infusing with contour balsa

Hi Steve, I'm familiar with flame treatment, plasma and corona treatment. Yes its the best way at present to prep sufaces for bonding/painting/printing. It cleans the surface and adds compounds to the surface that increase its surface energy (not its surface tension only liquids have surface tension). We do water break tests to check that the surface wets prior to bonding and preheat to remove "dew". The surface (adherand) to be bonded ideally should have a higher energy then the adhesive(resin). Water has a surface tension of about 70dyne/cm and epoxy has about 45dyne/cm so if water wets the surface epoxy should. The printing industry uses dyne pens to check surfaces before they paint. They are available in steps from 30-70 dyne and I'm about to get some to see if they are better than using distilled water.
http://www.accudynetest.com/pentest.html

Cheers Peter S

 

hi Andrew - Its fine to infuse over an existing laminate just be very good with your prep. Laminating resins however are not designed for secondary bonding but due to the vacuum environment and usually the very big surface area involved they are quite good (then you need my plasma cleaner! comes with steak knives to chip off those rough edges) . The joint line suffers from lack of thickness however due to the clamping pressure so I would suggest(similiar to the aerspace industry and prepreg bonding) that you wet the surface with a tough epoxy/resin let it get quite stiff then start your stack and go through to the infusion if possible with firm resin at the interface so the fibres are not touching. Some of our parts are 40mm thick so we build them in two goes. A 20mm thick dry stack is about as thick as you can get in tight corners and things before it tends to pucker and wrinkle and carry on. When it debulks it looses surface integrity. So we have done parts in two goes . Cheers Peter s

 

My latest test. 25 layers of glass and Proset epoxy. We just infused it about 3 hrs ago so I will have more detail about how it turns out in the morning. The second feed lines were probably not necessary but since it is our first experience it seemed prudent.

 

Ok the test piece came out very nice, we did suck a little air in where I didn't clamp off one of the feed lines properly but it was only briefly before we noticed it. We did have some issues though which i will detail below.
1/ when checking for leaks the bag sealed itself to the table between the vacuum lines at the end of the part so we had no vacuum at the feed lines, we then peeled back the bag and extended the flow media out to the vacuum lines but made the mistake of running it over the vac lines, this led to a string of problems later.
2/ the infusion went well although the mesh we used was more open than the greenflow we thought we were getting so it filled very quickly but no real issue as it did wet out thoroughly.
3/ we ended up leaving the pump on at full vacuum for 18 freakin hours, I chut it off and peeled a corner and the damned epoxy where the glass was thin at the end still transferred to my finger, I didn't check further inn where it was thicker, I just resealed the bag and applied vacuum again then valved off the resin trap and shut the pump off.
4/ we had a perfect bag, zero leakage and it held what seemed like full vacuum for another 16 hours, i say seemed like because we don't have a gauge on the pot yet but it was just as hard to lift off the job as when the pump was on.
5/ my son Jason stripped the bag and the part was cured.
So, the problems we encounted were the insanely long time to gel, its a good thing we were able to use the new pump, i would not have wanted to use the robinair for that long. We sucked out too much resin, i expect due to the flow media at the end cancelling out the resin break allowing the pump to suck it into the trap and also the resin staying fluid for so long. The dry glass stack weighed 2650 g and the finished test part weighed only 3650 g so only about 28% resin which is nowhere near enough, the peel strength is very poor, in fact i expect we could peel the thing apart. Another thing with the epoxy is that it is very brittle, i made the mistake of not putting a catch pot in the trap so we had to chip out at least half a liter of resin and it came out like broken glass, the part is the same. We are planning to do another smaller test with a faster hardner to address the ridiculous gel time although it will still be much longer than i would like, and also to try to get the resin content more like 35-40% and see if that fixes the peel strength problem. I much prefer VE so far.
I would much appreciate any thoughts on controlling resin content, ive never experienced a too dry laminate.

Steve.

 

Hi Steve; as you know there is no perfect resin, the very long gel time does give you the option to degass and to deal with unexpected issues that may arrise.
Too early to know if the peel strength is not good enough, for most long gel time systems elevated post cure temperature is mandatory not an option. What are the process requrements for yours. Your sample may have very good properties when fully converted (cured).
But in general as I mentioned earlier I think ~35% wt fraction for the common ve or epoxy resin that we use is a better target than 30%.

