Muffle furnaces are a design whose heating elements allow one to achieve the temperature necessary for firing ceramics, melting metals, and hardening steel in a personal workshop. Jewelers and other craftsmen whose workshop is at home understand the value of such a design. And given the high cost of factory-produced muffle furnaces, a similar furnace made independently takes on special importance.

Based on the type of heating elements, muffle furnaces are divided into:

electrical;
gas.

By purpose they are divided into:

for metal melting;
for firing ceramics;
for melting glass;
for hardening metal;

There are also industrial and homemade muffle furnaces.

But it is impossible to make gas-powered stoves at home, although gas is cheaper than electricity, since such experiments are prohibited by law. Electric oven control provides convenient temperature control.

According to their design type, muffle furnaces are divided into:

horizontal (the simplest);
vertical or potted type;
bell-shaped;
tubular.

Heating can be carried out in air, in a vacuum or in gas environment. At home, it is only possible to design a furnace with heat treatment of products in an air environment.

When you make a muffle furnace yourself, you can give it the desired shape and volume, and decorate it in a style suitable for the interior.

Main parts of the structure

List of materials and tools for assembling the structure

grinder (machine for grinding and cutting materials) with cutting wheels for metal;
welding machine;
sheet steel >2mm thick;
metal corners;
fireclay fire-resistant brick;
fireproof mixture;
heat-resistant silicone;
basalt thermal insulator (cotton wool, density 200 kg/m3) or perlite;
safety glasses and respirator;
nichrome (fechral) wire with a cross section of 1 mm;
wire cutters or metal scissors.

Muffle furnace construction technology

The procedure for performing work on a horizontal or vertical muffle furnace is similar, the difference lies in the arrangement of the furnace elements.

Important! When cutting bricks, be sure to protect your eyes and respiratory organs from dust with goggles and a respirator. Work must be carried out outdoors or in a well-ventilated area.

We place the resulting brick pipe in a housing with a layer of thermal insulation.

But first, you need to make a spiral with a diameter of about 6 mm from a coil of nichrome or fechral wire. To do this, we wind the wire on a base (pencil, welding electrode or a thin metal rod). We take out the bricks and again lay them out on a flat surface in a row.

We apply a spiral, make markings for future grooves, which we will cut out in the bricks with a grinder. We check the correctness of the lines with a building level. Ultimately, inside the workspace, the wire will be laid in a spiral from the bottom to the top of the workspace. It is important that the turns do not touch each other, otherwise there will be a short circuit.

To bring the ends of the wire outside the working chamber and connect them to the machine, insert three thin long pieces between two adjacent bricks ceramic tiles with thin channels cut into them for wire.

The use of such ceramic leads in the future will make it easy to produce renovation work muffle furnace.

Electrical switching with three power levels

for the first power stage it is necessary to connect two spiral circuits in series;
the second stage involves a separate connection of the lower spiral;
the third power stage is the parallel connection of two circuits.

When turning on the spirals, be sure to ground them!

We place the finished structure of the working chamber in a housing with a layer of heat-insulating material and one brick laid on the bottom, coating it with fire-resistant (kiln) clay or fire-resistant glue.

To bring the ceramic channels outside the body, we drill holes in it.

We make the lid from sheet steel, cutting it to the size of the oven and securing it to it with oven clay fire brick. We weld the latch, handles and canopies on top. To ensure a tight seal, we apply a layer of heat-resistant silicone along the edges of the lid and on the adjacent walls of the muffle furnace, having previously thoroughly degreased the surfaces.

After the oven has completely dried, we connect the wire to an electrical machine with a stabilizer and carry out a series of tests, adjusting the filament power of the coils and the temperature in the working space by increasing or decreasing the network voltage.

Important! To make sure that the oven is completely dry, it must be turned on at maximum power and make sure that there is no steam evaporating from the surfaces of the oven.

The door must be locked tightly while the oven is operating.

Video - homemade muffle furnace

What is a muffle furnace? And what is it for? Let's take a moment to turn to the memoirs of the court carriage master of France late XVII– beginning of the 18th century. Jeanto. Yes, the same one who invented the steering linkage. That’s what they call it – Jeantot’s trapezoid.

He invented something, but he couldn’t put the idea into action; the axle shafts of the prototypes kept breaking. Jeanto was in despair; he expected huge profits from the invention. Carriages with front wheels on a solid axle with a king pin would overturn every now and then, and the nobles with their beautiful companions, right up to their majesties, would fly onto the road filled with manure. The craving was alive, for oats.

And then a certain, unfortunately unnamed, Russian nobleman visited the workshop. Peter the Great was just preparing the embassy of Bekovich-Cherkassky, famous in history, to Bukhara. The star of the gifts to the khan was to be a luxurious carriage; This is what our compatriot came to order from the best European master.

So, he, having looked at the fracture of the part and tested its hardness (“The nobleman did not disdain to pick up a file!” exclaims Jeanto), gave advice: “First, cement it in a muffle with ground cattle horn, and then calibrate it. Then the inside of the steel will remain viscous, not brittle, and a strong, hardened crust will form on the outside and will not wear off for a long time.”

There is nothing to blame for Jeanto; he was a carriage maker, not a metallurgist. And the fact that Peter’s officer knew about metal processing is also not surprising. But it is immediately clear what the muffle is for - a heat-resistant and impenetrable container in which the part undergoes heat treatment. Its role is to isolate the part from external influences, incl. and from the flame, firstly. Secondly, ensure uniform heating.

Jeanto's masters most likely heated the muffle in an ordinary one. But over time, the requirements for the quality of heat treatment increased, and new technologies were developed. Already at the beginning of the 19th century. In industry, special muffle furnaces were widely used, designed exclusively for heating the muffle chamber. In the age of plastic, low-temperature muffle furnaces appeared in which composite parts undergo hot curing.

