Plug modification needed?

General Composites Discussions Composites Talk
#1

As you know I’m busy with a mold for a VW rabbit hood. Now the outside mold is finished and it’s time to move on to the inner part (ribbing side). Must I make a mold impression of the inside looking exactly like the OEM part? I guess I have to, because all the mounting point for the hinges and latch are on the existing ribbing. And if the inside is off by a hair thickness then the carbon part will have uneven panel gaps.
Now ribbing of the OEM steel hood is partially spot welded and the rest is bonded with a type of putty to the outer skin. That means the ribbing part is elevated by about a half a inch from the outer skin. I’m planning to tape these panel gaps with masking tape sealing off any gaps for gel coat bleeding, and then layer it with FB.

This hood has rolled edges, but it’s rolled in a square fashion. I believe it’s going to be in my way when the flanges are glued onto the outer side, for mold preparation. Is it a good idea to cut them flush with the outer side of the plug?
Remember the mold for the outer part still has them, so the final outer carbon piece will have them.
Here is a pathetic drawing of what I’m trying to illustrate.

#2

Just make shure that the vertical part is not important to you structurally as it is there probably to make the part more rigid on the edges as the reinforcements are not all the way to the bonnet edges from what i understand. I had a simmilar problem recently with Opel Corsa bonet where inner reinforcement was 2-3inches away from the edge so i made an 3mm coremat insert along the edge to gain rigidity and make it look a bit thicker. Works just fine, but make sure to predict what this offset will do to other gaps you will need to fill with glue when bonding the upper and lower half.

And for filling the gaps on the inside you should try modeling plasticine or clay. Is much easier to get the perfect shape then with masking tape.

#3

I think its going to be best to split the upper and lower halves. the molding for the upper half is done, to mold the inside ribbing while attached to the outide is going to cause too much problems. im going to mould the inside the conventional way without any flanges. so that menas I wont be able to infuse it. Im going to do a wet lay up and bag the whole thing, like a letter in an envelope.
I demolded the plug from the mold yesterday and was really impressed on my decision to wax the part with 15 layers of wax! It popped out without only one pull. that without any wedges!

#4

Make sure you dont mess the reinforcement shape while taking it off. The "bondo" like glue can be a bit of a problem especially if there are places in which you can not reach to cut it. If you bend it and make the mold like that you will have some tension when two parts are glued together and that may cause some problems. Thats why i take the mold of a complete underside, and flanges will come handy for wet lay up as well… And i can`t afford messing the factory part.

take a look at this, it might encourage you:

http://www.facebook.com/photo.php?fbid=365942986761862&set=pu.107646032591560&type=1&theater

So after doing this i am quite confident saying i can make any shape, you just need the right materials for it and then the modeling goes easy…

#5

Thanks for the response, I understand the risks involved. I will be as careful as possible and won’t use any force to split the two. The type of putty that is used on the stock steel hood is not really like bondo. It’s a funny type of putty. Its stays soft and doesn’t hardened. You can scratch it off with your nail, sorry for the description but it reminds me of snot! So no effort is needed to remove it, a simple credit card can slice through it. One of my biggest fears is the alignment. I’m just afraid if I bond the two halves together that the hood panel gaps won’t align once fitted to the car. So how do I prevent this scenario? Thanks for the help!

#6

well, if you miss by little, let`s say up to 3mm, you will be able to correct it on the hinges-mount points as factory parts have that tolerance left for setting on assembly.

The way to avoid it is to make reference points on reinforcement and take measures from reference point-line on the mold flanges (you do that before splitting the factory part! )

Than you glue your carbon halves in the mold - the outer skin will be sitting in its mold (on wich you have your references) and you put the reinforcement with the glue and than align with the measurements you took before from factory part. We usually apply thick PVA coat in between mold and outer skin and let it dry for couple of hours. It becomes like a glue and holds the outer skin to the mold so glue can`t leave the dent or result in a weavy bonnet. It will also add extra protection to the flange from the glue which might be squeezed. Make sure you use 0%shrinkage glue or your reinforcement will leave the trace which will be seen on outer skin. And which doesn’t change its volume with temperature variation because the same will happen in the sun or when engine heats the bonnet.