You can control this in a number of ways, as you gather by now my prefered is to decrease the vaccum after the infusion. Or you can simply infuse at a lower vaccum. Third option is to incorporate CFM matt into your stack, this can only be done with thicker laminates. CFM matt has a high void content/high resin wt fraction, but the resin fraction will not be uniform through the stack. But an advantage is that it can eliminate the requirement for over the top flow media.

Regarding the infusion; as you discovered you do not want the flow media going up to the vacuum lines. In fact you should hold it just short of the job especialy for thick laminates where you have a large lag time in the bottom resin front compared to the top.

You may have chosen to have parallel feeds for experimental reasons, but on a long and narrow job I would have used just the one inlet and feed line along the long side.

 

resin content

Hi Steve,
28-30% by weight is not too little resin. The peel problem is a function of the epoxy strength not the resin content. You need to speak to proset and determine their recommended post cure, using a different hardener so you get short ambient cure. And you need to post cure to get the required peel srength. Proset are a good mob and will have all the technical data you need to be successful. I do not recommend CFM for structural parts. It runs at 50% by weight resin and is the same strength as chop strand mat laid by hand. Peter S

 

Steve - The moment you opened the bag and found that the resin was still wet, your entire job was completely ruined. Once the laminate decompresses, it sucks in a huge amount of air and becomes completely porous. You cant just reseal the bag, suck it down again and expect anything but a complete disaster. Its nothing like a wet layup vacuum bagging process - other than it involves the use of a vacuum pump.

Infusion is a rather "scientific" process, in that mistakes like this just cannot be allowed to occur. Everything must be correct, just 1 thing out of place, and the job is likely a write off.

The transfer of resin at the inlet, requires a medium - i guess you now know

At the vacuum end - ideally you want almost the opposite, something that inhibits resin flow but maintains a vacuum path. This is why most people use peel ply only here.

Another major problem you will likely encounter, is accounting for likely racetrack paths, recognizing them before you mix the resin, and correct them before you begin.

 

I'm using a med hardener,70 min pot life.At 65F it takes 48 hrs under vac for it to be tack free.

The red flow states that it consumes less epoxy then the green,i found it to be the same.What were your results Steve?

 

Hi guys, i guess ive been spoiled by the user friendliness of the VE, i love that stuff, the only reason we are playing with the epoxy is the potential higher mechanical properties which for the particular repair job we are doing every bit helps.
The test panel is 48" x 10" which is 1/10th of the actual job, this was actually the second test panel, the first was a complete disaster, it was the same size but with just a single feed to see if we could pull the full 24" in each direction without intermediate lines, we used no flow media on one side and 30% shade cloth on the other,i marked the table every 3" in each direction and interestingly both sides progressed at exactly the same rate. we were trying the fastest of the 4 hardeners offered because it should have given plenty of working time relative to the VE we are used to but it did not as we had a meltdown in the pot, we did however manage to infuse more than half way before meltdown without flow media.
So after consulting with Proset we did this next test with the 3rd of 4 hardners as well as adding intermediate feed lines and we used the flow media, as you may remember we had been using the greenflow 75 with the ve which i felt was faster than we needed but i thought it would be good with the more viscous epoxy but what the supplier sent was an orange stuff which is a bigger mesh, with this test setup it flowed very fast but i don't really see any issue from that though with the slower hardner and mesh we really don't need the extra feed lines but we will still use them on the actual job. I don't think we did any harm releasing the bag at 18 hours groper, i believe the stack was mostly set up but as the test is an actual cross section of the repair job the layers are staggered to simulate the scarf into the hull so where i touched it was thin so it would not be set up as much as further in where the exotherm was more, after we finally debagged the part was totally set and very brittle. When my son put it in his car and closed the hatch he hit a corner of it and started a peel situation which im sure we could peel right apart. Peter, have talked to proset again yesterday and they were not at all surprised at the length of time to cure, they agreed with me that the resin content is probably too low and likely a cause of the poor peel strength but also said that the brittleness that alarmed me was due to the stage it is at in the cure cycle and it should get less so further into the cycle. We will be post curing the actual job at 120 F for 8 hours. Even if we go back to the fastest hardner we will still have a very long gel time at ambient, i would not want to be running my old hvac pump for that long. The new setup i have is very nice, the pump is 20cfm, 1 1/2hp and will run for as long as we want without spitting oil or vapors and the resin trap is setup like the vacmobiles with 6 fittings in the lid so we can segment the vac lines if we want and a nice glass viewport, we will be using a cut down 5 gal bucket as a catchpot and can extend the hoses into the pot to keep the inside of the trap clean. We will do everything we can to keep from needing the trap bit it makes a nice distribution center.we have the Robinair pump plumbed and valved in ready to go as a backup and a gasoline generator in case of power failure.

