High-temp epoxy curing oven

[Tominator 2]

Glad that it is working out for you. Looking forward on seeing the rest of the build.
-Tom

 

[cwbullet]

Great thread. I have been waiting for a good build thread on a curing oven.

Time will tell if the foil continues to stick after heat is applied to the inside of the oven. It will probably be ok.

 

[dixontj93060]

OK, restart on the foil. I played with the top cover putting it on/off about a dozen times. Nice snug fit and again happy with the foam-to-mineral wool adhesion, but the foil adhesion was flimsy. I pulled off the foil and found that the high-temp caulk wasn't really cured. It may be because I am building in my basement and it is a bit cooler down there, but regardless it made me search for another solution. So I removed/discarded the old foil and headed to my local hardware store. They suggested the stove gasket adhesive shown below. It is made to adhere to metal (and stove gasket material reminds me a little bit of mineral wool). I tried it overnight and it now has a solid hold on the foil and mineral wool, better yet, according to the product info, the adhesion will improve even more after a couple heat cycle cures.

 

[dixontj93060]

OK, now that I found a good adhesive combination, I am slowly working around all six inside faces allowing the better part of one day to get a full cure. Installed one end this morning, one more end to go. I am also cleaning up the Gorilla duct tape outside edge/corner protective wrap. Finished one side, one side to go (can't be seen in picture).



I have also punted on the computer fans. As CarVac brought up they likely would have reliability issues sitting in the oven chamber. After trying to disassemble them with no joy, and since they are only like $3 each, I thought about going ahead and sacrificing them and using them until they croaked. Problem is, once I have the temp cycles going I don't want to open the oven and I have no way of really knowing if they are still working if they are inside the chamber (well I might get an indication from more erratic or more frequent temp cycling, but then it may be too late with significant hot spots). I then took a look at a fan I have in my office that is used to cool a shelf of computer equipment. Nice, small 4" with a lot of air movement and also easy to disassemble and extend the metal fan blade shaft. So I went and ordered two from Amazon (should be in early next week).

 

[dixontj93060]

Installed the final internal panel last night and then decided to put the whole structure on a scale. 49 lbs! This thing is substantial and I am now glad I didn't pursue a wood frame. It would have not provided as much insulating power and it likely would have added even more weight (at least with the design I had sketched up).

Next up installing the heating and control elements.

 

[dixontj93060]

I have a couple here.

Fans came in ahead of schedule. Nice and quiet; move quite a bit of air too. Now I have to figure out how to extend the shaft. Burner, taking a look at your pics above, I cannot quite tell what you did to extend the shaft? It looks nice, like all one piece?

Right now the shaft is about 3/4" long (1/8" diameter) and I need to extend it 5 to 6 inches. Any thoughts on how to go about this?

 

[Burner]

I used a section of aluminum tube with an 1/8" ID to extend the shaft. I tapped one end to secure the fan and keyed the other to keep the motor shaft from spinning inside the tube. I also ran a section of aluminum tubing inside the oven walls to act as a bushing for the shaft to spin in.

Fans came in ahead of schedule. Nice and quiet; move quite a bit of air too. Now I have to figure out how to extend the shaft. Burner, taking a look at your pics above, I cannot quite tell what you did to extend the shaft? It looks nice, like all one piece?

Right now the shaft is about 3/4" long (1/8" diameter) and I need to extend it 5 to 6 inches. Any thoughts on how to go about this?

View attachment 117899

 

[dixontj93060]

I used a section of aluminum tube with an 1/8" ID to extend the shaft. I tapped one end to secure the fan and keyed the other to keep the motor shaft from spinning inside the tube. I also ran a section of aluminum tubing inside the oven walls to act as a bushing for the shaft to spin in.

Great. Thanks.

 

[dixontj93060]

OK, the fans. Although nice, I didn't finesse it like Burner did with the tapped hole and keyed shaft. I simply found in my workshop stock brass tubing to meet the need. One length of 3/16" O.D. and another one 7/32" O.D. both with 0.014 wall thickness (the fan shaft fits tight so I'm guessing the shaft is actually greater than 1/8", maybe metric, but I didn't mic it before assembling). The sleeve fit is perfect allowing a nice free spin without any tilt. Sleeves were cut 4" long to extend the full thickness of the walls. The shafts were cut 6-1/4" to make up for the blade mounting. Given that these fans are "single purpose" I simply used JB Weld to attach the shaft extensions. The fan blades were made to fit by expanding the center hole to 3/16". Tightening down the set screw into the brass tubing to make a slight indentation holds the fan blades very securely.

