Oven Options for Prepreg Curing (2023)

Building with prepreg materials is great, but you need a freezer for storage and an oven for curing. Of these, the Blast Furnace is often the biggest obstacle and the one with the scariest failure mode. A fire in the prepreg oven (okay, any fire) is bad news! This article will describe your options for choosing furnaces and how to build and equip one yourself.

Note: The information presented here reflects my opinion and I make no representations as to its accuracy or usefulness! Use of this information is at your own risk and that of EC! (and I, Chris) shall not be liable for any injury, damage or other undesirable consequences. These things (heat, electricity, gas, etc.) are dangerous and you must be very careful and get professional help if you are not qualified. Not even!

general planning

Which type of oven you choose depends on four main parameters:

1. How hot do you have to go?
2. How big are the things you need for cooking?
3. How much thermal control/consistency do you need?
4. How Much Money Can You Spend to Build or Buy Your Oven?

Generally, in order to raise any of the numbers 1-3, you need to raise the number 4!

If you are getting into pre-pregnancy, take a look at my article:LAMINATION WITH PRE-PREGSfor an introduction and many details.

parameters of your oven

Here are some things to consider...

Temperature

Probably the biggest factor in your oven decisions is the temperature requirements. If you need more heating (120C/250F or more), that takes you out of the practical realm of "do-it-yourself". There are of course ways to do this depending on your level of craftsmanship, but chances are when you are building your kiln that you are (or should be) running lower temperature prepregs. There are many excellent resin (epoxy) systems that cure below 100°C and you should look at these and determine the lowest temperature you can get away with - why not? If cure speed is an issue to reduce cycle time, see if you can get a resin system that cures faster at a lower temperature.

Above 120°C, I recommend buying a furnace (new or used) from a manufacturer that has safety features, is made of refractory materials, and has a good circulation system. As the temperature differential between ambient air and furnace air increases, one must focus more on thermal performance and insulation than just sealing the furnace shell. More energy is required to raise the temperature and more firepower is required from the heater. This gets tricky and requires some carefully thought out circulation and temperature monitoring systems. Again, it's not that you can't do this - it just gets harder - and more dangerous if you get it wrong.

But a low temperature (100°C/212°F) prepreg oven is very similar to a sauna. If it's big enough, you can go inside for a short time. The insulation must withstand a temperature difference of 60-70 °C (140-160 °F). You can use standard building materials and even wood to build a low temperature furnace. Some circulating fans can be used in ambient temperatures around 100°C - you have to buy the right ones. Later I will discuss considerations for building a low temperature prepreg oven.

How large?

If you're building a big one, you need a big kiln - and big kilns require big heaters! Many manufacturers can supply walk-in ovens for curing composites, and this is a great option for large businesses with daily scheduled baking. They are very expensive and require gas installations, firefighting systems and sophisticated recording and control computers. This is great if you can justify the cost and for high temp prepregs you would benefit from the fast rise rates (how quickly it gets hot) and overall well rated quality. Your safety and fire inspections will get better too!

If you are building a large one but using low temperature curing prepregs (up to 100°C), there are several options to build a custom disposable oven or set up a modular system to cure different sized parts, depending on your need. is necessary. With a background building high quality custom boats this has always been the way to go. At its simplest, boat shops quickly built "rooms within rooms" with stud and plywood walls and fiberglass or foamboard insulation, and running gas heaters through holes in the walls. Temperature was recorded with a series of thermocouples and temperature controlled by manually adjusting the heating elements. It's not efficient and labor intensive to run, but if you only need to cook large pieces a few times a year, it's very economical. For smaller stoves that are used more frequently, there are electric heating options that are safer and easier to control.

