Another newb making a hood - a few questions

General Composites Discussions Composites Talk
#1

First I must say this site has been a godsend. It took me a while to find but it was worth it. :slight_smile:

As some background I have built a low 11 second 1/4 mile Dodge Shadow and one of my goals with the car was a 10 second daily driver. I am putting the car on a diet which will give me the chance to learn more which I love. This time it will be with composites. I have exprerience with fiberglass and a bit with carbon fiber on our university Formula SAE team but not a whole ton. I should also note that challanges don’t scare me in the least and I love jumping in to something new.

I would like to make sure I am on the right track with my learning so far. Making the mold seems pretty straight forward so I think I have that down. I am planning a carbon fiber hood using 3 layers of 3K twill followed by 1/2" nomex honeycomb and another 2 layers of 3K. It will be a wet vacuum bagged layup using US Composites 635 expoxy resin. So here are some questions and please let me know if I haven’t include enought info.

  1. I haven’t had time to do any computer analysis on the laminate but does the 3 layers-HC-2 layers seem like enough? No one will ever sit, stand, or for that matter man handle my hood so it just needs to withstand the aerodynamic forces and careful removal-reinstall.

  2. With twill, is there a 0-45-90-180 or is it just 0-45 for alternating layers. I was planning to do the first layer at 0 for cosmetic reasons and then 45 but do I need to make sure the next is 90 or is 0 OK again?

  3. I don’t want the black carbon look but would like a bit of weave so I was going to do a candy tinted to the Monaco Blue body color and then clear. From what I read doing a clear gel coat is the way to go before the topcoat to eliminate the risk of pinholes. Is this correct?

  4. I’m concerned about weight but not really as it isn’t an F1 car. I haven’t used gelcoat yet but I can’t imagine an extra coat of clear gel would be adding all that much weight. Am I correct in that thinking?

Sorry for the long first post but I really appreciate the info I have already found in this site and just had a few dangling ends. Thanks in advance

For anyone interested here is my current build log…
http://www.turbo-mopar.com/forums/showthread.php?39999-93-2.4l-DOHC-Shadow

DJ

#2

0: you can gain extra weight if you don’t do the honeycomb right with wet layup. The resin on the top side layers will slide down into the cells, thereby drying out the layers.
2: twill is a 0/90deg weave. So there is always a 0 and 90, or a 45/-45, depending on which way you hold it. If you do a 4 layer stack, 0/90 being the first layer, the next 2 should be 45, and then the final layer should be a 0/45. This way you have a symetric layup. 0-45-45-0, verses 0-45-0-45
3: people do that, but you are still dealing with black fiber. if you tint clear coat with clear dye, it will just have a blue hue to it. You can use an opaque gelcoat, and add color, but then you won’t see the weave.
4: clearcoat will add weight of course. Thinner it is, the less weight. But along with #3, you can paint the hood, or use an auto based clear, and sand it down, and polish.

good luck!

#3

Right then. What riff said is correct. For the best strenght from the materials used you really want to orentate the cloth from layer to layer, as riff said. Two outer plies then your honeycomb then two more plies will be plenty stong enough for what you are doing, providing that it is all layed up correctly.

Using the nomex honeycomb will add a lot of stiffness to your part for relitivelly little weight (compared to extra plies for carbon). However is is important that a few rules are adhered to with using honeycomb, firstly the edges of the honeycomb will need a chamfer to prevent the subsequent cloth plies from bridging (which would make the laminate weak).
The honeycomb will soak excess resin into the cells, this will add weight and can lead to the part being brittle if this is excessive.
However saying this it is likely that addid the honeycomb to the parts that you are talking about will be a positive and will give you lighter and stiffer parts than if you did not use a honeycomb( because you would need more carbon plies to get the requires stiffness).

#4

If you are going to paint the parts i personally would not use a gel coat as this will add aditional weight to the part. Clearcoating after would fill any pin holes that you have on the surface.

Thats just my personal opinion on it however, there would be nothing wrong with using an epoxy gel coat on the surface. It would make any post finishing on the parts easier if you do gelcoat the parts.

