Plastic peel ply

[dust]

To me that just looks like there was a small bubble in the plastic, it looks fine, if you hit the area and the surrounding area with a little sand paper, it will all look the same, it doesn't look "white, like no epoxy, just a buble and the weave is showing instead of compressed fiber surface.

 

enjoy the build

 

dust

[John Slade]

Ouch! Those are big images. next time crop them a bit to help the bandwidth challenged.

 

I agree with Mike. The layup looks just fine. As he says, those areas are where the plastic didn't stick down to the glass. Maybe you got the plastic a bit hot there and wrinkled it up. Either way, keep going. You're doing fine.

John

[Rui]

OK, thanks guys. My only concern was that it wasn't immediately appearant after cure. It was only when I put pressure on it did it "flake off". Before that it was as shiny as the rest of the layup.

 

I suppose this is just new builder anxiety, so please bear with me.

 

John, there are small versions of the files on there as well. Just thought that if they weren't detailed enough for you to tell you could also use the large versions.

[dust]

ok, i figured it out, the flaking off was a little epoxy thet was on the plastic, non event, it was not a epoxy void glass spot it was the rest of the buble that formed between the plastic and the layup.

 

you will also get marks like this from peel ply creases and bobles

 

enjoy the build, take a deep breath, relax

 

dust

[douglasrfix]

...

No. What Bulent and I are talking about here is the use of clear 6 ml plastic sheeting, as found in home depot, on top of the finished layup and any standard cloth peel ply. The technique has been described as "poor mans vacuum bagging". The plastic sticks to the wet epoxy and shows up bubbles very well. Squeege over the plastic with a little help from a hair dryer and you can watch the bubbles run along in front of you're squeege and out the edge. Wet the squeege with epoxy for a better slide over the plastic. The air can't get back in because of the seal caused by the plastic. Lots of excess epoxy can be removed this way for a very tight, compact and lightweight result. Do not press too hard because air can be sucked back into the layup through the foam - you'll see this when it happens because the area goes dark when you squeege it, then goes white again. If this happens lift the plastic and add more epoxy. After cure the plastic comes off in an instant and you have a very smooth, almost moldlike finish.

...

 

I stumbled into this the other day while performing activities that did not include cozy building

 

Article 8299 of alt.surfing:

Path: sol.ccs.deakin.edu.au!munnari.oz.au!news.uwa.edu.au!rhino!rd005.251.54.192.in-addr.arpa!conrad

From: conrad@jtec.com.au (Conrad P. Drake)

Newsgroups: alt.surfing

Subject: FibreGlass FAQ Part 2 of 2

Date: Mon, 17 Oct 1994 05:23:32 GMT

Organization: QPSX Communications Ltd

...

===== Fibreglass repair FAQ 0.2 17 Oct 94 ===== Part 2 of 2

...

...

regular rec.aviation.homebuilt contributors. Tune in there for more info (unfortunatley

they don't seem to have an all singing/all dancing composites FAQ). Also try rec.autos.racing.*

rec.boats,.building and rec.models.rc

...

...

 

Composite workshop review (by Daid Parrish)

-------------------------

This weekend, I attended the composite basics workshop put on by

Alexander Aeroplane in Griffin, Georgia and I thought that others on

the net would like to hear my impressions on the workshop and what I

think are some very useful techniques presented.

 

First off, a little about the workshops and in particular, the

instructor for the composite workshop, Stan Montgomery...

 

Stan Montgomery is a very good speaker, has a masters in chemistry, so

he knows his resins, and was a military pilot and has built numerous

homebuilt aircraft, so he knows airplanes and composite construction.

He's also VERY passionate about composites. This has caused some - er

- disagreement with illuminaries such as Dick Rutan, but anyone that

can make a spar shear web out of heavy, finely woven bid glass and

achieve 40% resin, BY HAND, and still use peel ply should be listened

to seriously.

 

For those that don't have the foggiest what I just said, some extra

information. The spar carries the weight of the plane through the

wings, and the spar web carries the load between the upper and lower

spar caps, forming a strong and very stiff I-beam inside the wing. In

fiber/resin composites, the fibers carry the load while the resin

keeps the fibers together. If there is too little resin, the fibers

don't stick together as well, and the part is weakened. If there is

too much resin, which is much weaker than fiberglass, then the resin

starts taking some of the load, and produces a weaker product.

 

The problem is, most homebuilders think that if resin is good, more is

gooder. But anything above 60% resin, 40% glass, is actually weaker

than 50-50 or 40-60 and is both heavier than it should be and more

expensive, since any resin, from $20 a gallon polyester to $100 plus a

gallon epoxies are expensive to buy.

 

Bid glass is a fiberglass cloth that has nearly equal number of fibers

both along and across the bolt of cloth. Peel ply is a light weight

and finely woven nylon or polyester cloth that is used as the last

layer in a layup. When peeled off, it fractures the resin surface for

a stronger bond with subsequent layups without using sandpaper, which

damages the glass fibers on the surface.

 

Back to the workshop.

 

... next Stan showed us how to a layup with 40% resin, yet still

be fully wetted out. First he cut out two ply of glass cloth to

approximately the right size, weighed them, and then placed them on a

piece of plastic and poured on a weighed amount of epoxy to an exact

40% resin, 60% glass by weight ratio. He then put another piece of

plastic over the layups and worked the epoxy into the glass. To evenly

distribute the epoxy, he would occasionally fold the glass and work it

some more, making sure not to crimp any folded glass and carefully

applied heat from a hair drier to thin the epoxy out. When he was

done, the layup was totally saturated with no white streaks indicating

dry spots. Once this was done, it was applied to the canard core and a

sheet of peel ply was squeegeed onto the surface, further reducing the

epoxy content of the layup.

