Various Questions...

General Composites Discussions Composites Talk
#1

As my first thread, I have compiled a few questions that I could find answers really and was hoping some experienced people could help me out with.

I have just taken up carbon fiber as a hobby and would like to try my luck at making parts.

One question I have is when making a mold of a car part would silicon be sufficient or would I need to make it from fiberglass? I know that a steel or aluminum would be better but as a hobbyist I would like to keep some costs down if possible.

Second, I read that vacuum bagging is a necessity for a better final product. Would putting the vacuumed bagged item into a home made oven be helpful? And if so, could I use pre-pregs in this scenario?

Lastly, I haven’t really seen an answer to this but after the final product is taken out of the mold, what do you do to the edges? Do you just sand it down and have it smooth or what? Also if I were to use nomex hc, obviously you would not use it to the edges…I would just seal the nomex mostly in the center of the item such as a hood between layers of wet layup of carbon?

Hopefully my questions are clear. Also, thank you for any responses to my questions.

Zac

#2

To further drill into a home oven, if I were basically to make a big steel box and put heating elements into the box with it controlled through a PID would that be fine enough to be considered as an autoclave or no?

#3

Your oven would work but it wouldn’t be an autoclave. Autoclaved are large ovens that are also pressure vessels.

Here is a thread about homemade ovens
http://compositescentral.net/showthread.php?t=3487

Using an oven will reduce cure times of wet layups but they will cure at room temperature. It is mandatory for layups with prepreg to cure them.

#4

Also thank you very much for reading around before posting questions!

#5

Thanks for your reply Canon, yea I found that thread about the home made oven but there wasn’t enough information in it. I’m curious though, if you were to use a vacuum with the oven would it be similar to the effects of the autoclave where I can just purchase prepreg carbon?

I found my answer for the edges, and what I found was rolled up edges. There are some terms I don’t understand, but what I got out of it was that I should incorporate the “rolling” edge into the bottom piece of the mold and just bond them together and sand and smooth it out to have that look. (Starting to think for my own application it’s just a waste of time and weight…lol)

Still looking for an answer to the mold material. Is silicon a good idea or something of harder material?

Sorry but one more question… Can you create a mold over say… a hood of a car and mold with modeling clay or is there something I can put on the hood to create vents and remove after the mold is created?

Thanks again guys.

#6

As a home builder you cannot recreate an autoclave. An autoclave is pressurized, and ramped to temperatures above 400f. The best you can do as home builder is make a good oven to cure prepreg or learn vacuum infusion. I don’t know whether silicone is a good mold making material or not but if it’s stiff enough I don’t see why not. I don’t understand your last question. Do you mind rewording it?

#7

If your really considering prepreg you should make the mold out of the same material as the part is going to be or use a material that will closely match its CTE. For example if you are going to do a prepreg carbon part you would want your tool made from carbon fiber and high temp tooling resin or a low CTE metal such as Invar. However there are many low temp intial curing prepregs you can get these days that can cure as low as 122F then can be demoulded and post cured to get the properties needed. For a tool curing a lower temps like this you do not need to worry as much about your molding material since the expansion would be very minimal.

#8

A bit far off there! Autoclaves generally run pressures in the range of 40-100 psi. 150-200 is about the max, and that generally requires an autoclave specifically built for that.

#9

My mistake and corrected. I think I got pascals and psi confused but now I won’t anymore. Thanks!

#10

Thanks for the replies.

As for my earlier post “Can you create a mold over say… a hood of a car and mold with modeling clay or is there something I can put on the hood to create vents and remove after the mold is created?”

Sorry about the wording. I was wondering if there is a material that can be easily removable after creating a mold from an oem hood. Say that an oem hood has just a basic concave curve but I was to add a few increasing “ramps” of material.http://bulletproofautomotive.com/catalog-detail-image.php?img=V+Hood+1.jpg&item=2297

#11

This is very interesting… I will have to look more into this. I can understand that you would want a mold to be the same material that you want to make due to material compatiblity.

So a home crafter can make use of prepreg material. After so much reading I know that I would like to play with prepreg since it is lighter and stronger than wet layup.

Thank you for the information!

#12

Just be aware that it is a lot more expensive than other methods. Both the prepreg and tooling get very expensive. composite companies can justify the higher cost in the savings in labor cost, repeatability, accuracy, strength/weight, and lower skill level generally needed. For a beginner it may not be too practical to start off with prepreg because you will not be able to purchase large enough quantities to get good pricing. Also your oven needs to be constructed well, needs good circulation and controls. My suggestion is start off doing wet laminations/vacuum bagging, then infusion. Then move on to prepreg after. But if you can afford it go strait to prepreg its a lot better method.

#13

I second what Hojo said. Prepreg creates better parts but it is also much more expensive. If you’re making hoods it doesn’t really matter how strong the material is and even how light it is to an extent.

#14

Thanks for the information. I think you are right on the direction where I should go to wet layup and then go to infusion. If items come out good who knows then…lol.

Thanks again for the info. If I were try to make best quailty parts what would be the best material for the molds then?

#15

To keep cost down and for learning sake just start off making fiberglass molds using vinylester tooling gelcoat and resin. You can make them pretty cheap and quick, they work well for vacuum bagging and infusion and it will give you experience. Theres not one way to make a part so its always good to be well rounded. Try to learn as many processes as possible.