Frequently asked questions

If too little test material is filled in, the test stamp is already in the measuring section during the melting time. Particularly with higher MFR/MVR values, this can lead to significantly lower results, as the acceleration phase is also recorded.

If too much test material is filled in, the measuring section may not be reached until the end of the maximum measuring time of 25 minutes prescribed by the standard. This can lead to material changes (cross-linking or degradation) in the material due to the long dwell time.

In general, the measuring section should be reached approx. 1 min after the end of the melting time. To increase the repeatability of melt index measurements, the same amount of material should always be added.

Air inclusions lead to higher values, particularly with method A, as the material is extruded as a “foam”, so to speak, and therefore flows faster. Method B also results in higher values, particularly due to the effectively reduced density. Our melt index testers are capable of correcting the falsification caused by individual air bubbles. Nevertheless, it is always better not to include any air during filling.

As standard, our plastometers are operated with the displacement transducer according to method B and collect up to 40 measuring points in the measuring section. The machine automatically sorts out values that deviate from the mean value by more than the air bubble factor. This prevents the rapid drop in an air bubble from falsifying the final result.

Experience has shown that factors between 1.5 and 2 provide good test results. With significantly higher factors, such as 9, the function is practically deactivated.

The force that should be used for pre-compaction depends on the material and internal specifications. In most cases, we use a significantly higher force than the test load used, i.e. usually 10 kg or 21.6 kg. With the MI-3 and MI-4, up to 21.6 kg can be applied as standard. The mi40 melt index tester can generate a total force of up to 60 kg thanks to an integrated drive.

Normally, it should be filled in 3-6 portions. The material should be compacted by hand between the portions. This is the only way to ensure that the material further down is also sufficiently compacted. Ideally, the material that has not yet melted is pressed into the molten material below.

Materials with a high filling level or a high melting point, such as PET, PC or PA, which melt late and are therefore difficult to compact by hand, are problematic. In these applications in particular, a machine with automatic compaction is recommended. This greatly reduces operator influence during compaction.

Automatic compaction using a higher weight is an option with the melt index testers in the mi2 series. MI-3, MI-4 and mi40 are capable of this as standard. The mi40 melt index tester can also use the drive for force-controlled pre-compaction.

The melting time for measurements according to ISO 1133 is - 5 minutes. For measurements according to ASTM D1238 - 7 minutes.

In order to test the thermal stability of a material, measurements with longer melting times of 20 minutes, for example, can also be carried out for comparison. This makes it possible to assess whether the material changes during the dwell time of a processing procedure. If this deviation from the standard-compliant procedure is described on the test certificate, the test still complies with the standard.

Cleaning the test device is extremely important for accurate and repeatable measurement results! All our plastometers are delivered with manual cleaning accessories consisting of a tool for cleaning with a cloth, a round brush, a hand brush and a die-reamer. After the measurement, the remaining material is pressed out. Then the piston is removed and cleaned with brush or cloth. It is recommended to clean the test barrel with a cloth using the tool while the capillary is still inserted. Then the capillary is removed and cleaned with reamer and brush. The test barrel is then cleaned with several cloths and, if necessary, the round brush. In the end the barrel surface must be smooth like a mirror. The capillary should fall freely with a clear "click" onto the capillary holder.

We recommend the optional battery-operated cleaning device with cloth-tool and brush attachments for greater ease of use, especially for materials that are difficult to clean. Even stubborn contaminations can be removed quickly and easily. All cleaning tools can be obtained directly from us (product description "Options for melt index measurement").

CAUTION: With the corrosion-resistant but soft barrel material 2, only soft hand cleaning tools approved by us may be used to prevent damage to the surfaces.

We recommend cleaning the test barrel, the piston and the capillary immediately after each measurement without exception. Material residue that has not been removed and is exposed to higher temperatures for a longer period of time often degrades and has significantly different properties. Material residue can encrust, hinder filling or lead to additional friction. This can falsify the measurement results. Experience shows that the total cleaning effort also increases exponentially, especially when thermally decomposed material has to be removed.

First of all, it has to be checked whether the test barrel is really clean. A clean barrel has a completely smooth surface like a mirror.

It is also possible that corrosive (e.g. fluorine containing) material has been measured in the test barrel and that the barrel has been damaged.

At the capillary there are sometimes stubborn encrustations which are difficult to remove. We recommend that the capillary is cleaned of material residues by pyrolysis at a maximum temperature of 550°C, especially when changing materials.

The software basically allows the selection of up to eight different weights per barrel filling. To generate sensible data, each weight must rest on the test material for a certain time or for a certain distance. In order to measure with eight weights, some measuring points must be taken outside the standard measuring range. The mi40 can take measurements along the entire barrel length.

In the ISO standard measuring range of 50-20 mm in front of the capillary, typically three or four measuring points can be taken.

This is often caused by different additives such as lubricants or flow promoters, which can have an influence on the flow behaviour even in the smallest quantities. These are deposited on the barrel, the piston and especially on the capillary.

It is therefore possible, that the melt indexer is still contaminated with lubricants from previous measurements, which are only flushed out by the first measurement of the measurement series.

Otherwise it is also possible that the material with which the test series is run contains lubricants. These might only be sufficiently effective after they are spread on every surface by the first test.

The standard deviation is calculated from all test points. The selected final result is calculated as the mean value of the measured values within the standard deviation. The values outside the standard deviation are not included in the calculation of the final result. The selection algorithm can also be executed multiple times.

The FRR is calculated by forming a quotient of the MFR/MVR measured with different weights. The FRR can be determined with the mi40 with one filling of the barrel using a multi-weight-measurement.

The FRR is a measure of the shear thinning behaviour and thus of the molecular weight distribution.

miConnect can be retrofitted on all mi40, mi2.x from 2014 onwards and MI-3 and MI-4 from February 2014 onwards with an update of the plastometer's firmware. For older machines the retrofit is sometimes possible by hardware modification. Please contact us!

The capillary plug prevents the material from flowing out during the melting time and during filling. For powders, the plug can be helpful above an MVR of approx. 50 ccm/10 min. As standard, we recommend the capillary plug above an MVR of approx. 100 ccm/10 min.

ISO 1133-1: Determination of melt flow rate (MFR) and melt volume rate (MVR) of thermoplastics, part 1: general procedure

ISO 1133-2: Determination of melt flow rate (MFR) and melt volume rate (MVR) of thermoplastics, part 2: procedure for materials which have a sensitive time or temperature history and / or moisture

Procedure A: Strand cutting at a constant time intervals and determination of mass; result: MFR (Melt Flow Rate) in g/10 min

Procedure B: Determination of volume at constant time intervals; result: MVR (Melt Volume Rate) in cm³/10 min

Procedure C: Usage of a half die with dimensions L/D=4/1,05;
for low viscosity materials with MFR >75g/10min or MVR > cm³/10min.

Procedure D: Multi load measurement; automatic weight selection allows tests with different test weights with one barrel filling, as descripted in ASTM D1238 procedure D as Flow Rate Ratio (FRR)

Normally ISO 1133-1 or ASTM D1238 is used, as these are usually not sensitive to moisture. However, it is important to note that in any case a corrosion resistant steel grade is selected for the test barrel and piston. Standard steels can already be damaged by corrosion during a single measurement due to the acids released.

In procedure A, the strand is cut off every few seconds and the melt flow rate (MFR) is determined directly. The operator must remain with the machine during the measurement to collect the strand sections and to evaluate the result. Especially during cutting, errors can occur if the sections do not separate cleanly from the knife.

In procedure B, the melt volume rate (MVR) is calculated from the piston speed. In this procedure, the data-acquisition runs fully automatically and is therefore more convenient and less prone to errors. The determined MVR can be converted into the MFR using the melt density.

Due to the higher ease of operation and the lower error rate, we generally recommend the use of procedure B. All our melt index testers are equipped as standard with the necessary sensor for measuring the piston speed.

The direct result of a measurement with procedure B is the melt volume rate MVR. According to the standards both values MFR and MVR are equivalent. Some materials are also specified with an MVR value. However, if a MFR-value is absolutely required, the melt density is necessary for conversion.

It is important to use a melt density at test temperature and not the solid density, which is often found on material data sheets!

For unfilled and unmixed polymers, the melt density can typically be taken from the literature (e.g: PE @ 190°C ρ = 0.7636 g/ccm and PP @ 230°C ρ = 0.7386 g/ccm according to ASTM D1238). If your GÖTTFERT Melt Flow Indexer is equipped with a cutting device, the melt density can also be determined during the test according to procedure B using a balance.

Due to the higher automation and fewer sources of error, procedure B is generally preferred and more accurate. With method A, material loss during cutting or incorrect weighing is a common mistake.

