Composite mold making questions Options

[TonyAKAVW]

I want to make a diffuser/engine pan for my car, and last night I started gluing foam sheets together to make a mold. My plan is to form the foam into the shape I want and then seal it with body filler, and then primer and polish. I have been reading up on making molds for composite work and most seem to talk about making a copmosite piece for the mold rather than what I am planning. If I make my mold as described will my composite part rip the mold apart when I seperate the two? (assuming I use a PVA mold release film).

Also, if I make the part with carbon fiber, how do I finish the edges of the piece when I'm done?

I would like to vacuum bag this part because the part will be visible from the top and the bottom... Can I get away not bagging it and get a decent finish? (I'm not super concerned that it has a perfect surface really).

-Tony

Finished part will look something like this...

With the long straight edge meeting up against the firewall and the two tunnels exiting right under the bumper.

[BahnBrenner914]

First of all, SWEET! (IMG:style_emoticons/default/w00t.gif) I'm an ME student and aero-junky (IMG:style_emoticons/default/thumb3d.gif) , so if you pull this off, I want one too! (IMG:style_emoticons/default/biggrin.gif)

As for the composite work, I dont know much about making it, just that it can be made in any shape the engineer (me) wants it. (IMG:style_emoticons/default/av-943.gif)

[john rogers]

A couple of questions, do the exhaust headers drop down each side or run inside the diffuser then out the back and where will the engine cooling air exit from? If you check Patrick's web site, he has a 914 vintage racer that is "canned" in the rear to smooth out the air flow underneath and I remember him telling me it was done as an experiment and the results were not worth all the work involved. Since the firewall is about the lowest part of the car I would want the diffuser to start there?

[URY914]

Second of all:

WHY, To catch the oil?

[TonyAKAVW]

BahnBrenner: If my mold lasts I can make another copy.

John: My exhaust sits up almost against the floor of the rear trunk. I did this knowing that I would eventually put in a set of tunnels. The exhaust will exit either through my bumper (a pair of holes, one on each side of the licenes plate) or through the gap between the lower edge of the bumper and this diffuser plate. (which will sit 6-7 inches below the bumper, with the top of the tunnels almost touching the bumper ) So my exhaust headers turn up, right after the engine, which is at the point of maximum clearance from the tunnel roof.

My design has the enitre panel attach to the firewall/floorpan point. So there will be no gap between the diffuser and the floor pan.

Paul: Subaru engines don't leak oil (IMG:style_emoticons/default/smile.gif) At least not much.

There are a few reasons for doing this. First is an expired reason. I had planned on using this as part of my mid-mounted radiator cooling system, as a way to exhaust the air better. Since I moved the radiataor up front, that is no longer necessary.

Another reason is that without a rear valence, the car looks uncomplete. I don't really want to put a rear valence on and wanted to do something different. It will actually serve to keep a little road grime out of the engien bay area too..

And lastly, I have always wanted to make something out of composites, so this being a relatively simple shape, seemed like a neat first project.

I don't actually expect it will make much difference in terms of driving. First of all I don't drive fast enough, and if I did I would be worried about the front end lifting.

[BahnBrenner914]

Well I thought you had a stock-type teener, so there's no way I could use what you have. (IMG:style_emoticons/default/sad.gif)

I still love the idea though (IMG:style_emoticons/default/clap56.gif) I'll be very interested to see what kind of aero you get with it installed. Whether it reduces/increases drag, gives you some downforce, or just sucks itself right off the bottom of the car at 100 mph! (IMG:style_emoticons/default/av-943.gif)

[TonyAKAVW]

Here's kind of what I anticipate the fitmet to be like, although probably more clearance between the diffuser and the headers.

-Tony

[neo914-6]

Tony,

Appears you are actually making a part with composite (sandwiched) molds for a single layer CF part. The foam should work fine, I've even seen it done with cheap house insulation blue foam. The guy used cheap wall plaster "mud" and sealed it since the resin would melt it.

I would carve and form the male side, finish it and then make dams to pour liquid urethane to make the female side. You would treat it with a mold release like PVA. Finish that side and be ready to press the resin and CF cloth. The finish will depend on the amount of mold finishing. You can also lay in mylar if the surfaces aren't compound.

I'm not sure but cutting the ends should be like cutting fiberglass. Kevlar is the material that's difficult to cut.

