Do you HAVE to use everything in order, mold, prepreg, peelply, breather, bag....

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JustLearningKristin

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Do you HAVE to use everything in order, mold, prepreg, peelply, breather, bag.... I am asking because we have been playing with other ways, in our shop, of making a part. We have been playing with other methods to reduce the amount of time spent sanding, filling, resin coats, more sanding etc... one of the methods we tried was applying the prepreg to the aluminum mold and spraying chemlease in the vacuum bag and using a small piece of breather towards the bottom of the mold so the vacuum can preform its duties, then just attaching the vacuum and putting it in the oven and curing it. It seems to be working because the part is a lot more smooth out of casting and is requiring a lot less time prepping it for prime and paint. The pin holes have nearly become extinct. Now I have been trying to find information about why we have to use the peelply? Is there someone who knows why these layers have to be used?
Thanks!
Kristin :)
 

wsimpso1

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Geez. No magic exists in making parts with molds and a vacuum pump. If you have the resources and willingness to experiment or different goals, knock yourself out. Success is something that you measure...

What follows are the opinions of one guy who has methods that work for him. You are getting these opinions for free, and they might even be worth what you are paying for them. I do know that if you were working with my material set, my tools, and my goals, these opinions would save much learning time and make min weight parts. YMMV.

That being said, the standard methods you find written up are used for a reason. They make light parts and they work. Do something else and you will be experimenting, which may allow less expense or more appropriate parts for your purposes. You will also spend money and time on your science project. Maybe it is worth it, maybe not. Me, I tried to get as close to the tried and true as I could to minimize my experiments and get on with making parts.

Here are some examples:

Orion has written that he can not trust any peel plies to leave contaminant free surfaces, so his shop sands surfaces that had peel ply pulled from them before subsequent bonding or filling.

Some folks will spray or brush a gel coat into the mold and let it firm up before laying up the rest of the part. They want to reduce subsequent finishing and are willing to put up with the higher weight of a gel coat instead of filling with dry micro.

One standard way to build a sandwich part is to build it up in several layup-cure cycles. The outside skin facings exactly reflect the mold, and you might reduce finishing a little.The other method is to build it in one layup cycle and be done. I prefer this, as there is all sorts of external stuff I have to fair and finish later anyway, and in this way, I know that all of the pieces in that part have sold chemical bonds between them. My wing skins and part of my fuselage were done in one cycle. My stack is peel ply, perf ply, peel ply, cloth, core, cloth, peelply, perf ply, batting/breather, and then bag. I punch a pattern of tiny holes in the cores. The two layers of peel ply between part and mold and the holes in the core are to allow the excess epoxy in the outside layer of cloth to migrate out of the part. I found that I needed the holes and the double layer with my materials and process. You do what you must.

I found that my batting could draw more resin from my lamination than was good - dry layups. Perforated ply lets excess be squeezed off, but keeps the batting from drawing resin.

I found that on my highly curved fuselage parts, which were made on male molds, I had to do cloth against the mold, and cores (and peel ply) and stop there for the first cure cycle, then do the other facing in a separate layup. Again, you do what you must.

When I was making pass tubes for my controls to pass through, they had to be fuel tight. First try was conventional with peel ply and perf ply and batting. It made a better filter than fuel tight tube. So, I skipped peel ply and perf ply, and just wrapped it in visqueen before bagging. A little heavier, but absolutely fuel tight.

I am not sure of what you are doing. Putting your cloth against the mold may reduce your finishing, and may be worth it. Since I will ice everything with dry micro, I like the shape of the min epoxy surface to get the micro into. I do not know why you are spraying chemical release on your vacuum bag - they generally come free from cured resins pretty easily without release... Now, putting breather only in a small part of the mold may not end up applying full vacuum to the whole part, which may or may not be bad. It sounds like you may be leaving what most of us view as excess resin in the part, which ends up filling the cloth weave on both sides with straight resin, and no doubt leaving some more excess resin in the part as well. Normally, you don't want any excess resin on the inside (It is just extra weight and somewhat reduces the strength of subsequent bonding). On the outside, you are filling with stuff that weighs 11 pounds per gallon and is hard to sand instead of using stuff that weighs 4-5 pounds per gallon and is easy to sand. Yeah, you may be making what I call "heavy parts". Now maybe the weight gain is OK with you. You get to decide.

