In our last tech tip, we started a three phase discussion about viscosity. The first phase talked about how capillary rheometers are used to characterize the viscosity of plastics at a wide range of shear rates and temperatures. However, due to a variety of reasons most people in the injection molding industry prefer to use a simpler device when talking about the viscosity of a plastic – the Melt Flow Index machine. One should be aware that even though MFI is the dominate method used to characterize how plastic will flow in a mold, it does not actually provide a measure of viscosity.
Melt Flow Index (MFI) is a measure of how many grams of a polymer flow through the die in ten minutes. The test is performed at a given temperature depending on the plastic. The force used to push the plastic through the system is supplied by a weight which sits on top of a ram. Gravity then takes over and forces the plastic through a die. The test sample is then collected and weighed. The results are then extracted to determine how many grams of plastic came out over a 10 minute period. Therefore the units we use for the test results are “grams/10 minutes”. The test method is described in more detail under ASTM D1238. A typical MFI machine and collected sample are shown in Figure 1.
Figure 1
Due primarily to its low cost, ease of use, and easy to understand terminology (grams/10 minutes), this method has been adopted by our industry as a preferred way to think about viscosity. The terminology of grams/10 minutes has even been shortened to “melt” when coupling it with a number. As an example, when someone is working with a Polypropylene (PP) that has a melt flow index value of 20 g/10 minutes, they will often refer to that particular grade of plastic as a “20 melt PP”. MFI values are meant to be inversely proportional to the viscosity of the plastic at the given conditions of the test. The higher the MFI value, the lower the viscosity is supposed to be. But, the ironic part is that the MFI test was never intended to be used as a measure of viscosity, so it can sometimes be very misleading.
For a given plastic, the MFI test is performed at one temperature, under very low and uncontrolled shear rate conditions during a steady state isothermal condition. For a PBT having a MFI of 10, the shear rate experienced during the MFI test is approximately 20 1/sec. Contrast this to the shear rates experienced by a melt during actual injection molding as calculated using injection molding simulation (see Figure 2). The viscosity of plastics will change as the temperature and shear rate conditions change, and also as the frozen layer of plastic continues to develop throughout the filling and packing stages of the process. Therefore the MFI test actually has little value when trying to relate it to the actual viscosity seen during a typical molding process.
Figure 2: Shear rates are shown at various locations within an eight cavity cold runner mold.
Additionally, for the 10 melt PBT referenced in Figure 2, the volumetric flow rate through the MFI machine was 0.017 cm3/sec. For the mold shown in Figure 2, that would equal a fill time of 15 minutes. This is approximately 1,800 times longer than the actual fill time of 0.5 seconds.
Now, consider you have a new project and your customer is looking for advice on which nylon to use. The part has a long flow length and is relatively thin walled. You and your customer have concerns about being able to fill the part. One grade of nylon is a 7 melt, and the other is a 24 melt. Based on the MFI values, most people would choose the 24 melt nylon. However, in this case under higher injection molding shear rate conditions, there would be very little value to choosing the 24 melt nylon. In fact, the 7 melt nylon may actually flow easier than the 24 melt nylon (Figure 3).
It is still difficult to say for sure which material will actually flow easier because even with the capillary rheometer data we are still not seeing the effect of heat transfer between the melt and the mold. Additionally, this data does not provide us any indicator as to the resistance to flow that will result from changes in wall thickness or geometrical shapes since those variables are not taken into consideration with either the MFI or capillary rheometer tests. Both of these methods characterize a material using a single diameter die, essentially run as an isothermal extrusion process.
Figure 3
The obvious question then should be…”is there a better way?” More specifically, is there a better way to characterize the viscosity of plastics for injection molding that considers the effects of different velocities, shear rates, temperatures, and geometries while also taking heat transfer into account. To answer that question, we will take a look at ongoing research and newer techniques available in our next tech tip.