[TonyAKAVW]

Felix: Not sure about what you mean in terms of what I'm making. The mold that I'm working on should end up looking like this:

Basically its a foam piece (with sealer and primer, etc. on the surface) and then I will layer the cloth on top of that. I will probably use some kind of mat between CF layers to reduce cost, but this large foam piece won't be part of the finished product. The whole foam piece will be glued to a 1/2 inch MDF board. Which in turn will be supoprted by 2x4s. I figure that should help during the vacuum bagging, to prevent mold warpage.

-Tony

[iamchappy]

I might be easier to make it out of aluminum or stainless sheet metal. I just took a couple of pieces of paper and folded one up quite easily. A sheet metal fabricator could
bend one up I assume very reasonably.
Something to think about.

[GWN7]

Chappy's right....it would be easier to make it out of sheet metal as their are no complex curves/hidden angles. Once made out of metal you could use it as the master to make a glass one if you so desired.

[JPB]

Its not to hard to do Tony. If your mould is smooth and blemish free, assuming the foam part you are using is the mould and not a plug, it will be easy to get a good finish. What I do is lay up just resin or epoxy like three times for a good build up to sand later to a glass finish. It should get some acrilic clear on it to keep them UV rays off so it will get a good grit sanding before that also. For me, the hardest thing is to get the CF as thin as possible and as strong as it needs to be. The best time to trim anything is when the stuff gets geled and not yet hardened since you can cut it with a razor knife but you can use a grinder to get the shape you want when its completely hard.

[neo914-6]

Felix: Not sure about what you mean in terms of what I'm making. The mold that I'm working on should end up looking like this:

Basically its a foam piece (with sealer and primer, etc. on the surface) and then I will layer the cloth on top of that. I will probably use some kind of mat between CF layers to reduce cost, but this large foam piece won't be part of the finished product. The whole foam piece will be glued to a 1/2 inch MDF board. Which in turn will be supoprted by 2x4s. I figure that should help during the vacuum bagging, to prevent mold warpage.

-Tony

The two part mold would be similar to a die essentially replacing bagging which helps drive out all the air pockets and forms two finished sides. You can accomplish the same with laying the layers on your one piece mold and then a piece of mylar and squeeging out the air although you wouldn't control the thickness.

Structurally you can probably use one layer to take advantage of the CF strength.

Looks like a fun project...

[Brando]

Get ahold of the pan/diffuser for a 964 or 993 and make it as close to that as possible. They only really work at highway speeds (60mph+).

[TonyAKAVW]

From the reading I've done, it seems that 10-12 degrees for the tunnel is optimum. For a 6 inch high tunnel exit that means 31 inches long. It will probably make a difference at high speed, but it will be far from optimum..


Here's a nice diffuser... (~$1100)

[okieflyr]

My experience is vacuum bagging 60" composite model sailplane wings panels.
We use mylar for the surface finish which can come out beautiful when doing paint design transfers.(no pinholes)
My suggestion is to bag ALL your individual precut pieces flat, and lay in your spars and predetermined attatchment points within your lay-up.
Then you can assemble the pieces. You may have a hard time getting the bag to conform to your concave 90's and get some flooding. (not the worst place though).
Blue or pink extruded foams work well with the epoxy finishing resins, and there is no sealing requirement, but it won't deal with direct heat well.

Kevin

Felix: Not sure about what you mean in terms of what I'm making. The mold that I'm working on should end up looking like this:

Basically its a foam piece (with sealer and primer, etc. on the surface) and then I will layer the cloth on top of that. I will probably use some kind of mat between CF layers to reduce cost, but this large foam piece won't be part of the finished product. The whole foam piece will be glued to a 1/2 inch MDF board. Which in turn will be supoprted by 2x4s. I figure that should help during the vacuum bagging, to prevent mold warpage.

-Tony

[Katmanken]

I've used plaster of paris before, casts smooth, and it can be as massive as you want.

Kwn

[TonyAKAVW]

First of all, thanks for all the tips! I'm going to put this stuff in a progress thread soon.

So about heat... I am goig to be curing this inside my garage, which means I can't really take the temeprature up above say 100 degrees for maybe an hour before my propane heater runs out. Its my understanding that there are room temperature curing resins. Are they far inferior to heat cured resins or is it a matter of time?

As far as a vacuum pump goes, what are the cheap options? I've seen some people using refigerator compressors.. Can I use my air compressor inlet? How much of a vacuum really needs to be pulled anyway?

-Tony

[Katmanken]

If you are careful (NO FIRE OR FLAMES) , a cardboard box can be heated well above 100 degrees F. That and more is quite attainable with a hot air gun venting into an enclosed box.

[URY914]

I've been kicking around bilding one for my race car and have collected several dozen pictures. here are a few