Prepreg Mold Suggestions

General Composites Discussions Composites Talk
#1

So I will be making my first “on my own” part in the next month or so. I am fortunate enough to have access to prepreg carbon fiber which I plan on using for this project. The part itself is used to protect a battery from rocks and debris with little to no structural strength requirements. I plan on doing 3-4 layers for this project.

Attached is a PDF file of the part I am trying to make and the outer surface is the visual side. (Look at it upside down and I will be laying up inside that theoretically).

I have a couple of ideas for the mold, but am open to suggestions/recommendations as I have very little experience with composite materials.

1st. Make a wood plug of the part and layup over top with fiberglass to make a mold. I was told if I need to do this, I would need to “reinforce” it (Not sure if I understand 100% what he meant), I am thinking it means use thick matt on the back to add more rigidity?

2nd. Mount 3 pieces of rectangular tubing to a plate and fill (form a radius) at the base with some sort of filler that can handle the autoclave temperatures.

3rd. Split the design into 3 pieces and form them out of sheet metal, and screw them to a plate trying to match up the edges so no seam is shown.

Thanks for any input guys.

Edit: By the way, only a single part is required from this, however I am under the assumption that I may need to redo it a couple of times as it is my first time doing it alone.

#2

Cheapest one off part (or 2-3 depending on your luck) would be bending sheet metal. Prepreg needs to be cooked, so you can not use something that will burn/degrade at the temperature you are cooking at. Fiberglass molds need hightemp resins and gelcoats. (though, for a part like this, you don’t need a show quality surface) Metal sheeting can work well if you bag it well. the autoclave pressures are hydrostatic, so if you envelope bag it (ie, have bagging over every part, no gaps), you should not have any warpage issues.

I just noticed the size of the part. sheet metal would be a good way. There are pros that can bend small parts like this. Else, try a thick walled tube idea like your 2nd idea. Make sure your seal the ends of the tubes, or the bagging will burst!

#3

I am waiting a response from someone regarding the use of MDF as a mold material. I know they tried it last year and they ended up pre-heatining it prior to shaping to reduce any possible chance of stresses. If it worked out for them I think I would use the same method.

I was playing around with the idea of sheet metal today and have had troubles with the center of the part (the already formed piece would jam itself into the break). Going to look at other ways of bending it to produce the desired shape.

Thanks for the tips though, much appreciated.

If I can’t figure out a cost effective way of making a mold for this, I might just cave and do a wet layup.

#4

There is a few ways of doing it…usually though, marrying materials for expansion rate in an autoclave become an aspect…carbon mould to carbon part, glass to glass, etc…this way the mould expands at the same rate as the part.

This may sound stupid, but why not cast a mould for it…make the plug out of wood, spray it with DT EZ sanding primer, wet sand the plug down, stick the plug to a piece of glass, or PET, acrylic, steel table etc…model clay in the gaps from the plug to the table or whatever, wax it (a lot) and then build a box around it and pour like a replicast type product in there.

Be sure to de-gas it though before pouring it, if not, you will get bubbles for sure even at the surface of the part. There is a casting resin we use, I’ll have to look it up but it only cures in an autoclave, we use it in our small autoclave…

You’re a student right? We did some work for one of the colleges up north on a wind turbine project that students designed. Wasn’t your school though, another one…

Good luck, if you need some help figuring it out you can PM me, we’re also hiring :slight_smile:

#5

I will definatly look into a casting process for this, I never really thought about it…(Would be able to use this in the final report as well).

And am at Confederation College (Aerospace Manufacturing Technology). What company do you work for? I, along with about 15 other students will be looking for work in the new year around April.

Edit: While looking at other carbon fiber parts, I stumbled upon a link about a kleenex cover (Looks pretty cool and kind of gave me an idea). Might try and make something like this if things start to get too complicated. Any Ideas are welcome, including to that of changing the actual part design. As long as it is able hold a part that is 53mm x 53mm with about an 1/8" of padding around, it will work. However I have a bit of a height restriction, which is around 2 3/8" which will leave me around .100" to the rib above, and the center one is .100" less. Attached is a picture showing what I mean.

http://www.carbonfibergear.com/carbon-fiber-aluminum-tissue-box/

#6

I interviewed 2 students in July who graduated in April from that course I believe, we don’t do aerospace stuff much but a fair amount of work for aircraft and geo equipment. I’ll PM the company name over to you.

#7

You say it is a battery cover or shield from rocks… use kevlar and core material in there… core adds thickness and strength, better impact absorbtion too. The same with kevlar, it helps with impact issues.

#8

It won’t really be exposed to any real impact damage, just to keep pebbles and stuff. I am thinking right now it might be best for me to do a wet layup for this piece. I was thinking a bit more about the design and think I need to alter it a fair bit. (Going to try and make a “box” around it to keep it waterproof(ish). I appreciate all the help/ideas though!

#9

Just wondering if any of you guys have any tips to making some Louvers (Vents). I found one DIY that turned out really good (IMO) but am wondering if anyone else had any other method. I need this to keep most of the water from comming inside the box, yet still need ventelation to keep some electronics cool. Any ideas are more than welcome.

http://www.luxury4play.com/car-audio-accessories/92680-carbon-fiber-ventilation-slots-diy-2.html