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Troubleshooting Bad Melt Flow Index Results

Troubleshooting Bad Melt Flow Index Results. Presented By Harry Yohn. Melt Flow Rate (MFR).

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Troubleshooting Bad Melt Flow Index Results

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  1. Troubleshooting Bad Melt Flow Index Results Presented By Harry Yohn

  2. Melt Flow Rate (MFR) • Melt Flow Rate (MFR) also known as Melt Flow Index (MFI), Melt Index (MI). The MFR test is the most common test performed to determine the melt flow properties of virgin & compounded resins at a particular shear stress (related to the applied load) and temperature • WHY? • Resin Suppliers QC Check - Look for variation of MFR due to differences in polymerization and/or compounding • Material Selection - Marketing & Sales - can be used to differentiate between different grades of materials • Resin Buyers Inspection of Incoming Resins - Can indicate degradation caused by improper transport, storage or preparation (drying)

  3. ASTM D1238 versus ISO 1133 • Basically measure the same property but slight procedural and equipment differences can lead to differences in results – Changes coming in late 2011/early 2012 • Both offer a manual method – Procedure A - also known as Method A • Both offer a automatic timed flow measurement – Procedure B – also known as Method B

  4. Procedure A or Procedure B • No general consensus which method is best • Procedure A – • Best for organizations that test infrequently, use a wide range of materials, use differing additives in their materials, or use regrind/recycled material • Procedure B – • requires a “melt density” value • More automated test means less chance for operator error • Best for organizations that test same material repeatedly

  5. TROUBLESHOOTING • Understanding how MFR is affected • Correct MFR results can normally be assured by following proper testing procedures, by periodic machine verification and by frequent testing of control materials • However, “bad” MFR results attributable to the equipment and/or procedures may still be encountered

  6. Topics of Discussion • What are the specific machine components and factors contributing to “LOW” and “HIGH” Melt Flow Rate results? • How can these machine components and factors be checked?

  7. Reference • The following is based on ASTM D 1238-10 “Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer” • Note: Refer to ISO 1133 for appropriate dimensions when testing in accordance with this International Standard.

  8. Factors Affecting the Accuracy of a Test • Is the machine clean? • Is the machine in good working order? • Has the machine been calibrated/verified by a certified metrologist? • Temperature • Physical dimension • Distance and time measurements

  9. Factors Affecting the Accuracy of a Test • Machine Cleanliness • Machine must be cleaned after each test • Clean cylinder, piston foot and die with cotton patches and/or brass brush • Make sure piston guide slides freely. Clean with brass/bronze brush, crocus cloth and cotton cloth.

  10. Factors Affecting the Accuracy of a Test • Check Level • Level on piston rod

  11. Equipment Factors Affecting the Accuracy of a Test • Orifice (Die) • Is bore clean?

  12. Equipment Factors Affecting the Accuracy of a Test • Orifice (Die) • Is bore clean? • Is entrance chipped or rounded ?

  13. Equipment Factors Affecting the Accuracy of a Test • Orifice (Die) • Is bore clean? • Is entrance chipped or rounded ? • Is I.D. 2.0904 to 2.1006 mm (0.0823 to 0.0827”) • Verify with Go/No-Go Gage • Replace as necessary

  14. Equipment Factors Affecting the Accuracy of a Test • Barrel Condition • Barrel must be cleaned after each test • Die should fall to bottom of barrel with an audible “click” • Barrel should have a mirror finish, free from scratches, rust or other imperfections. • Is I.D. 9.5428 to 9.5580 mm (0.3757 to 0.3763”) • Is barrel calibrated/verified with a traceable bore gage?

  15. Equipment Factors Affecting the Accuracy of a Test • Check Piston Rod • Is piston rod straight?

  16. Equipment Factors Affecting the Accuracy of a Test • Piston Foot • Is foot diameter 9.4676 to 9.4818 mm (0.3727 to 0.3733”)? Use micrometer • Is land smooth? • Any damage to edge of foot??

  17. Equipment Factors Affecting the Accuracy of a Test • Temperature • Currently requires verification at 2 places in the bore • 10 mm above the die is required to be within +/- 0.2oC of the set point • 75 mm above the die is required to be within +/- 1% of the set point. • This requirement appeared in ASTM D1238 in the mid 1990’s, so older equipment may not comply • Is machine calibrated/verified to traceable source and properly set according to manufacturer’s instructions?

