Help understanding how core materials work

General Composites Discussions Composites Talk
#1

I’m getting ready to get my feet wet and try some core material in my lay up. I understand the concept of the added thickness making the part stronger with less weight but everything else im uncertain of.

-Is there a different core material for wet layup vs resin infusion or are they all the same?
-When you laminate the core material, does resin saturate it? What happens when you cut through a laminate through the core? will it flex when you pinch it? or are you suppose to make the core slightly smaller and cut around the core?
-how does honeycomb work? In my mind the voids would just be filled with resin making the part even heavier…
-Can you use foam core from an arts/craft store? I did a test layup with this and it seems to work very well, but im worried at a larger scale, the piece will act almost as if it was hollow sins the foam between the poster boards are not dense…

Also anybody have links/pictures of the layup process (wet layup)

thanks in advance!

#2
  1. You can use the same type and density of core for both wet layup and infusion. When infusing a cored laminate the core should be perforated to allow the resin to saturate both skins. You can also get the core with cuts or grooves to allow it to conform to complex curves and or eliminate the need for sacrificial flow media.
  2. Yes the resin will saturate the core and depending on the material, density and whether the core has been knife cut, saw cut, etc this will change the amount of resin that the core will absorb.
  3. When you put a hole in a cored laminate you obviously expose the core and it must then be sealed. If you know ahead of time where you will need a hole you can eliminate the core in this area or use a higher density core.
  4. Not quite sure what you mean with “will it flex when you pinch it?”
    5.I have never infused honeycomb core and but I don’t think I would recommend it if you are a beginner.
  5. Obviously you CAN use whatever material you like for a core material but there is a large variety of cores that are commercially available and designed for use in composites. Shear strength, shear modulus and compression strength are the typical properties to be concerned with. End grain balsa is cheap, readily available and has high mechanical properties with the downsides being it can rot and the properties vary a lot more than man made cores since it is a natural product.
#3

5.I have never infused honeycomb core and but I don’t think I would recommend it if you are a beginner.

Is there a way to infuse honeycomb?

#4

Yep, you can infuse Honeycomb core… but I’m sure it’ll just fill up all the cavities with resin. :slight_smile:

#5

You can use pretty much anything as a core, it just depends on what you want to achieve, and the usage of your final part. Doing a quick plug I have used plain corrugated cardboard to add some strength. I’m sure I have some photos, but this doesn’t sound like what you want to achieve. :stuck_out_tongue:

Depending on the final use of the part, you may be fine using a basic foam from a craft store. Basically the core material is just there to separate the two skins, like a steel “I” beam in construction. Foam cores generally aren’t very strong by themselves, so they don’t add much strength to the finished part. It’s the distance between the skins that adds the strength.

Then again, depending on your application you may need the increased shear strength of a honeycomb etc. I could be wrong, but my understanding was that honeycomb or similar cores do only add shear strength?

#6

Well i tried the art craft foam core boards since i had it on hand. it definitely stiffened it up to my needs. but since the foam core board is basically poster board sandwhiching foam, so when you squeeze it, it can flatten the foam core. now if this was in a laminate i was wondering it it would have the same effect, but i guess it depends how thick the laminate layers are…

also for laminating core, does it have to be equal layers? eg 2 fg layers on the botton/ 2 fg layers on the top? or can you have 1 fg layer on the bottome/2 fg layers on the top? (referring to wet lay up)

As for honeycomb, I dont make anything that would require the use of honey comb, i was just asking to help understand the process. So as far as wet layup core materials, im assuming balsa is probably the best bet since it will not soak up resin as much as a foam core would?

thanks for all the responses!

#7

The compression strength of the core can be and important factor. The skin under compression often want to buckle towards the core. If the compression strength of the core is too low the core will crush as the skin buckles inward. If bond to the core is poor or it’s tensile strength is low the skin can buckle away from the core. For this reason, many of the craft and hardware store foams perform poorly.

#8

Has anyone use coroplast (corrugated plastic) as a core material? I believe something of similar nature is used for alumalite (for making signs) it basically acts at the core material between 2 aluminum panels… Im wondering if that could transfer over to the use of composites since its fairly cheap

#9

plascore also makes a thermoplastic honeycomb core. You can get it with a scrim over the cells for use in infusion. I think it’s made of ABS. I’m sure it’s great. I use their kevlar/nomex honecomb all the time but it’s very expensive. Obviously you get what you pay for.

Not sure what the plastic honeycomb costs, but it’s probably not cheap.

THe other alternative is using structural foam. You can get urethane foam core sheeting for relatively cheap prices. There are a few sites that will sell you a half sheet or full sheet. Worth getting it to try it out, maybe like $40 plus shipping.