 

[dixontj93060]

A lot of learning today, both good and bad, on this project. First to get the construction up to date... Yesterday I installed the heating elements on each end of the oven. This was done by drilling three holes (in the same mounting locations as the original heater frame) into half inch conduit. Conduit was installed so that a minimum of 3" spacing to any wall was maintained. As can be seen from the pictures, I chose to go ahead and utilize the original heater grill and mounted that over the elements/reflector combo. I don't know if this helps or hurts the heat transfer, but seemed like a decent safety move.



I then used 16 gauge wire and crimped new connectors on each heating element terminal. The three wires were then snaked through a 3/8" hole drilled in the conduit and out one end/side of the oven. I used normal conduit terminators to attach the control boxes, cut the wires and soldered them to the heat element selector switch. The only hiccup in this process was that I had intended to have both control boxes on the same side of the oven, but, I made the conduit pieces identical instead of a mirror image of each other--small inconvenience though.



Now on to the learning... Let's start with the good. I put together a first test set up with the temp controller. First test was just with one (the bottom) heating element on for one side of the oven only with accompanying fan pushing air. Set the control for 100 degrees top end with a 3 degree deviation (i.e., on/off sequence). Worked great -- moving up to 100 degrees very fast. Noticed though that there was about a minute delay moving up in temp due to the time it takes the heating element to warm up. Also after hitting the upper limit of 100F the element would turn off, but would overshoot by six or seven degrees to 107F as the heating element cooled down. Of course, all this can be adjusted for when actual set up is done. One really nice thing was the heat containment as the heat on/off cycles lasted over 15 minutes so very little heat was escaping from the oven.

Next a second test was done with the bottom element on on the opposite side of the oven utilized also and a new limit of 200F. Again, fast heating and great heat containment. Obviously the ends of the conduit were hot to the touch, but I couldn't detect any heat escaping from the oven except slight warmth from the hole where the temp sensor entered the chamber and at the seams of the oven lid. Again overshoot on the 200F test was also ~7 degrees and cycling was similar between the two tests at about 15 minutes.

Now on to the "bad" learning. This really involves the fan solution, or lack thereof. In fact, this is why my testing was limited to just the bottom element on both heaters (remember the heaters have two elements each, but alas testing up to 400F will just have to wait). Probably just as easy to itemize the issues...

1) Given the volume of the oven, the 4" fans are inadequate. At first I thought I just had the fan positioned wrong (a little bit too far over the heater element) and tried moving it back. No real difference. They just doesn't push enough air.



2) The sleeve mounted in the wall of the oven for the extended fan shaft did not have enough tolerance. Given the extension and the variation in mounting, the sleeve had to be made bigger to stop excessive friction on the fan shaft. I did so by changing out for some thicker, larger I.D. aluminum tubing.

3) Finally the mounting of the fan is problematic. I thought I could simply take the back of the fan cage and screw/glue it into the face of the outside Fomular wall. But even after curing the fan assembly just had too much vibration/movement to be held by glue caulk in the foam. Next incarnation will probably be done by hanging some aluminum stock over the edge of the oven wall and screwing the fan frame into that assembly.

Given the above, I will be doing a second take on the fans likely moving up to a 6" blade version. But, in general, I am happy; a couple steps forward, one step back.

 

[TheTellurian]

Hi dixontj93060

Long time lurker here.:handshake: If I may opine: the fan blade is too close to the wall and should be extended out much more at least one prop diameter with two being better. The reason is that air volume needs to get behind the blade to blow freely. Being too close causes a huge vacumn that immediately sucks a large amount of the exhaust back in behind. I would mount it just in front of the heater using the crossmember as a support.

My interest in this thread is that I'm building a vac former and the oven part is causing gray hairs. :shock: The glue I used smokes and is acrid so back to the drawing board.:blush:


Richard

 

[dixontj93060]

Long time lurker here.:handshake: If I may opine: the fan blade is too close to the wall and should be extended out much more at least one prop diameter with two being better. The reason is that air volume needs to get behind the blade to blow freely. Being too close causes a huge vacumn that immediately sucks a large amount of the exhaust back in behind. I would mount it just in front of the heater using the crossmember as a support.

