Anyone using VARTM?
 

I began experimenting with VARTM (vacuum assisted resin transfer molding) about a month ago. After tons of experimentation and many different matrix / substrate combinations I finally got it to work. The results are simply astounding! They claim that the VARTM method will yield results comparable to pre-preg / autoclave-cured laminates and I believe it. I'm using it for CF laminates approx. 0.125-0.250" thick. It's cut my resin usage roughly in half compared to hand-layup / vacuum bagging techniques. Much less consumable materials used...no mess either! I'm currently transitioning all of my production to this method. The downside? It's very complicated and the most minute oversight or leak in your system will trash the part.

If anyone would like to discuss this technology and it's related equipment or materials let's do it either here or via email. I'm looking for information on increasing flow rates and distances. I can get about 15" of flow distance through the CF laminate now...when I began experimenting I could only get 3". :) I'd love to be able to flow about 36".

I would suggest reading the primer at the fiber-glast site if you are new to VARTM. They call it 'vacuum infusion' but we'll call it VARTM for sake of discussion mmmkay?

Let's swap info!

-Tom

RE: Anyone using VARTM?
 

A few years back, when I was involved in open-mold, wet layup manufacturing of carbon fiber automotive panels, one of the production guys got very good results by adding a small percentage of acetone to his epoxy resin mix, for the application of the first layer of CF, up against the mold surface.

At first, I didn't feel good about adding any acetone to the mix, but it did help with reducing the viscosity during the working period, and helped the resin clarity surrounding the C/F.

Have you tried this? My first thought concerning your situation, is that a lower viscosity resin might penetrate further, and you might also experiment with a slightly lower vacuum level, during the initial infusion stage, so the stack of fabric is looser, and then crank it back up, once the area is fully infused.

RE: Anyone using VARTM?
 

This is the first I have heard of this method, and am not quite sure what it is. I would love to see a discussion of it here.

Just checked the fibre-glast site. I have done wheel pants and cowls. I have also vacuum bagged a whole shuck load balsa skined foam wings.

I am currently working on a plug for a 1/4 scale SU 31 fuselage. I wonder if this method will work. I'm sure that it would be a job for 3m77.

RE: Anyone using VARTM?
 

Vacuum infusion in RTM (Resin Transfer Molding) is a technique where the dry, cut fabric layers are stacked and bagged as normal, then the resin is added through one or several ports, which have valves installed, to retain the vacuum.

The vacuum draws the mixed resin into position, between the filaments of the stack. No breather mat or porous mylar sheet is used, since the resin is slowly moving from the infusion ports to the vac ports.

Some manufacturers also use a UV-sensitive resin in this process, which will not gain viscosity until special UV lights are turned on, to start the final curing phase.

It's becoming a more common method in Southern California and Florida yachtbuilding, where control of solvent volatiles in large quantities is essential, in order to comply with state and county regulations that involve out-gas emmissions from plastic laminating companies.

RE: Anyone using VARTM?
 

What resin are you using right now? I'm VERY interested as well, but definitely have to go more than 12-15 inches; more like about 24 to 30. I'm also interested in how you distributed the vacuum. Having no experience, I envisioned a gang of tubes attached to needle valves (e.g. fish tank type air ganging if I remember right), that way you could play with the needles to vary resin transfer rates at different locations on the mold.

How about experimenting with a liquid other than resin to get rates approximately right? Like a thickened water of some kind, that way we could rinse and dry the cloth, reset and test again-I really don't like the thought of ruining enough cloth, carbon and kevlar that wold make up a 5-6 foot mold that I need to do.

How about next up some pic's?

RE: Anyone using VARTM?
 

These days, I'm not working in RTM. I just have some experience from a few years ago, when we investigated these techniques for an application we had.

As Tom stated, this is a very tricky process, a bit more complicated than regular bagged wet laminates.

But perhaps Tom will return with some answers to your questions, since he said he was interested in sharing info.

RE: Anyone using VARTM?
 

I'm already using a very low viscosity resin that's formulated for VARTM. The mixed viscosity is around 280 cps whereas 'normal' laminating epoxies (West Systems, etc.) is around 1000 cps. I'm using Jeffco 1320 / 3138. So far so good, but a post-cure cycle is essential. I just 'stress tested' a part that had cured about 12 hours at 72 degrees and wasn't too happy with the results...then I read that this system took 7 days to fully cure at room temperature. I'm now post-curing it at 130 degrees and will leave it for 6 hours or so...4 hours should be enough according to the specs.

