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MFI for Different Materials…

I received this question last week…

ATR
I am trying to find out why the injection pressure of PP is lower than the injection pressure of PC in terms of its MFI. But the MFI for PC is at a different temperature and for PP is at a different temperature.

My Response
Essentially, Melt Flow Index (MFI) is not an objective test. MFI uses different specifications for testing different materials. The pressures and shear rates are different than those encountered in production, and the temperatures are  typically not the same either.
The benefit to this test is that it is easy for anyone to conduct, and it provides a single number result.
Additional Thoughts
The most effective use of the MFI test, I have encountered, is to compare different batches of the same material grade for material scheduling.
Objective testing such as capillary rheometry or spiral flow are much more involved, but can provide much more comprehensive results. 
I discuss this in more detail in these posts:
-Andy

Processing Rule #1…

There are many rules to good processing, but this one is paramount…

Manager
What is the most important thing to look for when evaluating a technicians ability to process?
My Response
The most important factor in good processing is ‘Rule #1 – Make One Process Change at a Time’. When a change is made, it’s imperative to wait for the results to take place before making another process adjustment. This rule ensures you know the specific cause and effect of any change made to the injection molding process.
For example, if you increase both the injection speed and packing pressure at the same time, you may eliminate a short shot, but while doing so – you’ve created flash, part sticking, burning, and cracking. Because you changed two parameters at the same time, you don’t know which change corrected the short shot and which parameter resulted in causing each of the new defects.
Anyone changing more than one parameter at a time is not approaching the process systematically.
-Andy

Two Plateaus During Gate Seal Determination…

Recently a technicians asked me this question…

Tech
During a gate seal determination (part weight vs. second stage time), we saw the part weight almost stabilize and then it shot up until it finally leveled out. What might have caused this?
My Response
When doing a gate seal determination, you might see this two-plateau result. Essentially, this is causes by material shrinkage in the gate area allowing more material to flow through the gate before the gate finally freezes.
The material can either flow through the center of the gate as the material shrinks around itself, or the material can flow around the gate as it shrinks away from the mold surface.This is most common when you are molding parts with thick gates or materials with especially high melt flow rates.
Additional Thoughts
Since the first plateau is caused by stagnation of flow before a second flow of material into the mold cavity, it is always good to verify that such a process does not result in delamination, brittleness, or low mechanical strength.
-Andy

High Back Pressure…

This question arrived as a follow-up to a previous post: More About Back Pressure…

Saylan
If I have to work with an odd back pressure like 250 bar, does it create any damage to the equipment or machine?

My Response
Assuming this is the plastic pressure, this would not necessarily cause machine damage. This will consume a large amount of energy and create a large amount of shear during recovery. The potential for material and additive degradation may be high depending on the material being processed. 
You may wish to consider a new screw designed specifically to address the complication you are encountering during recovery. For instance you may need a longer transition zone if the shear is necessary for melting, or you may need a mixing element in the metering zone if the back pressure is used for additive mixing.
Additional Thoughts
This may be  great application for a shot-pot style of molding machine. This would allow a properly designed material-specific screw to be used to melt and convey the material while a plunger is used to inject the melted polymer.
-Andy

Maximum Pressure Setting During Injection…

This question was received earlier this week…

CD
What should be the minimum pressure during switch over time of injection?

My Response
If you are switching over through the use of screw position, then you should ensure the maximum pressure setting is significantly higher than injection pressure at transfer.
Additional Thoughts
It is critical that you have enough pressure available to maintain the desired injection speed. 
More about this can be found here: 
-Andy

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