Tellurian, thanks for the comment. And I do agree. I happened to snap that picture at the end of the testing. It is the third position I tried. First was right behind the heat reflector, one about halfway between the heat reflector and the wall, and then the final one in desperation right at the wall. None worked very well. I never thought about putting the fan in front of the heating assembly, that may work better; I guess I didn't go that direction as I'm trying to save the maximum amount of room for the actual "warming area", but we'll see when I get the new fans in.

 

[dixontj93060]

New 6" dia. fans came in today. Very much the of the same ilk as the 4" Holmes fans. Should be easy to disassemble. Traveling for work now so will have to be in a day or two.

 

[Reinhard]

You could try some basic duct work to control the air stream. I guess that even a short piece of tubing, that just connects the fan with the heating element, might make a difference.

Reinhard

 

[CarVac]

You could try some basic duct work to control the air stream. I guess that even a short piece of tubing, that just connects the fan with the heating element, might make a difference.

Reinhard

That would require drawing cold air in from outside, which is less efficient and might limit temperatures. Also, internal velocity will drop dramatically from the ducting.

 

[Reinhard]

That would require drawing cold air in from outside, which is less efficient and might limit temperatures. Also, internal velocity will drop dramatically from the ducting.

This is not what I had in mind. I was thinking only about a connection between the fan and the heater. There would still be an open space between the fan and the inner wall of the oven, where air gets sucked in. The idea is to force all of the air that gets moved by the fan through the heater (just like in a hair dryer). I suspect (but don't know better) that a significant amount of the air that gets blown by the fan, just flows around the fan to it's low pressure side again and doesn't reach the heater.

Reinhard

 

[CarVac]

This is not what I had in mind. I was thinking only about a connection between the fan and the heater. There would still be an open space between the fan and the inner wall of the oven, where air gets sucked in. The idea is to force all of the air that gets moved by the fan through the heater (just like in a hair dryer). I suspect (but don't know better) that a significant amount of the air that gets blown by the fan, just flows around the fan to it's low pressure side again and doesn't reach the heater.

Reinhard

If I'm not mistaken, this design has the fans completely internal: they just circulate air inside. This entails no airflow into or out of the oven.

 

[Reinhard]

If I'm not mistaken, this design has the fans completely internal: they just circulate air inside. This entails no airflow into or out of the oven.

This is also my assumption. I'm not proposing something else - just an optimization of the internal airflow (assuming this is an effective optimization).

Reinhard

 

[dixontj93060]

Just to clarify... Fan blades inside, motor outside with minimal opening (leakage) areas.

But after restatement I now understand Reinhard's point about getting more directed airflow on the heater reflector.

 

[dixontj93060]

New 6" fans disassembled and modifications of shaft are now curing (JB Weld). After some thought, I believe one of the reasons I was seeing shaft movement with the last set was that the shaft extension was simply brass tube stock. This has some flex. In this round I cut some 8-32 all-thread and inserted it the full length of the shaft extension sleeve to add stiffness. This also provides a more solid material for the fan blade assembly set screw to "bite" into, thus again, resulting in a more solid assembly.

 

[dixontj93060]

Finally got all the parts. Main item was finding a wide L-bracket from the HVAC section at Menards. Came as one piece and I used snippers to cut it into two equal pieces. The top lip (L) is large enough to overlap the top of the wall of the oven comfortably. The brackets were then marked and drilled with a 5/16" center hole and four 1/8" perimeter mounting holes. In general there is a bit of squeaking as the fan shafts slightly touch the aluminum sleeves, but not too bad. And the 6" blades are really pushing a lot of air. I think I'm about ready to do the high temperature cycling tests.

 

[dixontj93060]

An update here... The high-temp limit testing was delayed as the three-wire RTD sensor I purchased from eBay didn't seem to be compatible with the temperature controller. I found a number of two wire versions for sale from between $25 and $250 dollars. Not knowing the subtleties and/or having the time, I punted and just purchased the RTD controller directly from Control Products (the maker of the temp controller). A bit pricey, but much lower risk. It took them less than a week to get it to me which is good, because time is running out before Thunderstruck 4!!!!

