Bistable Composite Help

[NightmareGoggles]

Hello all,

I am currently trying to make a bistable composite using the paper below as reference.

https://core.ac.uk/download/pdf/187093671.pdf

The current layup I am using is B6-a, and my setup is as follows.

• I have a custom oven which reaches 225 C for 4 hours.
• I have a brass tube 1 inch in diameter
• I wrap my layup around one third of the tube in order to get a third of the tube.
• My fiber glass is +45/-45, 300gsm (couldn't find 200gsm)
• I have experimented with the following layups (From outside to inside)
  • 0.5 PP/300 GF
  • 0.5 PP/2x300 GF
  • 0.5 PP/3x300 GF
• I have ptfe coated glass fabric paper sandwiching both sides of the layup with a tight winding of heat shrink tape to hold it all together and apply compression in the oven

The problem I am having is that almost all of the composites I have produced have ridges from the tape where they did not compress fully or enough and thus the plastic is not fully impregnating the fiberglass at some places, leaving me with a composite that is not fully bistable in some parts. If anyone has any suggestions on how to fix this, or even general suggestions relating to my process, or perhaps even somewhere I could get these manufactured for me, I would greatly appreciate it.

[John Kimball]

31 minutes ago, NightmareGoggles said:

Sorry about the delay.

While I did initially start out making my composites with resin, I would always end up with something too rigid and on top of that I did not have a proper mold. However, once I found the paper linked in my original post and read how they melted polypropylene into the fiberglass to make the bi-stable composites, I made a DIY oven and used the same process they did in the paper. Here are some pictures of my current layups with descriptions.

https://imgur.com/a/O42oaAz

Your pics help a ton. Not knowing what temp you need to melt the PP, but you could greatly reduce your textured surface by overlapping your shrink tape. We only juggle over about 0.100” per revolution. So in other words, a 1” wide tape would have 10 overlaps in that inch. This will not only reduce that look you have, but will give you 75-100x more pressure during the cure. Keeping the wraps and overlaps consistent will reduce ridges.

You can see the spacing of the wrapping in the photo below.

[lo_0l]

1 hour ago, NightmareGoggles said:

Hello all,

I am currently trying to make a bistable composite using the paper below as reference.

https://core.ac.uk/download/pdf/187093671.pdf

The current layup I am using is B6-a, and my setup is as follows.

• I have a custom oven which reaches 225 C for 4 hours.
• I have a brass tube 1 inch in diameter
• I wrap my layup around one third of the tube in order to get a third of the tube.
• My fiber glass is +45/-45, 300gsm (couldn't find 200gsm)
• I have experimented with the following layups (From outside to inside)
  • 0.5 PP/300 GF
  • 0.5 PP/2x300 GF
  • 0.5 PP/3x300 GF
• I have ptfe coated glass fabric paper sandwiching both sides of the layup with a tight winding of heat shrink tape to hold it all together and apply compression in the oven

The problem I am having is that almost all of the composites I have produced have ridges from the tape where they did not compress fully or enough and thus the plastic is not fully impregnating the fiberglass at some places, leaving me with a composite that is not fully bistable in some parts. If anyone has any suggestions on how to fix this, or even general suggestions relating to my process, or perhaps even somewhere I could get these manufactured for me, I would greatly appreciate it.

Hey @NightmareGoggles! Welcome to Composites Community 🙂

Do you have any pictures that you can provide?

[John Kimball]

As stated, could yo \u provide some pics of the issues? Also, if you could tell us more about the resin you are using.

[NightmareGoggles]

Sorry about the delay.

While I did initially start out making my composites with resin, I would always end up with something too rigid and on top of that I did not have a proper mold. However, once I found the paper linked in my original post and read how they melted polypropylene into the fiberglass to make the bi-stable composites, I made a DIY oven and used the same process they did in the paper. Here are some pictures of my current layups with descriptions.

https://imgur.com/a/O42oaAz

[NightmareGoggles]

Thanks John,

That is definitely very helpful. Originally I tried to overlap as little as I could but as I continued in my experiments I overlapped more and more but not as much as you are suggesting, so I will definitely have to try that as well as the amount of rotations, as the max I did was about 3. Also Polypropylene melts at about 225 C and my main worry is whether or not the temperature will make it to the PP with that amount of wraps but my brass tube is hollow so that may help.

[John Kimball]

I don't know which shrink tape you are using, but Dunstone only makes one version that can go above 225ºC, so consider that as well when choosing a shrink tape.

https://www.shrinktape.com/products/hi-shrink-tapes/

The temp will make it there, but you may need to slowly heat up the part instead of going straight to 225º. That will give it a chance to heat through thoroughly and evenly.

[NightmareGoggles]

Hey John,

Just put a new composite in the oven and am waiting for the results. To follow up what you said about the tape I got this one for my composites when I first started and it handles the temp well https://www.mcmaster.com/6820A13/, but I will also look into the Dunstone one you mentioned. What's strange about what you say about letting it come up to temp is that in the paper I linked they put it straight in at 225 C so I wonder why they did that. I will try a few with the Dunstone as well as letting it come up to temp. Thanks for your help.

[lo_0l]

How'd this one go @NightmareGoggles? Any pictures of the newest run at this?

[John Kimball]

On 9/16/2023 at 2:54 PM, NightmareGoggles said:

Hey John,

Just put a new composite in the oven and am waiting for the results. To follow up what you said about the tape I got this one for my composites when I first started and it handles the temp well https://www.mcmaster.com/6820A13/, but I will also look into the Dunstone one you mentioned. What's strange about what you say about letting it come up to temp is that in the paper I linked they put it straight in at 225 C so I wonder why they did that. I will try a few with the Dunstone as well as letting it come up to temp. Thanks for your help.

The tape needs to shrink first, so a quick hit at 150ºF to shrink it, then let it cure naturally and slowly with a controlled ramp. I use a heat gun to do this.

 

[NightmareGoggles]

Hey guys, sorry for the delay I had a family vacation right after the latest run so I missed the pics. Here are some pics (Layup number 1 is 1 layer of GF and 2A is 2 layers). What immediately pops is just how much overlapping the shrink tape a ton helps, the surface is so much smoother and the tape is much more mechanically uniform. This has also lead to me realizing I cannot continue using the 300gsm gf I have as it is too thick and the PP is not fully impregnated as you can see in pics 1C and 2C. In the original paper they used 200gsm gf but I had trouble finding it so if anyone could help with that I would greatly appreciate it. I have also been experimenting with non composites for my project but as that is off topic I won't go into too much detail aside from the fact I am trying PA 6/6 as a replacement, but am running into problems with it creasing when folded in half.

Again thank you guys so much for your continued help.