If you are doing layers of filling and sanding to finish, you are working WAY TOO HARD and I can bet that you are getting lousy results at the same time. Look up George's method on Cured Composites.com, and the Prime Directive among the Cozy builders. This set of techniques fills with minimum weight, makes smooth parts, and the neat epoxy trick at the end results in pinhole free finished surfaces with a minimum of fairing and finishing time.

I look forward to more details on your methods.

Bill
 

orion

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As Billski pointed out, in learning about these processes, you will discover what works for you and what doesn't. You will also find the materials you like to work with and which make for the best parts. All this goes into the proverbial mix and creates that which for you is the ideal process.

In our current work, most of what we lay up goes directly onto the tool surface - the planes are going to have a finishing process anyway so why add the unnecessary weight up front, especially when much of it will have to be sanded off eventually anyway. The laminate (we do wet and prepreg) then gets a layer of peel-ply, then breather and then bag.

As far as the peel-ply is concerned, the reason we sand is that a few years ago the Boeing Company encountered some rather embarrassing (and expensive) failures of secondary bonds. A close investigation revealed that most of the peel ply fabrics are coated with a number of compounds (including Silicone) that may come off and actually adhere to the resin during cure. Since the original purpose of the compounds is to prevent the Dacron from sticking to the part, now that the parting compounds were a part of the laminate, anything that was bonded to the part in a subsequent operation was compromised.

But we still use the peel-ply (and ours we think is uncoated) because it is much easier to sand that surface than a shiny one.
 

JustLearningKristin

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Apr 29, 2012
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Thank you both for the info... I ordered some of the perforated peel ply to see how that changes what we are doing... We are also working on some silicone wraps or sleeves to help give the outside a better surface... we have to finish the parts with a "Class A" finish, so the less pin holes and flaws we start with, out of casting, the better. Does anyone know what kind of material is the best to use for making a mold and/or reverse mold? We use prepreg so whatever we use must be able to withstand temps of @ 120f.
Would you guys say that prepreg gives the best finish out of casting or would a wet lay up be better?
Any suggestions about getting the best (least pin holes) surface out of casting would be awesome!
 

orion

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Prepreg cure at only 120 deg. F?

Regarding best mold materials, it really depends on what you're trying to do and how many parts you want to produce. The best production molds use graphite and a high temperature resin - these are similar to those used in aerospace and if properly cared for, can last for hundreds of parts. Making them is a several step process since they are taken as a splash off of a high temperature capable plug or at times, even machined from a thick laminated billet. They can be designed for as little as a 250 deg. F cure or in excess of 350 deg. F., depending on your needs. But as you might expect, they are expensive.

You can also do your own tool, taking a mold splash off of a stable plug. In this example you'd make the mold from a quality high temperature Vinylester (good for 300 to 350 deg. F), which can be laminated as a room temperature cure product. For a best surface finish you can polish the plug and then apply a Vinylester based tooling Gelcoat before laying down the mold laminate. We've already had several threads here where the process of this type was described in pretty good detail so you may look around using the built in search function to find the particular discussions.

In order for these tools to be best supported, you can build either a graphite support structure or framework, or even a steel one seems to work pretty well.

As far as taking care of pinholes, you can look at a couple of ideas. First, before applying the laminate, you can spray on a thin layer of Vinylester primer or Gelcoat. This will give you a very nice surface (as nice as your mold surface finish of course) and the pin holes, if any, wont be an issue.

If you're after just having the graphite show, the only thing you can do is increase the amount of resin in the laminate. Two ideas can work here. First, you can eliminate much of the breather in the vacuum bag. If you just use PeelPly you'll still get a pretty good suck but you wont be absorbing the excess resin away so the number of pinholes should go down. then, you can also order your prepreg with a higher resin content. If I recall right, most prepregs come in at about 33% resin - if you order maybe around 38%, your pinhole issue should get better.
 

berridos

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I was surprised to read recently a guy that build, i believe it was hockey sticks, in a mold and that avoided gel coat due to the weight. Instead he sparyed 2k urethane paint into the mold and laminated afterwards. He told the result were absolutely satisfatory for a fraction of the weight and I would say adequate uv resistance.
 
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