Prepreg MDF mold issues.

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#1

I have a mold that is presenting some difficulty in bagging.
The mold is about 5 feet long, It’s mostly made from MDF then sealed with high temp epoxy filler and then baked for 24 hours to get moisture out of it.
It’s a 2 part clam shell mold where there are 2 cavities along the length of it and the cavities are mirror images of each other. It’s for a 2 part (left and right half) carbon fiber recumbent bike frame. like a long beam.
I’m using a mix of 12k twill weave, 3k pw and unidirectional prepreg that i have been giving generous amounts of time to gel and to really flatten out in this dual cavity mold but when I pull the parts out there are still spots in the corners that have allot of pinholes. I have to use higher temp bagging film which is not as stretchy as the stretchlon stuff so I add lots of pleats.
Even with extra pleats it just looks like I am not getting enough pressure in the corners which I understand is a common problem in working with prepreg, It really loves tons of pressure and if you can’t get it then you will get pinholes.
most of the part is fine but it got me thinking about using some kind of medium that would take up the space the bag needs to drop into the cavity, Like sand for instance…So the prepreg would be laid up in the mold, then coated peel ply is added, then a perforated release film then fill the entire cavity with sand so that the sand is actually above the parting line, a couple more layers of peel ply, (to keep the sand from being drawn into the vacuum lines) and then the bag and a couple of air filters in the vacuum line itself to make sure nothing gets into the pump.

The idea is to have air pressure do some of the work but by leaving enough sand in there so that the bag sits up a bit I could then clamp a long and thick board over the entire length to add some more physical pressure to the mold.

Is this just stupid? or should I just accept the pinholes and fill them with filler? Just trying to figure out if there is a cheap way to get more pressure on this mold without going to a bladder molding system which I have no experience with.

#2

I have not idea if the sand will work but it will be pretty messy.

Do a search of “composite intensifier”.

An intensifier is basically a silicon or urethane plug that fills the tight corners where the bag is bridging. The intensifier goes into the mold before the vacuum bag so the bag pushes them into the corner.

#3

Thanks, That was just what I was looking for.

#4

is it pultrusion die ?

#5

Hi Mando,

I recently completed a riveted aluminum monocoque recumbent and am thinking of graduating to carbon. (See http://goo.gl/orWGZ and http://goo.gl/6Vn74.) As such, I am very interested in your experience.

How did you make your MDF molds? Did you add draft? How did you finish the surface, and was it difficult to pull parts off them? Did you make your bike frame in two halves and join them, and if so, did you add a joggle or did you use some tape to join them? Do you have pictures of your process? What did you use for an oven? Where did you purchase your prepreg; do you store it in a regular freezer; and did you “kit up” your layup to minimize out-time?

These are a lot of questions, I know, but again – you’re doing pretty much exactly what I have planned to do…

Kind regards,

Ihab