To achieve a repeatable process, check the press.
In order to understand the interaction between melt-flow rate (MFR) and process consistency, it is important that we appreciate the meaning of the MFR measurement and what it is designed to do.
The MFR test is governed by specific procedures covered under ASTM D 1238 or ISO 1133 and involves heating a material to a specified temperature in a cylinder of a particular length and diameter. Once the material has been heated to the desired temperature, a piston and the appropriate weight are placed on the column of molten material. This load causes the material to extrude through an orifice of a standard size positioned at the bottom of the cylinder. The amount of material extruded through this orifice in a particular time is normalized to a flow rate expressed in g/10 min or dg/min.
This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe, our analyst and author, is an independent materials and processing consultant based in Sedona, AZ. Mike has provided analytical services to material suppliers, molders, and end users for 20-plus years. You can reach him at [email protected].
In this test, the pressure on the material is a constant and the rate of flow is the result. Since the pressure cannot change, any change in the viscosity of the material will result in a change in the MFR. But the change in MFR is not a true representation of the change in viscosity. An accurate measurement of viscosity requires a controlled shear rate, which in turn means a control over the flow rate of the material. The MFR test controls the shear stress, but it does not control the shear rate. Consequently, the MFR test exaggerates the real difference in viscosity between two materials.
In order to quantify this exaggeration, we can look at plots of viscosity vs. shear rate for materials of the same polymer type with different MFRs. The graph below shows these plots for two polypropylene materials with MFRs of 4 g/10 min and 22 g/10 min. The plot covers a range from 1-10,000 sec-1 and across this range is an evident decline in the viscosity of both materials. The actual viscosity values for both materials are noted at the lowest and highest shear rates on the graph.
The MFR values suggest to many processors that the real viscosity of the 4-melt resin is 5.5 times higher than that of the 22-melt material. However, there is no point on the graph where such a difference in real viscosity exists. The largest difference in viscosity between the two materials occurs at the lowest shear rate, where the values are 3460 and 1005 Pa-sec for the 4-melt and 22-melt resins, respectively. This is a ratio of 3.44:1.
Note that as the shear rate increases, the difference in viscosity between the two materials decreases. By the time the shear rate has reached 10,000 sec-1, the ratio between the two viscosities is less than 1.4:1. If we extrapolate these plots to higher shear rates, we will see that the real differences in viscosity become smaller as we inject faster.
What MFR really tells us
If the viscosity vs. shear rate plots do not reflect viscosity differences that agree with the MFR values, how do we reconcile the disparity between the two measurements? The answer to this lies in two factors: 1) the lack of control that the MFR test imposes on the shear rate and 2) the relatively low shear rates that are involved in the MFR test.
We can calculate the shear rates associated with an MFR test simply by knowing the volumetric flow rate of the material and the size of the orifice in the test instrument. Shear rate is governed by these two factors as expressed in this equation:
S = 4Q/pr3 where Q is the volumetric flow rate and r is the radius of the flow path.
In the MFR test, the volumetric flow rate actually defines the MFR value. Simply knowing the MFR, therefore, allows us to calculate the volumetric flow rate. If a polypropylene material has an MFR of 4 g/10 min, this will be associated with the extrusion of 0.111 in3 in about 210 seconds. Given an orifice diameter of 0.0825 inch, this results in a calculated shear rate of approximately 9 sec-1. To calculate the shear rate of the 22-melt material, we can simply multiply by 5.5, the factor that distinguishes the flow rate of the two materials from each other. This produces a shear rate of approximately 49 sec-1.
On the graph, we can see where these two shear rates, indicated by the arrows, fall on the viscosity/shear-rate curves. It is clear that the MFR test does not make a direct comparison of melt viscosity at equivalent shear rates. In fact, the nature of the test method ensures that the measurements are made at different shear rates.
The actual viscosities associated with the respective shear rates at which the MFR tests are conducted are 1905 Pa-sec for the 4-melt material and 354 Pa-sec for the 22-melt material. This is a ratio of 5.38:1 – very close to the ratio of 5.5 suggested by the differences in the MFR. But this is not a reflection of a real difference in viscosity. Instead, the MFR test exaggerates the real difference in melt viscosity because it is not designed to maintain a constant shear rate.
This shows that the conditions under which MFR tests are performed are very different from the conditions at which first-stage filling in injection molding is conducted. As an example, the shear rate on a PP resin that fills a part weighing 500g in 2 seconds through a round gate with a diameter of 0.080 inch is more than 230,000 sec-1 at the gate as opposed to the range of 5-200 sec-1 at which most MFR tests are performed. The MFR test is not designed to be a measure of how the material will flow under actual processing conditions. So why do so many material manufacturers list MFR as a key property on their data sheets?
