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Written by Administrator
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Thursday, 05 February 2009 05:57 |
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Among the first instruments plastics molders, extruders, and compounders are likely to consider when outfitting a new lab is a melt-flow indexer or "melt indexer." Known as an extrusion plastometer in more technical jargon, this apparatus has long been used to determine the melt-flow rate or mass-flow rate (MFR) of virgin and compounded thermoplastic resins. With the increased pressure on plastics processors to comply with ISO 9000 standards for quality management, more of them than ever are using these instruments to evaluate incoming material and test finished products. The melt-flow test detailed in the ASTM D1238 and ISO 1133 standards quickly measures one point on the viscosity curve under standard conditions. While the relative simplicity of this test has remained the same, melt indexers have evolved considerably since they appeared in the 1950s. Computerization and automation have reduced operator-to-operator variables, yielding results with greater accuracy and repeatability. Why measure melt index? Which method to use? More & better features |
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Last Updated on Thursday, 05 February 2009 05:58 |
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Written by Administrator
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Thursday, 05 February 2009 05:57 |
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The melt indexer consists of a heated barrel and piston assembly to contain a sample of resin. A specified load (weight) is applied to the piston, and the melted polymer is extruded through a capillary die of specific dimensions. The mass of resin, in grams, that is extruded in 10 minutes equals the MFR, expressed in units of g/10 min. (This value is also commonly called the melt index, MI, or melt-flow index, MFI.) Some instruments can also calculate the shear rate, shear stress, and viscosity in centipoise. |
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Last Updated on Thursday, 05 February 2009 05:59 |
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Written by Administrator
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Thursday, 05 February 2009 05:56 |
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ASTM 1238 and the ISO 1133 test methods are "technically equivalent" but there are some minor differences. Both ASTM 1238 and ISO 1133 offer two variations of the standard melt-flow test: Method A and Method B. Method A describes a basic manual melt indexer. It involves making "cuts" of the extrudate at timed intervals as it exits the extrusion die. The "cuts" are weighed on an analytical balance to determine the average mass and this value is extrapolated to the mass that would be extruded in 10 minutes—the MFR. Weights range from 1.2 to 21.6 kg to permit measuring materials ranging from very low to very high viscosity (more viscous melts require more weight to force them through the die). The load and temperature to be used are determined by the material and the ASTM test method. |
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Last Updated on Thursday, 05 February 2009 05:59 |
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Written by Administrator
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Thursday, 05 February 2009 05:56 |
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Price differences reflect mainly options. Temperature controls may include an automatic preheat phase as either standard or optional. Digital encoders are available to measure piston travel for Method B with much higher precision than the older optical technology using photocells. The latter can take 12 data points in 1 in. of piston travel, but a digital encoder can take up to 40 data points per inch. Motorized lifting and lowering the weight onto the piston is another newer option. |
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Last Updated on Thursday, 05 February 2009 05:59 |
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