Yes, such a capillary or die can be ordered from us together with the necessary piston. The half-die with L4D1.05 mm may be used according to ISO 1133 from a MFR > 75 g/10 min or MVR > 75 ccm/10 min (measured with normal capillary!). According to ISO 1133, the half die can be used with both procedure A and procedure B.

In ASTM D1238, the half-die measurement is described as Procedure C, and is presented as an alternative for materials with an MFR > 75 g/10 min. Procedure C is similar to Procedure B (path length measurement).

The result is determined in the same way as with the standard capillary and the numerical value is typically smaller by a factor of approximately 8. The result is always reported and marked with a small sub-script "h", e. g.: MFRh (190°C/2.16 kg) = 20 g/10 min

In principle, we cannot make any binding statement here, as the necessary test cycles depend on the regulations of the individual customer.

If not otherwise specified, we recommend maintenance every one or two years, depending on the testing volume. In addition, the basic functionality should be tested much more frequently (depending on the frequency of testing, weekly - 3 monthly) with measurements using reference material. We offer a stable and durable reference material with test certificate.

We also offer maintenance contracts (also according to ISO 17025), which are prioritized when appointments for maintenance are made.

During maintenance, the entire testing device is checked for conformity with the melt index standards. This includes: Temperature distribution in the test barrel, time measurement, displacement measurement, alignment, capillary geometry, piston geometry, barrel geometry, weights.

As a last step, standard measurements are performed using our reference material.

We specify the HDPE reference material M80064 at the test conditions of 190°C usual for PE with a weight of 2.16kg. However, the material is very temperature stable and can still be measured with good repeatability up to approx. 250°C.

Basically, PP is not a moisture-sensitive material. However, under certain circumstances it is possible that moisture is present on the surface of the granulate. In these cases, bubbles can be seen in the strand due to the evaporation of the water. This can lead to higher measured values, as the density decreases.

Especially due to the higher stiffness of the glass fibre reinforced material, air inclusions can also occur due to too weak pre-compression, which then lead to higher measured values. If that is the case a higher, automatically applied pre-compression load might help.

The geometries of the piston and the barrel have very fine tolerances and the gap dimensions at the piston tip have a great influence on the measurement result. Changes in surface roughness can also lead to falsified measurement results. In any case, we recommend to repair or replace an apparently corroded test piston or inspection barrel.

The piston speed is determined by stopping the time the piston needs for a certain distance. For this length, called resolution by us, three pre-settings are stored on the machine. The normal resolution of 0.7 mm/test point was chosen to record 40 measuring points in the ISO 1133 measuring range with a length of 30 mm.

The number of test points can also be reduced, for example to shorten the measuring time or to limit the measuring range. This can make sense for thermally unstable or especially fibre-filled materials, where the measured value changes significantly over the barrel length, due to fibre orientation and fibre deposition. By limiting the measuring distance, the reproducibility can be increased in some cases.

Such special weights are basically possible. The integration of the special weight into a weight magazine such as the mi40 may also be possible, but must be checked by our design department in each individual case. Please contact us.

For particularly low viscosity polymers we generally use method B. Since these are mostly pure, unfilled polymers, the melt density is known from the literature. The test barrel is filled completely to approx. 120 mm above the capillary. A capillary plug is necessary to prevent material loss during filling and during the melting time. We have already performed MFR-measurements with values of up to 4000 g/10 min on our mi40.

Typically, a capillary plug is used to prevent the material from trickling out through the capillary during filling and pre-compression. The capillary plug is closed before filling. The material is usually filled and compacted in portions. The capillary plug is opened after the melting time.

In cases with extremely sticky and fine powder, an inlet can also be used. This is a test channel insert which is filled in cold condition on a press and is only inserted into the test barrel at the beginning of the melting time. This option is available for the mi40. Please contact us!

The maximum service life is generally not limited, but depends strongly on the materials tested. The standard test barrel is suitable for polyolefins or other unproblematic polymers and typically shows little wear even after years of heavy use. If material with a high fibre content is regularly measured, a particularly abrasion-resistant test barrel material should be selected to significantly increase the service life. Especially for fluoropolymers, a special corrosion resistant barrel has to be used. A standard barrel is destroyed by the acids released after only a few measurements. The test barrel of our plastometers MI-3, MI-4 and mi40 can easily be exchanged in a few minutes.

The test barrel should always be smooth like a mirror after cleaning. A matt finish indicates damage to the surface. The test barrel and piston are measured by our service staff during maintenance and compared with the tolerances permitted by the standards.