#5

Thanks riff. Are there any tips to doing a wet layup honeycomb? I wanted to stay away from an adhesive layer since that seemed to add a lot more complexity. My plan was to gel coat, let tack, lay down the first layer, wet it out and squeegy,then the same for the second and maybe third. After laying in the HC I was going to wet the next layer on a table, sqeegy it out and transfer it to the mold followed by the last in the same manner. As long as I’m careful to have well saturated but not dripping wet layers should I be OK? Most of my process has come from reading Paul Bird’s write up on his mustang parts.

I know the tint won’t change the black and that is what I want. I just want some of the base color to tie it in with the rest of the car. It is a very dark blue to start with so I think that will be in my favor.

Thanks for clearing up the fabric orientation for me.

Thank you for the advice. I figured there would be some resin drip into the open cell and thought there would be no way to avoid it but just do my best to minimize it. I was originally planning 2-HC-2 but after reading about non HC builds and using upwards of 5 layers I got worried that the 2 on the outer wouldn’t be enough even though a HC construction will be much stiffer. If a double 2 layer is the general consensus I will be happy to get rid of the work, money and weight of a third layer. :slight_smile:

I was also at one point planning to chamfer the edges but thank you for the reminder. I really need to start writing all the tips I find here and there down.

It seemed from reading that many people experimented quite a bit to get pinhole free results. I guess I thought with my minimal experience that a gelcoat would be the “easier” way out. That being said I pride myself in always trying to do it right so if I should man up and stop trying to cheat then I’m all for it. Can I go back once the part has been pulled and fill in the holes with some more epoxy? I painted my whole car in my garage but I don’t feel like making a booth again just for a hood so a buddy will be doing the top coat. I want to be sure all he has to do is spray and hand it back to me.

#6

technically, you would want to do it in the 0 direction as opposed to the 90 degree, right? The warp direction has sliiightly different properties than the fill.

P.S. I would also add layers to your mounting points, be it pinned areas or where it bolts to a hinge

#7

This youtube video might be useful to you - [ame=“http://www.youtube.com/watch?v=sA8kIEbQ4uY”]http://www.youtube.com/watch?v=sA8kIEbQ4uY[/ame]

Its the first of five segments, they also have a good mold building series of videos too, albeit a little old school.

#8

What do you mean by warp and fill directions?

Ya, I was going to do a few more layers at the front for the pins and along the edges. I was thinking of adding a couple aluminum blocks for some aluminum hinges to bolt to but haven’t completely decided yet. Being a street car I want it to hinge but still want to be able to remove it easily at the track so I need to come up with something there. I have a couple ideas floating around in my head I just need to solidify. In the pin location how many extra layers should I put down?

#9

If you look at a roll of fabric, some of the yarns run in the direction of the roll and are continuous for the entire length of the roll. These are the warp yarns. The short yarns which run crosswise to the roll direction are called the fill yarns.

Very nice work by the way. I always love seeing something “different” being built. :smiley:

#10

Rotorage has it right

hmm, I would say maybe 2 or 3 extra layers… if at pins, then maybe 2"x2" squares or bigger if you aren’t sure where the points will be exactly.

#11

  1. Even with Gel you can still get pinholes. I havn’t colored anything like you describe. But if I was to try I wouldn’t put on a gel coat. I would just spray the candy on after it was demolded and clear just like a normal body panel.

  2. If weight is a concern go bare, it’s the lightest possible. But like I described in #3 I would just clear it and not gel. The weight difference wont be that noticble either way. Isn’t the general rule of thumb that 100lbs is = to .10 second in the 1/4?

#12

Excellent, thank you. I hadn’t found that series yet and it changed my opinion on doing a 2 stage bagging schedule. I didn’t know I could just use some more epoxy resin to adhere the honeycomb to the first layer once it was cured. I thought I would have to track down some of that adhesive sheet which also seems quite pricey. (I know, by this point what’s a few more bucks. :rolleyes: ) But knowing that will work I would much prefer to do it that way so I can get the best skin laminate before the honeycomb is layed on.

Perfect, that clears it up nicely. Thanks for the compliments. I almost like having a car that no one makes any parts for as it give me an excellent excuse to learn how to do it myself.

