What is MFI? - Mold Flow Index
Melt flow rate versus melt flow index
These are interchangeable terms, right?
They basically mean the same thing.
Am I right?
Am I right?
Yes, there is some truth in this statement.
The terms are used in an interchangeable manner when discussing material flow.
But. . .
They’re not the same.
Let’s look at the two acronyms and their respective units.
What’s up with the strange units?
MFR: melt flow rate. Units: g/10 mins
MFI: melt flow index. Units: %
The extreme difference in units gives you a clue that MFR and MFI are not the same.
Bet you are more curious about the differences now.
Aren’t you?
Here’s the plan.
We will answer the following questions for both MFR and MFI.
At the end of this exercise you will have a clear and nuanced understanding.
Why does MFR/MFI mean?
Where do the measurement units come from?
Where and how is MFR/MFI tested?
What equipment is used for testing?
Are there any special testing procedures?
What is Melt Flow Index?
It’s a flow test used to assess material degradation due to molding stress.
In every molding process, the material gets heated, melted, rotated, squeezed and pushed.
All of this abuse stresses the material causing some breakdown.
The breakdown is referred to as “material degradation”.
When the material gets stressed and degraded, several material performance factors suffer.
Two of the main breakdowns are . . .
Mechanical
Splitting
Degraded material will have lower mechanical properties.
It may perform badly in drop, functional and environmental testing.
Material ingredients
Sometimes, the stress on the material forces it to separate into its component parts.
Typical ingredients in plastic resin include……
Raw resin
Pigments (color)
Plasticizers
Flame retardant
Lubricants
Pigment separation can cause all sorts of funky cosmetic defects.
Simply put . . .
It’s good to keep material degradation to a minimum.
Again.
Every molding process stresses and degrades the material.
The key is to manage part design, tool design, process and material selection to make a functional, beautiful part.
Strange % units explained
MFI is used to measure and monitor this degradation.
It measures the % change in flow before-molding versus after-molding. Resin pellets versus molded part.
The test shows the increase/decrease in flow rate.
Hence . . . %.
Some materials flow more when degraded. Others, less.
Where and how is MFI testing done?
It’s done at the molding vendor facility.
Flow is tested using a rheometer (same as MFR).
Let’s take an imaginary trip down to molding shop and walk through the testing process from start to finish.
Sounds good?
Right?
Let’s go.
Rheometer Device
Let’s go for a walk to the molding floor
To recap . . .
MFI is a comparison of material before-molding
versus after-molding.
That means we need to get test samples of before and after.
Ok. Let’s mentally walk to the molding machine.
Before-Molding
Samples taken from the neck of the hopper (see pic)
Three samples collected
Samples stored in sealed containers
Material Feed on Machine
Collecting the second stage samples
After-Molding
Samples taken from the molded part(s)
Plastic sample is cut from the part
Sample is chopped into pellet sized bits
Three samples produced
Samples stored in sealed containers
Before and after
Both sets of samples are taken to the on-site test lab.
Samples are tested using the rheometer.
Here’s the condensed sequence
Test the flow of the three “BEFORE” samples
Calculate the average value.
Test the flow of the three “AFTER” samples
Calculate the average value.
Check the difference between BEFORE and AFTER average values.
The general target is to be under 30%.
If the difference is 30% or higher then this triggers a debug process to lower the MFI.
Lowering MFI is a whole other story.
I will develop this point is a future article.
Bullet point takeaway
MFI
Testing done at molding site
Samples taken before and after molding
Three samples of each collected
Samples stored in sealed containers
Tested using rheometer
Average values taken
Average values compared
Units: %
Now. You have a nuanced understanding of MFI.
Well done.
Congratulations.