Check with your local fire inspector and ensure any built-in continuous furnaces are compliant. Built-in sprinklers and fire alarm systems may be required - because a built-in stove is a room! The last thing you want is a fire – and the second to last, having to make costly last-minute changes to comply with the rules and the risk of fines or business disruption. Modular ovens tend to sidestep these issues as they are temporary.

thermal control

The keys to thermal control are good temperature sensors, good airflow, and enough heater power to make rapid changes in oven temperature. Circulation is probably the most important thing when it comes to actual baking performance. A room-sized stove can easily get 10 degrees hotter at the top than at the bottom - especially if the floor is poorly insulated. You really need to move the air and blow the hot air from top to bottom. Some heating systems are designed to draw in air from above, heat it up, and blow it back down, creating rapid movement and an even air temperature. Good stoves should have airflow that feels like being outside on a windy day - a really hot day!

After the circulation decreases, the temperature measurement can be made at fewer points in the air. When using an automatic heater controller to set the temperature, it is important that it operates on the air temperature and not on a thermocouple attached to a part. You want the air to be even, and you want the air to be even to heat the part and tool being fired. You can then monitor the temperature of the part at many points to ensure you are heating as needed and maintaining the required cure cycle.

Wait, can you use a direct warm-up strategy?

Sometimes it makes sense to heat your tools directly with pad warmers or silicone pad warmers. This can be much more efficient, faster and safer.

Pad or cartridge heaters require good regulation and control - and they heat up quickly. For aluminum molds, they can be a great solution!

Silicone pad warmers can be found in hot glue kits and 3D printers. They can be purchased with high temperature adhesive or bare and are available in a variety of dimensions and power levels.

This image shows a silicone heating pad for a 3D printer attached to a piece of 1/2″ (12mm) MIC-6 cast aluminum plate that has a removable coating on the opposite side – here upside down shown. This little table heats up quickly and works well for temperature controlled layups and even low temperature curing before soaking. The control thermocouple is fixed on the top and the bottom part is exposed to avoid overheating.

It's worth taking a look...

So buy or build?

My advice is to buy if you can and build if you must. Unless your job is "stove builder," your efforts are likely to be better spent elsewhere. But necessity is also the mother of ... furnace construction. Big ovens are expensive and huge - if you need to cure composites but can't afford the money and space that an industrial oven requires, you only have one option.

If you need to build a large oven, it may be worth looking for a smaller commercial oven for everyday use with smaller parts. They are more efficient, safer and you don't have to worry about them. People use homemade electric ovens - like baking a cake - but the temperature control is not good. You can buy used industrial furnaces at a reasonable price and (especially when it comes to electric furnaces) easily install them. It can even be on wheels! I bought a 3'x3'x5' Grieve box oven from Craigslist, drilled some holes for vacuum connections and thermocouples and used it almost daily for years. Not just for curing pre-preg - for filling or speeding up gluing jobs that would otherwise have me waiting overnight for the next operation - very handy!

buy an oven

If you are considering buying an oven, you have many options. Small ovens are typically mobile (not fixed to the floor) and this can be an important factor in ease of use and flexibility. Large furnaces are almost always bolted to the floor of the building in which they are located, and fire suppression systems often need to be fitted. Larger ovens are usually gas powered and require special gas piping and safety systems. If you are at this level you know more than I do about what you need! Large furnace suppliers have sales teams and application engineers who can help you. See Appendix A at the end of this article for a list of oven manufacturers commonly used to cure composites.

For small shops and people looking to build a kiln I would recommend looking at a used industrial kiln. Between Ebay, Craigslist and surplus resellers (my favorite -HGR excessin Ohio, USA often have plenty to choose from) and just ask around - there's plenty to do. Many industries use ovens for drying, curing and heat treating, and the processing requirements of composites are quite modest by comparison. The only downside is that some of the used ovens you will find are very dirty and have old control circuits. Furnaces are pretty basic at heart, and parts are commonly available or upgradeable. Old temperature controllers can be upgraded to modern PID units with ramp and soak cycles.

