First vacuum infusion. Please Help!

[millan]

Hello guys. I just did my first vacuum assisted resin transfer infusion using three plies of unidirectional carbon fabric and epoxy resin. The resin-hardener mixed viscosity is about 440 cps at 77 degree F and I did the infusion at the temperature of 68 degree F without heating the resin. I cut the infusion mesh about 1 inch short of the fabric and I also provided about half an inch of peel ply break. I did the leak test and vacuum drop rate was about 0.5 inches of Hg per day. Please see the the setup of the process.

First I did not degas the resin and during the infusion I could not control the infusion speed so before I could know the resin already reached the halfway mark within 3-4 seconds and I could see lots of bubbles throughout the laminate. To reduce the infusion speed I pinched the feed tube. I think I pinched it way too tight and the result was that bubbles increased in the laminate. Due to this the bubbles also started forming in the feed tube. The feed tube was completely immersed in the beaker containing the resin, so I thought it happened because I almost closed the flow to the laminate to control the speed resulting in vacuum pulling air bubbles right out of the resin in the feed tube.

Second when the resin hit the vacuum line the vacuum dropped suddenly. Is it a normal thing for the drop in vacuum once the infusion process reaches reaches its end?
When I removed the part from the mold the upper face had obviously voids but the bottom part of the laminate facing the mold was even worse. I looked like the bottom face was resin starved having lots of dry spots. I am using peel ply on both sides of the carbon for the secondary bonding of the composite to tabs for the purpose of doing the tensile tests on the specimens.

I have searched a lot on whether degassing is required or not. And it seems that there is no consensus on this. I know next time I have to control the infusion speed because ideally the resin should reach the bottom at the same speed as it is advancing forward. But I am finding it difficult to adjust the hose pinchers as slight mistake can ruin the part. So I am thinking of wrapping the inlet spiral tube in a peel ply to slow down the speed. Any suggestions to improve the process? Thanks a lot.

[Ro_Yale]

A couple of things I would suggest is 1st get the temperature up to about 80F. Also infuse across the shorter path(side). Probably doesn’t make a lot of difference on a small part but it’s a good practice. Also restricting the resin flow does help to get all the material wetted out. I also see bubbles at the restriction when I do that. I seem to get the best laminates when I raise the temps though. I never infuse below 70F. I hope this helps.

[hojo]

UD is much more difficult to infuse because the fibers are nested very close together. You need to have very low viscosity resin, and infuse slowly. Making sure your glass plate is warm as well will help, if its cold it can cause a quick viscosity change in the resin even if resin is warm going in.

[millan]

@ Ro Yale; Thanks for the reply, Sir. I infused in the direction of the carbon fibers. It is often said that the unidirectional carbon fabric should be infused in the direction of the fibers as it is considerably faster to infuse unidirectional fibres this way. The resin is able to run freely along the fibre length, as opposed to trying to force through the fibre mass perpendicularly.

[millan]

@ Hojo; thanks for the reply, Sir. Yes, you are right that when the unidirectional fibers are put under vacuum they will close together to create barriers for the infusion resin, stopping it from saturating the laminate fabric stack. That is why to reduce the risk of this, stitched unidirectional fabrics are recommended where the stitching facilitates resin flow. You can see in the third image that there is some dense stitching done to facilitate the flow of resin.

I will make sure next time when I am infusing to raise the temperature of the resin and also keep the glass plate warm.

[hojo]

You will have much better results finding a infusion resin with lower viscosity, or increasing the temp of this resin to decrease viscosity. 200cps or less would be better for UD. Good luck

[Ro_Yale]

I appreciate the lesson of infusing uni. I’ve never done uni before but will be in the near future.

[AllThumbs]

Dont know whether or not you were degassing. I had very similar results on several pieces in the begining. I was degassing my resin at a full bar, for five minutes. I kept increasing my degass time and getting improved results. I now degass my infusion resin for 13 minutes at a full bar vacuum. My results are now perfect. Good Luck.