MDF mold for low temp prepreg

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#1

Hi everyone,

I’m planning on making a carbon fiber part which is about 2 metres by 1 metre (6.5 feet by 3.1 feet). This part is mostly flat except for a few curves on one end of the part. It will be of constant thickness (approximetly 6 plies thick).

I was originally planning on doing a wet lay up, however, I have recently come across some very low temp cure prepreg (can be cured at 70 degrees Celcius (160F)). Do you think I would be able to use an MDF mold successfully at this temperature without running into warping or other problems?

My plan was to seal the mold using a Frekote sealer (CS-123), use Duratec gray primer and sanding to achieve a good surface finish and finally apply frekote 710LV as a release agent.

Any feedback is greatly appreciated!

#2

Is possible, but beware temperature distribution in your mould. Seal it all around with epoxy, then primer, then release products, and your good to go. But keep an eye out on the temperatures when curing :wink:

#3

May I ask why everyone recommends sealing with epoxy? do sealants like Frekote CS-123 not do a good enough job on very porous materials like MDF? Sorry if this is obvious, this is my first time working with MDF.:o

#4

semi permanent sealers such as Frekote are design to seal solid surfaces of their micro porosity. MDF is basically pressed paper that is super thirsty. The epoxy will get soaked in to the MDF and prevent moisture from later being sucked in. After sealing with epoxy you can spray primer on top of this, sand, followed by semi permanent sealer to fill micro porosity, then a release agent

#5

I use it a lot of MDF when making patterns. It is very sensitive to moisture content. A change in humidity can have a serious effect on the pattern. I learned this the hard way. I sprayed a fairly flat pattern with Duratec on one side and the next day came back to it and it looked like a potato chip. The Duratec sealed the surface on top and the bottom expanded due to a change in the weather (increased humidity). Since then, any flat patterns I make I build with a torsion box type structure. That helps keep everything where it’s supposed to be.

On the sealer, I’m assuming you mean to Duratec the surface first and then apply the sealer. If so, I think it will work for you.

#6

some of my coworkers tell me that they used mdf molds in making production parts. They were making wing and control surface ribs… not sure what they did to seal them but, they were using prepreg and making many parts out of them.

i’m gonna make a simple mold using mdf soon. I think I’ll hot coat it with epoxy, seal, release. I’ll use a lower temp pre preg as well. A small amount of cte expansion shouldn’t bother my application.

#7

I was indeed considering reinforcing the bottom of the MDF with something like you described, if only to ease transportation.

About the problem you encountered of it absorbing humidity, if I were to soak epoxy into all the surfaces of the board, do you think that could prevent the warping?

#8

I’m trying to picture sealing the MDF with epoxy, but all I can imagine is ending up with a very uneven surface. Do people typically seal with epoxy and then put it back on a router/machine to finish it? Or am I just being paranoid about the surface finish?

#9

It can swell a little, but swelling should be minimal, or even unnoticeable. Depends what you are trying to achieve for final tolerance of the mould. Another option is to buy tooling board as it is very stable, and you dont need to seal this with epoxy. Just CNC it, then lightly sand machine marks away and add release. Of course cost is much more, but final tolerance will be as good as your CNC is capable of.

#10

I actually have tooling board available, however, it would require bonding 4 pieces together whereas a single MDF board is big enough. I was concerned the tooling board might not survive being moved as it would effectively have 3 separate butt joints.

Of course, now 'im thinking the hassle of MDF might be worse than simply building a reinforcing structure below the tooling board…

#11

Yes, a simple wooden or metal structure, and making sure to not stack the butts in the same location (do it like stacking bricks) would probably be the best, most accurate way IMO

#12

for the sealing, just a light coat of epoxy, then sanded… haven’t personaly made molds this way, but i know others who have.

We’ve used molds that are made of tooling block from multiple pieces, once it’s bonded together (using vacuum) the blocks are as good as one piece. I haven’t seen issues with the block bonds. the blocks get bonded and then machined and we’ve moved them and haven’t had much issue. Though some of them seem to distort a good amount under heat but, return to their shape once cooled. That could be an issue, maybe to do with bonds? I think i’ve only seen this on the really long spar molds.