When buying a used or possibly rebuilt oven, make sure that the safety functions work! Furnace controls should be designed with a main heat controller (PID or plain old style thermostat) and also a superheat controller with oneandersThermocouple that shuts off power to the heaters when the over-temperature threshold is reached. There should also be a way to turn off the heaters if the fan or blower stops working. Most electric ovens use a series of relays to control power in a loop configuration, with each relay tripping (current flowing through the switch) when a specific logical condition (input voltage) is met. Ensure that the relays are normally open (N.O.) with no power and that the system logic does not allow leakage conditions. When in doubt, consult an electrician or other qualified and experienced person for advice on any modification to a control system. Often original manufacturers can provide schematics (or you can find them online) even for fairly old ovens. Anyway - try your over-temperature shutdown system!

build an oven

So your heart is set on building a furnace...okay, great. If this is what you're looking for, I'll share what I know about building stoves and how to do it effectively and safely.

Safety note: do not do this!There are many ways to seriously screw this up - fire, electrocution, fire, broken parts, fire, gas explosion...did I mention fire? Be careful! Also hire an electrician if you are not one.

So there are four key things to figure out:

1. Envelope: how will you close and insulate the volume of your furnace?
2. Heat Source: How will you heat your oven?
3. Control: How do you control your heating?
4. Supervision: How do you know what's happening to your pieces as you prepare them?

Oven Envelope

Building an oven is like building a house. You have to build the structural part that holds everything together and then you have to build the thermal parts that keep the heat in and the cold out. Unlike a house, you don't have to worry about rain! There are ways to build that combine structural and thermal requirements into one solution (e.g. SIP panels for buildings), and other ways that keep the two separate. Our main focus here is thermal performance. You just have to make sure it doesn't fall on you, your friends and your belongings.

If you're just trying to cook prepregs at low temperature, or post-cure cast-on or wet parts, you'll probably only cook to around 100C. This is hot, but not so hot that you have to worry that the hot air will actually set the furnace materials on fire - but it can melt them if you're not careful in choosing. The saunas people actually go to can be 100C hot - and in dry air it's uncomfortable but not painful to be in 100C hot air - just don't touch anything metallic!

The gold standard for this is metal clad foam board stock (Kingspan is a common brand). Commonly used in commercial freezers, garage doors and many other industrial situations, it consists of thin sheets of metal wrapped around a foam core. Care must be taken to find foam sheets that can withstand the high temperature. Ovens can be built on internal or external welded structural frames (often mobile - with wheels!) - foam panels are mounted on top of the frame with gaskets to seal air leaks. Metal is beautiful because it is fireproof and easy to clean.

Similar to metal panels, I have seen 'stick-built' stoves with metal studs and rockwool insulation, which were then coated with external metal roofing material. This can also work for a modular or mobile oven. The main idea is structure, more insulation, relatively more airtight shell. You can come here as you wish.

If you're considering a modular oven, you might as well do it in sections - or better yet, with collapsible snap-in sections! This is a preferred option for boat builders who need to handle long, thin parts, but not as often. A collapsed furnace or a section of a larger furnace can be used to harden smaller parts. Versatility is excellent!

Remember that there are two ways to lose heat: through a lack of insulation and through air leaks. You want to focus on both. If you have 6″/150mm insulation but big gaps, that's no good. Best with 2″/50mm insulation and gaskets or tape over the joints to prevent heat escaping through the holes.

Wood-fired ovens are suitable for low-temperature cooking. Wood is cheap, light and can be painted with flame retardant "intumescent" paint. It has the advantage of having a low thermal conductivity, so it doesn't extract as much heat as metal. However, it is good to think of your oven as a well-insulated house. And think of fire. Wood burns easily and is a poor choice for use with open flame heaters.

Here is a picture of a wood stove I had many years ago. It was constructed from 1/4″/6mm plywood over wooden frames with fiberglass insulation. The heaters were electric "monster hair dryers" and there were a couple of circulation fans (high temp ducted fans) that kept the air spinning. Some makeshift seals made of breathable fabric and laminated insulation can be seen on the floor. Parts are held off the floor and vacuum hoses pass through a hole near the floor. A thermocouple was hung from the top so that it hung about 30 cm above the parts and several thermocouples were attached to the parts to record the curing temperature.