I was thinking of using the Aerocatch pins which will give some more surface area to distribute the load but 3 extra layers in that area doesn’t seem like it will be a problem.

So the consensus seems to be scrap the clear gelcoat. No problem there and thank you everyone for the advice. Would I then spread on a layer of epoxy and let that tack or go straight to wetting out the first layer like in the video series that shoe posted?

Ya, they say a 1/10 per 100lbs. I am sitting at a PB of 11.32@126mph. My goal is to shed as close to 300lbs off the car as I can with a CF hood and dash being two of the parts. I figured the hood was the easier part to start with, hence this thread. Right now the car is putting about 500hp to the front wheels and I think I have a good 100hp left in the setup as it stands. So with that, the weight reduction and a couple other tweeks I shouldn’t have any problems hitting my 10 second goal. :slight_smile:

#13

I would be careful following that video, the techniques used are very crued and could easily lead to some serious issues

#14

Can you elaborate? Just saying it is crude won’t help me do a better job.

#15

i use g10 for hard points.i cut out the nomex where the hard points are.you can buy g10 in whatever thickness you need to match your nomex.

#16

Remember do not put honeycomb where you will be putting any holes through the panel. cut the honeycomb clear of that area before you lay it up, two reasons for this, first the honeycomb will crush if you try to bolt anything through it, secondly if the honeycomb is not sealed around the edges it will soak in water and will degrade the laminate.

Puting a coat of epoxy on the mould surface or not??? well that is really a personal preference thing depending upon the way that you like laying up… some people do… some dont.

#17

Most of the time, in PW and twills, the warp and weft are equal. When you get into harnesses, then yes the tow counts are not equal, and there IS a 0 and 90 direction. I think the 0 would be where you have most of the tows visible, and the 90 is the direction where you can’t really see the tows well.
I think.

Few other points:
Hardpoints/boltdowns/hinge areas. Yes, you do not want HC core there. You can use metal (treated for corrosion, or painted and wrapped in glass!!!), G10/FR4, or even Delrin or another hard plastic. You can also add up those “pockets” and hard points with a large amount of layers. Make sure you don’t get too thick or you might not be able to close the hood :slight_smile:
You can also fill the core with a corefill adhesive. It just fills in it with a hard resin, and then you can mount directly on that. Might be the easiest.

Good point about the water and honeycomb. Any holes in the core, and exposed to the rain…will soak up water. If you HAVE to drill a hole for some reason through the core, you can get an epoxy filled syringe and 1: plan ahead by filling those cells where you will drill. 2: after drilling, poke the needle in the surrounding cells, and fill it in the best you can. Fill the hole, and then just redrill.

As for vacuum bagging with open cell core like HC…hmm… I really don’t know how well that will work. most of the time people use prepreg, prebonded layers on the core, or making skins, and secondary bonding the core inside with film adhesive. Someone might know better.

#18

Because they werent controling there RC or vac level. I would suggest doing test panels first simulating the laminate and time it would take and do a destructive test. I would guess that car they made would start falling apart after a couple weeks, but maybe they got lucky.

#19

Wet lay and vac bagging works fine but you do have to control the amount of resin that you are putting into the laminate carefully. you will need a little extra resin than if you didnt have a honeycomb but I really do mean a little.

I do quite a lot of wet lay vac bag layups using nomex honeycomb and it works just fine, whilst not quite as good as if using pregreg its not far behind if done corectly.

I have found that tufnel is a good material for hard points in honeycomb. some people use woods but i have found in the past that woods wear and you can drill a series of holes close together which you can do with a tufnel hardpoint.

#20


Dallara front wing endplate F191 by dobsonm, on Flickr

This wing endplate uses 3mm nomex honeycomb with tufnel hard points where the bolt holes are the double row of holes are 4mm dia and have about the same gap from edge of hole to edge of hole so they are very close together!

The pic is not very high quality and the face closest to the camera has been flatted ready for priming (hence they might look quite pin holey). They have now done a season of racing and are only showing signs of wear on the bottom edge that contacts with the ground which has a titanium rubbing strip bolted to it.