Now muffle furnaces have enough work:

Remelting of valuable metals and alloys, when the introduction of impurities or thermochemical interaction in a flame is unacceptable.
Heat treatment of metals: cementing, hardening, annealing, tempering, normalization (aging).
Firing ceramic products, especially artistic ones, to obtain an even surface tone.
Drying of electrically conductive materials; As is known, microwaves can only dry dielectrics.
Combustion to mineral components by sublimation of volatiles (ashing). In this regard, there is such a sad duty as cremation.

In addition, muffle furnaces are used in assay analysis for the so-called. cupellation, for growing single crystals and in many other areas. Eg. in medicine – for high-temperature sterilization.

Make or buy?

With your own hands you can make a furnace with a muffle with a volume of up to 5-7 liters. If you ask about the proper quality of processing, then this work is not easy. Therefore, first let’s see which of the ready-made ovens are suitable for home use and how much it will cost to purchase them.

Laboratory assistants, scientists, and just craftsmen are well familiar with the honored veteran - the PM-8 electric muffle furnace (see figure) with a muffle chamber volume of 6.5 liters (in the figure on the right, 190x120x300 mm). The secret of the “peemka” is in the chamber; it is made of heat-resistant ceramics using the slip casting method. The PM-8 muffle is eternal, for it separately at auctions they ask for half the cost of the furnace.

PM-8 is available in 2 modifications, PM-8K with manual temperature preset and relay control, and PM-8M with digital control. The factory price, accordingly, is about 20 thousand rubles. and from 23 thousand rubles. The number 8 in the designation is the number of temperature control steps, from 100 to 900 degrees. Power consumption in heating mode is up to 2.4 kW, i.e. The "pemka" can be plugged into an apartment socket. You can take a used PM-8 without fear, the construction is oak. True, the price will not be reduced much.

The only drawback of PM-8 and PM-10 from the same series is that the door gets very hot. Laboratory technicians working with the PC can be recognized by their constantly burnt fingers.

The range of modern muffle furnaces on sale is quite wide. For example, Russian SIKRONS (two left positions in the next figure) no longer fire, and the price is at the same level - from 23,000 rubles. But they are conditionally muffle-type; the heating coil is located in the chamber. Therefore, without anything in them, you can only fire ceramics, and all operations with metals must be carried out with great care. Especially for remelting; SIKRON is not recommended for it at all.

SNOL stoves, middle positions. in Fig. – full-fledged muffle. True, it is possible to burn the SNOL if you handle it awkwardly; opening the door from the side does not guarantee against burns. But they are produced in a wide range of standard sizes, from the smallest. But the prices are clearly not Russian: the cheapest SNOL costs less than 30,000 rubles. can not found.

Of the domestic models of “apartment” power, MIMP and “Nakal”, 2 right positions, diverge well. in Fig. They are cheaper than PM; buy a used one for 8-9 thousand rubles. is quite real. However, their chambers seem to be made by sintering ceramic powder, so these stoves are not intended for regular use for decades. However, they are quite suitable for occasional use in a home workshop.

Another thing worth mentioning is the Israeli kiln specifically for Shenpaz ceramics. For established handicraft production with stable sales, this is a good option. Although it costs a lot, from $3000, the products look exactly the same. Shenpaz is readily accepted by dentists; it costs much less than Focus, etc., and works just as well.

If you do it, how?

Most of the designs described in the sources will give such a quality of processing that it is easier to put the finished muffle (see below) into any furnace or supercharged furnace. And to “bring them to mind”, you will need to spend no less than buying a good factory used one. Therefore, further we will focus on stoves for small, delicate and important work, about which the general public probably knows nothing at all. Let's first understand the key components, and then move on to the structures.

The design diagram of an industrial muffle furnace is shown in Fig. Simplifying it and reducing the cost can be started right away with the flame tube: for chamber volumes up to 10-20 liters and, if the furnace is not intended for ashing and cremation, a small hole in the rear wall of the muffle will be sufficient.

This oven is a chamber oven with horizontal discharge, which is simpler and safer to use. But in some cases, shaft furnaces with a vertically located blind muffle are easier to manufacture and cheaper. In addition, it is easy to provide heating in them without air access if the muffle has a lid. But in any case, the key components will be the heating system, the chamber itself and a thermocouple for temperature control. With thermal insulation and general design We'll figure it out as we go.

Heat source

They say that a muffle furnace can be made even with wood. Well, if you can find firewood with a calorific value of more than 14,000 kcal/kg, then you can. But no - it is useless to cover a small muffle with a huge woodpile; most of the heat will go to increase the entropy of the Universe.

In principle, the necessary concentration of heat can be created by stone or charcoal, but only in a supercharged forge. Then there is no point in building a whole furnace; it is enough to place the muffle in the forge as needed, which is what was done in the Jeanto forge. Liquid fuel gives more than enough heat per unit of its volume, but it itself evaporates when heated. If these gases are not captured and the muffle is not forced to heat, the heat will also evaporate to no avail.

What remains is gas, either natural or from a gasifier, and electricity. The first is cheaper per unit of heat release, the second is easier to handle and manage. We will continue to work with these heat sources.

Thermocouple

Without continuous temperature control, a muffle furnace is simply an expensive and dangerous toy; the degree of heating often needs to be monitored in a closed volume, and the experience of a blacksmith will not always help here. The only option for a temperature sensor up to 1000-1300 degrees is a thermocouple. But there is no need to think about where to “get” platinum. Here are two options for quite affordable thermocouples, operational up to 1100 degrees:

Iron constantan, thermo E.M.F. 53 mV/deg.
Nickel-iron, thermo E.M.F. 34 mV/deg.