Saying the above, today I began the high-temp tests at 250F, 300F and 350F. I wanted to do 1 hour soak on each and observe the ability to reach, then hold temperature consistently. Through 300F I only had to use one of the two heating elements on the radiators (400W setting). To get up to 350 though, and maintain that temp, I had to turn on the second element on each side.\





Each temp level was achieved quickly; in about 10 minutes or so, thus if you wanted ramping, you would have to manually move in steps to stretch out the duration of the temp cycle. This is not required for my application, but was a question posed earlier. In any case, a few pics at 350F, again, held for 1 hour with no problems. At each setting the temp cycling at the set limit temp +/-4 degrees held for somewhere between 5 and 10 minutes--if not obvious, the lower timeframe was on the higher temp setting. Overshoot on the cycles was typically about 2 degrees additional heating as the elements cooled off and while the fans were not running.

 

[dixontj93060]

A couple of other points, and one area of concern.

The temperature was cross referenced with a digital thermometer purchased at Harbor Freight. Through the cycle testing the sensor and thermometer differential varied plus/minus about 4 or 5 degrees. Depending on the temp level and whether it the chamber was raising or lowering in temp, the differential would shift on the plus/minus side. I believe it was a good enough cross-reference to feel the internal temperature was pretty consistent with my controller readings.



Another point where there was an interest in temperature was on the cross bars holding the heating element. This was of concern because the bars rested directly on the pink foam insulation (outer frame of oven). I was worried that this point might get too hot and compromise the oven frame. This point varied between 240F and 245F at the 350F chamber setting. There was no visible damage/compromise to the foam at this temp level.



And the point of concern... After the 1 hour 350F cycle, it was noticed that the pink foam of the lid exhibited about a half inch warp on (concave and up on each end). This was not enough warp to compromise the seal internally, but may be an issue long-term. I believe it will work for my project at hand using the ACP Composites EHT epoxy, but may require some changes to protect the lid in the future (some upper reflector or more insulation maybe; or add / reconstruct the lid using a wood frame).



Oh, one other point of interest, even after more than two hours of cool down from 350, the chamber is still holding nearly steady at 195F--I'd say it's pretty well insulated.

 

[dixontj93060]

OK, now almost ready to go after going through a dry run on the vacuum system with new line assembly (something to survive inside the chamber with the high temperatures). Used copper tubing and flare connectors. Yeah, I know, it looks wasteful on the vacuum bagging materials, and it is, but I have never vacuum bagged a fin can before, so I wanted to make sure I was on the right track before really applying the laminate/epoxy. As can be seen from the picture, I'm doing two pieces at one time, the fin can, and the nosecone, thus have more than a few joints in the vacuum line assembly, thus needed to test the integrity of that also.

 

[Burner]

Awesome job!

 

[dixontj93060]

Couple questions about your vacuum system. Since I don't see any switches, are you running the pump continuously, or are you closing the valve once the vacuum builds? How long does the vacuum hold if you are shutting off the pump? Does the pump vent any oil once the vacuum builds?

I picked up a pump from harbor freight, have hoses, valves, fittings, bagging material and tape. I've seen some build their setup with reservoir, switch, relief valve, oil catch, and on, while others use a rotary vane pump run straight to the bag with nothing but a hose and gage. So I think I am ready to go but am wondering what is really needed if I'm going to leave this thing run for 12 hours.

All good questions, answer is, I don't know on most of them since this is the first time I have done this... First off the vacuum set up is being reused from the one I have for doing research motor mixing. In that application the vacuum never runs more than an hour or so continually. In that application the overall pressure draw is regulated with a relief valve so as not to draw the propellant mix up/out of the bowl and into the vacuum (although I do have a catch reservoir in line with the vacuum hose). In the case of vacuum bags for laminating, it seems that the seams, although secure, would only hold pressure for 3 or 4 minutes without the vacuum applied. This experience matches what I have observed on vacuum bagging videos I have watched on YouTube, ShadowAero series and other sources. Others may have better luck holding vacuum. My previous method utilizing Foodsaver bags certainly hold vacuum for days, but alas, those bags won't survive 350F+ heat. So yes, the plan is to have the pump run the full 8 hours curing cycle. I will be watching the oil level. Also, the relief valve will be left slightly open as I found in my testing I could pull 90+psi but at that point it pops a hole in the vacuum bagging material--so regulating the vacuum pull at 40 to 50psi is more practical.

