Help: First Time vacuum Bagging Problem

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#1

Hi,

I tried wet lay and vacuum bagging of carbon fiber composites for the first time for a friend. Fuselage for RC planes and canopy for his old quad copters.

Process:
Made fiber glass moulds for the parts
Used either hand lay up or vacuum bagged them.
Slow 24 curing epoxy resin, no heating or any other curing process.
Fabric: 240 gms to 280 gsm twill CF
FRP release wax used as release agent.
Another liquid release agent also applied over wax (probably PVA but not 100% sure)
For vacuum bagging used both traditional method (vacuum on one side, mould on another). This method caused the mould to flex or warp.
Other method was to put the mould and fabric directly in a bag and seal open end. In this case vacuum pressure is on both sides. No flex or warp of the mould under vacuum, no leakages.
First I apply release wax (at least thrice per mould with 15-20 min interval). Then added a thin layer of PVA by hand/sponge. When I use PVA surface finish seems worse, tends to stick to the body.
Pure wax release agent without PVA is much better.
For the fabric, I tried both methods, first wherein we drape the fabric in mould and then apply resin. Second much better method was to apply resin to fabric first and then drape the wet fabric into the mould. Apply excess resin as required.

Please have a look at the pictures.
Issues:

  1. Wrinkles. No matter how careful I am I still get wrinkles in what I think are perfectly simple surfaces. This is a problem in vacuum bagging

2)Fabric not sticking to the mould. Even under vacuum bagging with no leakages, there are certain areas where the fabric just does not stick to to the mould. Even on relatively smoother curve surfaces the same happens.

  1. Finish:
    Had pin holes initially. Started applying resin to the mould and the problem solved. But it started another problem of sometimes the extra surface resin sticking rather onto the mould then the actual part. So get some shiny resin rich areas and some mat finish type areas.
    Also seem to get cloudy surfaces.

  2. Fabric overlap areas: Since fabric does not drape well over all surfaces, I tend to make cuts at points of wrinkles. The cuts look shabby externally, specially in wet layup. In wet lay up they also tend to stick out and I got a bad cut while handling it. Any method of avoiding messy cuts or making cuts cleaner?

The finish is quite ok as a mat type finish at some of the better areas but I want a glossy finish. Adding Lacquer or clear coat will be better but I need to sort out the above issues first.

Thanks in advance.

#2

First off, you may not benefit from the infusion process if you only intend to produce a few parts for a project. As you just discovered, infusion can be a tricky process which often requires a great deal of trial and error to get the vacuum and resin flow lines in the correct places, control of resin flow, mold design, etc.

Not to discourage you from attempting or learning the infusion process, but you may want to try to lay up a part by hand and see what the results are. If you lay up a part by hand, you will benefit from a top coat, which will do two things to improve the look of the finished part. The top coat will give you a bubble free clear finish, and it will also give you a tacky surface to press the first layer of carbon onto. The initial top coat will need to be applied an allowed to cure until it is tacky, at this point you will begin to apply your cf and wet out each layer. The first layer is critical since it needs to be in direct contact with the mold surface, this initial tacky top coat will help with this.

I have also attempted to lay up a part by hand, followed with vacuum bagging to compress the laminate, which can work great as well, you just need to make sure not to allow the laminate to shift when you are setting up the bagging material.

#3

You didn’t read his post at all did you? :stuck_out_tongue:

#4

I guess not well enough…:amuse:

#5

Using enough breather? Should be thick and not fully saturate during the pull

#6

Gloss - suggest putting a thin resin coat on the mold and allow to set to a tack then proceed with reinforcement (below). Oh - either use chemical release or spray the PVA over wax. Dilute with water (a little at a time) to get it to flow. Mist the first coat to prevent beading on the wax. Be careful with resin application, a sticky brush will lift the pva.

Pinholes, I coat mold with resin, then saturate the reinforcement on saran wrap and then lay on the mold. The second reinforcement is applied dry then stippled. The double coat method will ensure saturation. Allow it to soak a bit before adding more resin.

Apply peel ply or perf, then breather to absorb excess resin.

Wrinkles - Insert into the bag. Evacuate slowly and make sure the film does not pull tight anywhere, make pleats as needed. Progress the vac slowly as this is ensured.

On your relatively flat parts this should eliminate the wrinkles, give a gloss surface and good resin coverage in the part.

I don’t always make perfect parts, but this should help you make progress.

#7

@BillL

Thanks BillL

Can you elaborate more on the wrinkles part?
I guess my pump too powerfull and need to adjust the ball valve to make it slower.
Regarding vacuum bag pulling tight , I have relatively deep draw in my part (Imagine a U shape with depth about 105mm/4 inches and mouth opening of 108 mm )
The bag I got from supplied wouldn’t touch the bottom under vacuum.
Changed the bag to some polythene bags , they worked better but still not clean enough. Result was the bags would be very tight at the top edges.
I have an idea of adding lots sponge/foam cubes of very soft kind between breather and vacuum bag, so maybe the bag would compress the sponge , the sponge would add pressure to the fabric/mould.
Also maybe if sponge put correctly, it will help reduce pressure on the edges.

Thanks

#8

@ smp4616:

Yes I guess that’s the problem, Usually my breather is full or completely saturated.

Thanks

#9

Your vacuum bag isn’t big enough, you need more bag with more pleats. 105mm isn’t deep at all, you will easily be able to bag that.

Use a bag that’s twice the footprint of your mould.