Probably the simplest and cheapest oven is a box made of foil-laminated insulating panels. The leaves are stiff and can be cut with a razor. I've had success using 2″ thick foam taped to the seams. You can make a box with one side open and place it on top of another sheet containing the product to be cooked. You should place your pieces on the bottom of the oven so they are surrounded by hot air. The easiest way to heat a box like this is to cut a hole in one side near a corner and line it with a metal pipe (or a tomato can with the bottom cut off!) and point a heat gun at the hole. If you place the hole in a corner, the air will swirl in a circular pattern around the stove instead of having a less predictable flow pattern. You can place multiple heaters in opposite corners of the oven and "blow around" the outside.

Make sure the heaters are not pointed at your parts or the oven walls! Because fire.

You'll also need holes for vacuum connections and thermocouples - and this can be as simple or complicated as you like.

heat sources)

Once you have a well-insulated oven shell, you'll need to blow some hot air into it. But how much? I'll do this in feet and Fahrenheit - but calculators and similar equations for the correct units are easy to find. This is probably one of the simplest ovens out there and doesn't account for air leakage. Attention: simplification!

Heat Loss (BTU/hr) = Area (sf) * Temperature Difference (F) / R-value

So imagine an 8'x4'x4' box made out of 2″ foam sheets with an R-value of12. Let's say the room temperature is 70°F and the oven temperature is 200°F. Our total area of ​​the six sides of the box is: 32+32+32+32+16+16=160 square meters. The temperature difference is 200F-70F=130F. So our heat loss looks like this:

1733 = (160*130)/12

So to keep this oven at 200 F in a 70 F room would require 1733 BTUs per hour. To convert BTUs to Watts, divide by 3.41. Then…

1733 (BTU/h) / 3,41 = 508 Watt

Now 508 watts isn't much - the average hairdryer can do that - but maintaining the temperature isn't the game - we need to get it up to that temperature in a reasonable amount of time and then hold it there, even if someone opens the lid to take a look throw, or if there is 300 lb room temperature tool block (or another - see below) to heat. So we're going to need a lot more, but it's a good number to have for reference. At least double that could do in a tightly sealed small oven, but it would rise very slowly. For practical reasons, four to six times the heat loss is a reasonable amount of energy for a low temperature furnace. For reference, the little Grieve oven I had could power a 3'x3'x5' box at 6,600 watts - and it scales like crazy!

To get an idea of ​​how much energy is (theoretically) required to heat objects:

• 500 pounds of aluminum from 70F to 200F at 1 degree per minute requires 2110 watts
• 500 pounds of Invar from 70F to 200F at 1 degree per minute requires only 1108 watts
• 500 pounds of epoxy/carbon laminate from 70F to 200F at 1 degree per minute requires 2088 watts

Now that you have an idea of ​​how much power you need, let's discuss where to get it. The image below shows three levels of heat source. On the left is a "super hair dryer" type heat gun, which is usually powered by a regular outlet. Capable of delivering up to 2,000 watts or more, they have built-in fans to extract heat from the metal chassis. The ones I use are frommaster flow. There is a Swiss company calledLeisterit makes the larger high-performance ones, but I don't own any - I hear they're good. The intermediate heater is a gas-powered type that is often used for temporary heating on construction sites. They have an electric blower and ignition system, and use compressed gas (propane, LNG, etc.) from a cylinder or on-board gas line. On the right is a much larger circulating air blower with an electric heating system. This is a special product fromElectroHeatin Sweden, which have an output in the range of 20-30 kW and require an industrial-scale power supply.

Of course, there is a very wide range of heating options that can be used to heat the air. These are the ones I've seen for user built ovens in all price ranges. If gas heating is an option, it can be inexpensive compared to electric heating, but has the two main disadvantages of blowing flames and exhaust gases into the furnace volume. Flames can be mitigated with careful aim and the use of metallic "blast plates" to deflect the jet of super-hot air away from combustible materials. A built-in system can use a heat exchanger to completely solve these problems. The main advantage of electric heat is the degree of its controllability and the fact that electricity is far less dangerous and complicated to conduct in your installation.

Ovens need attention. For all but the best and most designed "DIY" stoves, it's always nice to have someone around when things heat up. I've had issues with fires myself and it's super scary! Every large shipbuilding shop I've worked at ensures that there are two people present for every great parts cook - both to oversee the process and to make sure things stay safe. You may be tired or just want to go out for dinner, but don't! There are so many potential problems, and unless you've designed and built your furnace from refractory materials very carefully - and tested it a ton - you never know what's going to happen. That's a good thing about autoclaves - the problems are in a thick metal shell - but autoclaves have other sources of drama!

Combustion fumes are a serious hazard. Gas heaters need to be operated in a well-ventilated area to avoid carbon monoxide build-up - because it can kill you. A quick survey suggests that a quarter of all propane-related deaths are caused by carbon monoxide poisoning. You can buy smoke and carbon monoxide alarms - and you should!

controls

The goal of an oven controller is to apply enough heat to keep things at the right temperature, and do it without unnecessary variability. You need a way to monitor the temperature and then control the heaters to have a stable (ideally controllable) internal temperature. The simplest control of all is a "chimney" - you know, like a hole in the top of the stove! If you have a constant supply of heat, such as B. an electric heat gun or propane blower, simply vary the chimney size to vent the heat as you blow it out and leave just enough in the furnace. This is surprisingly effective if you're careful - and you have to be careful! The chimney can be as simple as a hole with a board placed over it that you can move to more or less cover the hole. You simply balance "heat in" with "heat out" to keep the desired temperature stable.

Well, if you do this for work or think someone might see you, you're going to want a real electronic programmable controller with a small screen and buttons. It's best to buy it as a unit from someone who knows how to construct electrical things. You can buy simple controllers used in many industries where people control temperature - from brewing to ceramic kilns. If you're thinking about building your own controller - and I can't dissuade you - there are a few things you should know. I'm not an engineer or electrician, so take this for what it is - the ramblings of an unsuspecting amateur! This is how a digital oven control works:

First you have a thermometer - this will likely be a thermocouple - that will read the temperature. If it's too cold, turn up the heat. If it's too hot, either do nothing or, ideally, look back on what you just did and plan to do less in the future. Then he waits a bit and reads the temperature again - and repeats. You can get a simple temperature controller that will just read the temperature every few seconds, and when it's below a threshold, it turns the heater on and runs until it's warm enough, then turns off - and repeats. These can be mechanical - just a dial, or digital with a "set point" and an offset at which it kicks in. You could set it to 200F and say the max error is 3F; This works well in many cases, and since your parts and molds have mass, they stabilize the heat your materials "feel" - but if you register the temperature you'll see that it looks very sharp - because it is!

The ideal temperature controller has some nice qualities: first, it's programmable, and second, it has a plan for when things get out of control. Safety first: Your controller must have an over-temperature alarm and a way to turn off the heaters. Things don't work properly and relays can fail and your heat can get trapped. When that happens, you need a SEPARATE system to say, "Oh shit, oh shit, it's getting too hot - unplug!"

On the programmable side, you're in luck - there's something called a PID algorithm - which you can find in any "proportional-integral-derivative" controller you can buy. It's a small computer that takes readings and applies error correction to stabilize the temperature and rate of temperature change. Many of them can handle so-called "ramp-and-soak" profiles - a graph of temperature versus time that can be programmed.

You can think of a PID controller as a "cruise control" for your heating system. In a car, cruise control can vary how hard it accelerates and brakes - which makes it really effective. The simplest ovens use a control system where there is only one accelerator - and it's 100% or nothing. It would be difficult to drive at a constant speed if the gas pedal was an all-or-nothing operation - but thankfully this binary option can work much better for a heater than for a car!

Some of the PID controllers you can buy are great, but they are very difficult to program. You really need the manual and it takes a long time to figure it out. mistakes are bad There are great touchscreen options with much better interfaces (Watlow makes one) that make programming and operation easier. These things are always improving, so if you hire other people to run your oven and you're cooking high-end stuff, it might be nice to get an easy-to-use controller!

Your temperature control thermocouple must ALWAYS be in the air section of the oven - never in or in a section! The air heats up quickly and the rest of the solid material takes a long time to warm up. If you try to use a hard-to-heat part to control the oven, the heater will continue to pump out heat and the air will get too hot. Use the heaters to heat the air and let the hot air heat the parts. You can make really bad things happen if you don't! Be careful!

The PID controller has an input thermocouple that reads the temperature of the air in the oven and an output that controls a relay that turns the heater on or off. A relay is just an electronic switch that can handle the high current loads associated with electric heaters. There are two types of relays that you will see in ovens - mechanical contactors that turn on and off with a bang, and solid state relays that do nothing more than flash a small light when they turn on the power.

Contactors are suitable for switching operations that occur less frequently - for example once a minute, solid state relays can switch on and off every few seconds. Depending on which type you use, you'll need to adjust the way your PID toggles. The beauty of a solid state relay is that it can switch so fast that it behaves as if it were variable - the PID can vary how much it keeps in heat and how much it turns off over time, to operate the foot switch effectively in the heating system. Combined with the PID algorithm, a solid state relay controlled oven can maintain a much more consistent temperature over time.

Hence the overall picture is an interlaced set of check stitches. "External" is an over-temperature alarm. It can turn everything off if it gets too hot - but it usually stays on. A second relay runs inside the control circuit, which takes over control – but only as long as the safety circuit is working. All of this has to be in other loops that have an on/off switch for the entire system. And fuses - fuses are needed. Really - find an electrician or buy a ready-made control box!

monitoring

When pre-baking or even post-curing a large, high-quality part, you want to be sure you're getting the part temperature you want. This requires you to attach a "thermometer" to the part itself - probably many! The solution to your problem is the thermocouple!

For more information on setting up thermocouple logging, see this post:TEMPERATURE MEASUREMENT WITH THERMOCOUPLE

A thermocouple is a junction of two dissimilar metals that produce a very small voltage that changes with temperature - the "thermoelectric effect" is what's happening here for you scientists. Therefore, there are standard combinations of metals used to connect thermocouples that are denoted by a letter value. A very common type of thermocouple is the "J" type (also called "L" type in Europe).Wikipedia), which is a combination of iron and constantan (a nickel alloy). Type J thermocouples can be used between -40 °C and +750 °C - perfect for furnace monitoring! You will also see Type K (Chromel-Alumel) which has a slightly different range but is also very common.

A problem with thermocouples is that they require a reference temperature, called the "cold junction," to provide a stable voltage that can be compared to voltages that vary with changing junction temperature. This is basically just another type of thermocouple housed outside of the hot room. Here's an image showing the effect of a reference junction on a cook's recorded temperature:

See this line below - the pink one - which is the cold spot that is monitored along with the temperature of some parts in an oven. The blue trace is the air temperature - just a thermocouple hanging over the part in the oven. This cook had a “ramp” to 80°C (the initial steep section – it sounds steep, but it actually took the pieces 2.5 hours to reach 80°C) followed by a 3-hour “soak” at 80°C This was a low temperature prepreg in a thick section so the bake profile was designed to prevent the exothermic cure reaction from getting out of control and increase the cure temperature. You can see the part temperatures (green and red) "run through" through the air and outside of the mold in about 3 hours. This is the "exotherm" that occurs during curing and can be detrimental to tool and part stability if things cure too quickly. Here we kept the temperature low and then ramped it up to 95°C for 6 hours just to make sure the cure was complete - even in thinner areas of the part that may not have generated much heat of their own.

So about this cold spot – it also tells a story. This was baked on a hot day and the shop door was open and some air was blowing through. We started cooking in the morning and you can see that the ambient 'cold spot' temperature has risen and fallen throughout the day. Around 9:30 at the chef's it was late and getting cold - so someone locked the door. The control box was discarded when the internal temperature around the hot furnace rose again without a breeze to cool it down. You can see how the cold junction instability affects the "temperature" measured by the thermocouples! You can also bet that the actual oven and piece temperatures didn't fluctuate that much between 9:30 and 10:30 when baking. After the day's fluctuating room temperature, the rest of the cook was calm. Keep this in mind when using thermocouples for temperature monitoring and control – they are a relative measuring instrument and only work as well as their reference junction and calibration allow.

