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Measuring the melt viscosity in practice - Part 2

04.2021| 8 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 we share how the measuring process goes.

 

 

Process of measurement

Before the start we need ensure the raw material is dried and the right nozzle is assembled. We check the predetermined cylinder temperature and dosing path for the production, but it is better to take a bigger dose (3-4 D). The basis of the measurement that we set the injection unit into rear position and the melt is injected into the open at different injection rate, even at several material temperatures. We register the set velocity in cm3/s and monitor the pressure required for injection moulding. On some machine types it is possible to use this exit function in automatic mode and the machine records the maximum pressure. On other machines we need to read from the control unit the current pressure value, which makes the measurement difficult. It is important to increase the pressure limit of the manual/automatic injection moulding to at least 1000 bar. If the measured pressure reaches the 70 % of the pressure limit, we increase it further so it does not effect the velocity of the screw and therefore the measurement.

 

 

Determine and record input parameters

To identify the measurement, it is recommended to record the following basic data:

 

- Injection moulding machine identification, type
- Material type, drying time
- Injection screw diameter
- Dose size

 

The variable input parameters required for the measurement is summarised in table 1. To determine the different setting possibilities, we use the recommendations of the material datasheet (cylinder temperature), and the injection rates used in practice as a starting point. The injection rate can be changed quickly, so we recommend using 3-8 setting in the range 10-80% of the machine’s maximum injection rate. It is important to know the maximum dose size to determine the maximum velocity. The following general formula can be used for this:

 

 

 

Table 1. Input data used for measurement (example)

 

Nozzle geometry

Cylinder temperature (every zone)

Injection rate (% of maximum velocity)

Length

Radius

Nozzle

Cylinder 2

Cylinder 1

10, 20, 40, 60, 70, 80 %

(taking the dose into account)

4, 8, 16 mm

1.25 mm

-10 °C, Original profile, +10 °C


 

When we determine the velocities, we need to consider the machine ability and the time required to read the data. For example, if we set a velocity which the injection moulding machine is not able to keep or there is not enough time to read the data, we should decrease the velocity. The set and real injection rate can be obtained from the first time derivative of the screw movement, which can be visually displayed on the injection moulding machines (Micrograph).

 

 

 

Figure 3. Injection moulding machine used for measurement

 

 

 

Record output parameters

 

For the measurement we dose with injection moulding machine (back pressure decreased to zero) to at least 3-4D position, then we read the measurable pressure in manual mode. The goal is to record an average value, this can be read in the stabilized phase. The reading accuracy will be decreasing with the increase of the injection rate.

 

 

 

Figure 4. Pressure required for injection moulding as a function of time

 

 

The basic measurement data in our case

 

- Person performing the measurement:     Kiss Ádám

- Date:                                                                   2020.10.28

- Machine identification:                                 CEHU1-BOYXS-10T

- Material identification, drying:                    PA66 – 70G30HSL NC010, 4 óra, 80°C

- Injection cylinder diameter:                        D14 mm

- Dose size:                                                         6 cm3

 

 

Input data used for measurement

 

The maximum dose size for our injection moulding machine is 6 cm3, so using the previous formula (4) the maximum injection rate is 12 cm3/s. This empiric value helps us to read the result more easily.

 
 

Nozzle geometry

Cylinder temperature

Injection rate

Lenght

Radius

Nozzle

Cylinder 1

Feeding

1 cm3/s

4 mm

1.25 mm

275

275

40

2 cm3/s

8 mm

1.25 mm

285

285

40

5 cm3/s

16 mm

1.25 mm

295

295

40

8 cm3/s

 

 

 

 

 

10 cm3/s


 
 

Measured data evaluation

 

In the next chapter we show the results in function of injection speed and we examine two parameters, the effect of material temperature and nozzle length. Both parameters have significant effect on the injection moulding process and it is important to know these

 

The effect of material temperatute on the viscosity

 

The results are summarized in table format in function of the injection rate. The measurement was done at 3 different nozzle geometry (length), so different tables contain the data.

Table 2: Pressure in function of injection rate and temperature, with 4 mm long nozzle

 

Vfrcs
[cm3/s]

275°C
material

[bar]

285°C
material

[bar]

295°C
material

[bar]

1

212,00

200,00

160,00

2

250,00

230,00

205,00

5

295,00

275,00

240,00

8

330,00

315,00

260,00

10

390,00

340,00

280,00

 

 

First step is to make an injection rate – injection pressure diagram from the measured data.

 

 

 

Figure 5. Pressure in function of injection rate with 4 mm long nozzle

 

Increasing the injection rate, the injection pressure will also increase, although the degree of the pressure increase is continuously decreasing. At 5 cm3/s injection rate 250 bar pressure is required, increasing the velocity, at 20 cm3/s injection rate 550 bar pressure is required for the injection moulding. So, although the injection rate is increased to four times of its original value, the required pressure value increases only to its double, this can be explained with the polymer’s pseudoplastic behaviour.
In practice the ratio of the pressure and velocity is usually illustrated as function of velocity, this shows the shearing sensitivity of the material. The more the viscosity decreases with the increase of velocity, the more the ratio of pressure and velocity decreases (Figure 6.)

 

 

Figure 6. Ratio of pressure and velocity as function of injection rate, material PA66, 4mm long nozzle

 

Using the data of measured pressure and nozzle geometry the apparent shear velocity, apparent shear stress and apparent viscosity can be determined. The flow curve shows the shear stress in function of the shear velocity. The material’s shearing sensitivity exponent can be obtained by fitting a straight line following the power of law to the results.

In practice it is recommended to fill the cavity the shortest possible time, so with the biggest injection speed possible (machine, material, product quality). The increased injection speed results in decreased viscosity. The high speed allows for the heat transfer between the mould and the injected material to decrease, therefore the cavity pressure can be reduced during the filling process. In case of moulds with thin inserts, the number of insert breakage and lifespan of the mould can be improved.

 

 

Figure 7. Apparent shear stress in function of apparent shear speed, Material PA66, 4 mm nozzle

 

The shear stress factor was n=0,24 at the material temperature of 275°C, which is in line with the value found in the literature. So at this material, increasing the injection speed significantly decreases the viscosity. If this value is near to 1, then increasing the injection speed does not decrease the viscosity. By increasing the material temperature to 295°C, the shear stress factor changed to n=0,227. It can be stated that this material becomes slightly more sensitive to shear speed at increasing the material temperature. At higher temperature the viscosity decreases more by the increasing injection speed than at lower temperature.

 

 

Figure 8. Apparent viscosity in function of shear speed, Material PA66, 4mm nozzle

 

Based on the viscosity curve there are two things to be considered. The stability of the injection moulding process and the beneficial effects of the reduced viscosity. It is recommended to set the injection speed where the viscosity reaches the minimum value, but the injection moulding machine is still able to repeat the process stably. If the injection speed is high, then at the change of viscosity the measurable pressure increase/decrease will be smaller, than at a smaller injection speed. Showing this through a concrete example, if the viscosity is examined at the apparent deformation speed of 1300 1/s (2 cm3/s) and ±100 1/s, then the possible change is in the range of ~500 Pa*s. If the injection speed is increased to double, then this change is in the range of ~200 Pa*s. So the higher speed can help to increase the stability of the process.