 

[Issus]

Dixon, thats a really nice looking oven.

For your bagging setup take a look at this: https://www.veneersupplies.com/products/Project-EVS-Auto-Cycling-Pump-Vacuum-Press-Kit.html

I have one of these to use with my pump and use several 1m long 50mm PVC pipes (bigger pipes/end caps here were too weak). Pump cycles on and off very nicely, and with the pressure control I can even vac bag foam without crushing the core material out of existance lol.

There are several set of plans there, I had trouble getting the vacuum regulator and mac valve elsewhere.. shop around though you can probably find the bits and pieces cheaper elsewhere.

 

[Pat_B]

I do a lot of bagging, and yes, you pretty much need to keep the pump running. If you are able to get out the leaks in the system then you might be away with around 5 minute intervals in between when the pump runs. That helps keep the pump from getting too hot.

I have to keep a close eye on where I place my vacuum pump so that it doesn't creep along the floor from the vibration of running.

 

[dixontj93060]

Well finally got back to this after a week in North Carolina. Yesterday I did the layup and went through the 10 hour, 5 temp set cure. Cool down was prescribed and when I opened the oven this morning it was still at 95 degrees (from turning off the heat elements at 350F at ~10pm last night). Here are the pieces as they came out of the oven. The high temp bagging material was kind of "crispy" in the end but did hold a vacuum at least through 6 hours when I turned the vacuum off.



In general, I would say this first laminating attempt was successful. I was worried though. After about half way in, I was wondering whether I would open the oven to an airframe that had a couple sagging fins and/or the airframe itself would go out-of-round or bend. Although I rested the airframe on two 75mm casings, I had inserted two 54mm motors, one on each end with about a 3" gap in between. In hindsight it didn't make any difference, but probably would have been better to have just one longer 54mm running the length of the airframe. Below are the parts after being cut from the bagging/release material. Clearly lots of trimming and rough sanding to do, but in general a decent outcome, especially with respect to the airframe and fin airfoil contours I was trying to achieve.



The problem areas are shown in the picture below, namely the high temp bag seal on the airframe and the forward 54mm casing that was inserted. In hindsight, it seems both of these would have been obvious to me, but no. To provide an airframe seal, the rear and forward casings were first wrapped with packing tape and then on the rear casing I applied bag seal. The rear casing was easy to remove as I believe the bag seal somehow insulated the packing tape from the heat of the 75mm casing it was resting on for support. This wasn't the case for the forward casing as the packing tape completely melted away and the gummy mess left behind made it very hard to remove the forward casing. The second issue which is worse (in background below), is that I had applied the bag seal directly to the airframe forward of the fins. That was a big mistake as the bag seal is bonded to the airframe (I believe). Again in hindsight, it would have been better to put down packing tape on the airframe first, then the bag seal. Oh well, even if I have to sand it off, worst case there will just be less of the bare carbon showing on the finished rocket, which is OK I guess. BTW, if anyone knows something that will dissolve the bag seal material, please let me know.



Oh, one final comment on the high temp epoxy I used from ACP Composite. In the end, I would barely call this a laminating epoxy. When mixed it is pretty thick and it is almost gel-like. The best way to apply it was not with a brush or roller, but using a tongue depressor/popcycle stick and "squeegee" it on. In the end, I guess it does achieve lamination, but really I think the best way to use it would be to first do your multi-layer lamination with normal epoxy and then put on the final layer of laminate with a 6oz or less FG using the high temp epoxy.

 

[CarVac]

Oh, one final comment on the high temp epoxy I used from ACP Composite. In the end, I would barely call this a laminating epoxy. When mixed it is pretty thick and it is almost gel-like. The best way to apply it was not with a brush or roller, but using a tongue depressor/popcycle stick and "squeegee" it on. In the end, I guess it does achieve lamination, but really I think the best way to use it would be to first do your multi-layer lamination with normal epoxy and then put on the final layer of laminate with a 6oz or less FG using the high temp epoxy.

If you really need heat resistance, you need resistance to heat soak, so you want the entire layup to be out of high-temp epoxy. It seems most people have sufficient success with just a coating of high-temp epoxy that you don't even need to put on a surface layer of fiberglass with it.

This is one reason CCotner and I got PTM&W 5712 epoxy infusion resin: it's very thin and wets out cloth with zeal.