The part will be infused in a female mold.
I am thinking it can’t be done in a single piece without cuts in the fabric. I don’t think visible seams will look too good either. Any suggestions?
For reference, the part is about 26" (66 cm) at the longest points and is 9" (23 cm) deep in the center of the part.
If it was me I would probably put nice cut seems in the corner. Problem with putting in one piece is that the carbon gets all distorted as it sleeves to that shape, which doesn’t always look good, and can cause bridging.
I use a single piece of twill weave cf for the surface layer of almost everything I make in a one-piece mold and the stuff I make usually has more complex curves than the piece in the pics.
I would create a smaller male impression of the piece (like a plug, but smaller). I’d achieve that using sheet-lining wax to simulate part thickness and then cast the smaller male impression on top of the wax in your female mold. You can use any casting or sculpting material for that but I would use epoxy putty or fiberglass.
I would then sandwich the cf between the female mold and the male impression using weighs or clamps to compress the cf between the female and male tooling. This will stop the material rising at the corners which would otherwise be a problem with using a single piece of cf.
You might have to make some incisions in the cf to make it conform to the corners but nothing on the cf that actually goes into the mold (just the perimeter).
I’d use diy pre-preg instead of an infusion to make it easier to lay the cf in the mold without disturbing the weave. It will look better that way and it will be quicker without the hassle of setting up an infusion. I would probably not use heat set resin to make prepreg for a part that big.
Regular room temp resin will be fine for prepreg on a part like that if you don’t have a curing oven large enough.
If you don’t want seams, that it possible imo, if you use the correct method for the job.
IMO, the only way you’ll achieve that is with something to physically hold the cf in place on a part that shape.
Alternatively, you can hide seams by carefully laying the cf so the pattern is continuous. Make sure all edges are neatly cut. Then vacuum bag it with enough pressure to hold the edges together and the seams will not be visible. Personally, I would find that method harder and more time consuming.
Are you saying seam at the green lines in the image below? I am now thinking about making it in 3 pieces and using a structural adhesive to bond where the 3 parts come together at the green lines. It would seem to make things easier.
Zebra, you bring up another big concern of mine - keeping the fabric tight in the corners and eliminating bridging.
I was thinking about doing a wet hand layup for just the outer cosmetic fabric, letting it harden, then going back and laying up and infusing my structural layers on top of the cosmetic layer. The wet layup would enable me to get the outer cosmetic fabric tight to the corners and not worry about bridging. Any thoughts on this idea?
When you say diy pre-preg Zebra, are you speaking about just wetting out fabric on plastic sheeting and then installing it into the mold?
Exactly as you have shown with the green lines
Simples
Doesn’t matter how you do it in one there will be distortion,this way you have perfect weave and can follow it through or herringbone on the joins
There’s absolutely no way that part can be done using a 2x2 twill in one piece without distortion of the weave, the fabric just won’t allow it. It would be the same whether you used wet-lay, infusion or pre-preg; it’s not the process that is the issue.
No need for secondary bonding though, that laminate would be easy enough to construct as a single piece, simply with cuts in the fabric as you have marked with the green lines.
Thanks guys.
My original thinking was to do this in quarter sections (as shown above) and bond the quarter sections together to form the final part- see image below to get an idea what the final part looks like. Where the quarter sections meet, there are heavy aluminum mounting brackets bolted thru the CF, so I was not worried about the CF not being continuous there.
When I mentioned doing the part in 3 pieces, I was referring to the image below. I am re-thinking doing 4 quarter sections, and instead doing 3 pieces (like below) to make the final part. The middle piece would be time consuming as it would require a 4 part mold.
So instead of bonding 4 quarter sections together, I would have 3 parts to bond together as shown below. If the best place for seams is where f1rob stated, then this makes more sense to me to arrive at the final part.
Just for a sense of size, the part is almost 1 meter across.
Does this seem like a better idea to arrive at the final product?
What kind of finish can I expect at the bond lines?
If the thing is 1m wide, I still see no reason you couldn’t build it in one piece. Multiple piece mould for sure, but if designed well then you could still layup each section individually before bolting the mould together, applying a bit of extra reinforcement to the joints to give them some overlap, and then bagging it all together. Of course this would only really be feasible with infusion or pre-preg, I wouldn’t attempt it wet-lay.
Worst case, I would at least do the four sides and the top as one structure, then design your bottom piece to have a ‘joggle’ to locate it and give you a very sound structural bond.
As for what sort of finish you can expect at the seams, I will find some photos of parts I have made where every side was an individual section of mould and every edge has a seam.
