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Old Aug 15, 2012, 05:04 PM
2012 NZ Speedcup - 231 MPH
DownUnderPilot's Avatar
New Zealand, Tasman, Richmond
Joined Mar 2006
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Help!
Surface finish

Help!

I am making my own folding carbon props, from my own moulds. These are coming out OK, although I'd like to get a better finish on the prop.

My moulds are very smooth and polished. I use release wax and PVA applied with a sponge brush, but it's difficult to apply uniformly with one stroke and I am going to switch to spraying it on.

Occasionally I get tiny craters on the moulded part. Sometimes I get bigger ones. Can anyone give me some tips?

I am using West 105/206, which although clearly not the best, does work very well for props as it has a very slight amount of flexibility which seems to be about perfect for these sized props (7x10). The mould halves are clamped together for curing.

Any help would be appreciated!
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Last edited by DownUnderPilot; Aug 15, 2012 at 06:31 PM.
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Old Aug 16, 2012, 03:48 AM
Composites guy
North OC, Ca.
Joined Jun 2005
1,187 Posts
Difficult to tell if this is a air bubble or a PVA bubble. more pics perhaps?

How are you introducing the resin to wet-out the part? What draws the air out of the part?

Is there any dry material at all in the tool?

The curvature may act as a pocket at that location.
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Old Aug 16, 2012, 05:31 AM
2012 NZ Speedcup - 231 MPH
DownUnderPilot's Avatar
New Zealand, Tasman, Richmond
Joined Mar 2006
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OK... the resin is poured onto the cloth and then rolled/scraped to wet out. It sits on paper backing (impossible to apply otherwise).

What do you mean by "dry material in the tool"?
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Old Aug 16, 2012, 09:35 AM
Registered User
Australia
Joined May 2010
32 Posts
Hi,

What composite layering are you using? E.g 3/4 then 3 ounce then the carbon?

I also use the west system and am trying to get a servo mold working at the moment, it has complex curves as well. And have seen similar problems, but believe the issue that I have is the female and male mold aren't accurate enough, and it creates slight voids.

One other area that I am going to look at is resin viscosity, might try thin it down with methanol. As I am using a vacuum system instead of clamps.

Hope this gives you a few ideas.

Cheers,
.adrian
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Old Aug 16, 2012, 11:19 AM
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Old Aug 16, 2012, 11:42 AM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
USA, OH, Worthington
Joined May 2002
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Some ideas:

Use a clearcoat lacquer or two part in the mold first. That can help the epoxy "lay down" better than it sometimes does on a PVA'd surface directly.

Wet the outer layer or two of cloth out before you put it in the mold. You can ensure it's fully wetted by placing it between sheets of waxed paper and rolling with epoxy, then rolling it with paper towels to remove excess.

Possibly wait until the epoxy starts to get tacky to put it into the mold. On some complex layups I mix a small amount of epoxy and a faster hardener to get the outer layers "sticky" so they stay put in the mold while I lay up the inner layers with another batch of resin.

Just some ideas. When you're laying wetted cloth that likes to trap air against a sealed surface without a bleeder, bubbles can be hard to eliminate.

BTW... don't thin the resin. That likely won't help, it will just reduce the physicals of the finished product.
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Old Aug 16, 2012, 02:18 PM
hot air rises...
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Pleasant Grove, UT
Joined Jul 2005
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If it's surface finish you're looking for I would suggest trying a semi-permenant release (like Freekote) followed by a sprayed on clear coat.

I never did get hang of getting a nice surface finish using PVA, but without a doubt Tom has the PVA thing figured out.
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Old Aug 17, 2012, 10:39 AM
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Australia
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Quote:
Originally Posted by ibuild View Post
Hey, where is the methanol when the epoxy is cured?

Regards
Hi ibuild, my understanding is that it just flashes off.

As I haven't yet tried it, I will bow to Tom's experience as this is pretty new to me. I also need to do the strength tests as I was told that it doesn't weaken the bonding. But again something that needs to be checked.

Cheers,
.adrian
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Old Aug 17, 2012, 09:03 PM
Composites guy
North OC, Ca.
Joined Jun 2005
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Okay so you are making wet-preg: pre-impregnated cloth on a paper. When that goes into the tool, it is easy to trap air against the tool ( think of folding over scotch tape with out squeezing out the air). The air may become trapped against the tool and may not want to come out.

It may help to put a layer of resin onto the tool first and then the wet-preg.

My comment about the dry material is to use a light glass(.7 oz/yd^2) against the tool and let that act to bleed any air out of the part as it becomes impregnated by capillary flow.

Scott
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Old Aug 19, 2012, 06:54 AM
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United States, Death Valley
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Do try spraying the PVA. I have never applied PVA with a brush, seems like many do though.

When you spray it you can get it like glass and perfectly uniform, and the parts finish will only looks as good as the PVA coat that was applied to the tool. if you look at the tool's gloss after the PVA has dried, thats the exact finish your part is going to have.