I hadn't tried varying the amount of vacuum when introducing the resin. I'm pulling 25.5" Hg and let the pump run continuously (power outages are NOT a good thing). The problem I see is introduction of additional unwanted resin to the layup...BUT it probably wouldn't be that big of a deal. I'll have to try it. I might be able to reduce or eliminate the interlaminar flow medium (FG surfacing veil...continuous strand mat). It would simplify things if I didn't have to use the veil...but I just ordered 100 yards of it! Grrr.....

I wonder if there would be other problems with varying the amount of vacuum. It might be possible that the excess resin wouldn't be fully eliminated or would be eliminated at a disproportionate rate. The resulting laminate may be thicker near where the resin was introduced. Hmmmm...only one way to find out. Thoughts???

-Tom

RE: Anyone using VARTM?
 

When I speak of lowering the vacuum, it might just be 5"HG less, that might do the trick.

Perhaps do an experiment with a dry stack of your layup schedule materials under vacuum, at increasing, then decreasing levels and measuring the height with a digital vernier caliper, to see how little pressure drop it takes to 'open' the stack a bit, for better infusion.

RE: Anyone using VARTM?
 

On the Jeffco products-that's actually what I've been shopping, so it sounds like I'm at least approaching the material selection about right.

On a 5-6 foot part, do you draw (infeed) multiple tubes and vacuum multiple to get an even flow rate across the mold? It would seem to me that you'd need to vary the vacuum (or resin in-feed) in some manner to get an even resin tranfer front. I'm guessing the goal is to get the resin transfer front to arrive at the vacuum side all at the same time to provide even wet out.

Post curing at levels below 200 is no big deal - I'm already at 100 ambient now (well 95)! A folding hot box is easy enough to make.

I'm hoping to experiment over the weekend, but will have to use a smaller part and thin the resin - I'll get a viscosity cup and thin it down to about 300 cps. I can't remember what the Pro set starts out at with the slower set hardener, but I thought it was less than 1000. What is the optimum viscosity?

RE: Anyone using VARTM?
 

Mel, I'll give that a try...maybe later today.

Chuck, I'm trying to get an understanding of the layup dimensions...about 72" x 30"? If this is the case then you need to flow through 30" or use multiple resin distribution lines and suction lines. If the decreased vacuum = longer flow distances theory holds true, then you may be able to flow 30" with an ultra-low viscosity resin. Or you may be able to put one resin distribution line right in the middle so that you'd only have to flow 15". Depends...

Optimum viscosity? IMO, 1 cps...the viscosity of water. The thinnest system I've seen is around 140...which I may be trying next. The stuff I'm using now works well at 280cps but 140cps would be even better. I can't think of a reason why you wouldn't want the thinnest matrix you could find. Debulking the resin becomes a much quicker chore, flow rates are increased, microscopic bubbling of surface layers would be reduced if not eliminated, the list goes on and on...

-Tom

RE: Anyone using VARTM?
 

From what I've seen of RTM molds, even pressurized RTM systems, you definitely want to introduce the resin at a mid-point on your mold surface.

I don't know what your present resin supply lines are like, but mounting the resin above the mold on a stand, like an intravenous drip, with the supply coming from the base of the container and going to the central port might help also.

You'd then have the standing liquid weight in that line, adding a bit of positive pressure during the infusion cycle.

RE: Anyone using VARTM?
 

Thanks guys, it's making a good picture now. I've got some molds of a plane and the largest mold is roughly 1/2 of the fuselage, excluding nose cone. It is roughly 5 foot in length and I'm guessing a circumference of around 30", probably a little less. I'm going to end up buying some resin and am not adverse to experimenting, so I'll buy as thin as is available.

Supplying the resin n the center of the 30" dimension makes sense as it will have less distance to travel. How do you de-bulk? Are you using a vacuum line hooked up to a solids/liquids capture tank before going to the vaccum resevoir? At that point, I'd guess you'd really have to have a good feel for what is too dry a lay-up, or are you looking for particular indicators like on a standard lay up?

Lastly, do you save any time? It would seem to be a whole lot easier to produce pieces one you've got the process down pat.

RE: Anyone using VARTM?
 

Have you tried real Flow Media? I'm sure you would get better flow if you used true flow media instead of veil & strand mat. (check out [link=https://www.airtechonline.com/htm/defaultIE.htm]Airtech Online[/link])

https://www.airtechonline.com/airtec...rflow%2060.jpg

For quantities by the yard, instead of a full roll, contact Evan at [link=http://www.icancomposites.com/]icancompposites[/link]. Even though the site doesn't show it, he can supply the flow media in smaller quantities and a great price.