First, the approach of doing the cosmetic layer first and separately, then building up the structural layers after is feasible. I have used that approach successfully. The potential issue to watch out for is delamination of the surface layer from the rest. Make sure to double check for this around the edges and apply more resin in the gap to seal it if you find a problem.
Doing the cosmetic layer first also has the advantage of you not wasting all of your cf if there is a problem with the surface layer (which is the most difficult). It’s obviously more time consuming though.
Second, on DIY prepreg, there is slightly more to it that wetting out the cf on a plastic sheet. Specifically, you want to wet it out and sandwich it between two release liners and use a squeegee to remove all the air. I run it through a cold laminator to save time but it works fine with a squeegee too. I would squeegee it on a flat surface like a large mirror or glossy plexiglass. Then, most importantly, you want to wait until it gels before trying to use it.
This is where some trial and error and technique comes into play. You want the diy prepreg to gel to the point where it becomes tacky but not cured so much that you have trouble removing the release liners without disturbing the weave.
If you have a friend to help you, that would be better because carefully removing the release liners and moving a partially cured and highly sticky piece of cf of that size will be tricky on your own. It will stick to everything it touches including your gloves and clothing etc…
As the prepreg has already gelled between the release liners, no more air will be able to get into the resin in the mold and, if you coat your mold release wax with PVA properly, you should have no trouble making it stick to the sides of your mold for the lay up, even without any type of vacuum or inserts (on a shape that big).
However, if you do have issues with the cf raising at the tight corners or over other fine details, you will want to add some type of inserts to hold it in place.
Making the inserts can be tricky. If you just use the mold to cast them, you might not have enough room to fit them in behind the cf. If you make the inserts smaller than the part then there may not be sufficient pressure to hold the cf against the mold surface evenly and with no gaps.
I sometimes use the mold to cast inserts using something soft like silicone. The silicone inserts can be forced in behind the cf to sandwich it against the mold, even if it’s the same size as the actual part. Your issue with that approach is the size of your part and mold. It would cost ten billion dollars to fill that mold with silicone (approx :).
If it were me, I would use a thixotropic silicone product like MoldMax Stroke to make the inserts and then back the silicone inserts with something rigid like epoxy putty to make it more cost effective.
I would try it without the inserts first though, because if you don’t need them, you save a lot of time and cash.
I like to use a very fast curing epoxy resin for this method. It gives you less working time but, the way I look at it, the less time it has for gravity to pull the cf away from the mold surface, the better.
If it only takes 15-30 minutes to cure, I can stand over it and physically hold a corner down by hand while it finishes curing if I have to, to prevent me having to start over again. That only makes sense as a one-off though. This whole method is impractical for a multi-part production line.
If you are making multiple parts, I think the correct process, if it is a closed mold, would be to use an inflatable bladder. If the mold is open, I would use compression molding or bagging.
I could do it with a single piece of 2x2 twill without damaging the weave. I’ve done it with far more difficult shapes than that. That part is relatively large (compared to what I usually make) and I find it a lot easier to make the cf conform to larger curves.
Where it gets difficult is when the cf is stretch across a corner. I have to make a bunch of carefully placed incisions in the fabric around the perimeter of the mold. You can’t always start with a square or rectangular piece of cf and hope it fits rights.
I really don’t like the use of multiple pieces of cf to be visible on the surface of a cosmetic piece so I can usually find a way to avoid it for one-off or small volume projects.
I would invest in some cf cutting equipment if I had to do it for any volume…
I am sure it can be done in one piece, but I don’t have the confidence to try. I know I can make the 3 separate pieces and have them come out good. If I mess up, we are looking at about $700 in cloth, resin and consumables. My biggest concern is getting the cloth to stay in all the corners and everywhere else as some of the layup will have to be upside down. A clear epoxy gel coat would be ideal for creating a tacky surface for the cloth, but I can’t find one that is readily available. Spray adhesives are going to show on the surface. I don’t know what to do to get the fabric to stay and give myself a reasonable time to get it all laid up. So I don’t have faith that I can get this part built as one unit.
The layup, due to the strength requirements, is going to be
204 gsm 2HS
5 layers 193 gsm uni (IM7)
810 gsm bi-axial
Soric 4mm
810 gsm bi-axial
5 layers 193 gsm uni (IM7)
204 gsm 2HS
I have had good success infusing with the soric, so I want to stick with it. The only negative is that it does show a little print thru.
Those parts you bonded look pretty good Hanaldo. I assume there was a clear coat put over the finish product.
