Pre Preg Oven ideas????

[Starfish]

Anyone have any ideas for making a 'oven' to use on a
smaller sailboat made from Pre Preg? (around 20-26ft) Any home build ideas would be welcomed.

Also I was speaking with someone that said a water blanket? No clue
about that.....Any info?.... would be great!


Thanks

[KnottyBuoyz]

I've seen a couple of homebuilt ovens. One was made of the blue foam insulation board with the mylar backing. Readily available at Lowes etc. They were baking rowing shells so it was really long. All that was needed to raise the temps to 140 or so was a 2500 watt heater. The other was a greenhouse type of arrangement with two layers of plastic. The two layers were sealed and inflated to form a thermal break and the heaters, probably about the same size as the one mentioned above heated the tent. I think they had two heaters, one at each end with it venting out the top near the middle.

[Starfish]

Thanks,

kind of along the lines i was thinking, For something i can use a few times,
I was thinking a steel box with a few vents' some high heat fans blowing,
with 1-2 industrial space heaters (blowers, gas, propane/kerosene)



Anyone have suggestions on what to make the mold supports out of,
for reuse in higher heats?


thanks

[Boatbuilder BC]

insulation board is better than the plastic tent for pre-preg ovens, you will lose to much heat and unless you use a high temp plastic it will start to melt or the joining tape adheasive will give way. the blue board also distorts a lot and is unusable afterwards. i have used Kingspan ridged insulation boards 40-50mm thick, foil backed. can use them again and again. using a steel box also has its problems absorbing too much heat, its not easy getting the oven to a steady 85-90 and keeping it there with propane or space heaters or getting your dwells right. elec with thermostats is much more accurate.

[Starfish]

Originally Posted by Boatbuilder BC

insulation board is better than the plastic tent for pre-preg ovens, you will lose to much heat and unless you use a high temp plastic it will start to melt or the joining tape adheasive will give way. the blue board also distorts a lot and is unusable afterwards. i have used Kingspan ridged insulation boards 40-50mm thick, foil backed. can use them again and again. using a steel box also has its problems absorbing too much heat, its not easy getting the oven to a steady 85-90 and keeping it there with propane or space heaters or getting your dwells right. elec with thermostats is much more accurate.

thanks for the suggestions, what i was looking for. I'll look into those for sure.
Electric should not be too hard to do. Thanks again.

[Herman]

I have some customers using steel panelling, the sandwich type: 0,5mm steel, 50mm PU insulation, 0,5mm steel.

Usually it can be bought in "B" grade, meaning that the stuff is slightly off-colour. (who cares...)

for heaters: I see a lot of sauna heaters used.

Use a fan as well, to distribute the heat evenly.

Insulate the floor (very important, lot of heat goes into the floor) and of course the roof as well. No venting, you do not want to lose heat, or create drafts. (only possible with electric heaters).

Use a thermostatic system to be able to control temperature ramp rate. Control is the key to a succesful prepreg cure.

[fiberglass jack]

i have a bunch of aluminium moulds that i vac bag, I need to get some heat into the mould to cure the resin, what i did was ought some stick on engine heaters, you just peel the backing off and stick , they do a wonderfull job at heating the moulds if you had enough spaced along the mould you should gdnerate enough heat, they come in 75 watts to 250 watts

[Herman]

With Aluminium moulds that is a great option. The heat will be spread through the mould quite evenly, so even if there is a small area without heaters behind them, they will still reach the desired temerature. However, with the glass sheet that we have, with heat tracing behind it, you will only get the heat at the direct contact area of the heater. Same goes for composite moulds. When heat tracing is used on these, try and space the tracing evenly, and close to each other, and fill everything between with a mixture of resin and aluminium powder. Then back up with some glass, and insulate. Do not forget to heat the flanges as well, there can be resin there that also needs to cure.

An oven to stick whatever mould in is more flexible, but heated moulds come in quite handy as well.

[SamSam]

I have an older book mostly about building kayaks. It has instructions on building heated molds from GRP. Electrical resistance wire from an electrical supply company is embedded in the mold during construction, about halfway through the lamination process. Looped back and forth and spaced about 1" apart, when done, the wire is hooked up to a device (rheostat?), plugged in and the temp adjusted by the rheostat.

[Starfish]

Originally Posted by SamSam

I have an older book mostly about building kayaks. It has instructions on building heated molds from GRP. Electrical resistance wire from an electrical supply company is embedded in the mold during construction, about halfway through the lamination process. Looped back and forth and spaced about 1" apart, when done, the wire is hooked up to a device (rheostat?), plugged in and the temp adjusted by the rheostat.

Sure would like some more info. about this way for doing it, if anyone has seen it done?

Sounds like good idea.

Also like the alum. mold option, with heaters. More info. on both of these methods would be great.

[marshmat]

When I worked on the solar cars, we had a couple of prepreg ovens that were good to 120 C plus (approx 250 F). They were really quite simple, yet worked wonderfully for all sorts of aerospace-grade prepregs. Each is basically a sheet aluminum box, surrounded by another sheet aluminum box a few inches bigger on each side. The space between is filled with Roxul mineral wool, a cheap and safe insulation that's very fire retardant. Eight 100-watt shower stall heat lamps in each 1-cubic-metre oven gave us fairly quick ramp-up times, and a thermocouple taped to the vac-bagged part would provide the signal to cycle the lamps on and off to hold the set temperature.

If you go with one of the in-mould heaters mentioned, I'd advise wrapping the moulds in Roxul or something similar. Uniformity of temperature is important to getting these things to cure properly. Thermostatic control- ideally via several thermocouples at various points on your part- is also necessary.

[Herman]

Fro the resistance wires, visit someone who supplies heated floors. They have various types of wire, and the electrical gizmos to control things. although the temps of a floor are quite low, they at least have the knowledge and the suppliers to help you.

Between the wires, put down putty which is aluminium filled, to give a better spread of heat. And indeed, insulate the mould on the outside. You do not want to heat anything else than the mould and the product. Putting some blankets or some insulation over the vacuumbag helps as well.