With almost no loss of accuracy, iron can be replaced with low-carbon structural steel, i.e. regular soft steel wire. Nickel wire is on sale, and constantan is used for winding school rheostats.

There is no need to weld the ends of the wires; just twist them tightly with pliers. They still burn quickly, not platinum. Then the end is bitten off and a new twist is made. Of course, the wires should not touch each other except when twisted.

Temperature control - with a regular household digital tester. E.D.S. at the ends of the pair will vary within 0.53 (0.34) - 5.3 (3.4) V, the digital camera reliably catches such voltages. In a small chamber, one thermocouple is sufficient.

Muffle

The ceramics for the muffle are not ordinary ones, but ones with good thermal conductivity. It is given by additives of rare metals. Therefore, it is better to make the muffle yourself and not try. We’ll see where to find a ready-made one a little lower, but for now we’ll show you why homemade products are bad.

On the left in Fig. – a stove with a homemade muffle made of refractory clay. Firstly, clay, even fireclay, conducts heat too poorly for uniform heating of the muffle. Secondly, for firing a workpiece, a temperature is required not lower, but much higher than the working temperature. Where can you get 1400-1500 degrees on the farm, and even 1900 for fireclay? What ignorance of these conditions or neglect of them leads to can be seen there.

A faceted chamber made of shaped fireclay bricks (on the right in the figure) is already more durable, but heating it from the outside in any way is useless. Although chamotte conducts heat better than red brick, it is rather weak for a muffle. It remains to place the open electric spiral inside, but, firstly, for household appliances this is strictly prohibited.

Secondly, and most importantly, the muffle will no longer be a muffle. Hot metals noticeably evaporate; anyone who has been to a foundry remembers the characteristic smell. And the vapors of the spiral alloy have nowhere to go except into the part or product. As a result, a valuable alloy may completely lose its value, aged or cemented metal may not gain strength or become brittle, and the figurine may become striped or stained.

So where can you get a muffle?

The best option– ceramic chemical glassware, it is sold in laboratory equipment stores. Shape, size - to choose from, supplied with lids. You can choose a stove with both horizontal and vertical load. High thermal conductivity, excellent chemical resistance.

For furnaces with a horizontal chamber open at both ends (tube furnaces), pieces of tubes from broken powerful fuse links are perfect. They are made from electroporcelain. Its other name is zirconium ceramics. Exactly the same as what goes on PM muffles. And also cast.

Such large fuses are used at transformer substations (TS) of electrical networks or distribution areas (RU) of power lines. The internal diameter is from 5 to 30-40 cm. They are not subject to strict accounting, so you can buy or beg without fear of prison.

It’s easier for ceramic artists: 800 degrees is enough to fire ceramics. Therefore, any earthenware or porcelain vessel will be suitable for the muffle of a ceramic firing furnace. suitable size and configurations. Even the flush cistern from an old toilet.

About school rheostats

It is useless to try to turn a school rheostat into a muffle: its frame is fragile, porous, and the winding, when heated, short-circuits. But the rheostat as it is keeps the temperature at 250-300 degrees. This makes it suitable for making homemade carbon parts from satin-weave carbon fiber and hot-curing resins. Veteran aircraft modelers, for example, use this method to form fuselages, wing edges, spars, ribs and stringers of their aircraft championship classes.

About Electric Power

In order not to get bogged down in calculations, let’s immediately determine the power consumed by the furnace. Let's hope that the reader will be able to calculate the resistance of the spiral, the diameter and length of the nichrome wire himself, or will refer to the manuals on this topic. So:

Furnaces with a closed muffle type PM, horizontal or vertical – 450 W/l.
Firing bell furnaces for small ceramics – 650-700 W/l.
Tube stoves, horizontal only – from 1.5 kW/l.

In all cases it is assumed that the thermal insulation is properly installed and working temperature 1000 degrees. And keep in mind that the power required for the furnace is proportional to the FOURTH degree of temperature! The length of the stove-pipe must be at least 3-5 of its internal diameters.

Some homemade products

Shaft furnace on gas

The diagram of a gas vertical muffle furnace is shown in Fig. It is based on a ready-made muffle-chemical vessel 1. The dimensions of burner 3 are shown conventionally; a large one from a kitchen stove will ensure heating of a 3-liter muffle to 800 degrees. The general proportions are approximately the same as in Fig. Some deviation is not critical.

To heat the stove we buy separately gas burner for household hob with a jet and fittings for it. We install the burner on a steel sheet and power it from a cylinder with a reducer. The heating power is regulated, as in a stove, by turning the nozzle knob.

The requirements for housing 3 in a steel shell and on legs are not as high as for the muffle, so you can make it yourself. You will need fireclay (fireclay, kaolin or blue Cambrian) and fluffed asbestos for filling clay mortar and thermal insulation. If desired, asbestos can be replaced with better quality, but more expensive expanded vermiculite.

The clay for molding the body is first soaked (“fermented”): the clay powder in a bowl is filled with water to the top of the filling and left to “sour” for a day. Next, the solution is kneaded until the plasticine is thick and 25% by volume of fireclay sand is added to it (for fireclay clay) or clay sand the corresponding variety.

Note: Clay sand is ground up ceramic scrap made from this clay. Available for general sale.

Then add 1-2 volume parts of fluffy asbestos, and the solution is thoroughly mixed. By this time, you need to prepare a wax model (dummy) for the inner conical part of the body. The outer parts (cheeks, protrusions for the muffle rim) are formed either by hand or on a potter's wheel. Under steel legs ovens, we immediately make holes. Or we form the legs from the same mass, only then we will have to adjust them with a grinder so that the stove stands level.

Note: There is no need to do a fat test as for. For a small, solid and mechanically almost unloaded part, the natural fat content of good clay is sufficient.