 

According to Boeing, this is impossible. They can achieve a 37-63

ratio, but only by using multi-million dollar autoclaves. While a

40-60 ratio takes quite a bit of experience, I was able to do a 50-50

layup with no problems on my first try...

 

I do have one nit to pick with Stan on epoxy though. Being a chemist,

he wants exact molecular ratios of resin to hardener, therefore the

only way to do this is by weighing both the resin and hardener before

mixing instead of using an epoxy pump, which does do ratios by volume.

For me, working alone, all that extra weighing of resin and hardener

just takes to much effort and time. Assuming the pump is working

correctly, the volume ratio is based on the weight ratio of the two

components and the only weight ratio change would be from the

DIFFERENCE in the expansion rates of the components with temperature.

Since most epoxy systems has a 5% margin of error, I'm not overly

concerned about this difference, but I am going to retest my pumper at

various stroke lengths. Also, if you have a scale that can only

register to 2 grams and you're doing a batch of ten or twenty grams

for a small layup, you may end up with an error greater than 5%

anyway. Knowing the weight of the glass and the epoxy used in most

layups I totally agree with.

 

Another neat technique he showed us was with unidirectional spar cap

tapes. The tapes are only a few inches in width and produce a thick

layup. The rovings are held together with a sparse cross thread, but

any weaving in a glass cloth reduces the strength. What he did was

find the single fine thread on the edge that held the cross thread in

place and removed it after the tape was placed on the canard. Once it

was removed, the cross thread was carefully removed, leaving straight,

flat fibers in the spar cap. That one even surprised the epoxy

manufacturer that sat in on our workshop on Sunday.

 

Recommended tools were scales, layup rollers and a hair dryer. The

scales are used for weighing the glass and epoxy to calculate their

weight ratios. The rollers are for working out air bubbles and to

distribute the epoxy. He was against using a paint brush to remove air

(a process known stippling) because it tended to break up bubbles

instead of removing them. Layup rollers are shaped something like a

small paint roller, except the roller is plastic or aluminum and has a

grooved surface that allows entrapped air to escape. The hair dryer is

probably his favorite tool. With it he can drastically thin the epoxy

to improve wetting of the glass and speed up the setting time.

 

Another point he made was that all epoxy layups should be post cured

at an elevated temperature. All epoxies have what's called the glass

transition temperature, where it looses it's strength. They all have a

maximum transition temperature, such as 190 degrees Fahrenheit, but

the actual temperature that it weakens is only thirty or forty degrees

above the temperature the resin was cured at. If it's 60 degrees when

you make a wing, the wing will sag when the skin reaches only 90 or

100 degrees. Not good if you fly down to Sun 'N Fun!

 

To fix that, after the initial cure is done, Stan post cures the part

by heating it to 130-150 degrees for a few hours with the part

supported so it doesn't bear any weight. This can be done by painting

the part with black tempera and leaving it out in the sun or by

putting it in an 'oven' made out of cardboard boxes and a small

forced air heater.

... I cut the tapes to length, weighed them, and

poured an equal weight of epoxy over them on plastic film. With the

layup roller, I spread the epoxy out. With narrow bid tapes, this can

be a bit difficult without the tapes distorting, but the roller did a

good job as long as I didn't move the epoxy ahead of the roller too

quickly. They're rather expensive, but I think they do a better job

than stippling with a paint brush.

 

Once wetted out, I cut the film to rough size and carried the whole

thing over to the plane and put it in place. One thing though. The bid

tape conforms to the surface much better than the plastic film that

keeps it from stretching, so you have to carefully peel the cloth from

the film as you put it in place. Once in position, I used the corner

and layup rollers to press the cloth in place and covered that with

peel ply squeegeed in place.

 

When I peeled the peel ply off the next day, there were streaks were

there was no epoxy between the bulkhead and the bid tape, mostly on

the vertical surfaces of the bulkhead. The problem has to do with the

surface of cured triax cloth that was used on the bulkhead. In triax

cloth, there are three layers of fiber bundles, stitched together like

a quilt instead of woven, giving it greater strength. But it also

makes the surface more uneven, with valleys between the bundles of

glass fibers. The streaks I saw were the valleys that had not filled

with resin.

 

My mistake was not heating the layup with a hair dryer to thin the

resin out. I had even prewetted the triax with resin on one side as a

test before taping and it didn't seem to make much difference. As a

second test, I cut out a 3 by 3 inch piece of bid and laid it over the

original streaked tape and out onto the bulkhead, this time using

heat. When I peel the peel ply off this time, the dry streaks were

OVER the bundles of glass, indicating I'd used too much heat and

pressure and had worked too much resin out of the cloth. The valleys

were filled nicely though. Ah well. Live and learn.

 

One thing I feel fairly certain about is the actual best resin to

glass ratio will depend heavily on the weight and weave of the cloth

and how much work you want to put into thoroughly wetting it out.

 

The next time I try layups like this, I'll try the other technique he

showed us. In it, he marked off the size and shape of the layup on the

plastic film with a Sharpie pen and cut the cloth to approximately the

correct size and shape. After weighing the cloth, he poured an equal

weight of epoxy directly on the film and spread it evenly with a

plastic bondo squeegee. He then laid the cloth on the spread epoxy and

worked the epoxy into the cloth. When the glass was completely wetted

out, he used a razor blade to cut both the cloth and plastic film to

the marked line, leaving a layup ready to be used.

...

David Parrish