If its warm (like 80*f or above) I thin it a little with water.
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Old Aug 19, 2012, 04:49 PM
2012 NZ Speedcup - 231 MPH
DownUnderPilot's Avatar
New Zealand, Tasman, Richmond
Joined Mar 2006
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Quote:
Originally Posted by sarmoby View Post
Okay so you are making wet-preg: pre-impregnated cloth on a paper. When that goes into the tool, it is easy to trap air against the tool ( think of folding over scotch tape with out squeezing out the air). The air may become trapped against the tool and may not want to come out.

It may help to put a layer of resin onto the tool first and then the wet-preg.

My comment about the dry material is to use a light glass(.7 oz/yd^2) against the tool and let that act to bleed any air out of the part as it becomes impregnated by capillary flow.

Scott
Great ideas Scott, I'm going to try putting a thin layer of resin on the tool first. The light glass also appeals as well. I'm making one change at a time so I can understand what is the cause of the problem.

Incidentally, I used a lot more clamping pressure with my latest trial and it completely eliminated the extremely tiny craters. I still got one large buddle or void, but I'm wondering if that's the resin's capillary action pulling the resin off the tool when clamping pressure is applied.
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Old Aug 19, 2012, 04:51 PM
2012 NZ Speedcup - 231 MPH
DownUnderPilot's Avatar
New Zealand, Tasman, Richmond
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Quote:
Originally Posted by SilentHunter View Post
Do try spraying the PVA. I have never applied PVA with a brush, seems like many do though.

When you spray it you can get it like glass and perfectly uniform, and the parts finish will only looks as good as the PVA coat that was applied to the tool. if you look at the tool's gloss after the PVA has dried, thats the exact finish your part is going to have.

If its warm (like 80*f or above) I thin it a little with water.
SilentHunter I watered down the PVA by about 50% and used a very fine brush and this appeared to work very well. I got a great blemish free dried finish on the PVA. Release was as good as the undiluted PVA.
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Old Aug 19, 2012, 11:44 PM
Thailand
Joined Aug 2010
527 Posts
I only ever use wax. I never use pva or anything else.
I never have release problems.
The thing I use is a spray on paint as a gel coat. It is a 2 part polyurethane that has to dry quickly , very quickly infact. You can spray on a layer and in 10 seconds it's touch dry.
There is also a bit of a technique to spraying on wax but easy enough.
Probably lucky here in Thailand as it's so hot and paint does go off quickly.
It never sticks though and we recently did a Jet ski hull that just popped out very easily.
Jim.
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Old Aug 20, 2012, 10:56 AM
G_T
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Joined Apr 2009
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Currently I thin my PVA, but not 50%. At that level, the resultant PVA film is rather thin and tears easily. I prefer to add not more than 30% water, generally less. Put it on wet, then blot out the excess from your brush and collect the excess that pools in the bottom of your mold. Don't fiddle with it too much. It should flow out pretty well. Let the PVA thoroughly dry. My hatches come out with a mirror finish this way. That is probably roughly the same size as the parts you are making. Large surfaces are more of an issue because the PVA starts to tack and you have to work quickly. If you brush over it when it has started to tack, the odds are you will add brush artifacts which can be seen in the final product.

I do not wet out any fabric before putting in my mold. I paint a coat of epoxy on the mold tooling surface first. Then I lay in the first layer of fabric, which for my hatches is light S-glass. I put enough epoxy in the mold to wet out this layer completely, but not really any more than that. I blot the layer of glass down onto the epoxy and it wets out. Once wet out, I bruch it to make sure it is evenly wet out. That can be necessary due to vertical surfaces. The epoxy by itself will try to flow to the low points. The longer it takes to get the fabric in the mold, the more the epoxy will have flowed away from where you put it.

Then I add additional epoxy over the first layer of fabric, for wetting out the second layer. Then I blot in the second layer. Repeat process until done.

This way, the epoxy starts at the mold surface. One never creates a situation where you have wet fabric going on top of wet fabric, which can trap bubbles which then need to be worked out. Big bubbles are easy enough to work out, but small ones are hard to see and harder to remove. Better to avoid the methods which create that situation IMHO. But you pay for it in the extra time this methods takes, vs just wetting out the fabric first.

In areas where there are sharp compound curves, fabric will not conform well. These areas may need an extra patch of very light glass and a bit of splooge (epoxy + cabosil + possibly a little graphite if the fabric is carbon). This is used to build a paste region with less abrupt curvture that your fabric can conform to without undue stress.

If you still have problems, you can thicken the first layer of epoxy that goes into the mold with some cabosil. It stays in place better as it is thicker. I don't do this now but I have done it in the past.

Gerald
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Old Aug 20, 2012, 05:49 PM
2012 NZ Speedcup - 231 MPH
DownUnderPilot's Avatar
New Zealand, Tasman, Richmond
Joined Mar 2006
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Great stuff guys, much appreciated. I tried one last night by firstly painting on a very thin layer of resin and letting this gel/tack up. The I applied the carbon cloth and wet it out. I got ZERO bubbles and craters! This seems to be a valid approach, now to see if I can replicate it.
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