The methods are described in ASTM D1238 or ISO 1133.

The standard specifies the preheat time for PE is 5 mins at 190deg.C and you are only going to need 5 to 10 g of material.

https://en.wikipedia.org/wiki/Melt_flow_index

Dear Mohamad,

I will remind that the melt flow index (MFI) is the ability of a plastic/polymer-based material to flow in limited interval of time (g/10min) under standardized conditions of temperature, as determined using a specific melt flow tester/index equipment (please consider ASTM D1238 or ISO 1133 to have information about the temperatures, die, barrel, load, experimental method, etc.).

We will obtain some indirect information about the rheology of polymers that will be processed. Generally, polymers with mean/higher fluidity (high MFI values) are better for injection molding, while for extrusion a lower MFI will be required. MFI could give some indirect information about the molecular weights for the same polymer (e.g., higher or lower molecular weights), about the fluidity of polymer at a choice temperature, and so on.

For MFI experiments we will use only few grams of material for testing (5-10g), while the time for heating (few minutes) is usually connected to the nature of polymer and its thermal characteristics (heat capacity, thermal conductivity, melting enthalpy (if it is the case), etc.). On the other hand, to calculate the optimum heating time of 300g HDPE sample, MFI values will not be helpfully in determination of energy for the heating/processing of your material at a selected temperature.

Good luck in your study and best regards,

Marius

PS: For eventual help some selected information:

A. V. Shenoy, D. R. Saini & V. M. Nadkarni (1984) Melt Rheology of Polymer Blends from Melt Flow Index, International Journal of Polymeric Materials and Polymeric Biomaterials, 10:3, 213-235, DOI: 10.1080/00914038408080271

https://www.dynisco.com/userfiles/files/Practical_Rheology_Handbook_full_version.pdf

http://www.mat-ing.com/en/instruments/mi-tech/all-instruments/download/519/349/19

https://www.smartflow-usa.com/documents.htm

https://www.smartflow-usa.com/scientific-cooling-calculator/

Synonyms of Melt Flow Index are Melt Flow Rate and Melt Index. More commonly used are their abbreviations: MFI, MFR and MI. Confusingly, MFR may also indicate "melt flow ratio", the ratio between two melt flow rates at different gravimetric weights.