There is no clear-cut reason to prove which method is ideal for a particular organization; however there are certain guidelines that can be used to make a choice.
Procedure A is highly useful for organizations that test infrequently, utilise a wide range of materials, use a range of additives in their materials, or use regrind/recycled material.
Procedure B needs a “melt density” value and is ideal for organizations that test the same material several times over and want to reduce the chance for operator error. Some organizations may find that the alternative Melt Volume Flow Rate (MVR) procedure, offered by both standards more useful.
Factors that Affect Test Accuracy：
Irrespective of the method chosen, two organizations may test the same material and obtain two different test results, wondering why the difference arose.
The precision and accuracy of a test are affected by a range of factors that include the following:
The melt indexer needs to be in good working order
The machine must have been calibrated by a certified metrologist, checking temperature, physical dimensions, and distance and time measurement accuracy
The machine cleanliness needs to be maintained
The technicians must be properly trained and use the same testing technique
Testing procedures and requirements need to be closely followed by periodic testing of standard reference material (SRM) or control materials.
Hence it is important to examine the actual industry test standards. A trouble shooting guide is offered by ASTM D 1238 in the appendix of the document and certain salient features are highlighted below.
Equipment Issues that Affect Test Results：
In order to ensure good test results, it is important that the testing machine and auxiliary equipment such as scales and micrometers are verified using equipment traceable to national metrological standards.
Certain measures that need to be taken are listed below:
The verification procedure will check the machine dimensions, temperature control and distance measuring devices installed on the machine, for conformance to the relevant test standard.
The frequency of the verification is determined by an organization’s quality program. Annual calibration is a common practice within the industry but regular inspection of the critical components of the test instrument must be done, especially for consumable items that include dies, piston rods, and piston feet. These should be replaced when required.
Before conducting any tests, a visual inspection of all components of the testing machine shoud be carried out. The furnace of the instrument, which contains a heated metal cylinder with a defined bore, is leveled using equipment typically supplied by the equipment manufacturer.
The equipment must be located in an area free from vibration and excessive air currents. Dimensional checks are to be made when the machine is cold.
The test machine components must be cleaned after every test. No residue from prior tests must remain on the surfaces of the metal parts involved in the test. These surfaces are cleaned with cotton patches and cloth and/or a brass brush. Solvents are usually not needed or recommended.
The barrel finish must be mirror-like and free of rust, scratches, and imperfections. The barrel is cleaned by repeated swabbing with a cotton cleaning patch using tools normally provided by the equipment manufacturer. Some material is a little more difficult to clean and may require the use of a brass brush to ensure a clean surface. After cleaning the barrel, a clean die must fall to the bottom of the barrel and make an audible “click.”
The die outer surface is cleaned with a cotton cloth and the die bore is cleaned using a drill and/or a brush. The bore diameter, defined in D1238 as 2.0904 to 2.1006 mm (0.0823 to 0.0827”), must be periodically checked with a go/no-go gage. The die must be visually inspected to make sure that the entrance of the bore is not rounded or chipped. If the die fails the go/ no-go check or is damaged, discard it and replace it with a new, conforming die.
The piston rod is normally cleaned with a cotton cloth but use of a brass brush may be needed for certain materials. The piston guide (if present) must slide freely on the piston rod. It is essential to periodically verify if the piston is straight and that the leading edge of the piston foot is sharp and free of burrs or damage that would cause it to rub against the barrel wall. The foot diameter when measured with a micrometer — must be 9.4676 to 9.4818 mm (0.3727 to 0.3733”).
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