Allowable strain of polypropylene (PP)?

[cthompson68]

I have a questions about the material property of PP . I found this article regarding cantilever snap fits. My FEA software is not suited for plastics with non-linear stress-strain curves. I have a similar design manual from BASF Plastics and was looking for additional information about polypropylene.

• Calculations for Cantilever Beam Snap Fits:

http://engr.bd.psu.edu/pkoch/plasticdesign/snap_calc.htm

I have a question about the “allowable strain” for polypropylene (PP). The article provides a formula for converting elongation at yield into allowable strain based on a distinct yield pt or break pt depending on the material used. From MatWeb, PP has an average elongation at yield = 10% to 13% and elongation at break = 148% to 279%. What is the maximum allowable strain for PP? I suspect it should be higher than ABS or PEI (Ultem) as it is used in living-hinges.

 

[CoryPad]

How many times will the part be strained to this level? Snap fits usually are strained once, so you can use a high strain value. A living hinge may be cycled 100's or 1000's of times, so the allowable strain would be less.

 

[Demon3]

For a stiff part (not the living hinge) a rule of thumb is that you should not put more than 20-25% of the yield strength of a polymer on the part long term as it will lead to creep.

If you are asking about fatigue for many, many repeated use of a snap fit then you'll have to get actual data from a manufacturer of PP. Fatigue information cannot be guessed from a stress-strain curve.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[patprimmer]

The living hinge properties of PP is very dependant on the linear molecules being aligned across the hinge axis. The hinge must be thin and must be cycled through a full range bend cycle while the material is still hot from moulding and not fully crystallised. This property dos not apply if he crystal structure has not been oriented.

Regards
Pat
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[cthompson68]

The question was in regards to a removable cantilever snap fit (~ few hundred cycles). According to a document titled "The nonlinear viscoelastic behavior of polypropylene", the yield strain of PP is estimated as 13%.

MatWeb lists the average elongation at yield (PP) = 9.65% (

http://www.matweb.com/search/DataSheet.aspx?MatGUID=08fb0f47ef7e454fbf7092517b2264b2).

From the previous posted link (

http://engr.bd.psu.edu/pkoch/plasticdesign/snap_calc.htm:

The permissible strain for a snap fit design varies depending on the material properties. For materials with a distinct yield point, 0.7 times the elongation at yield can be used. For materials without a distinct yield point (usually fiber reinforced plastics), 0.5 times the elongation at break can be used. Some typical allowable strain values are listed in the table below.

Distinct yield point: 0.5ebreak = Design Strain
No distinct yield point: 0.7eyield = Design Strain

Material "Unreinforced Allowable Strain"

PEI 9.80%
PC 4% - 9.2%
Acetal 1.50%
Nylon 6 8%
PBT 8.80%
PC/PET 5.80%
ABS 6% - 7%
PET

Assuming the yield strain of PP = 13%, than the permissible strain of PP = .7 x (13%) = 9.1%. From the above chart, this is less than PEI (Ultem) and more than ABS, which is what I would expect.

I have not yet selected a contract manufacturer or material supplier. As the material would need to be FDA approved, what material supplier's would you recommend? Some suppliers I can think of are: Huntsman, and Bassell.

 

[Demon3]

I would recommend Borealis because I have had nothing but great experiences with them over the years.

You list the yield strain on PP as 13%. That value will vary quite a bit depeninding on what grade you are talking about. For example there are PP homopolymers and copolymers. In the copolymer area there are block and random copolymers. Get expert help to pick the right type.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[patprimmer]

As well as degree of modification by copolymer grades also vary greatly by molecular weight and nucleating agents. A fractional MFI blow moulding grade will have substantially different properties to a 60 MFI fast cycle injection moulding grade. You need data from the raw material suppler for the particular grade you intend to use

Regards
Pat
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for site rules

 

[cthompson68]

Chris,

Can you provide a contact at Borealis? I have not yet received a reply to my request from them, nor have I received a reply from LyondellBasell. SABIC (GE Plastics) has let go of some of their staff recently. I do not know if these other companies have been effected by lay-offs as well.

Another engineer suggested Eastman, so I will try contacting them as well for information.

 

[Pud]

ct68: Where are you?

H

http://www.tynevalleyplastics.co.uk

 

[Demon3]

I can't post e-mail addresses here. If you are in Europe I would suggest you call Borealis Austria and ask to speak to my friend Werner Posch.

http://www.borealisgroup.com/about/locations/europe/austria

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[Demon3]

He is based in Linz.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill