Pinholes in surface after resin infusion 2

[crud]

Please help me before i go insane... After every infusion i do (practcing on glass using carbon fibre and plain weave e glass) There are small gaps between every tow in the weave. I have tried everything i can think of to stop this. I am using carbon that is stored in bubble wrap, epoxy infusion resin, i acheive full vac and no loss of vacuum. As i do it on glass i can see where the problem lies.... During infusion it works perfectly, but as soon as i clamp the feed line small voids form in the weave. I also de gas the resin for 10 minutes first. Any help would be very much appreciated.

 

[MotoMike]

Chris and B.E.,

First, good luck Chris with the new setup. Make sure you keep some resin on the backside of the feed tube clamp after infusing. It helps seal off air. Timing the clamp off is important. With practice you will learn to clamp a little early to let the resin wetout and the vacuum migrate it. This will give you a thinner and lighter part. What is the gel time of your epoxy?

Second, B.E. I guess it depends on your resin system. I have pulled 29 in Hg on parts for hours without degassing and have not found a problem with boiling off the volatiles in my system. Hopefully I won't run into that. I was wondering how decreasing the vacuum would give you 10 in Hg in the tube but still maintain 20 in the bag? I would think it would be the same, 10 throughout the system. As I said I'm still learning and can use any advice on this stuff.

Thanks

Mike

 

[berkshire]

Moto Mike
Here is how you get 10" Hg in the tube and 20" Hg on the bag. Its called double bagging.
With redundant vacuum chambers about the perform comprising:a mold;a preform located on the mold;an inner bag sealed to the mold and enclosing the preform and forming a first vacuum chamber,an outer bag sealed to the mold and enclosing the inner bag and forming a second vacuum chamber,at least one vacuum pump coupled to the first vacuum chamber and the second vacuum chamber to evacuate the second vacuum chamber to a pressure greater than the pressure in the first vacuum chamber; and a resin source coupled to the first vacuum chamber to infuse resin into the preform while maintaining the pressure in the first and the second vacuum chambers.
Be aware that some, of this stuff is patented by various people. and goes under various trade names such as: Scrimp,Vartim, Vec.
B.E.

 

[MotoMike]

B.E.,
I was wondering right after I submitted the post if you were referring to using two layers of vacuum bags. Neat idea. I wonder if it wouldn't be easier to vacuum the outer bag and use positive pressure to infuse. Probably a whole different set of problems with that as well. Thanks for the info.

Mike

 

[Compositepro]

Th double vacuum-bag process is a very effective process for making void-free, non-autoclave parts but it is not relevant to this discussion. "Double vacuum-bag" is a confusing misnomer. One of the "bags" must be a hard, structural shell which would more appropriately called a vacuum chamber or vacuum-oven. One iteration is a vacuum-bag over a steel pot. In this case, the vacuum-bag is used as a gas seal but the steel pot is used to keep the outer bag from applying any force to the inner bag. This is essential to making the process work.

A vacuum-bag in ambient air is under 30 inHg at all times, whether or not you are pulling vacuum. 30 inHg is the barometric pressure and is equivalent to 15 psi. It exists because the column of air over every square inch of the earth's surface weighs 15 pounds. Pulling vacuum under the bag will only reduce the fluid pressure. That fluid is resin or perhaps air. So if the fluid pressure under the bag is 10 inHg of vacuum (20 inHg absolute pressure) and the fluid pressure above the bag is zero gage pressure (30 inHg absolute pressure) what is keeping the forces in balance on both sides of this thin plastic film? It is 10 inHg of compaction pressure on the fibers.

 

[crud]

Moto mike.

The pot life is around 90 mins.

Gel time is around 5-6 hours at roughly 20-25 Degrees c.

It is epoxy resin specifically designed for infusion.

I'm sure we will get it sorted soon. I think there is much more to it than i thought. But i never quit...so ill keep trying untill i get it sorted.

It seems when i have been doing this that as soon as the resin blocks the path of air near to the vacuum port, that the vacuum reduces from the vacuum side back towards the feed pot. I think this happens because as soon as the resin blocks the path of air the fibre glass appears too lighten in colour very gradually as if its loosing its compression. When the part comes out, it then looks dry and suffers from the problem of voids inbetween all the tows.

 

[MotoMike]

Chris,

I would suggest going back to basics to determine a few things first. 1) Figure out if you are vacuum boiling the resin. To test this I suggest doing a traditional wet layup.

Try This.

1) Layout your tacky tape perimeter and cut the appropriate vacuum bag to fit.
2) Cut a few layers of fabric, one layer of peel ply, and three layers of breather to fit inside the perimeter of the tacky tape.
3) Mix up your epoxy and with a brush wet out the layers of fabric on your glass mould.
4) Put the peel ply on top of the fabric and resin followed by the tree layers of breather.
5)Place your vacuum tube wrapped in breather on top of the layers of breather.
6) Place your bag on top of all this and pull vacuum until it cures.
7) See what you have and if you have bubbles in the fabric.

2)Now if the test shows that the epoxy didn't vacuum boil then try this.

Do the setup with the breather and peel ply just like you did above but don't wet out the fabric. Add your infusion tube at one end and your vacuum tube at the other end - right on top of the breather. Now for geometry - orient the part so that the inlet is lower than the outlet and let the filling pull against gravity. This should help avoid any air bubbles.

You can add a little heat with heat lamps to speed up the cure.

Let me know how it goes.

Mike

 

[GregoireMC]

Is it possible that it is due to a gel time too long?
After infusion of the fibers the inlet is clamped; then the vacuum is still pulling the resin out of the fibers and thus creating little dry spots or void bubbles?

 

[Compositepro]

Gregoire, it is possible to look at it that way but it is not very helpful for solving the problem. It is not practical to vary the gel time for every different size of part. But you are correct in that this small part is more difficult because the gel time is so much longer than the infusion time. If the part were much larger this problem may not occur. But that depends on what the actual problem is. And that has not been determined yet.

One factor that should be measured is the final part weight and initial preform weight. That will give the resin content, which will give some indication if there is too much resin bleed out of the part(causing boiling). If the resin content is not low (less than about 35%) then the problem is trapped air. But as I said earlier, both issues are involved to some degree. This is what can make solving the problem difficult, and requires an understanding of what is going on.

 

[MotoMike]

Chris,

I believe I have the answer for you. I use a similar epoxy to the one you described. I have made hundreds of parts without experiencing the problem you are having...guess what...I don't degas the epoxy.

I tried a test today and degased the epoxy..and guess what happened? The epoxy was filled with tiny little bubbles. I let the epoxy sit to cure and did not shoot a part. I checked it repeatedly and even though it looked like the bubbles were gone, if I put a stir stick in and just pulled it back out it was covered in a thin layer of epoxy that was filled with tiny bubbles.

Take care in mixing your epoxy by hand and do not degass. I believe that the viscosity or surface tension of the epoxy may prevent it from collapsing the bubbles.

Good Luck!

Mike

 

[crud]

moto mike.

What size was the part you did the test on? And how many layers did you use?

I think it has something to do with the size of the part. And pulling full vac.

I have just set a piece going, and infused at full vac but just as it has infused i reduced the vacuum to 21hg in's.

I have noticed that all the air appears to be removed from the part but there are no bubbles at all in the part (it wont cure untill tomorrow).

I have done this before and the part came out perfect. I think that if the part was maybe 1m2 that it would cope with full vac. Although i dont understand why this happens it seems to work.

Dropping the vacuum tho 21hg in's doesnt seem to drop much compaction pressure either so should be ok on a contoured mold. Ill post tomorrow after i remove it and see what it looks like. But i have a feeling it will come out perfect.

 

[crud]

moto mike.

To answer to your prevoius post, i have tried degassing and not degassing and it always comes out the same.

Its not so much tiny bubbles but voids inbetween the tows in the weave as if the resin is being sucked from the gaps. Hence the reason i am testing dropping the pressure again.