The dried body is first fired in the Robinson Crusoe way: it is made from bricks laid directly on the ground to dry it. Underneath it is surrounded by a fence 0.7-1 m high, made of steel mesh or perforated sheet, so that later the coals do not spread and there is access to air. Place the workpiece on the floor, cover it with any ceramic dish and fill it, only carefully, with fuel (coal, firewood) to the top of the fence.

The fuel is ignited from below. When it burns down to coals, add more and more until the workpiece is completely hidden under a mass of hot coals. Then, adding fuel little by little, the process is maintained for 4-6 hours. Next, we wait until all the coals burn to ash and the ash cools down. Only now can the workpiece be removed.

Note: firing must be done under a canopy. Not a drop of water on the fire!

The final firing of the body is carried out in the assembled furnace, with thermal insulation laid between the cheeks and the shell put on. We put the muffle in its place, only empty. Turn on the burner to the lowest heat, and gradually, over 30-40 minutes, bring the flame to the highest. We heat the stove like this for an hour and a half, after which it is ready for use.

Furnaces for ceramics

The simplest kiln for firing small ceramics is shown in Fig. Household electric stove, clay flower pot the right size - that's all. You just need to place a piece of fireclay brick under the product so that the spiral does not heat it directly from underneath. The power is regulated by an LATR or thyristor regulator.

It is not necessary to insert a thermocouple into the hole of the pot; the process with such a small oven is easily regulated visually: at dusk or on a cloudy day in the room, the sides of the pot should glow dark red. When this glow appears, we begin counting down the firing time, 4-12 hours, depending on what is being fired. Overshoot is visible - reduce power.

For a larger furnace (next fig.) you will need a larger muffle. The best option is an earthenware barrel (see the picture on the right below), the pickles in them still turn out to be unimportant. At the bottom of the barrel diamond drill drill a hole for the thermocouple and the outlet of the expanded air.

Now we need a pod. It is assembled dry from fireclay bricks on a steel sheet and fastened with a frame from a corner. Inside, closer to the edge of the barrel’s neck, make 2-3 circular grooves with transitions and place a spiral or heating element of the required power in them.

The last stage is the outer galvanized shell. It is removable and is put on after the product is installed on the floor and covered with a muffle. The gap between the shell and the muffle is filled with asbestos or vermiculite.

The firing procedure is the same as in the previous case, but the temperature is controlled only by a thermocouple. The barrel is large, its walls are thick. Even if you do not make shells with backfill, until it lights up, the product will burn out.

Tube furnaces

For an electric furnace-pipe of a sufficiently large diameter (say, from a fuse-link body), we will first give a recipe for coating the heater. The composition is the same as for the body gas oven, but, firstly, the clay is kneaded until the thickness of good sour cream or liquid cottage cheese. Secondly, the thermal insulating filler (asbestos, vermiculite) is provided in 4-5 volumetric parts relative to clay.

The next point is winding the spiral. On the root (initial along the winding) end of the pipe, tightly, close to each other, we wind 5-10 turns of nichrome and grab them tightly with a metal clamp and a screw with a nut and a spring washer (grower). The “ears” of the clamp must be elongated and have holes for current supply and heat removal; For more information about this, see below, in the section on mini-ovens.

Note: the first/last turns play the role of current collection. Nichrome has the property of partial self-insulation, which is why it is used on heaters. Simply pressing the end of the wire with a clamp can make poor contact, and to fix the problem you will have to break the stove and do it again.

Next, we wind it into 2 wires: next to the nichrome wire we place a copper wire of the same diameter. At the end of the winding we make a current collection and secure it with a clamp, as described above. Now we remove the copper and start coating. Apply the coating in a layer of 4-12 mm until the solution has settled. Then - a week of drying, an hour-long run at half power (voltage is 1.4 times less than the nominal one!), and final assembly of the oven, similar to that described below.

Note : If you know how to wind windings with variable pitch, then it is better to drive the nichrome turns towards the middle of the pipe and accelerate it towards the edges. This way the stove will be much more economical and durable.

Mini-muffle

For minor works as a ready-made muffle with a heater, you can take a PE or PEV wirewound resistor of 50, 100 or 150 W (see figure).. Its power is indicated on the housing after the type, for example. PEV-150. The internal diameter of PEV resistors is respectively about 20, 30 and 38 mm. About - because electroporcelain tubes from different factories in different times Several different ones were used.

The resistance value for voltage 220 V for 50 W is 910-1000 Ohm, for 100 W – 430-470 Ohm, for 150 W – 330-360 Ohm. There is no need to force heating by decreasing the resistance; anyway, such a stove will not produce temperatures above 900 degrees, and the resistor will quickly burn out.

The weak point of PEV as a muffle is the contact clamps; they are made of brass, so they require a heat sink. It is made from copper strips with a thickness of at least 2 mm, as shown in the following. Fig.. An enameled copper busbar is very suitable for the windings of powerful transformers; enamel will provide additional insulation.

After installing the heat sinks (aka legs), power wires are connected to the resistor terminals, and the heat sink legs are insulated with 2 layers of glass tape with a 50% overlap of turns. The insulation is impregnated with heat-resistant (not lower than 130 degrees) varnish.

The last stages of assembly are to put a thin steel shell on the stove and fill it with thermal insulation. The edges of the shell must be knocked out in advance or bent inward at a right angle so as not to cut through the insulation of the legs. There is no need to make a complete flange inward, this way the insulation will be better kept in the shell. That's it, the resistor stove is ready for use immediately after assembly.

Finally

Muffle furnaces, especially small ones, are still not very common among homemakers and individual entrepreneurs. But in vain, these stoves can do a lot.