The MFR specification is used because this measurement is a good relative indicator of the average molecular weight of the polymer. Molecular weight drives performance. So when a material supplier sets a nominal MFR of, for example, 9 g/10 min and provides a window around that nominal value of 7.5-10.5 g/10 min, they are not doing so because of concerns regarding processability. They are using this number as an indicator of whether or not they have control over the molecular weight of the material.
The ability of the MFR test to exaggerate real differences in melt viscosity makes it very sensitive to changes in molecular weight. As the polypropylene materials in the graph show, these differences in MFR should be minimized at normal processing conditions. A machine set up to run with an abundance of first-stage pressure should be able to self-adjust in response to changes in the MFR of the raw material by simply regulating the amount of pressure called for to achieve consistent mold filling. And if, at a shear rate of 10,000 sec-1, the difference in viscosity between a 4-melt and a 22-melt resin is less than 40%, imagine how small the differences will be from lot to lot for a given grade of material.
But let’s suppose that, either because of machine limitations or the manner in which the press is set up, the process cannot run with an abundance of first-stage pressure. If the process is pressure limited, then the molding machine operates like an MFR tester. It becomes overly sensitive to changes in the viscosity of the raw material. And since the pressure the machine can generate is capped, the only possible response to an increase in melt viscosity is to slow down the injection speed.
As the flow rate declines, the real viscosity increases, which amplifies the real difference in melt viscosity in the same manner that the MFR test does. The molder then makes process changes to compensate for the difference in the viscosity of the material. Since the machine is pressure limited, it cannot inject at a higher speed. Therefore, the processor will rely on the other process variable that influences melt viscosity – the melt temperature. If the process is already operating at the upper end of the melt temperature range for the material, then the processor is backed into a corner and the material supplier gets the phone call about inconsistency of the raw material.
Before you pick up the phone, examine your process to ensure that it is not pressure limited. First, check the actual injection pressure at transfer against the first-stage injection pressure setpoint. If they are the same, the process is pressure limited or the transfer position is too far forward and the cavity is being filled completely during first-stage injection.
To check this latter possibility, disable the pack-and-hold portion of the injection process and examine what is referred to as a “fill only” part. If the part is not slightly short or at least noticeably sunken, move your transfer position so that less of the shot fills the cavity on first stage. The part should be 95-99% full by volume at transfer, not 100%. Once a proper transfer position has been established, recheck the actual injection pressure at transfer against the setpoint. If the actual injection pressure still equals the setpoint, raise the setpoint until you achieve a differential between setpoint and actual.
As you do this, you will likely notice that the injection speed increases. This is due to the fact that, as more pressure is made available, the machine becomes more capable of achieving the fill rate setpoint. If the injection speed changes, it may be necessary to revisit the transfer position setpoint since a faster fill speed will result in more material entering the cavity during first stage. This is especially true of a hydraulic machine, where the actual transfer point is determined to an extent by the inertia of the screw moving forward rapidly and then trying to slow down without the benefit of a positive braking mechanism.
Art to science
If, after raising the first-stage injection pressure setpoint to the maximum pressure the machine can generate, you find that the actual pressure continues to equal the setpoint, then the press is not capable and the best solution may be to find a press with a higher available injection pressure. Ideally, you will be able to achieve a pressure setpoint that is 200-300 psi of hydraulic pressure above the actual pressure required to perform first-stage filling. This provides the process with the necessary pressure differential to overcome the inertia of the system. This is the pressure you would need to move the screw if the injection unit were empty.
Once the process has achieved a sustainable pressure differential between setpoint and actual, the machine can operate on velocity control. This ensures that the shear rate remains the same and as the viscosity of the material changes from lot to lot or within the lot, the first-stage pressure will automatically adjust to compensate.
If this objective cannot be attained and because of scheduling constraints or considerations of press size a different machine is not an option, the last resort is a study to determine the source of the pressure bottleneck in the system. This exercise, called a pressure drop study, is designed to identify areas in the flow path where an excessive amount of the available machine pressure is being lost.
This pressure loss can occur anywhere between the nozzle and the end of the cavity. If it occurs in the flow path leading up to the cavity, it may be possible to enlarge the sprue, runners, or gates to reduce the pressure loss. In some cases these studies have shown that the maximum machine pressure is attained before the material even fills the runner! Pressure loss varies inversely as the fourth power of the diameter of a flow path with a round cross section, so small increases in the size of the flow path can go a long way.
If, after these efforts, you still find yourself with a pressure-limited process, then you will continue to struggle with process consistency because your molding machine will operate like an MFR tester; it will accentuate the real differences in viscosity from lot to lot and the physics of polymer flow will condemn you and your staff to a career of process tweaking.
Enjoy it while it lasts, because the day is fast approaching when injection molding as an art is replaced by injection molding as a science. Our competitive environment demands that this happen. Process control as babysitting is not a sustainable activity in a high-wage nation like the United States. But at least once you understand the fundamentals of the problem, you can reduce your company’s phone bill with the calls that you won’t have to make to your material suppliers.