process conditions, for example polypropylene:
Below 4 mfr is low flow and suggested melt temperature range of 475-525F /246-274C
4-10 mfr is a medium flow and suggested melt temperature range of 445-475 / 228-274C
10-20 mfr is a high flow and suggested melt temperature range of 400-445 F / 200-228C
Above 20 mfr is an extra high flow and melt temperature range of 385-420 F / 196-215 C
The above illustrates that there is more heat necessary to flow a material that is of lower melt flow than that of higher melt flow, basically the thick viscosity requires more heat than that of thin viscosity. The important thing to remember here is that polypropylene is a semi crystalline material and also that the data above is generic. Could the polypropylene listed be run at lower temperature, probably. A side note here is that given the data it could be possible to jump up the flow of material via the melt flow and lower the melt temperature and thus reduce cycle time due to lower heat content to the material if in fact the physicals of the newer material meet requirements.
Understanding the test for melt flow is important. The definition is grams of material per 10 minutes. The standard test conditions vary for each type of material thus comparisons can only be done per material type under the same conditions. There are set weights, orifice size and temperatures per test condition. As has been written the melt flow test is a great test to determine the molecular weight of a material, and to that extent basically whether it has a long chain or a short chain. It is also great at determining whether the processed material has been degraded by comparing the original material and ground up parts to each other and comparing the flowabilty of the material. I.e. did processing destroy the materials physicals. This based on chain length, basically comparing the numbers on the melt flow test, is the finished products melt flow within 15% of the original virgin pellets melt flow.
And yes as a molder or processor they will look at melt flow and say it should or shouldn't work in our mold, or if they are having issue with a low flow material will jump to a higher flow material, which in the end is a viscosity change.
Point: For Newtonian fluids, viscosity is a temperature- dependant constant, regardless of flow rate. For Non-Newtonian fluids, the viscosity varies not only with temperature but also with shear rate.
Now with injection molding the material is sheared. Depending on this shear rate there is a change in viscosity of a particular material. This issue is called non-Newtonian flow. Basically water is Newtonian in flow, while thermoplastics are Non-Newtonian flow. If plastics are injected faster the material tends to flow easier, this due to shear and alignment of the molecular chains. Basically think of the fountain flow effect, one end of the chain attaches to the wall and as it flows stretches out to become elongated thus not tangled, the other now act similar and we get orientation in flow which now cause the chains to flow easier (they slip over one another). Understand that speed of injection, gate size of our mold (or smallest orifice in molding system) are what now act as our shear point causing orientation / elongation. This will in effect lower the viscosity. Note that if we stop the flow the stress / elongation of the molecules will come to play and re-tangle and shorten up. This can be seen in poor injection control from fill to pack when there is a hesitation in packing and the result is a short shot. Basically the switchover time is too great and when finally applying pressure to material it has lost its orientation thus does not flow as easily or at all in some cases.
Now if using a Campus data base for material it will provide the viscosity curves of the material, or sometimes the material supplier will provide. These are using lab instruments and results are in a log to log scale which produces a straight line on a graph. The data is great information on flowabilty, but to someone on the floor it is hard to interpret.
One way of doing things is to create a relative viscosity curve using the injection molding machine and a mold. It is stated as relative as it is for that mold and that mold only.
What is done is that the machine and mold are used to generate the numbers / data so as to do the math and than plot the curve. What this shows is that the faster we push the material, (short fill time) we have a lower viscosity number and the slower we push the material (long fill time) we have a higher viscosity number.
F x T = RV
F= force in plastic pressure
T= time to travel a set distance
1/T = shear rate
Now the calculations are such that we multiply the force (F) times the time (T) and plot this over the reciprocal of the time (Shear Rate) on an XY chart. There are certain precautions that need to be taken and standards that should be met in order to perform the test. One issue in performing the test is pushing only a set volume of material into the mold each time, thus this becomes a constant for that mold. Basically the shot size is never altered once it has been established.
The results show visibly what is happening to the viscosity of the material when plotted out on the graph. It shows that pushing the material faster the material reduces viscosity and pushing it slower the viscosity increases. If in fact the test is preformed at 2 different temperatures it would show what actually happens, and where the temperature of material can play an effect in viscosity of the material. If the machine is using different speeds of injections, profiling, it shows what happens at those speed settings in a relative manner.
In the end it is the shear rate, temperature of material and mold which have an effect on the flowabilty of the material, given that the equipment is robust enough to handle what is asked of it. This flowabilty is referred to as viscosity yet most will still go by melt flow or melt index which in effect is a result of low shear grams per ten seconds and compare to what is going on in the molding machine which is shooting at time grams per seconds.
To be continued
