Inj mold - Melt flow effects on cycle time

[brayfield]

Is there a rule of thumb for the effect of melt flow rate on cycle time in injection molding? For instance - assume I have a 20 melt flow PP polymer making a part in 25 seconds. If I change to a similar property 30 melt flow can I assume a certain amount of cycle time decrease?

Or are there two many other variables like packing and what not to make any generalizations?

 

[Demon3]

No relationship at all. MFI/MVR is measured at low shear rates, injection molding is done at high shear rates. This means that (because polymers are non Newtonian) there is no relationship. In fact MFI and MVR are only used more or less because they are easy to measure. If you want to correlate with injection molding you will have to do measurements on a real rheometer.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

Higher MFI generally means lower molecular weight.

This might, but not necessarily mean you can fill the mould at a lower temperature and therefore reduce cycle time.

There are many variables that effect cycle time, but cooling time is normally the most significant, but for example, with long flow path thin PP parts, screw back time might exceed cooling time and therefore less cooling time is pointless.

Regards

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[Demon3]

I have seen two grades of PP with totally different MFI and the same cycle time This surprised me at first until I realised how complicated it all is. As Pat said often it's the limitations of other parts of the cycle that dominate.

For a PP part what could help a lot more than MFI would be:

1. Adding a nucleating agent (Miliken had good ones)

2. Adding filler, such as CaCO3 to increase thermal conductivity and thus shortening cooling times


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

We use the Miliken nucleating agent and it certainly gives some very good results including reduced cycle time, however this only works if you also decrease the melt temperature as the Miliken product also improves flow.

Their web site has some good info.

The product is Hyperform and this is a link

http://millikenchemical.com/chemica...~~~~~~~~Home?OpenDocument&SubSiteID=Hyperform

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[brayfield]

Thank you very much gentlemen!
If anyone has other advice, it would be welcomed!

Bill

 

[Demon3]

Any good supplier, for PP I would recommend Borealis, will make mold flow calculations for you and this may help you to calculate any cycle time differences.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell