Need Help!- Manufactured a custom composite oven for post cure

[c.my.m3]

Hey everyone,

I don't post much, however, I could really use some of your advice or comments. We have a 8ft (depth) x 6ft (width) x 6ft (height) oven we manufactured to cure a specific part. This composite piece (made in carbon fiber) needs a post cure each time we make them to increase production on it. Presently we are VARTM'ing the part which works fine, however it needs to sit overnight so we don't pull it to quick from the mould.

Anyways, we figured if we post cure the part we can turn it around a little quicker to maintain production levels.

Here is my issue, I am a total Noob when it comes to figuring out how to heat the space. The initial idea was to use an electric duct heater but I am second guessing that decision now. I have had some comments from companies I spoke to trying to source a unit that it would work perfect, then other companies have told me it is a bad idea and will take a month of Sundays to reach the heat point.

I know a lot of composite shops have built there own ovens, especially in the boating market. Anyone have any idea's or recommendations, roughly what size (KW) do you think I will need - My assumption is at least 18KW, also what fan size should I get to blow the air in.

I would typically cure at 80 to 250 degree's but there is the occasional need to go 350 degree's

Any help would be appreciated, if you could imagine you had this structure, how you heat it is basically what I am asking...need some idea's

Thanks!

[mcphill]

You won't want to pay the power bill to get an electric heater to heat that volume to 250 F, 350 F is WAYYY expensive! As an easy comparison, how long does it take your oven to get to 350 F? 15 minutes? That volume is probably 2.5 x 2.5 x 2 ft, or about 12 cubic feet. You want to heat 6 x 8 x 8 ft, or 384 ft^3 - that's 32 full size kitchen ovens worth of power... Once you are up to temp, holding it is a different matter entirely. Just as getting to 80F is not too bad, but you need to design for the max temp you want. How long do you want it to take, too? Is an hour OK, or do you need 15 minutes, or is 6 hrs OK? You will need way more power the faster you want it to heat up to temp.

I think you better look in to a gas furnace as a heat source. You won't get there with electrical systems unless you have money to burn...

[CNC Pro123]

I’ve done mold work for a composite shop, they used a simple plywood box (insulated) with a simple spot light as a heat source…

[c.my.m3]

Originally Posted by CNC Pro123 I’ve done mold work for a composite shop, they used a simple plywood box (insulated) with a simple spot light as a heat source…

Yea, I have seen those...the part is way to large for that. Something this size requires air flow otherwise there will be hot and cold spots in the mold.

[c.my.m3]

Originally Posted by mcphill You won't want to pay the power bill to get an electric heater to heat that volume to 250 F, 350 F is WAYYY expensive! As an easy comparison, how long does it take your oven to get to 350 F? 15 minutes? That volume is probably 2.5 x 2.5 x 2 ft, or about 12 cubic feet. You want to heat 6 x 8 x 8 ft, or 384 ft^3 - that's 32 full size kitchen ovens worth of power... Once you are up to temp, holding it is a different matter entirely. Just as getting to 80F is not too bad, but you need to design for the max temp you want. How long do you want it to take, too? Is an hour OK, or do you need 15 minutes, or is 6 hrs OK? You will need way more power the faster you want it to heat up to temp. I think you better look in to a gas furnace as a heat source. You won't get there with electrical systems unless you have money to burn...

My thoughts too...I wonder how bad the bill would be, anyways, to answer your question, on average 90% would be 200 to 250 ideally for 3 hours. Only when we do aircraft composites would it need 350 range. I'll look into a gas set-up. Maybe I could snatch a axial tube fan from a booth and furnace system from it also.

Any other ideas? Anyone?

[GabZip]

Gas is the way to go. I have in the past made them from a house hold heater

[CarbonKevin]

Consider heat blankets...industrial ones, obviously. If it were me, I'd rig up a reusable vacuum bag setup to hold the bag tight to the part - no need to pull infusion levels of vacuum, all you're doing is encouraging contact between the blanket and the part. The advantage would be more even thermal distribution and energy savings since you're heating the part directly.

[ckelloug]

I'd suggest looking on e-bay for an industrial convection oven. I got a used Grieve convection oven that is 48x36x36 for around $2000. It will go up to 375F. It has large heating coils at the bottom, an analog controller with a thermocouple for temperature maintenance and a 1/3 horsepower blower that circulates the air in the oven. You could potentially build a similar oven to what you want but it might cost more to build it out of new parts than to buy a used oven.

If you build, find something like a watlow controller, some heating elements, an insulated box and a blower motor as well as a contactor to control power to the elements. Get yourself a mechanical engineering handbook and work out the losses through the box and that will give you an idea of how much power that you need.