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> PP 20 melt vs 35 melt *
Rick Maston
Posted: May 7 2004, 08:58 PM


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Can someone explain to me the differents between 20 melt PP and 35
melt?
I've made some parts with 20 melt and they turned out great. But if I
give the living hinge a twist she breaks.
Would I get a non tearing living hinge with 35 melt?
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Ernie
Posted: May 7 2004, 08:59 PM


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In absence of other information, I have to assume that 20 melt means a melt
flow of 20 g/10 minutes, and 35 melt means a melt flow of 35 g/10 minutes.
In other words, the 20 melt flow flows less, that is, the 20 melt is more
viscous than the 35 melt resin. Assuming similar molecular weight
distribution for the two resin, the more viscous 20 melt flow should have a
higher molecular weight than the 35 melt flow. Higher molecular weight
resin of 20 melt flow is likely to be tougher than the lower molecular
weight 35.

But, the above assumes that the two resins are identical in all respect
except for melt flow (ask your resin supplier!). Furthermore, if your
molding conditions are marginal, (not enough packing pressure, time,
temperature) the 35 melt flow may give you better results than the 20 melt
flow resin.

Is the hinge a proven design? Are there notches or other stress risers in
the part? Your resin supplier may be able to provide you with known designs
for hinges.

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Rick Maston
Posted: May 7 2004, 08:59 PM


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Thanks Ernie for the reply. The living hinge functions great.It's only
when I twist it, then it fails. If I try to pull it straight apart
it's very strong, like trying to snap twine. I think the design is
good. A sales person told me that it would be more tear resistant if I
ran 35 melt. There's no notching, she folds nicely. I even baked it at
160 degrees and the hinge didn't fail. Am I being to critical?
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Rolf Wissmann
Posted: May 7 2004, 08:59 PM


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If your hinges break already during their very first use, you may have
chosen the wrong plastic and you ought to consider the use of H-POM instead
of PP. If the hinge gets only used once during the lifetime of the part, a
high-molecular (low MFI) grade of PP may eventually work, assuming the
design of your part & hinge are correct, but if the hinge gets used
frequently PP is perhaps not the right choise of material. With any plastic
you must also assure that optimal molding conditions are used ...

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