Allowable strain of polypropylene (PP)?
Allowable strain of polypropylene (PP)?
(OP)
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.p su.edu/pko ch/plastic design/sna p_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.
• Calculations for Cantilever Beam Snap Fits: http:/
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.





RE: Allowable strain of polypropylene (PP)?
RE: Allowable strain of polypropylene (PP)?
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
RE: Allowable strain of polypropylene (PP)?
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Allowable strain of polypropylene (PP)?
MatWeb lists the average elongation at yield (PP) = 9.65% (http
From the previous posted link (http:/
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.
RE: Allowable strain of polypropylene (PP)?
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
RE: Allowable strain of polypropylene (PP)?
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Allowable strain of polypropylene (PP)?
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.
RE: Allowable strain of polypropylene (PP)?
ct68: Where are you?
H
www.tynevalleyplastics.co.uk
RE: Allowable strain of polypropylene (PP)?
http://ww
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
RE: Allowable strain of polypropylene (PP)?
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