© Carl Hanser Verlag, Munchen. Der Nachdruck, auch auszugsweise, ist nicht gestattet und muss vom Verlag schriftlich genehmigt werden.

Melt Flow Index MEASUREMENT AND TESTING 1

Kunststoffe international 10/2016 www.kunststoffe-international.com

On the Pulse of the Polymer Melt

Precise and Real Time Measurement of the Melt Flow Index

State-of-the-art quality control systems for production require precise and real time measurement of control

parameters such as the MFI. Such measurements lead to optimized process control, product quality, and reduced

consumption of resources.

One parameter of quality control, found in every data sheet throughout the industry, is the melt flow index or melt flow

rate (MFI). It specifies how much polymer melt flows through a defined die configuration when loaded with a measurement weight (Fig. 1). Details are defined in the standard ISO 1133 [1]. The melt flow index thereby characterizes the complex polymer melt flow properties with a single parameter.

Conventional MFI Measurement

Standard laboratory measurements have the disadvantage of process-related inaccuracies and poor reproducibility of mea-surement results. Operator inconsistencies, poor cleaning or wear of cylinder, piston and die influence precision and repeat-ability of the MFI measurement. Among other contributing fac-tors this leads to a high coefficient of variation, which is specified according to the ISO 1133 standard [1] with a coefficient of varia-tion of about ± 5 % for one laboratory and ± 10 % varying be-tween different laboratories.

Another drawback of lab analysis is the long period of time required for collecting the samples and performing measure-ments. The delay between production and lab measurements results in significant amounts of downgraded material in the event of a fault.

Melt Flow Index in Real Time

An automatic melt flow index measurement device substantially reduces the delay time between lab and production. There are different approaches for online devices:

W Bypass- and side stream rheometers are directly flange-mount-ed to the production extruder. On the one hand the advan-tage is an immediate measurement on and in the process. On the other hand this means that the measurements are not performed on the finished product, the state in which the customer receives it. Furthermore, the polymer melt tem-perature typically deviates in the extruder and the lab mea-surement. This deviation additionally has to be recorded and

Fig. 1. Traditional MFI and MVI measurements in the lab: Details are established in the standard

ISO 1133 [1] (source: OCS)

Conventional melt flow index• Weigh out sample material

and fill into heated cylinder• Compress polymer in cylinder• Wait for preheating time• Load weight on piston • Start measurement, when piston is

at the correct height position

Measured quantities• MFI (melt flow index [g/10min]):

Weigh the amount of material which flows out of the die in a certain amount of time.

• MVI (melt volume index [cm3/10min]):Determine volume rate from piston movement.

(conversion via melt density ρ, MFI = ρ × MVI)

ISO melt flow index

Weight

Piston

Temperature sensor

Cylinder

Die

[VEHICLE ENGINEERING] [MEDICAL TECHNOLOGY] [PACKAGING] [ELECTRICAL & ELECTRONICS] [CONSTRUCTION] [CONSUMER GOODS] [LEISURE & SPORTS] [OPTICS]

© Kunststoffe

Fig. 2. Melt flow index measurement device:

The OP5 from OCS is configured for online

measurements (© OCS)

© Carl Hanser Verlag, Munchen. Der Nachdruck, auch auszugsweise, ist nicht gestattet und muss vom Verlag schriftlich genehmigt werden.

2 MEASUREMENT AND TESTING Melt Flow Index

© Carl Hanser Verlag, Munich Kunststoffe international 10/2016

Fig. 3. MVR data for steady state and a product transition between 4 and 5: The green line

re presents the high frequency online data of a OP5 rheometer, the yellow line represents the

low frequency in lab test results (source: Nova Chemicals)

Time

12

10

8

6

4

2

0

g/10 min

0M

FI1 2 3 4 5 6 7 8 9 h 10

High frequencyat-line-data

Low frequencylab-test results

© Kunststoffe

Fig. 4. Online system components: a) Film line

with camera and FTIR analysis (Fourier trans-

form infrared spectrometer), b) Offline-con-

tainer (left side), which supplies the film line

during a shutdown and container for collect-

ing lab samples, c) Pellet analyzer, for measur-

ing contaminations, color, throughput and

size and shape distribution, d) main rack and

control system for the pellet transport system

(© Nova Chemicals)

a)

c)

b)

d)

User ExperiencesNova Chemicals has been using the OP5 instrument for 30 months and – according to their account – has been very pleased with its performance, ease of use and reli-ability. The OP5 is a very reliable instru-ment with an up-time of > 99 %. The de-vice allows to increase sampling from the traditional 1 test per batch to 7-15 tests per batch. According to the company’s at-line system managers, the OP5 has required minimal maintenance. In operation the system only needs cleaning of the die and check-ing the pressure sensor offset once per month. OCS performs yearly calibrations on the pressure sensor and transducer. The test method of calibration is user- friendly and easy to maintain; one calibra-tion curve covers a broad range of prod-ucts, using the same die.

The AuthorDr. Christin Thalhofer works in the development department at OCS Optical Control Systems GmbH located in Witten, Germany; thalhofer@ocsgmbh.com

ServiceReferences & Digital Version

B You can find the list of references and a PDF file of the article at www.kunststoffe-international.com/1750872

German Version B Read the German version of the

article in our magazine Kunststoffe or at www.kunststoffe.de

© Carl Hanser Verlag, Munchen. Der Nachdruck, auch auszugsweise, ist nicht gestattet und muss vom Verlag schriftlich genehmigt werden.

Melt Flow Index MEASUREMENT AND TESTING 3

Kunststoffe international 10/2016 www.kunststoffe-international.com

taken into account for calibration. Another drawback results from the operation, maintenance, and installation at the pro-duction extruder.

W Alternatively, a transport system can automatically collect pellet or powder samples from the production and transport them to an online lab. For the online rheometer OP5 (Fig. 2) of OCS Optical Control System GmbH, Witten, Germany, the MVI measurement including transport time typically takes be-tween 5 to 10 min. Automatic measurement approaches usu-ally quantify the volume index, therefore, the measured value is called the melt volume index/rate (MVI). This allows to monitor the variability within batches and track product changes in the process (Fig. 3). For this, all measurement quan-tities are continuously sent from the online lab to the central DCS-system [2–4].

Besides of the melt flow index a number of additional test pa-rameters can be analyzed in an online lab. For instance, the Nova Chemicals Corp. (Fig. 4) additionally analyses gels, contaminants, density, additives, color and pellet shape and size distribution in their online lab at Joffre, Canada.

Melting the Polymer in an Online Lab

There are different ways to melt polymer samples for an auto-matic MVI measurement. The most common is to melt the poly-mer with a lab extruder. A different possibility is to use a ram ex-truder, as used in the OCS rheometer OP5: a piston compresses the polymer sample against the heated area of a melter. The molten polymer runs of through a circular orifice. This gentle and rapid melting process minimizes shear damage, cross link-ing, thermal and other degradation processes. Since the average molecular weight and the melt flow index are related to each other, the gentle preparation of the polymer melt (similar to the ISO standard) is an essential factor.

Besides of the melt temperature the two fundamental MVI measurement parameters are the volume or mass flow rate and the pressure acting on the polymer melt. In the standard lab de-vice the pressure is induced by a loaded measurement weight. A measurement weight of 2.16 kg corresponds to a pressure of about 3 bar. The melt flow index corresponds to the volume or mass flow rate. This is measured by the standard lab device ei-ther by weighing polymer strands leaving the die or by measur-ing the displacement of the piston. Automatic measurement de-vices typically calculate the volume rate from the rotational speed of a melt pump, which conveys the material through the measurement die or die block.

There are several contributions that add up to the total pres-sure drop in a conventional lab device: The largest part of the pressure drop typically comes from the viscous flow in the die channel. The pressure drops resulting from the melt flow above the die also contribute to the measurement result. Furthermore, there is a sudden contraction in the flow channel of the standard lab device in the entry and exit region of the die. Because of the melt elasticity this results in additional significant pressure drops. For a LDPE melt, for example, under a standard load of 2.16 kg the entrance and exit pressure drops add up to about 20 to 30 % [5]. There are systems for the automatic measurement of the melt flow index which measure the pressure drop along a capillary die of constant diameter with a number of pressure sensors. In

these systems only the viscous flow in the used die is measured. Therefore, in these configurations the elastic entrance and exit effects have to be additionally calibrated.

