Hi, I'm new and I apologise if this is a dumb question. I'm making things out of carbon fibre with compression moulds and they will easily fit into a kitchen oven. I googled it and nobody seems to be using their kitchen oven. Is there a hidden problem that I'm missing? Will the epoxy fumes kill me or something like that?

The reason I want to make the switch from room temp epoxy to pre preg is that I can't seem to get a neat weave. Will using pre preg help me keep the weave straight?

My oven goes as low as 60 celcius and my moulding material can withstand 300ish. So getting the right temperature shouldn't be an issue.

Any help would be greatly appreciated.

Hi Guys,

Homemade Prepreg by definition is easily possible to produce, by its very name it simply refers to the reinforcement fibre being impregnated (soaked) in resin prior to it being placed into the mould. However the type of prepreg that can be ‘home-brewed’ really cannot be compared to commercial materials.

Normally when the term prepreg is used it refers to a cloth impregnated accurately and consistently with epoxy resin that is at it’s ‘B’ stage of cure, to do this the specialist resins are heated to a liquid and the cloth is passed through the resin ‘bath’ and then a series of rollers all under perfect tension, heat and time control. This is then rolled with separation film sealed and stored frozen until use. The ‘out-life’ (time the material remains usable after being taken out of the freezer) varies from around 6hrs – 3months, 2-6weeks is typical for most common prepregs.

The resin in the prepreg is slightly ‘tacky’ but not wet when it is in it’s workable state at room temperature, the material can be temporarily softened with heat from a heat gun to aid working it into complicated contours. The layup procedure for prepreg requires accurate placement and particular attention to bridging and mould intimacy, These materials are then vacuum bagged and cured under heat (typically between 80 – 140 deg C) either in an oven or an autoclave (a pressurised oven). When the prepreg is heated the resin becomes liquid allowing it to flow and conform to the mould surface. After the curing cycle which is typically between 1.5 – 8hrs the part is complete and can be released.

In answer to your questions on freezing, I’m sure that the standard laminating resins can be frozen as long as they are fully sealed and are allowed to come up to room temperature before being removed from there sealed environment, if they are exposed to air while they are frozen the moisture from the air will condense on the resin and become absorbed which will cause issues with the cure.



Here are the potential problems with a homemade prepreg:

Resin Distribution and impregnation; Evenly applying the resin at the correct ratio (typically 60/40) and ensuring that it has fully penetrated the cores of the fibre tows can be tricky.

Getting to ‘B’ stage; the window where this material would be usable even with slow hardeners is small… you would be lucky to have more that 1 hour.

Sealing the material; actually getting a non-stick material to apply it to, so it can be sealed and frozen is trickier than you would think.

Working with the material; Having a tight time-frame and variable levels of tack the layup process can be made much more difficult.

Final finish; unless you have (like me!) the privilege of an autoclave which cost many tens of thousands of pounds, your surface finish is likely to be plagued with voids and pinholes.




The one method I had moderate success with:

In developing and testing the oven only pinhole free easy-preg system I did get one home-made method to work reasonably well, here’s what I did:

1.    Evenly saturate 1 ply of 200g twill with 160gsm of coating resin (double normal ratio) on a sheet of waxed paper.
2.    B-stage this for 12mins at 75 deg C, allow to cool and apply a second ply of dry cloth to this tacky surface.
3.    Seal into a poly-bag and freeze
4.    Remove from freezer allow to thaw and use in a chilled workshop (12deg C)
5.    Bag and cure laminate at 80deg C for 2hrs

This system produces virtually pin-hole free surfaces due to the dry plies of cloth acting as a core breather; a refined single ply version of this technology is used in the easy-preg with huge out-life and consistency advantages.

The impracticality, labour intensity, unreliability and wastage meant that we never even came close to implementing this system for any form of production, but for those experimenters out there feel free to give it a go!

In summary resin infusion will yeild much much better cosmetic and structural components than a home-made prepreg will.

I feel a prepreg video coming on!... Within the next month or so I will show you exactly how to use the easy-preg to get completely pinhole free, perfect prepreg components without an autoclave; it took us years to refine and I’m sure even the most experienced composites engineers will be impressed with what it can do!

And keeping on-topic – Yes you can use a domestic oven for curing, be-careful, never leave them alone, and verify all set temperatures with a thermometer. Other that that for small components they can be really useful.

Paul Statham
Easy Composites / Carbon Mods - Technical

Wow thanks Paul, that's a great answer. Thanks for the time you put into that.

I'm going to demould my homemade prepreg effort tomorrow morning (in about 8 hours) as it's been cold here today, I want to give it more time. I think if it works then I'm going to get some easy preg.

Update... I got impatient and just demoulded it. I got a very straight weave on most of my product but there were voids and pin holes in the tight corners. I don't think I can make homemade prepreg work for me.

Paul, I'm making a cover for my iPhone 4. I've used RTV2 silicone for the moulds, both plug and female. I've done it this way because I need a perfect finish on both the inside and the outside. I couldn't get vacuum bagging to work with the tight corners either. Do you think your easy preg system would work in a situation like this? I plan to put two layers in the mould and press the plug in and throw it in the oven.

Here are the voids I ended up with...



Also... do you think the easy preg will fix the way that weave spreads out like that?

I just realised that I'm getting confused between "Easy-Preg" and "Vari-Preg". Sorry about that. It's the vari preg that is in my price range. Do you think vari preg will help with the spreading out of the weave and the voids?