Explaining Melt Flow Index

I receive this question very often, and feel it would be great for the blog…

Can you briefly explain melt flow index, and how processors use it?
My Response
Melt flow indexing is the most popular, and yet least accurate way to determine material viscosity. The melt flow index (MFI) is the measure of how many grams of polymer pass through a standardized capillary under a standard load over 10 minutes. The value obtained through the melt flow index test is a single data point. The melt flow index only tests the material at one shear stress, and temperature. 
In general, a higher melt flow index indicates a lower material viscosity. This means that a material with a melt flow index of 20 flows easier than a material with a melt flow index of 5. Melt flow index information from different materials and material grades may be used for a rough comparison of flow characteristics for different materials.
Many processors use this data to qualify incoming materials and to help anticipate changes in the process. For example, if the lot of material you are processing has a MFI of 10, and a new lot has an MFI of 15… you can anticipate issues such as flash, over packing, or overweight product and make the appropriate adjustments.
Additional Thoughts
To obtain more accurate and relevant viscosity data… it is better to perform rheology tests using a capillary rheometer or a parallel plate rheometer. Many companies will also perform in-mold rheology tests using actual production molds.
  1. MJ left a comment on 2009/12/01 at 1:59 pm

    Hello Andy.
    Does any system exist that adjusts the process according to incoming measurements of material from lot to lot?

  2. Andy Routsis left a comment on 2009/12/01 at 3:54 pm

    If the process is established properly, the need for adjustments would be minimized. The important thing is to know when a change is about to occur so that you can look for changes and anticipate any changes.
    A process log is also a great tool since a processing history can be made and cross referenced.

  3. Dave left a comment on 2014/06/20 at 2:49 pm

    I am new to this MFI and was wondering, it says 0.67 so is that less than 1 or is it some conversion of maybe 6.7? the material that I am running is very watery like and I cant get it to run my pipe, once it enters the vacuum tank it keeps blowing open like a ballon.

  4. Andy Routsis left a comment on 2014/06/22 at 9:20 pm

    Hello Dave,
    It is very common in extrusion to use materials with an MFI below 1. Unfortunately, MFI does not give any useful data about the material except how they flow through an MFI machine. To determine if there is a problem with your material, you will have to compare the strength of this material with a material which makes good product. This can be done by your local testing lab.

  5. tysonrobert left a comment on 2016/11/18 at 2:35 pm

    I had to switch from a 20 melt material to a 12 melt material due to the manufacture being out of 20 melt. I am having an issue with 12 melt either my parts are short shot or have flash. Any suggestions on some changes that can be made? I did not have these issues with the 20 melt. I have changed pressures, dose size everything and no change. I appreciate any info I can get being the company I work for is kind of a “learn on your own” type place.

  6. Andy Routsis left a comment on 2016/11/19 at 10:57 am

    Whenever you are encountering shorts and flash on the same part, it indicates you are filling the mold too much during first stage injection. You should short shot the mold 5-10% and then use packing to complete mold filling and packing. You will have to use a higher packing pressure than before, but the process will be more consistent. The good news is that you will likely be able to reduce the clamp force to get better venting for your short shot.
    I recommnend you view some of our earlier posts on filling with a short shot.

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