Polypropylene molding

[myates]

Hi, I have been experimenting with "molding" polypropylene. I have been using a polypropylene suture material that is 0.010" and 0.007" in diameter. I have been tightly wrapping the suture around a fixture I made and then heating it in the oven to about 350 degrees F to keep its shape when removed from the mold. This has worked but does not keep its shape as well as I would like. I have done some research but could not find properties for just PP suture (I am assuming that the properties are different than raw PP). I also realize that this is a softer plastic and can be deformed easily but it seems to be deforming easier than I want. Is there anything that can be done to harden this PP suture after being "molded"? Keep in mind it is a product that will be used in a surgical procedure. I have tried coating it in a silicone with little improvement.

I am also currently looking to get this injection molded, knowing that is the route that we will go with it eventually, but having issues finding a vendor due to its complex shape and small size. If anyone has vendors that may be up to the challenge let me know.

Thanks.

 

[MikeHalloran]

It might be as effective, and faster, to plunge the fixture and fiber into boiling water, then cold water.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

PP suture (and I speculate) is likely to be molecularly oriented - i.e. when the filament is extruded, it is drawn out to a thinner section, axially aligning the molecules, giving a stronger thread. As your heating/cooling will not change this property, I would guess the filament is still trying to straighten.

You are unlikely to mould .010 diameter PP longer than about an inch - maybe two long (gut feeling). Polymers have defined calculable maximum flow lengths.

You will not be able to "harden" the PP.

I always assumed sutures were nylon?

H



www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[myates]

I was aware of the suture's molecular orientation, and it was very stringy when working with it, but I was hoping that with heating it to its molding temperature it would want to keep the shape I put it in. I was even able to melt the suture, which just ended up shriveling up and melting away.

At this point I am in desperate need of an injection mold company that can work with me with getting this part molded. Most companies I have tried says that it is too small and complex or they just don't do parts like this. I may just have to figure out a way to mold it myself.

 

[btrueblood]

Tough to visualize what you are doing, but one idea: can you knit or stitch or weave the filament onto/over your form, to add some 3d depth and knotting to help stiffen the structure?

Would your part be producible with a 3d printer - type process? Without more detail about what your part looks like (maybe a picture would help) and what features of the wrapped part need to be preserved, it would be difficult to give any advice regarding molding.

 

[myates]

I will have to abandon the polypropylene suture molding idea. I was not having much luck in getting the result I wanted. They did look pretty awesome though.
On the bright side, after further searching I was able to find a handful of micro molders, in which some are specific to the medical field. Some of these companies are:

http://www.mtdmicromolding.com/

http://www.micromoldinginc.com/

http://www.micromolderie.com

They were very anxious to help!

 

[Demon3]

It is essential to understand that the properties of a real fiber are totally different to what you are making. When fibers for sutures are made the polymer melt is drawn, this aligns the polymer chains so you get way higher strength and stiffness. For example I have seen PP with a modulus of % GPa whereas normal injection molded PP is around 1.5 GPa.

So, before abandoning this make sure you get some manufactured PP fiber which is readily available.

Dr. Chris DeArmitt - FRSC

http://www.linkedin.com/in/chrisdearmitt/

 

[myates]

I have tried looking for some manufactured PP fiber, but I have not stumbled upon any. Maybe I am not sure what to look for exactly. Could you point me in the right direction where it is readily available?

Thanks

 

[Demon3]

Carpets are made of it. Googled for "PP fiber" and got lots of promising hits.

Dr. Chris DeArmitt - FRSC

http://www.linkedin.com/in/chrisdearmitt/

 

[Demon3]

Searching Amazon for "polypropylene fiber" shows a $6 toilet brush with PP fibers you could try.

Dr. Chris DeArmitt - FRSC

http://www.linkedin.com/in/chrisdearmitt/

 

[MikeHalloran]

Making "something" out of "something else" usually just leads to frustration, because the properties you need may be artifacts of someone else's process, and you have no direct control over them.

Now that you have found some molders who interested, this would be a good time to back up a bit, and figure out what properties you actually need, and start designing a product within the limitations of the processes you can get.



Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

Just to clarify I don't suggest you make sutures from toilet brushes but once you feel that PP fiber you will see how much more stiff and springy it is that molded PP and what you could expect from PP fiber you have made for you.

Dr. Chris DeArmitt - FRSC

http://www.linkedin.com/in/chrisdearmitt/