So how do you use thermocouples? Well, there are many options! The easiest way is to buy a portable "reader" that you attach a thermocouple "probe" to and read the temperature. You can hang one of these thermocouples in your oven and monitor the temperature of the air inside. Just think of the reference junction and don't leave the reader on the stove! The multi-channel recorder is one step further. They are readily available from many manufacturers and can be recorded in internal memory, on a computer, or even on paper tape. The one I've had for a long time was made by Pico and has 8 input channels and plugs into a USB port on a laptop. You can buy loggers with any number of channels and logging options if you want to spend some money - but it's getting cheaper and easier all the time. I will list some links to these types of products in Appendix B at the end of this article.

So once you've made some thermocouples with plugs on one end and a junction on the other, you need to connect them to your parts. Most of the time you can just stick them on the outside of the vacuum bag and that's fine. I like to use a few inches of tape (sack mastic) wrapped around the end to avoid damaging the bag. A lot of tape doesn't hold up well to the heat, but I've found that good, thick, clear tape (not the flimsy brown one) works really well - just tape the thermocouple built into the tape - you can do X- with two Pieces of tape if he wants. If you must bury a thermocouple in your part, make sure there isn't a leak where you run it through the bag. Some thermocouple wires can leak through the wire insulation - but not too much.

Since voltage is generated at the junction - where the two wires join - it is important to ensure you connect them where you want them (at the end) and not where you don't want them (at the plug, somewhere in the middle where the insulation is damaged etc.). Test them by holding the tips with your fingers and watching the temperature on the logger increase. Thermocouples will wear out over time and the voltage will drop or become less reliable - but you'll probably mutilate them first. I like to buy lengths of thermocouple wire and make my own in correct lengths. You just cut it to length and connect a thermocouple fitting/connector to one end and remove and twist the wires at the connection end. You could also zap them with a TIG welder if you roll like that. Roll them up carefully when you're done using them and don't let the wires twist or you could end up with a dodgy splice in an unusable spot.

If you are also interested in registering the vacuum, read this article:HOW TO REGISTER THE VACUUM

Conclusions

This is definitely not all you need to know to craft or use a compound furnace. There are so many variables and ways to do things that it will be an ongoing learning experience... forever. But the basics are very simple. Ovens have been made for a long time, and for basic composite uses, you really think of a well-insulated space with the increased heat, rather than an "oven" in the sense of cooking your dinner. I have tried to show where things can go wrong and I hope you will be very careful. Good luck, have fun, and when in doubt don't!

If you've built a great oven and would like me to include it in this article for reference, I'd love to show you a variety of solutions. Get in touch and send some photos - I'll give you the link!

Appendix A: Oven Manufacturers

This is a list of companies that sell ovens suitable for processing composite materials. Look for used ones on Ebay, Craigslist and other sites when new ones are too expensive! If you know of brands that should be added to this list, please let me know.

• Complain
• Wisconsin oven
• to ship
• Lewco
• Azul M

Appendix B: Thermocouple Record

These are some companies that produce recording material for thermocouples. There are many more but these are the ones I used. Let me know if you have one that should be added to this list!

• Omega
• Watlow
• Start
• Pico Tecnologia
• MicroDAQ

Monitoring:

This article contains information reflecting my views - I make no promises of usefulness! Use of this information is at your own risk and that of EC! and the author(s) of the article shall not be liable for any injury, damage or other undesirable results. These things can be dangerous and you should be very careful and seek professional help if you are not qualified.

These articles may contain errors (please let me know if you find any!) and contain biases based on my/the author's limited experience. If you disagree with anything here, please contact us. It's not just about sharing my knowledge, but also about learning from others. I am happy to include additional information and differing opinions so that readers are better acquainted with the variety of "correct" answers!