Want Help in Picking a Suitable Material for a Injection Molded Part (~Plastic Cotter Pin)

[PressEquip]

I'm trying to determine what would be a suitable material to make this item which function likes a plastic cotter pin

Part Description: ~0.25"OD x 1" long cylinder (one at each end), connected by a 3/16"OD band
How its Used: Insert one of the plastic cylinder into the hole located on the shaft and then insert the other plastic cylinder into the opposite hole [on shaft, 180° apart] (the connecting band hugs the shaft and there is some tension in the connecting band which helps keep the plastic ends in place)

Part would be used in temperatures: -30°F to +40°F
Product would be exposed to: Water, Grease, Oil

Desired Properties of Material
-Suitable for Injection Molding
-Flexible
-Durable
-Impact Resistant

From my limited research I was leaning towards a Vulcanized EPDM Modified Polypropylene (such as Exxon Mobiles Santoprene) but am not sure it's a good choice

Any insight that can be provided is appreciated

Thanks



A Fan of EngTips, share the knowledge

 

[btrueblood]

EPDM won't hold up to grease and oil.

What tensile/shear stresses will the pin see? Or are you worried about the band (which sounds like an o-ring to me)?

 

[PressEquip]

It will be used in a mechanics shop type setting so it will be potentially exposed to some oil and grease but not submerged.

The pin will see very small shear stresses, it's intended to act more as a positioner

I'm more concerned about the band (it's like a half O-Ring, it only goes around one side of the shaft), since its so thin (~3/16" OD) and needs to have some stretch.

I want it to be tough enough so that someone can't easily pull it apart with their hands. The material should have a tensile strength of 550 psi or greater, the material might see impact loads of 1000 psi (impact duration ~ 5 seconds).

Since the intention is to use the part multiple times, it will see some cycles of loading and unloading, particularly in the band. I would like the material to have at least moderate fatigue strength (not start to crack after 20 cycles or less)

Thank you for your comment

A Fan of EngTips, share the knowledge

 

[btrueblood]

Even a little bit of grease will soften and degrade the EPDM. I'd use something like polyethylene for the pins, as it's pretty impervious to everything, as well as being relatively rigid and slippery, and so easier to insert into a hole. And good old el-cheapo nitrile rubber for the o-ring band. If you notch one end of each pin, then pop the o-ring over the notch, and around the shaft to the other notch, would it not work? A lot less trouble than finding a way to mold the whole thing in one go. I'm willing to watch you try and push a pair of rubber pins into a hole though, if I'm allowed to heckle and eat popcorn.

 

[PressEquip]

Thank you btrueblood for thinking about my request and providing some feedback

The end user really has their heart set on a one piece construction (only one item to keep track of).

Yes, pushing rubber pins (poisson's ratio = ~0.5) into a tight fitting hole would be problematic. From my understanding the pin to hole isn't a tight fit (pin=~0.25"OD, hole=~0.3125"ID). I would like to use a material with poisson's ratio of 0.3 to 0.4 (the lower the better, I don't want to eliminate too many materials by choosing a very low poisson's ratio requirement).

Any recommendations for a suitable material that could be used for a one piece construction?

A Fan of EngTips, share the knowledge

 

[btrueblood]

Nitrile rubber, or urethane. Picking a durometer that is a) stiff enough to allow the pins to be inserted, and b) flexible/stretchy enough for the band to stretch and snap back during insertion is going to be the trick for any elastomer. You might think about over-molding hard plastic parts (pins) with the elastomer band.

 

[susanz]

Material PP will be ok for your project, it is cheaper, flexible, wearable

http://www.lprosperitymold.com

LPmold is a plastic injection mold contract manufacturer with the full capability to work competently with our customers to transform their design into a finished product in the most of competitive prices.

 

[PressEquip]

Thank you susanz for your response

Do you have a recommended Polypropylene specs/grades? Such as desirable physical parameters (Durometer hardness range, ...)?

What do you think of Vulcanized EPDM Modified Polypropylene (such as Exxon Mobiles Santoprene)?

What are your thoughts on btrueblood's concern that EPDM won't hold up to grease and oil?

Thank you for your time

Kind Regards,

A Fan of EngTips, share the knowledge