If too little material is filled, the piston is already in the measuring range during the melting time. Particularly with higher MFR/MVR values, the values can then be significantly lower because the acceleration phase is also recorded.
If too much material is filled into the barrel, it is possible that the measuring range is only reached at the end of the maximum measuring time of 25 minutes prescribed by the standard. This can lead to material changes like cross-linking or degradation due to the long heating time.
In general, the measuring range should be reached approx. 1 min after the end of the melting time. For a good repeatability, the same amount of material should always be filled in.
Air inclusions lead to higher values, especially with method A, because the material is extruded as foam, so to speak, and therefore flows faster. Method B also leads to higher values, especially due to the effectively reduced density. GÖTTFERT Melt Flow Indexer are able to correct the error by single air bubbles. Nevertheless, it is always better not to trap any air during filling.
Which force should be used for pre-compression depends on the individual application as well as material characteristics. In most of the cases we use a much higher force than the test load, i.e. 10 kg or 21.6 kg. For MI-3 and MI-4 up to 21.6 kg can be applied as standard. The mi40 can generate a total of up to 60 kg due to the additional force of the motor.
Usually you should fill in 3-6 portions and always compress between by hand. This way it is ensured that the material below is also sufficiently compacted. Ideally, the material not yet melted is pressed into the melt 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 compress by hand, are problematic. Especially in these applications, a machine with automatic pre-compression is recommended. This greatly reduces operator influence during filling.
Automatic pre-compression by means of higher weight is optionally possible with the mi2 series. MI-3, MI-4 and mi40 are capable of this as standard. The mi40 can also alternatively or additionally use the drive for force-controlled pre-compression.
As standard, our plastometers are operated with the displacement transducer according to procedure B and collect up to 40 measuring points in the measuring range.
The machine automatically sorts out values that deviate from the average value by more than the air bubble factor. This prevents the rapid subsidence of an air bubble from falsifying the final result.
Experience has shown that good results are achieved with values between 1.5 and 2, but for significantly higher values such as 9, the function is practically deactivated.
The melting time is 5 minutes for measurements according to ISO 1133 and 7 minutes for measurements according to ASTM D1238.
To test the thermal stability of a material, measurements with longer melting times of 20 minutes, for example, can be carried out for comparison. This makes it possible to assess, whether the material changes during the residence time of a processing operation. If this is the only deviation from the standard procedure 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.