Muffle furnace made of ball screw (fireclay-fiber board)

This is a description of a budget muffle furnace built on the basis of fireclay-fiber boards produced by the Aprelevsky Thermal Products Plant. This is what is called cheap and cheerful. As of spring 2015, one slab costs 1,670 rubles. But you won’t be able to buy one slab; they sell it in boxes. There are 4 pieces in a box. Therefore, we will proceed from this quantity.
Slab dimensions 500x500x100 mm. This size suits me, so I proceed from this size, although there are also larger sizes on sale. On this page I will describe the design of the muffle furnace itself, without a control unit. Because I use one temperature controller in several ovens, I will devote a separate page to its description. The entire structure will dance depending on the size of the slabs, on the most optimal cutting of these slabs, taking into account the least waste. So, taking into account the original size of the slabs, we got a muffle with an internal volume of 27 liters.

This figure schematically shows the location of the ball screw plates, as well as the scraps of these plates. The dimensions of the oven, that is, the metal walls of the box, are drawn in red. One cell has 10 cm. That is, the full slab in this figure will be 5x5x1 cell. From 4 complete slabs we get: - door, ceiling and both side walls. The back wall and under (bottom) of the oven are made from scraps. Thus, one box of slabs was enough for everything.

Iron

Thus, the dimensions of the furnace body will be:
500 plate + 4 mm two thicknesses of iron sheet + 6 mm two thicknesses of angle 25x25. Total 510 mm.
Choose the height with legs yourself based on your height.

The middle photo below shows the back cover of the oven and the door blank. Please note that the depth of the box will be the same as the width and height. This is because there will be a ventilated cavity at the back for switching the power supply to the heaters, as well as a low-current thermocouple.
All the furnace control electronics will be in a separate housing, so I don't have to worry about cooling the furnace body. In the rear cavity of the oven there are perforations at the bottom, top, and also in the back cover. Therefore, air moves freely in this cavity and the area of the areas for connecting nichrome is quite large and therefore there is no need to install fans for cooling.

Cutting ball screw slabs

Processing ball screw slabs does not present any difficulty. You can do this with the same tools as for wood, and even more so for metal.

The first video shows the cutting of ball screw slabs. Two metal guides are taken and fixed on pre-marked lines. If you have the skill, you can simply hold it with your hand. In the first video, the task is complicated by the fact that the fireclay insert in the oven door is not at right angles, but beveled. But by pressing the lower guide to the table and holding the upper one with your hand, you can cope with this task.

The second video shows how to mill grooves in a slab for laying heaters. To show how, I used a piece of slab.

It should be taken into account that chamotte greatly dulls the tool, and if you use a hacksaw blade, you can neglect it, but it is better to use a cheap burr. The photo shows how a cheap wood cutter from the market worked. When milling a groove, remember to bury the cutter at an angle so that the heater does not fall out.

When marking grooves for heaters, the following considerations should be taken into account. If, when marking the hearth, the heaters should be positioned evenly (right figure), then when marking the sidewalls, the heaters should be moved down so that when calcining, say, a flask, they are heated more evenly. Because hot air will rise.
Air exchange in the muffle will occur using natural draft. Scraps are welded at the bottom of the door and in the ceiling stainless pipes by 3/4 inch. And then all this is mounted into the housing. The body is pre-painted heat-resistant paint

Electrics

Here I will not dwell on the calculation of spirals. I have already described earlier simplified calculation of heaters made of nichrome and fechral. I will only indicate the data of the heaters specifically for this stove. First of all, I made a calculation when all three heaters - right, left - are connected in parallel. This was done on purpose. If suddenly one of the spirals burns out, then the remaining two will complete the calcination cycle according to the given program. When switched on in series, the remaining one will not work.
Heater data:
- right, left - Nichrome 0.7 mm Length 21.8 meters, R=62.4 ohm, 775 VA.
-under- Nichrome 0.6 mm, 19 meters, R=73?9 ohm, 654 VA.
Total=2204 VA

The first photo shows the winding of a nichrome spiral.
A tube of suitable diameter is clamped into the drill. In this case, God sent me a red mark with a diameter of 8 mm. The end of the spiral is attached near the cartridge. If you don’t have the skill yet, I recommend it hand drill. If you have experience, then electric is better.
After the spiral is wound, it should be stretched to the required length. There is one trick for this. Take a piece of cambric of a suitable diameter, approximately equal to the diameter of your spiral. In this case, God sent me a silicone tube with a diameter of 10 mm. The tube fits into the groove. and the length is marked. The coil is then evenly stretched beyond the ends of the coil to the length marked on the tube. In no case, not in pieces or segments, but at the ends. Otherwise, there will be no uniformity, and it will no longer be possible to compress it back. Then the spiral is placed in the grooves, and the ends are brought out through the back wall through the drilled holes.

Installation of the electrical part can be carried out as you prefer, or as you are used to. For example, I did it in the same way as in the photo. These are, first of all, stainless steel tires. Even if they heat up, they will still not oxidize and there will be good contact. They are isolated from the aluminum frame using ceramic fasteners made from spacers for powerful transistors. You can also use ceramics from old sockets, this is how I connected the thermocouple to the compensation wire (on the left frame at the top). The jumper could have been made smaller :o). But it was just like that already. The nichrome findings were formed into mullite-silica tubes (MSR). If you don't, don't worry. Use some beads from an old iron. At worst, ceramic tubes from resistors and fuses. Nichrome just doesn’t like contact with mortar-based liquid glass, with which I covered the cracks.

I would like to add that it is better not to fasten the lining parts with any mortar. Furnaces do not like rigid connections. Dry lapped surfaces are better. If the two walls were not made of segments, I would have done so.

Door

I initially planned a rectangular door. But when cutting an opening in the front wall, a piece of sheet iron formed, which the toad did not allow to be thrown away. That's why the door turned out to look like a pyramid.

