FREQUENTLY ASKED QUESTIONS FOR MELT FLOW INDEX TESTER (MFI)
It heats and compresses thermo-softening resin, then measures resin extrusion rate through an orifice, under carefully controlled conditions;
- Precise and uniform control of resin temperature
- Accurately known applied weight
- Extrusion through a calibrated extrusion die orifice
- User-defined, totally reproducible automated methods
Rate of extrusion (typically g extruded / 10 minutes)
- Known as Mass Flow Rate (MFR)/ Melt Index (MI)/ Melt Flow Index (MFI)
Melt viscosity / flow resistance
• Measured at carefully controlled temperature and weight load
• High melt viscosity = High flow resistance
• Melt viscosity is inversely related to extrusion rate
• High extrusion rate = Low melt viscosity
Polymers consist of very long molecules, made by linking together hundreds, or thousands of small monomer molecules.
The basic structure of this long polymer molecule is known as the polymer chain.
Short, unbranched polymer chains slide past each other very easily leading to low melt viscosity.
More complex branched chains and greater chain lengths both make it more difficult for the chains to slide over each other, leading to higher melt viscosity.
For a particular polymer type, it is an indicator of mean molecular weight, (also known as relative formula mass)
High mass flow rate (MFR) → low flow resistance → low molecular weight
More broadly, it gives an indication of the expected physical properties of the finished component or product that is made from the resin.
The performance of a finished product or component derives from key characteristics of the resin. Testing resin is the first link in the Quality Assurance chain. So........
• Resin suppliers must test their resin melt index
• Their customers will test the resin as a material acceptance specification.
Resin MFR is a key indicator of the end product quality.
It’s also a fast and inexpensive way to compare vendor resins, so resin users can reduce the risk of failures at very little cost to their operation.
It can, but sometimes it doesn’t
Melt Flow Rate (MFR) values can change because of;
• Handling and transport
• Storage conditions
• Improper drying
• Resin regrind
MFR testing can pinpoint;
• Poor incoming materials
• Inconsistencies between batches
• Improper storage at your own facility
• Inappropriate drying
• Any negative effects of regrinding material
• Other problems with your processing conditions
An injection-molded polycarbonate component was found to crack when dropped. The virgin resin had an MFR of 28.5 g/10 min. Good resilient polycarbonate components were ground up and their MFR was measured, at 27 g/10 min, well within tolerance (+/- 30%).
A problematic, more brittle part had an MFR of 90 g/10 min. It was discovered that a defective batch contained a large amount of regrind, which had been thermally degraded in processing.
• ASTM D1238 ( Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer)
• ISO 1133 (Determination of the Melt Mass-Flow Rate (MFR) and Melt Volume-Flow Rate (MVR) of Thermoplastics.)
• ...... and various other international standards
These standards require the measurement of mass flow rate (MFR) under standard conditions. Their manual and automated methods (A and B) are rather similar.
Standard and motorized configurations of the Melt Flow Tester (MP1200) both comply.
A number of measures of resin performance can be made
• MFR (Melt Flow Rate), also known as the melt index
• MVR (Melt Volume Rate)
• FRR (Flow Rate Ratio)
• Apparent shear stress
• Shear rate
• Intrinsic viscosity
• Melt density
The MP1200 can be upgraded with optional features, including ISO 1133 tools and an encoder-based programmable piston displacement transducer (or PPDT for short), for testing according to Procedures B & C (ASTM D1238) or for volume measurement tests (melt volume rate or MVR) and melt density calculations
The MWLD makes life easier and enables weight to be added at a user-defined interval, when the resin is up to temperature.
Less user intervention and greater method reproducibility
• Flow Rate Ratio (FRR) attachment
• (requires optional automatic timing switch)
ASTM D1238 Procedure D requires that applied weight is varied during the test (up or down). This changes the flow rate at the extrusion and allows a flow rate ratio to be measured.
The weight change is triggered by the automatic timing switch.
With Horizon, the user can;
Store - an unlimited number of test method settings
Recall - any previous test results at any time
Generate - test reports and SPC control charts
Control - multiple melt indexers from one PC
Analyze - data from more than one machine
Read - data from other physical testing equipment
The most common procedures are denoted Procedures A and B within ISO and ASTM specifications. They are similar in each standard. Procedures C and D are a little more complex but this machine can perform all of them.
And there are others too.....
For more information on the procedures, please watch the below videos.
Nexus Analytics is Tinius Olsen (a leading specialist manufacturer of materials testing machines) Sole Distributor in Singapore & Malaysia.
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