Steve.

 

Hi Tungsten, whose epoxy are you using? I found with the greenflow that it retains about 1.83 oz/ft2 from memory, in this one test with the more open orange stuff the resin stayed with the peel ply so I havnt weighed it yet but I will. I would think that the bigger holes would retain more resin.

Steve.

 

Peter, do you have any figures for the mechanical properties of a cured laminat of epoxy vs VE vs PE. I really cant find much online for cured laminates. I am planning on making some composite chainplates off the job infused and then inserted over the top of the bulkhead bonded and bolted. I was planning on using the proset but am wondering if I couldn't just use the VE I love so much.

Steve.

 

Steve have Proset given you resin properties for different cure conditions? 120'F ~50'C for 8hrs does not seem enough for such a slow gel time system. I would have thought more like 80'C for 8 hrs.
Peter is the best person to give you examples of physical properties for laminates.
But as a general guide if you decide that you will only post cure to 50'C then resin with the best properties at that condition will be the best one for you. More than likely VE will give you better strength but lower elongation. You have to ask for full data sheets to be able to compare.
I made some tests for infusing chain plate laminates, at that stage I was still infusing at full vacuum and not backing off after infusion. I decided that the resin content was too low ~24%wt for the strength of the resin I was using. At the end just ended up wet bagging them.

 

Andrew, it was the tech dept at proset who told me to go 120f for 8 hours. The test piece did get a bit better as time went on but we were able to break it by elevating one end and jumping on it. The first test part we did which was with the fastest hardner which exothermed at over 250 f and melted the bag so retained all its resin at about 65% fiber was impossible to break the same way.
So, what we have learned from this is that we need to keep the resin content in the 35 to 40% range, a little higher resin content is much better than too little but how do we keep the pump from relentlessly drawing out the resin when it takes so long to gel? about the only thing I can think of is to use something like that vacuum hose that stops the resin from leaving the part, i don't remember the name. Otherwise it seems to me that it takes so long for the resin to cease being fluid enough to flow that by then too much has been drawn out of the part. We had as close to the perfect bag as its possible to get on the last test but it still drew the resin out, I don't expect we will be able to get as good a bag on the actual job. Im very frustrated right now as we have had such a good run with the VE.

Steve.

 

Control resin content with vacuum pressure. The lower the vac the lower the Vf. 80% vac should have you around 38% resin by weight. You can (should) still infuse at full vac, then back it off once it's filled.

 

Steve; was it the elevated temperature cure or the increase in resin Wt% that provided the apparent improvement in peel strength??? I suggest both.

Localy I found it very difficult to get full data sheets for resin properties at various cure conditions. It was only after many phone calls and complaints that I got some usefull data in writing. I am only guessing but you may have <95% conversion rate at that cure.
If you can not gett better data in writing do a simple experiment. Make a set of resin castings that are only ~2mm thick say in plastic bottle tops. Post cure these under different conditions, say 8hrs at 120'F, 24hrs at 120'F and 8hrs at 175'F.
Then put these into a pot of water starting at say 20'F below the expected Tg and slowly bring up the temerature while stirring. Fish the samples out one at a time and quickly compare how pliable they are to each other, continue with this untill they all become soft.
If the samples with higher cure conditions dont show higher heat destortion temperature then the 8hrs at 120'F was enough, but if they do then you know you do not have full conversion.