The ball screw insert sat on the same mastic. Processing the slab is just as easy. The second photo shows the inlet with a stainless insert.

In the first photo door hinge. You may not have one like mine. It’s okay, it’ll just come from the market. The main thing is that the door is made first, then the stove is laid on its back, the door is put on, centered, a template is made from paper or cardboard, a draft is made according to the template, and only then it is welded to the door. The second photo shows traditional constipation. The weight of the ball is enough to press the door. The main thing is without fanaticism.

Total

For most people, muffle furnaces are completely useless. At the same time, furnaces are necessary for those whose profession or hobby involves making jewelry, firing ceramics, or smelting metals. Moreover, muffle furnaces are used in the creation of single crystals, cupellation, and also in medicine for high-temperature sterilization.

The cheapest factory models cost about 30,000 rubles, which significantly hits the pockets of home craftsmen. But if you remember school physics courses, in particular thermodynamics, then it is quite possible to build such a structure with your own hands.

Depending on the design features of the furnace, there can be:

tubular;
cylindrical shape;
vertical arrangement;
horizontal arrangement (the simplest option).

Heat treatment can be carried out in air, vacuum or inert gas, but at home only the first option is possible.

Depending on the type of heating element, muffle devices are:

gas;
electric.

Operating gas appliances is cheaper, but making them yourself is prohibited by law. Electric ones are more expensive, but have a significant advantage - the ability to accurately regulate the temperature.

Important! A homemade muffle furnace can be given any shape and dimensions, and made in a style that would suit the overall interior of the house.

What will be required at work

The most convenient to use is vertical design. To make it you will need:

grinder, metal circles;
wire cutters;
welding machine;
nichrome wire, ø1 mm;
steel sheet 2.5 mm thick;
basalt wool;
corners;
fireclay brick;
silicone sealant;
fire-resistant mixture;
respirator, plastic glasses.

Main design elements

Important! A muffle furnace consumes quite a lot of electricity. For example, a device capable of heating up to 1000°C requires about 4 kW. The electrical wiring with which the oven will be connected to the network must withstand heavy loads. You will also need a machine with a 25 A stabilizer.

Important! Asbestos should not be used for this, since when heated it can release carcinogens.

Case manufacturing

A rectangle of appropriate dimensions is cut out of a steel sheet, bent to a radius, and the seam is welded using welding machine. The resulting cylinder is coated with fire-resistant paint and as soon as it dries, the bottom (a circle cut from the same sheet) is welded to it. The bottom and walls are additionally reinforced with reinforcement. The volume of the cylinder must be calculated in such a way that thermal insulation can be placed in it.

Important! When using, for example, an oven, the bottom also needs to be reinforced with corners.

Do-it-yourself muffle furnace: manufacturing instructions

Stage one. The inside of the cylinder is lined with basalt wool.

Stage two. Thermal insulation is being constructed, for which, as mentioned earlier, you only need to take fireclay bricks. The procedure is as follows: seven bricks must be joined in the body so that they form a pipe. In the future it will serve as a working chamber.

The bricks are laid out in a row, and marks are made on each of them at which they will be cut. Next comes the actual cutting, after which the bricks must be of such a shape that they can be connected into one hollow pipe. To simplify the procedure, the bricks can be numbered. At the end of cutting, the pipe is assembled, tied with wire, and the symmetry of the shape is checked. If necessary, adjustments are made.

Important! Cutting must be done on fresh air, preferably in a ventilated place, not forgetting about the means personal protection(respirator, overalls, glasses).

Stage three. Nichrome wire twisted into a ø6 mm spiral by winding it onto a base (used electrode, pencil, etc.). The brick is then removed from the body and placed back in a row.

Stage four. A spiral is applied to the bricks and future grooves are marked. The correctness of the lines is measured by the building level. As a result, the laid spiral should lead from the bottom to the top of the pipe. Contact of the turns is not allowed, as this may cause a short circuit.

Stage five. To bring out the ends of the spiral and connect them to the machine with a stabilizer, three strips cut from ceramic tiles with polypropylene and channels for wire are inserted between a pair of bricks. These channels will greatly simplify future repair work.

Stage six. The finished structure is placed in a steel case. In this case, one brick is placed on the bottom of the body, pre-coated with fire-resistant glue. To remove the ceramic channels, holes are made in the housing in appropriate places.

Important! The spiral can only be turned on after the structure has dried, otherwise the wire will be damaged.

Stage seven. The same sheet steel is used to make the lid. A circle is cut to the size of the structure, and for more reliable fixation, fireclay bricks (1 pc.) are attached to it on top. Then the handles, canopy and metal latch are welded. The edges of the lid in contact with the walls of the oven are covered with a layer of fire-resistant silicone. Silicone is applied only to a previously degreased surface.

Stage seven. Test run. To allow the structure to dry completely, place it in a warm, well-ventilated place. The use of any heating devices is prohibited as this may lead to cracking of the masonry. After drying, the coils are connected to a machine with a stabilizer, and the heating power and operating temperature are adjusted.

Important! To check the dryness of the masonry, you need to turn on the device at full power and see if it evaporates. work surface steam.

When operating the oven, the lid must be tightly closed.

Muffle furnace for ceramics

The image shows a primitive muffle kiln that could be used to fire small pottery pieces.

To make it you only need clay pot for flowers and electric stove. Half of the fireclay brick is placed on the tile (so that the spiral does not come into contact with the ceramic), the product is placed on it and covered with a pot. A thyristor regulator is used to regulate power.

Even in daylight, you will notice that soon after turning on the sides of the pot will glow dark red. From this moment the firing countdown begins, from five to twelve hours, depending on what is being fired. If overheating is observed, the power is slightly reduced.

Stove made of earthenware barrel

You can also make a larger stove from a faience barrel.

