MDF female molds for autoclave?

The situation: Project requires two female molds, each about 5m x 2m. Both are fairly flat. Parts to be built from prepreg carbon and kevlar, autoclaved and vacuum-bagged.

The idea: CNC mill the molds directly from MDF (medium-density fibreboard), laminated with tooling epoxy to the needed size. Fair and finish by hand. Autoclave parts at no more than 50psi, 120-180 Celsius.

Testing so far: We've done small parts (up to 1 metre) with MDF molds, it was quite successful. But that was at 120 Celsius cure, and at ambient pressure, not autoclaved.

The problems: MDF is wood based. So it might: warp, scorch, decompose, outgas, crack, etc.

Has anyone tried this before? Any thoughts?

 

I have done it with smaller but deeper parts.Expect your glue lines to be visible.If the budget for the project allows,take the moulds to the working temperature and pressure with a vacuum bag applied and take a look at them when they have cooled down.The type of surface finish applied can be significant.I have used melamine lacquer,acrylic primer and epoxy.They can all work but you will need to be sure that a a full cure has taken place in order to avoid problems with solvents or by-products affecting the release agent or inhibiting the cure.You will need to support the MDF on a flat surface as it tends to become slightly plastic.Do not expect to produce a huge quantity of parts as there may be a tendency for fragments of MDF to stick to the carbon.

 

Thanks wetfeet.
Any thoughts on a metal backing frame for the mold? Would that help the rigidity? Or are the thermal expansion coefficients too different, ie. would cause warping? (We've estimated the MDF molds to be roughly 3,000lb each.)

 

I would not recommend attaching a metal frame,because as you suggest,it will expand more than the MDF and cause bowing.If you can build a flat metal surface on a metal frame and seal the bag onto the flat surface,you should be OK.The expansion of the metal should not bend the MDF.You may find it difficult to obtain large sheets of suitable metal for the size of your project and joining with a guaranteed pressure proof seal is a challenge.In which case you will have to envelope bag the piece if you have doubts.

 

OK, so here's the current plan:
Sheets of MDF will be laminated together into a block of about 2m x 5m, with the rough shape of the part already cut into the middle. This block will then be CNC milled to the proper shape and hand-faired. It will then be sanded and finished with a tooling epoxy. The unit will probably need a couple dozen large castor wheels on the bottom to allow it to be moved, and will sit on the flat floor of the autoclave while cooking. The inside surface of the mold is thus airtight and the bag can be attached around the top edge of the mold.

Would this work? Or is the stuff too porous to bag this way? (Also, does MDF release any fumes into the autoclave when it heats up, or is it fairly stable?)

 

It will probably not work.I have found MDF to be too porous to establish a vacuum,much less a seal at autoclave pressures.If you have a vacuum gauge,you could try to run the mould up to working presure to verify its integrity-or not.I remain convinced that you should either seal on to a surface or envelope bag.

 

Thinking about it further,what about the possibility of using a melamine faced board as the bottom layer?If you did so and if you sealed the bag onto the bottom surface,you would only have to cover the joints in the board and the bolt heads of the castors with mastic tape to stand a much better chance of a seal.