I was just wondering if any members on here have played with microwave curing or know much about it? Ive been doing some reading on it and there seems to be quite the variation in results.
In theory, the microwave approach can offer significant energy savings compared to composites cured in a conventional oven and most studies agree on this – typically in the 70-80% range. The primary objective is to reduce costs through oven times, energy consumption and cheaper tooling in certain circumstances than autoclave curing, however the biggest variation seems to be in quality.
They studies do not seem to reference pressure used in microwaving cures and always compare high pressure autoclave cures with eg vac bag in a microwave. The resulting quality in an autoclave is therefore considerably higher in some studies and seems to be ‘similar’ in others in terms of void content for example.
Are high pressures not used in an attempt to reduce compressed air/nitrogen costs and possible cycle times or does higher air density have an effect on microwave penetration?
Could poor quality be due to resins or pre-pregs not designed to have low viscosity flow times that microwaves may have?
Has anyone worked in this are?Just curious, no other reason!!
The microwave cure ovens I"ve seen are just ovens. Since they’re an oven, it’s not pressurized which is why you would see a big difference in quality over autoclave. But comparing the two is probably not a good comparision. I guess it would really come to down to cost in energy between the two as a fair comparison. 70% reduction in energy cost is good. I’m not sure what the nitrogen costs per cycle, but I know that one cycle is super expensive and we used to get truck loads of nitrogen delivered several times per week. Granted for one of the autoclaves you could fit a bus in it.
I wish I knew. We have a microwave processing oven that hasbeen sitting in the back corner of a back room in one of the labs. Big thing. Might have been pressurized, but I’m not sure. LOTS of controls. Don’t expect to use a normal home oven, since you can’t control the true power. but that thing is so old, I doubt anyone here has seen it being used 
Could be interesting… if you can reduce power consumption for curing, it would defenitely help in making parts that are more efficient and more eco-friendly. ALso it sounds a little fun to cure airplanes in the micro!
DING “Oh, the vertical stabilizer is done”
Yeah Sammy, that’s what I was wondering - why they aren’t pressurised? So effectively its just another method of OOA curing. There must be a reason for not pressurising them though, maybe there are complexities with the air/nitrogen bag arrangement. I found this extract though that seems to suggest that some are looking at high pressure microwave curing: http://www.sciencedirect.com/science/article/pii/092401369290445X
Re the nitrogen, we pressurise ours with air (only 1.5m x 2m) to save substantial amounts on nitrogen.
Certainly an interesting topic isn’t it.
That article is about a high pressure process that uses no vacuum… so I think of a press? Or a two part mold? Or silicone expansion?
I don’t gleen much from the abstract, did you read the whole article?
Not sure what methodology their using but if it’s in a microwave for the heat, I’d think the mold would be only one side with a bag of some sorts. A two piece metal mold wouldn’t work for obvious reasons. Do microwaves easily pass through carbon? If so then do they just effect the resin?
interesting for sure.
ANd what pressure do you take the parts to without nitrogen. I know you can run a clave with airpressure only but there is a point where the air becomes dangerously flamable right? I guess when the 02 concentration is high enough?
No I didn’t read the full article as I couldn’t access it but you’re dead right with the metallic tooling, probably a similar story for various other types of tool so the frequency of the waves must need to be targeted towards the specific resin.
Our clave is rated to 10bar with compressed air, all in line with manufacturers spec and has been to 11bar for testing so Im guessing that concentration is above that. From the manufacturers perspective, 10 bar is fine so that’s good enough for me
I think the main reason why you use N2, is because at higher temps, the O2 will start to oxidize the insides and degrade the autoclave. Especially when you are going up to 720f and a few 100 PSI…you want a very inert atm conditions. Possibly if the part exotherms, there will be less O2 to burn, as well?
I was told that the concentrated oxygen can become combustable under high perssure and the added heat made it explosive. NOt sure if that’s true but I know oxygen is fairly unhealthy stuff in high concentrations. I belive this is why they mix helium into divers tanks when they’re doing deep operations. To dilute the o2. In clave they just take it out completely.
When we spec’ed ours I was convinced by the engineer that it is purely for limiting fire risk - for example, on a big autoclave where its very easy to leave a job card in the clave, its not hard for such things to catch fire at the higher pressures and temps. However, ours is only a metre or two so the risk is considerably lower given everything can be seen. Saved us a fortune so far and nothing has blown up…
yet…