Resin Infusion Voids

[RotorChef]

Hello all,

I am new to this forum and am excited to learn from the wealth of knowledge and experience from all of the members. I did some searching and found some similar issues regarding voids in my laminates while using VARTM methods.

I am using a sizeable aluminum mold that has a computer controlled heating element to control the curing process. My layup schedule is three layers of type 7781 fiberglass and a resin system designed for RTM. I have my fiberglass against my mold surface with a layer of peel ply on top of that, followed by one layer of flow media and then my vacuum bag. The mold is single sided at this point and has two vacuum ports and one point of infusion.

I am degassing my resin for about 15 minutes while preheating both it and the mold. The preheating of the resin is minimal via a hot plate but the mold itself is brought up to 50 C. I am pulling my resin through the two vacuum ports selectively to promote full saturation. Once the part is fully wetted out, I am switching my inlet port over to the vacuum side and pulling out excess resin/air from all three ports. At this point, the mold is then raised to 100C for four hours, per the resin's cure cycle recommendation.

I have vacuum gauges at my resin trap as well at my pump and they are reading identically without any drop.

The problem occurs once most of the excess resin is expelled, and voids show up in the form of very small air bubbles as well as leaving large areas of the fabric appearing to be dry, as seen through the vacuum bagging.

Am I promoting further degassing by heating up the mold in my later stages? Is it possible to be applying too much vacuum (I have read in another thread to lower the vacuum to 10 in/hg once the infusion is complete) which would result in creating voids?

If I had any types of small leaks would it be introducing the air back into the laminate in large volumes?

Any help is appreciated, thank you very much!

 

[Demon3]

There's a thesis on that topic from SiComp in Sweden:

http://epubl.ltu.se/1402-1544/2006/45/LTU-DT-0645-SE.pdf

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials

http://www.phantomplastics.com

 

[Compositepro]

Pulling a high vacuum on the resin after infusion in addition to heating the resin is almost guaranteed to result in aporous part.

 

[Demon3]

As pore size is critical perhaps modifying that would help. I.e. large pores mean a lot of stress concentration / weak zones and early failure. If you were to dispers some fine particles in there, the voids would grow on those and create many smaller pores that do not have such a detrimental effect on properties. This is what foam producers do, they add fine particles to give nucleation sites upon which bubbles form resulting in a more uniform cell structure and smaller pores.

Chris DeArmitt PhD
President - Phantom Plastics LLC

Consulting, ideas and training on plastic materials

http://www.phantomplastics.com

 

[RotorChef]

Thank you for the reply. Would you suggest that after part is infused, we lower the vacuum to evacuate excess air/resin immediately? We are pulling somewhere around 26" during the infusion which takes about 8-10 minutes.

The heat is then ramped up to 100c from 50 at this point. With the heat still at 100, we should try lowering the vacuum?

Just wanted to clarify, wasn't sure if you meant that the heat and vacuum were a problematic combo or if both were troublesome.

Thank you!

 

[Compositepro]

Both are troublesome, which makes the combo more problematic. Your problem has been discussed several times before in this forum. Search for those threads and try to understand them.

Here are some basic principles. Total pressure on the bag is equal to fiber compaction pressure plus resin hydrostatic pressure. This relationship always holds at any point. The lower the resin hydrostatic pressure, the more likely that a bubble will grow or form. The less the fiber compaction pressure, the thicker the fabric ply will be. Vacuum cannot remove air or volatiles unless there is an open channel for the volatile to flow through.

 

[cpinz]

Never change temperature during infusion, start the process at the temperature held possibily constant at the value prescribed by the technical sheet (generally T>15°C )
Slow down the resin flow close to vacuum line placing the vacuum tubing at 50-60 mm from the laminate edge ( dont' place it on the laminate !! ) and use some flow stopper/retarder below the vacuum tubes ( peel ply, dry fabric work fine)
Make proper use of resin trap that must be sized according to the volume of resin involved. Let the resin flow from resin tank to the trap without interruption of resin feeding from tank. Let the resin gel and only then decrease slightly the vacuum.

I hope it helps