In the previous two tech tips, we began a three part discussion on how our industry looks at the viscosity of a polymer. We reviewed capillary rheometers and melt flow index machines up to this point. We pointed out various benefits and flaws for each method. Now we are going to take a look at a newer approach to evaluating a polymer’s viscosity called Therma-flo™.
“Can I mold that part with this material in this machine?” It’s perhaps the oldest question in injection molding, but yet we have very little information available to us to help answer that simple question…especially without the aid of mold filling simulation (and even that is suspect at times when it is being performed by uninformed analysts). Currently most people rely on melt flow index values to give us some indication of how easily, or not, the plastic is supposed to flow inside of an injection mold. It is generally expected that a high-MFI melt should be easier to mold than one with a low MFI. But how often have you selected a material based on this type of data, only to find that it had little or no relationship to how the material actually processed?
In response to a growing need for plastic melt characterization for injection molding, Beaumont Technologies has developed its patent-pending Therma-flo™. This method characterizes the injection moldability of a plastic material through a broad range of mold geometries and process conditions using a high precision injection molding machine and a high speed data acquisition system which records in-cavity data in real time. This method has proven to significantly improve on the ability of a designer to make informed decisions when selecting or designing a part or mold for a given plastic material. It also helps the injection molder to determine whether a particular machine will be capable of filling the mold within the limits of their molding machines.
Therma-flo™ captures both the rheological and thermodynamic characteristics of a melt as experienced during injection molding. Additionally, it evaluates the impact that flow rates and geometries have under these non-isothermal conditions. The mold itself includes cooling lines to allow for the normal thermal exchange between melt and mold and the formation of a frozen layer. Both of these factors are significant aspects of injection moldability and are entirely missed by capillary rheometers and melt flow index machines. Additionally, Therma-flo™ uses realistic injection flow rates, melt temperatures and flow-channel thicknesses based on those found during normal injection molding.
Figure 1 shows Therma-flo™ results for a HDPE with a 6.9 MFI. Each of the Therma-flo™ curves shows the injection moldability of the melt though each of five different cavity wall thicknesses (from 0.030 to 0.100 in.). On the “Y” axis is an output of pressure per inch (psi/in.) of flow length. On the “X” axis you have in-cavity flow fron velocity (in./sec). This is very different from typical MFI or capillary rheometer data. There are no intangible units such as poise, Pascal seconds, reciprocal seconds, or g/10 min, that are normally reported by more traditional melt flow characterization methods. Therma-flo™ data is much more tangible and relevant to what a designer or processor would want to see. Additionally note that the 6.9 MFI value does not ever change, regardless of the geometry or process conditions. However, the Therma-flo™ easily demonstrates how those conditions drastically affect how the plastic flows inside of the mold.
There are a number of ways Therma-flo™ data can be used. Of particular interest is the ability to evaluate the relative injection moldability of two similar materials. Figure 2 shows Therma-flo™ data for two grades of PP, each with a melt flow index of 20. Each of the four windows in Figure 2 show the pressure per inch vs. velocity data at different thicknesses, for the same two materials. The MFI value would lead you to believe that each grade will flow the same inside of your molds. The Therma-flo™ data indicates otherwise and highlights the impact that process conditions and wall thickness have on the ability of each polymer to flow inside of an injection mold.
To put this into perspective, consider that material supplier B comes to you with a 20 melt PP that they say will perform just as well as the 20 melt PP you get from supplier A, but supplier B will sell it to you for $0.10/lb cheaper. So you decide to use the PP from supplier B only to find out that you are now pressure limited and can no longer make the part.
The influence of injection speed, part geometry, mold and melt temperatures and the thermal exchange between the polymer and the mold cannot be taken lightly. Plastic flow is extremely complex and should not be grouped under a single value to encompass how it will flow under all conditions, as is the case if you use melt flow index as an indication of viscosity.