Just my opinion & everybody's got one - Bob

RE: Anyone using VARTM?
 

I've been keenly interested in this, but unable to put into practice because of extremely complicated molds(lots of deep corners). One person I talked to pressurized his in-feed pot for better flow. Are you guys sealing to the mold(with that thick caulk type tape) or are you making a bag for everything (including the mold) to fit into.

RE: Anyone using VARTM?
 

Hot tip: Heating the mold surface, layup and resin to around 100 degrees F increases the flow rate considerably. I managed to flow 12" through a pretty thick CF uni / BD stack at 25.5" Hg without any flow media. I'm not going to use any flow media...no longer necessary for me. Anyone want to buy some surfacing veil cheap? Great for eliminating print-through during mold making...

Bob, thanks for the info. Originally I needed (or thought I needed) an interlaminar flow media. This would have introduced a 'weak spot' in my laminates so I scratched the idea and decided to adjust my processes instead. That stuff looks like it would work great though!

I agree wholeheartedly.

I use a pickle jar. Drill a 1/4" hole in the lid and shove your line through it then seal around the line with sealant tape. Pull a full vacuum for 5 minutes or so (depending on the viscosity of your resin) and the bubbles will grow, migrate to the top, then pop! Using the ultra-low viscosity resins at room temperature still requires debulking, however, I didn't feel it was necessary after heating the resin to 100 degrees F...the viscosity drops EASILY in half...it was like milk.

Absolutely. I use a big pickle jar with a suction line, the resin 'drip line' and pressure gauge mounted on the lid. Works great and when it begins to fill up with resin, just toss it and use the same size jar so you can reuse the lid.

My 2 cents: I don't see the need for pressurizing the resin feed line unless the feed lines are exceptionally long. The amount of resin that the layup will accept is dictated by the amount of suction...forcing more resin into the layup would simply thicken the laminate near the area where the resin is introduced. That being said, true resin infusion (a different process altogether) requires pressurized lines for injecting the resin...as well as high-buck heated molds able to withstand the pressurization without deflecting.

I've had to do quite a bit of McGuyver-ing in the past week...I'm shooting for perfect parts every time and a few more tweeks and lessons learned the hard way, and I'll have it nailed.

Anyone know where I can get a 'high vacuum' pump cheap? I'd like to pull an absolute vacuum.

-Tom

RE: Anyone using VARTM?
 

First, excellent thread!

Now, what do you consider an "absolute vacuum?" What is your target pressure inside the bag? If you pump hard enough, air will diffuse through the bag and the seals. Pump harder and the epoxy will boil... ;) Not meaning to be a dork... I work in a ultra-high vacuum (UHV) surface chemistry lab.

Are you looking for 26psi? 27?? 28?? 29??? [X(]

Please keep this excellent info coming! :)
-David

RE: Anyone using VARTM?
 

Hi David, I was hoping that you'd chime in!

Good point on the resin boiling. Last night I had bubbles magically (tragically) appearing in a production piece. I was sure that the system was properly sealed. I theorized that the resin could be boiling and since I was infusing at an elevated temperature this certainly seems like a possibility.

I thought (there's that word again) that a high (29" Hg) vaccum would be the way to go. Maybe I need to rethink that. I'll try pulling about 20" Hg for my next piece and see if the slightly reduced vacuum and elevated heat combination yields improved results.

The elevated temperature does wonders for flow rates but may create other problems. There is no free lunch!

-Tom

RE: Anyone using VARTM?
 

Just as a clarification for others following the thread - the Flow Media (Airtech) I posted is a surface type media not an inter-laminar media. In a normal layup it would go on top of the peel ply and then directly under the vacuum bag material.

- Bob

RE: Anyone using VARTM?
 

Tom,
I'm pretty sure the bubbles in the epoxy are not from boiling... the vapor pressures of epoxies are generally very low. (I was just being silly) Your viscosity is nice and low also, and should be even better at the elevated temperatures. How long did you pump on the bag before you let the resin in? I wonder if the bubbles are from residual air pockets in the dry laminate.

Is it possible the heat is causing the epoxy to cure too fast, resulting in exotherm boiling? That seems unlikely since 3138 is a ~60min hardener. How hot were you making the stuff?