Stage one. First, a small hole is made at the bottom of the barrel with a diamond drill to release the expanded air.

Stage two. Next you need to do under. To do this, a “foundation” of fireclay bricks is assembled on a small sheet of steel and connected metal corners. Then, in the upper part of the barrel, grooves are made for the spiral (no more than three or four) - the heating element of the required power will be placed in them.

Stage three. All that remains is to make the outer shell of galvanized steel. It is made removable, since it will be put on only after covering the product with a muffle (barrel). The space between the muffle and the outer shell is filled with asbestos.

The firing procedure is no different from the previous version, but the temperature is adjusted using a thermocouple. The walls of the barrel are thick; by the time they light up (even without the outer shell), the ceramic product will already be burnt.

Safety precautions

Only persons who are familiar with the safety precautions for operating electrical appliances should operate the muffle furnace.
You cannot start work without personal protective equipment - gloves, goggles, special clothing.
The structure can only be switched on with grounding.
Do not start work if there are chips, cracks or other structural defects.
The oven must always be supervised.
After switching on, do not touch the heating element (coil).

Despite numerous advantages, muffle furnaces, especially small sizes, are still not very common among home craftsmen.

For a more detailed look at the design, watch the video below.

Video - Muffle furnace

Today, muffle furnaces are in demand at enterprises of various types. They are indispensable for testing, acceleration chemical processes and other works with metal, ceramics, glass, etc. If you want to buy a muffle furnace, please note that using the equipment is allowed only after reading the manufacturer's instructions.

Muffle furnaces can have different loading chamber volumes and opening types

Electric muffle furnace - what is it?

A muffle furnace is equipment that allows you to create optimal conditions for laboratory research and production processes associated with heating materials and products. The vast majority of these units are powered by electricity. To ensure that Electric Furnaces optimally suit their purpose, they are made from high-quality materials.

When working with muffle furnaces, it is necessary to strictly adhere to safety regulations

History of muffle furnaces

Muffle furnaces were first used in France. Their appearance is associated with the name of the famous carriage maker Jeanto, whose family built carriages for the royal court and noble nobles. He invented the steering trapezoid at the end of the 17th - beginning of the 18th centuries, but its tests were constantly frustrated due to the breakdown of the experimental axles. The situation has changed in better side, when a Russian nobleman came to the master, who wanted to order the famous European carriage maker to make a luxurious carriage. It was supposed to be a gift from Peter the Great to the Khan of Bukhara.

After testing the axle's strength, this customer told Jeanto how to improve it. First, it was necessary to perform cementation in a muffle made from a material that included powdered cattle horns. And only after that carry out hardening. This processing technology allowed the steel to remain viscous on the inside, but have a hard, hardened crust on the outside.

The muffle furnace, designed by a French master, was called the axle construction system

Areas where muffle furnaces are used

The muffle furnace is used in many areas; its application is quite wide. The device is necessary for heat treatment, as well as storing a variety of samples and materials under certain temperature conditions.

Let's figure out why a muffle furnace is needed; the equipment is used for:

Hardening, tempering, melting and other operations with metals.
Thermal firing of ceramics.
Ashing and cremation.
Drying of all kinds of substances.
Analytical research.
Growing single crystals.
Production jewelry and souvenirs.

Muffle furnaces are used when working with dental materials, precious metals and other types of materials

Main types of muffle furnaces

All types of muffle furnaces can be divided according to the following characteristics:

Purpose (industrial, laboratory, jewelry, educational).
Type of construction (horizontal and vertical loading, tube furnaces).
Processing medium (air, gas, vacuum).
Weight (from 18 kg).
Working chamber volume (from 3 l).
Heating level (moderate, medium, high temperature).

If we take a closer look at the muffle furnace and what it is, then the models can be divided according to the type of heating into electric and gas.

Muffle furnaces can be used for laboratory and educational purposes, industrial, jewelry, etc.

Advantages and disadvantages of muffle electric furnaces

What is a muffle furnace? It is a heating technique that has a wide range of operating temperatures and ensures precise adjustment. In addition to the convenience of loading and unloading processed materials, the advantages of heat-generating devices include:

Uniform heating of materials without temperature changes.
Cooling rate of samples after heat treatment to proceed to the next production stages.

Sealed door closures minimize heat loss and increase efficiency

But the muffle furnace, the purpose of which is to influence high temperatures on the surface, it also has a number of features that some users consider disadvantages. These include:

Inability to use equipment at temperatures above 1300° C.
The relatively low thermal resistance of the muffle, which determines the number of thermal cycles withstandable (heating-cooling).
The need for additional electricity to heat the muffle itself.

Muffle furnaces are characterized by significant thermal inertia, which in different cases is considered as a plus or minus of the equipment

How does an electric muffle furnace work?

The working capacity of an electric muffle furnace is the so-called muffle. This is a ceramic or fiber tank of a semi-cylindrical shape, the heaters in which are built into the walls, i.e. are internal or closed. There are electric furnaces for ceramics and porcelain with stationary muffles, as well as those equipped with a stationary heating chamber and a replaceable muffle. In any case, when the heating elements are closed, the processed materials and objects are protected from direct radiation from the heaters. And those, in turn, are protected from exposure to aggressive vapors and volatile substances released during heating.

Muffle furnace design

What does the design of an electric muffle furnace consist of?

An electric muffle furnace is a device consisting of several elements:

Working chamber. Various substances and objects are heated in it.
Housings. Performs protective and heat-insulating functions.
Heating elements. Ensure the performance of thermal operations.

The wire or tapes are located in the chamber; they can be partially or completely open or completely closed.

