Measuring the melt viscosity in practice - Part 3
05.2021| 4 min
By Szabolcs Horváth
Purpose of measurement:
To acquaint machine setters and technologist with a method so they are able to determine the viscosity of the material in function of temperature and velocity using injection moulding machine.This to fing the right injection speed during filling phase of the cavity.
Introduction
In the first part of article about viscosity measurement we described the theoretical background of the measurement (see here), in the second part (read here) we shared how the measuring process goes. In last part we evaluate the results of the measurement.
Measured data evaluation
The effect of changing the nozzle geometry on the viscosity
At the previously set injection rates we performed the measurement with different sized nozzles. We examined the effect of nozzle geometry at a constant temperature of 285°C.
Table 3. Pressure required for injection in function of injection rate and nozzle geometry
V_{frcs} |
4 mm |
8 mm |
16 mm |
1 |
200,00 |
212,00 |
210,00 |
2 |
230,00 |
250,00 |
260,00 |
5 |
275,00 |
285,00 |
312,00 |
8 |
315,00 |
335,00 |
370,00 |
10 |
340,00 |
380,00 |
425,00 |
The pressure required for injection is illustrated in function of injection rate (Figure 9). As expected, increasing the nozzle length increased the pressure required for injection. The pressure increase consists of two parts, one is the increase due the geometry (longer hole) and the other is the effect of the temperature. The end part of the nozzle is usually unheated. In several cases there is stability problem during the production if the changed nozzle in the machine is longer than originally and the production is continued with the same settings and same sized heating capacity. The significant increase of the nozzle length entails the need for the correction of the heating size and capacity. During this measurement the temperature distribution along the nozzle length was validated thermocouple, so only the effect of the geometry can be seen.
Figure 9: Pressure required for injection in function of injection rate, PA66 material, material temperature of 285°C
The increase of the nozzle length resulted in the decrease of the apparent shear stress. The molecules are settled in the direction of the flow and due the orientation their resistance to the flow decreases, so the apparent shear stress will also be decreasing. Important to note, that the apparent shear stress is related to the given nozzle geometry, so naturally the pressure requirement for the injection will increase more the longer the nozzle we use, however the material suffers from lower shear stress.
Figure 10: Apparent shear stress in function of apparent shear speed, PA66 material, material temperature of 285°C
Increasing the nozzle length results in decrease of the viscosity at material temperature of 285°C. Worth noting the difference between the individual nozzle as a function of the injection rate. At low inkection rate (650 1/s – 1cm^{3}/s) with a 4 mm diameter nozzle the measured viscosity is 4800 Pa*s, while with the 16 mm nozzle this value is only 1200 Pa*s. The reason for this is that the molecules are more organised in the longer runner and the resistance to the flow will be smaller. Increasing the injection rate, the viscosity becomes less dependent on the nozzle geometry. The differences will become smaller between the individual nozzles. At 6500 1/s (10 cm^{3}/s) apparent deformation speed the difference between the measured valued is under 600 Pa*s.
The increased injection rate helps to minimize the differences caused by the different nozzle geometry and to reduce their effect on the process.
Figure 11. Apparent viscosity in function of apparent shear speed, PA66 material, material temperature of 285°C
Summary
The viscosity measurement can be easily and quickly performed on a traditional injection moulding machine in a couple of steps. The measurement allows us to compare the different raw materials and the properties of the same materials which were made under different production series.
One of the most variable factors during the injection moulding is the physical, mechanical, and structural properties of the used polymer. With the viscosity measurement method this can be tested and supported by data, therefore shortening the troubleshooting of the injection moulding process. The temperature change or the use of inadequate nozzle has significant effect on the entire production process. Based on the measurements, with increasing the speed the differences caused by the temperature and nozzle geometry can be greatly reduced.