I've been playing with pressure bladders lately... maybe now I neet to try VARTM? ;)

RE: Anyone using VARTM?
 

David,

OK, if the epoxy wasn't boiling from the 25.5" vacuum, do you think it would be possible to go with more vacuum? I thought about buying an HVAC pump that pumps down to 50 microns...29" plus of vacuum.

I don't believe that the epoxy was exotherming. The temperatures weren't exceeding 105 degrees F. Maybe I just had a tiny leak that I couldn't detect. Before resin introduction I shut off the pump (there's a check valve next to the pump) and make sure that the gauge isn't moving. I leave it that way for five to ten minutes to check the seal.

The latest test parameters were: 25.5" vacuum, no mold or 'stack' preheating, heated the resin to 100 degrees F. With the mold being cool (73 degrees F) the flow rate diminished considerably in comparison to the 100 degree F heated mold / stack. Still a decent flow, no bubbles, the parts look nicer than as any cured prepreg laminates I've ever bought. I think that preheating the mold and stack is a better way to go. I'll remove the heat immediately before introducing the heated resin and allow an initial room temperature cure before postcuring.

-Tom

RE: Anyone using VARTM?
 

Tom,
Sounds to me like you're doing everything right. How are you heating the mold/stack? Electric blanket or oven?? Or?

You can make sure the epoxy isn't having issues by pouring a ~1/16" deep pool of mixed epoxy into a small plastic can lid and trying to heat that to ~100F in your vacuum jar. That should be able to convince us that there's no exotherming or boiling going on.

Sounds like this most recent test worked better though.

An easy way to get to high vacuum is to gang two diaphragm pumps in series. Rotary vane type pumps can be costly, noisy and require maintenance (checking/changing the oil). What type of high vac pump were you looking at?

Yes, I think an initial room-temp cure is important. I have seen bubbles form in carbon props I have made when I cured them at >100F with room-temp epoxy. After the ~12-24 hour initial cure, ramp the temp up to your peak as slowly as you can. I've had parts soften and print-through if the ramp was too fast. Are you doing your post cure unsupported, or in the mold?

RE: Anyone using VARTM?
 

I'm getting there quickly!

I've been using an electric blanket to preheat the mold and stack. I don't think that the 5-10 degree variance in temperature is much of a concern.

I'll try to test a small batch of epoxy heated in the debulking jar.

The results were very nice! Would have been perfect if the resin would have flowed quicker.

I have two diaphragm pumps...one is weaker than the other. Should I use the strong one downstream of the weak one or vice-versa?

I was going to see if I could borrow one from the heating / cooling company down the street.

I plan on doing all post-curing in the mold. My molds will only take about 170 degrees but I should be OK.

-Tom

RE: Anyone using VARTM?
 

That's a good question. Umm... I think basically you want the strongest pump to be furthest downstream with the reason being that it will be the pump experiencing the greatest pressure differential. In other words, it will see full atmosphere on it's exhaust and near vacuum on its intake, whereas the first (upstream) pump will have near vacuum on its exhaust and a little lower pressure on the intake. I could be wrong though, so try it both ways and report back which results in the lower ultimate pressure in the bag. ;)

Keep up the good work!

Good luck!!
-David

RE: Anyone using VARTM?
 

Airtech has a new system out. Give them a call and ask for Tom Doty, tell him I sent you and what your doing. He will help you in every way. He's good people.

Make a promise and keep it that you will share with him everything you learn.

RE: Anyone using VARTM?
 

VARTM or Vac Infusion is a very easy process that yields comparable end product to prepreg. The trick is to pull a complete vacuum at 30" and using resins that aren't any thicker than 250cps. I've been doing vac infusion for over a year now and it's pretty much the process of the future. With MACT 04 coming near, most boat manufacturers are scrambling to switch from open wet layup. Keep in mind that VARTM is only effective in large parts. Setup time is longer so doing infusion on a small mold would be pretty inefficient unless all you're looking for is the increase in strength to weight ratio. A lot of manufacturers now are developing reinforcement materials that aids in the flow of resin. Continuous strand mat, triaxle and grooved core to name a few. Woven cloth can still be used with the aid of the flow medium. As far as resins, Epoxical makes a special blend of epoxy thin enough to infuse. Other than that most vinyl esters are naturally thin enough for infusion. Reichold in particular makes a blend of polyester based resin for infusion as well. You can pick from different blends with cure time ranging from 8 minutes and up. A good VARTM setup should only take about 4-5 minutes for the resin to infuse completely.

Evan