  18. Equipment Factors Affecting the Accuracy of a Test • Actuating Switch for Procedure B • Check accuracy with Actuating Switch Calibrator. • Check that test measurement starts between 48 and 44 mm (1.89 to 1.73”) above the orifice. • Run Procedure B test and compare result to simultaneous manual cut-off and weigh result

  19. Equipment Factors Affecting the Accuracy of a Test • Power Fluctuations • In some areas, unstable electrical supply may cause machine performance issues

  20. Procedural Factors Affecting the Accuracy of a Test • Sample Mass • Variations in sample mass may affect MFR • Use scale or balance to weigh sample • Scale or balance should be calibrated/verified

  21. Procedural Factors Affecting the Accuracy of a Test • Preheat time • Preheat is required to ensure material is sufficiently and evenly melted. • Is temperature stable within ±0.2 ºC after preheat? • ASTM specifies 420 +/- 30 seconds preheat time for most materials

  22. Procedural Factors Affecting the Accuracy of a Test • Moisture • Some materials, such as ABS, PMMA, PET and Nylon, are hydrophilic and absorb moisture • These materials needs to be dried in a suitable oven prior to testing – some material require drying under a N2 purge

  23. Procedural Factors Affecting the Accuracy of a Test • Sample Packing & Purge Time • Purging expels trapped air • Commonly used to move the lower scribe line of the piston closer to start point • Purging must be completed at least 2 minutes prior to the start of the test for MFR < 10 g/10 min • Best to avoid packing sample when charging bore if multiple operators or use mechanical means to ensure consistency • Follow suggested charge weights in Table 3 • Trial and error to determine sample charge weight to ensure tests start at the same time from test to test

  24. Procedural Factors Affecting the Accuracy of a Test • Test Load • Are comparative tests being made using the same test temperature and weight?

  25. Procedural Factors Affecting the Accuracy of a Test • Extrudate Cutting Technique • Procedure A tests • Precision • Auto cutters vs. hand cutting

  26. Procedural Factors Affecting the Accuracy of a Test • Piston Travel Distance • Procedure B tests • 6.35 mm (¼ inch) • 25.4 mm (1 inch)

  27. Procedural Factors Affecting the Accuracy of a Test • Melt Density Value • Procedure B tests • ASTM D1238 Table 4 lists melt density factors for generic PP & PE • More accurate to measure the actual melt density of the specific resin being tested • Combine a Procedure A and Procedure B in a single test run give both weight and volume data allowing the calculation of melt density.

  28. TROUBLESHOOTING • Standard Reference Material (SRM) • Limited selection available from national metrological institute i.e. USA’s National Institute Standards and Technology • Expensive • Not readily available everywhere • Only gives an indication of verification for that particular material • Using material to test the machine or using the machine to test the material?

  29. Good Testing Practices • Measurement Uncertainty • Possible to get measurement uncertainty for each individual verification step • ISO 17025 certified calibration vendors are required to determine • There is no recognized way to translate the uncertainties given on our certificate to the uncertainty of a melt index test

  30. Good Testing Practices • Machine Calibration • It is important to have melt flow indexers completely calibrated/verified on a regular basis • Yearly intervals are usually recommended • However, periodic inspections are necessary for consumable items such as dies, piston rods and piston feet • Replace as necessary

  31. Good Testing Practices • Inter-Laboratory Control Testing • Maintain a control sample • Periodically test samples using all operators and machines • Maintain SPC control charts • Deviation from standard an indication of a problem

  32. Good Testing Practices • Sources of deviation in these results will include: • Thermal effects • Environmental effects • Vibration, air currents, humidity • Dimensional effects • Piston feet, dies, cylinder, mass of test load, scale, etc. • Intermittent procedural errors • Material effects

  33. Good Testing Practices • Proficiency Testing Programs (PP, PE) • Voluntary participation in program • Gives you an indication of where you stand compared with other participants verifies your testing practices • ASTM runs a proficiency program • Collaborative Testing Service is a PTP vendor

  34. Thank You

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