Calibration

Principally speaking, the more similar the flow channel is to that of the standard lab device, the smaller the required calibration. But the sudden contraction in the standard lab device is not suit-able for a continuous measurement without permanent chang-ing and cleaning of the die. A different way to design the die en-try is shown in Figure 5. Here the rheological relationships are as close as possible to the lab device (ISO 1133 [1]). At the same time, the delay of the slowly moving polymer melt close to the walls is minimized, to ensure a good purging. The deviation between uncalibrated measurement results and lab results typically are only 5 to 10 % in this setup.

The full correlation between the uncalibrated (OP5 rheome-ter) and the standard lab measurements is ensured with an easy, straightforward calibration according to the following scheme: After the installation of a new OP5 system typically three to five representative resins are selected from a product line. These samples are measured in two ways – according to the ISO stan-dard and in the online device. In a double logarithmic plot this data is fitted via a linear regression. The fit parameters (slope and offset) are then used for calculation of the calibrated melt flow index values.

The calibration file is set up only once for a new product type and product line. It is valid for the other grades of the same prod-uct type from the same product line and is valid over the com-plete dynamic range of a measurement die. This means that the calibration file is valid during a material transition. Therefore, ma-terial transitions can be monitored closely (Fig. 3). This allows fast-er transitions with less low grade or waste material.

Measurement of Volume Rate and Pressure

The loaded weight stays constant in the conventional melt flow index measurement during a measurement. This corresponds to a constant pressure drop (in first approximation). To set a con-stant pressure drop in an online MVI measurement, the speed of the melt pump, or more precisely the volume rate, has to be controlled and permanently adjusted. In contrast to this, the OP5 rheometer of OCS measures with a constant volume rate. For this, the system measures close to the pressure set point in a two point measurement: the pressure is measured at two different, constant volume rates only after a certain stabilization time. From the results of the two point measurement, the volume rate of the selected pressure (measurement weight) is obtained.

Additionally, the measurement principle and quality of the pressure sensor are important factors for the long term stability and reproducibility of the measurements: Many pressure sensors work with a transmitting fluid so that the strain gauge can be positioned far away from the hot polymer melt. But this makes the pressure sensor sensitive to changes in ambient tempera-ture. The OP5 system has a special pressure sensor without trans-mitting fluid and it measures only at a constant temperature. This allows to set up a special calibration curve for the individual sensor and measurement temperature, which is not possible for

© Carl Hanser Verlag, Munchen. Der Nachdruck, auch auszugsweise, ist nicht gestattet und muss vom Verlag schriftlich genehmigt werden.

4 MEASUREMENT AND TESTING Melt Flow Index

production rheometers. In this way precise pressure measure-ments are possible.

The described measures result in high measurement re-peatability with a base-3-sigma level of typically ± 1 % in the OP5 system.

Conclusion

The automated measurement of the melt flow index allows to significantly reduce the delay time between lab and production

and it allows to increase the reproducibility of measurement re-sults. This enables a better process control, an optimized con-sumption of resources and the production of products of a con-tinuously high product quality. W

Fig. 5. Die block of the OP5 system (source: OCS)

Inflow of polymer melt

Pressure sensor

Die block

Die

© Kunststoffe

Special Reprint fromKunststoffe international 10/2016

INJECTION MOLDING

In-Situ Polymerization for

Fully-Automated Production of

Lightweight Structures

page 102

DESIGN FOR RECYCLING

K 2016:

Closed Loop for

Plastics Packaging

page 141

international

www.kunststoffe-international.com

SPECIALThe Industry Takes Stock in the

Plastics World Market

page 7

Magazine for Plastics 10/2016

Designer BLMBREAK your LIMITS

Masthead

PublisherCarl Hanser Verlag GmbH & Co. KG, Kolbergerstraße 22 81679 München

© Licensed edition authorised by Carl Hanser Verlag, Munich. All rights reserved, including re-printing, photographic or electronic reproduction as well as translation.

www.kunststoffe-international.com