Chris.

 

[MotoMike]

Chris,

Glad you figured it out. It may just be a problem with vacuum vs mass of the epoxy in your part. Trying to relate this to some avenue of science to explain what you are seeing would be nice but beyond the scope of this forum. I did not shoot my part with the degassed epoxy as it took took long to clear. It did start to clear up as it reached cure temp but it still had air bubbles around the stir sticks that I left in the cup. My parts range in size and layers. Usually 3 to 5 layers and up to 14" by 16". I can't say too much more about my system as I have been developing it for 8 months and I use a pretty unique way to infuse and then cure with elevated temp and pressure without an autoclave. I can turn parts out every three hours. Its been a lot of work and many 24/7 days, but it works really well. I don't let anyone into the shop to see the system, not even family or friends and definitely no cameras.

At this point I'll leave you to experiment on your own. I apologize if I didn't help you as much as I hoped.

Good Luck with your projects.

Mike

 

[CompChris]

I have the same problem, little tiny pinholes evenly spread in my test part. I have done tests for months and have no ideas any more. I am using a two sided rigid mold whith th VARTM technique. My injection pressure is curently 0.16 bar (0.16 kPa). The injection goes well, however the pinholes only come after some 45 minutes after infusion. I clamped the resin inlet off and put the vacuum of on the vent so that ambient pressure is everythere. What can be the reason why the bubbles occure? I used degased low viscosity resin and non-degased low viscosity resin.

 

[MotoMike]

A good resource to these problems is Resin Transfer Moulding for Aerospace Structures by T. Kruckenberg and R. Paton, ISBN 0 412 731509, Chapter 13 page 438 "Defect Formation"

 

[berkshire]

Motomike and Crud

Here is an article on resin transfer molding that may give you some additional pointers.
B.E.

http://www.vacmobiles.com/files/newsletter_11.pdf

 

[crud]

I found another thing that makes my test parts perfect quality... If i squeegee the part (flat test panel) i can see the air being removed from the part. Then it comes out perfect. However if i leave it to do it on its own the air seems to get trapped by the resin and will not move out the part unlesss it is helped.

So i think i must be getting air trapped in the part, but my next step is to find how to stop the air getting trapped in the first place.

 

[CompChris]

Hey MotoMike, I read the section and could not find any mistake I have done. What I tried is a test with different kinds of glass fibres in one part, the result was that there was still an even distribution of the tiny bubbles in the test panel. I think the bubbles come from the resin system. I don't know what I did wrong. Does anyone have an idea?

 

[MotoMike]

We seem to have a problem. That everyone is facing so it might be best to try to apply some scientific testing to this. I would propose some systematic testing to try to pinpoint the problem. Here is a rough idea of what I am thinking:

Bubbles caused in the resin/epoxy matrix could be caused by three things:
1.trapped air
2.trapped gas or volatile
3. Vacuum bubble

From these I would tend to believe that trapped air would be the best choice followed by a trapped gas or volatile.

To test your system look for air leaks. I did a test with a layup of breather fabric and water colored with different food coloring to look for leaks. I even submerged the whole system in water to test. The results were interesting but not totally conclusive because the surface tension of the water can plug small leaks. The use of smoke or some other aerosol would be better.

Another thing is to look at the basics of moving a liquid through a media with vacuum or pressure. Is there an even resin/epoxy front through the system? Use peelply, flow media, and breather to maintain even flow of resin/epoxy and distribution of vacuum through the system. Try different viscosity resin/epoxy and different temperatures. Practice releasing and closing the infusion tube to eliminate air. Try different fabric suppliers but be careful when testing with glass and then going to carbon as they most likely have different sizing and wet out characteristics. Also don't assume that because your system works with polyester/vinyl ester that it is an easy switch to epoxy. The polyester/vinyl ester matrix has a relatively low viscosity and surface tension which allows it to wet out quickly and then it gels relatively fast. Epoxies take a bit more R&D to get just right and you may have to work with different temps to reduce the viscosity and/or improve the gel time. Be cautious of your mould release and mould material max temps.

