Although melt flow rate testing is not very challenging and the equipment is not too expensive, only a few molders carry out the test in-house. Out of those who perform the tests, a comparatively small percentage of them know why they are performing it or what they are measuring.
Melt Flow Rate Testing
A majority of the material suppliers specify and control their products to a melt flow rate (MFR) specification, as they are aware that it is associated with the average molecular weight (MW) of the polymer they are manufacturing. If the MFR is constant, the average MW is also constant.
However, several molders who test their incoming materials to confirm the certifications obtained by them think that they are testing to guarantee process consistency. This thought is due to a poor understanding of the link between the MFR value and the actual viscosity of the material at processing conditions.
The capillary rheometry curves demonstrate that the viscosity of a polymer is governed by the shear rate. It is also known that the volumetric flow rate of the material is one component of the shear rate. The geometry of the flow path is the other component and it should be a constant for any particular mold. It is obvious that the shear rate changes with the location in the flow path. For instance, it is different in the runner compared to its value at the gate.
However, for any specified location within the flow path, it is expected that the cross-sectional area of the flow path remains unchanged. Thus, the flow rate of the material as it fills the mold is the only process variable that can have an impact on the shear rate. A machine parameter known to processors as injection speed controls the flow rate. The technician chooses the injection speed on the machines used currently by determining the suitable screen and entering a setting that is typically mentioned as a linear distance per unit time such as inches/second or mm/second.
For instance, consider that the setup document for a specific mold running in a specific machine demands a first-stage injection speed of 3 inches/second. In case the stroke distance from the starting of the injection process to the point at which the machine shifts from first to second stage is 6 inches, then the fill time should be 2 seconds. Although fill-time clocks accurate to two or three decimal places are the equipment conventionally used on modern molding machines, very few processors ever verify whether this is actually the case. Processors are likely to enter the setpoint and believe that the machine is performing its assigned task.
Lack of Pressure — Key Factor
Several factors could stop the machine from responding the way it should; however, ultimately, all these factors point to one main issue, which is the lack of pressure required to attain the preferred velocity.
Processes that function without the presence of abundant first-stage injection pressure are known as pressure-limited.
Viscosity can be described as resistance to flow. It can be regarded as the product of the pressure applied to the fluid and the period for which the pressure is applied. In order to move a higher viscosity fluid to a specific distance, it is essential to use more pressure over a specific period of time or apply the same pressure over a prolonged period of time. This relationship is reflected by the SI units for viscosity, Pascal-seconds (Pa-s).
In order to make a molding machine to function in a velocity-controlled mode, it must be configured to offer more pressure than what is required to transport the desired volume of material to the mold in a definite interval. Thus, when there is an increase in the viscosity of the material as a result of a variation in molecular weight, there will be a proportional increase in pressure but the time will remain constant. If the time required to carry the material remains unchanged, then the shear rate remains constant and will restrict any change in viscosity to the intrinsic effect of the higher MW. The latter contribution is comparatively small when the material flows at velocities characteristic to the first-stage injection.
If the pressure is reduced, either by the way in which the machine is installed or by the design of the molding machine, when there is a rise in the material viscosity, the time needed to carry the material to the mold becomes longer. Simply put, the injection speed slows down all by itself. A slower injection speed, in turn, causes a lower shear rate and this amplifies the difference in the viscosity of the material, as with the MFR test.
The MFR instrument is pressure-limited. Once a constant load is added, the operator just monitors the behavior of the material. The potential of the molding machine in the plant to “notice” the divergence in the MFR of the material is due to the fact that the machine is configured to be pressure-limited similar to the MFR tester. In other words, a velocity-controlled process decreases the impact of varying viscosity on the process stability, whereas a pressure-controlled process amplifies this impact. Normal MFR changes only affect the process if the machine functions with a limit on the available pressure.
Lot variations in raw material are not the only probable cause of a variation in the viscosity. Upon mixing regrind into the pure material, differences in the MW of the regrind or fluctuations in the amount of regrind can result in changes.
Furthermore, several polymers show variations in melt viscosity because of changes in their moisture content. For instance, the viscosity of a 30% glass-filled PET polyester dried to 50 ppm may be 10%–15% more than when the same material is dried to 200 ppm. Both the materials are assumed to be dry enough for a majority of the applications; however, they may be processed in different ways, based on how the machine is set up. This effect is more marked in nylons. Therefore, there are many myths in the industry about nylon materials being “too dry.”
In short, a properly configured molding process will nullify the effect of the usual fluctuations in MFR for a material of specific grade on the process or the parts resulting from that process. This is because the conditions of the MFR test are different from those of the molding process until the pack and hold phase is attained. By that time, the mold cavity or cavities should be nearly (but not completely) full if the processes are carried out properly. A robust process, correctly set up in a competent molding machine, should deal with variations in viscosity much greater than the usual lot-to-lot variation seen in a certain grade of material.
This information has been sourced, reviewed and adapted from materials provided by Dynisco.
For more information on this source, please visit Dynisco.