Also look at chemical interactions. Are dust, dirt, oils from your skin, release chemicals getting on your fabric and causing a problem? Fabrics are coated to work with different resin systems (I believe its called sizing) is your fabric sizing suitable for your resin/epoxy matrix?

My advice, start with vacuum and leak tests. Then work on resin/epoxy tests. Try different ones and at different temperatures. Be careful with polyester/vinyl esters when trying different temps as they can set up fast. Also be aware of your gel time and try to match it with the size of your part and rate of infusion. If your part is small and you can infuse in seconds but have to wait hours for it to gel you have a higher probabilty of getting a leak and pulling in a bubble.

So a few more basics. If you have bubbles in one area of the part, trace the bubbles and look for a leak or an area that does not have good vacuum. If it is evenly distributed throughout the part then you may have a major leak or more likely the resin/epoxy has dissolved gases, you have impurities in your components or chemical interactions.

Try a wet layup with a brush but pull vacuum as if you infused to see if the resin/epoxy is degassing or there are chemical interactions not related to infusing, this assumes that you can get a tight vacuum without the infusion tube.


Keep your system tight, keep everything clean, try different components, keep good notes, and good luck with your projects.

Mike

 

[MotoMike]

I had one more insight to try. After infusing and clamping off the infusion tube try elevating the tube so that it is higher than the part. If you clamp so that some resin is in the tube after clamping and you don't have an air leak the bubbles will migrate to the top of the resin in the tube. Although, if you are pulling the resin through peel ply it may not let the bubbles back through. Its something that I tried awhile ago and it seemed to work.

Mike

 

[MotoMike]

I would like to thank Chris for starting this thread and everyone that contributed because you led me on a path to some discoveries about my techniques. My setup is very similar to the one that Chris initially described with the exception that I use a 12 CFM pump connected to a resin trap with gauge that connects to a manifold that lets me pull 6 parts at once. I was not having any problems with the surface quality on my parts and thought initially that I had some answers for Chris. It turns out that the contributors that focused on vacuum pressure were on the right track.

In trying to figure out why I wasn't having the problem I connected vacuum gauges and small resin traps (made with pvc pipe) to each of my bags. Initially the vacuum for each part was 27 to 28.5 in Hg. Not the 29.5 that I had near the pump but I thought it was acceptable. After infusing the parts I was shocked to see that the vacuum dropped in each part to less than 15 in Hg. I was developing leaks after infusion which explains my surface quality versus Chris's. I sprayed some water around the setups and found that the corners of my bags were leaking as the resin flowed through the parts. I did some testing and redesigned the way the corners were taped and got the vacuum to stay consistent. When I demolded the parts I now had the pinholes that everyone has been speaking about.

My setup was fabric on the mould, peelply on top followed by breather fabric and vacuum bag. The infusion tube with spiral wrap at one end and the vacuum tube wrapped in more breather at the opposite end. I noticed that I was infusing and pulling vacuum from the backside of the part. I made up larger moulds and cut my fabric an inch bigger all the way around. I then placed a perimeter of breather fabric on the mould so that it would be directly under the extra inch of fabric. I left the breather off the back of the entire part and only used peelply under the infusion tube placed as close to the edge as I could place it. I put the vacuum tube on breather fabric at the opposite end that I stuck under the fabric and on top of the perimeter breather fabric. In this way the vacuum tube was off the part and connected only through extra breather fabric to the part (the vacuum bag was cut several inches bigger to fit all the extra fabric and breather. My goal was to use the extra breather around the perimeter to pull vacuum to the face of the part instead of the back. I trimmed off the oversize part and extra breather later. I did this for one part and under the maximum vacuum that I could get at the part 28.5, the part was perfect with no pinholes or voids. I made up 5 more moulds and have been running all six daily and the results have been superior, no pinholes, voids, or bubbles on the surface of any part.

Can Chris or someone else who has been having pinholes try this and let me know if it solves the problem for them as well?

Thank you!

Mike