Operating principle of a muffle furnace

Considering the muffle furnace, its structure and principle of operation, we can say that over the past centuries the equipment has gone through many transformations and has changed. Only the essence of its functioning remained unchanged. Parts or substances placed in the muffle are isolated from external environment and are heated evenly on all sides.

Harmful substances that may be released from samples during heating are removed using a ventilation system. In addition, the ovens provide natural or forced convection

Heating elements of muffle electric furnaces

As can be seen in many photos, the muffle furnace performs heat treatment thanks to heaters. They can be of different shapes and sizes. Depending on these indicators, different numbers of work items may be required. They are located around the entire perimeter of the loading block, and may differ:

Design(tubular, tape, spiral, etc.).
Manufacturing materials(nichrome, fechral, tungsten, chromite, etc.).
Arrangement method(closed, open).

The most common form of heaters is a spiral made of wire with excellent heat resistance and resistance.

Electric muffle furnace control system

A modern muffle furnace is a device that makes it possible to process various materials, easy to use thanks to control modules. Today, mechanical, analog models are used less and less; they have been replaced by electronic and programmable thermostats. With their help you can quickly and clearly:

Set any thermal values.
Set gradients and switching times.
Monitor current thermal indicators.
Carry out heat treatment with high precision.
Set heating times and temperatures.
Determine the holding period and cooling time, etc.

Modern modules allow you to program even complex operating cycles of equipment

Buy a muffle furnace or make one yourself

It is quite possible to figure out how to use a muffle furnace on your own, but how to make this device is not entirely easy to understand. Having considered even an incomplete list of necessary starting materials and a schematic description of the construction process makes it clear that independent production of muffle furnaces is quite a complex matter. The market offers big choice a variety of models, which allows you to find the best option.

Workers in laboratories, workshops and workshops know what a muffle furnace made in strict accordance with technological requirements means - the equipment not only speeds up many processes, but also allows them to be carried out with extreme precision.

Making a muffle furnace at home

How to make a muffle furnace with your own hands

Regardless of the design of the muffle furnace, the design of the device will consist of the following elements:

Muffle. It’s better to do it yourself and not try, because... you need ceramics with high thermal conductivity, which can only be provided by additives made of rare metals. Purchasing a ready-made product will help you get out of this situation. ceramic tableware at a laboratory equipment store.
Fireproof. As the first layer, you can put asbestos boards and fiber, as well as fireclay bricks, which will be the cheapest in cost. For the bond, it is better to take a fire-resistant mixture for masonry.
Frame. Many home-made refrigerators use old refrigerators, which must be strengthened, especially the bottom. This can be done by welding a steel rectangular angle.
A heating element. There are ready-made spirals, but if you wish, you can wind them yourself from nichrome or fechral wire.

Basalt wool is used for reliable cladding of the body

Tools and materials for making a muffle furnace

The use of a muffle furnace is relevant in various fields. Depending on the operating conditions and the type of samples being processed, the following are used for the production of equipment:

Structural steels. They are used to make the body (casing), support, door and some other elements.
High alumina materials and fire retardant fiber. They are used to construct working chambers - muffles.
Nichrome, fechral, tungsten, molybdenum, tantalum. These and other materials serve as the basis for the heating elements of a muffle furnace.
Fiber compressed blocks. They form a highly effective thermal insulating shell (lining).

A prerequisite for the materials used is environmental friendliness, heat resistance and impact resistance

The following tools are used to create an electric muffle furnace:

Drill and set of drills.
Grinder, cutting and grinding wheels.
Welding machine, electrodes, clamps and magnetic corners.
Hacksaw.
Screwdriver.
A tape measure, a metal ruler and a construction square.

To construct a muffle furnace you will need tools for working with metal and wood.

Step-by-step process for making a muffle furnace

Before you start designing, prepare a drawing of a muffle furnace, all the data in which will be derived using formulas and precise calculations. Make sure you have all the necessary components and accessories. After preparation components begin their adjustment, among the many stages of which:

Laying bricks on the bottom and walls of the future muffle furnace.
Door seal, which can be done using silicone, and sometimes a screw locking device is created.
Heating coil gasket etc.

When choosing the optimal volume of the working chamber, consider the size of the objects that will be processed

Basic calculations for the manufacture of muffle furnaces

Based on what the muffle furnace is intended for, its optimal characteristics and capabilities are selected. In order for the unit to operate productively and safely, many calculations need to be made. You will need to determine:

1. Parameters of the electric heater of the electric muffle furnace

What a muffle furnace looks like - a chamber with a door, inside of which there is a heater. Whether the unit will function depends on whether you select the components correctly. Among other things, it is worth paying attention not only to the estimated power of the heater, but also to its:

Manufacturing materials.
Accommodation.
Constructions.

It is necessary to determine the cross-section and length of the wire that will be used to create the heater. For example, the minimum diameter for spirals from:

Nichrome– (7÷10) × d.
Fehrali– (4÷6) × d.

All calculations must be carefully checked to prevent rapid combustion of heaters and fire hazards

2. Power and current of muffle furnace

In order for the muffle furnace, a description of the functioning of which can be found on various thematic forums, to fulfill all the assigned tasks, it is necessary to calculate:

Power. You can find the optimal indicator based on the volume of the furnace. The table below shows the recommended ratios of indicators.

Current strength. The formula used for calculation is: P÷U , where P is power, U is voltage.

The volume of the chamber is measured in liters, while the specific power is in watts

3. The optimal length of wire for an electric muffle furnace

Considering what the muffle furnace is used for, it is necessary to select the correct wire size. Having the value of its total and specific resistance, used as heating coils, you can find the length of the conductor.

Resistance heating element(R), determined by the formula U÷I , where U is voltage, I is current.

The current power affects the choice of wire cross-section - if components with insufficient capabilities are used, too much current will quickly overheat them, melt them or cause burnout

4. Specific surface power of the electric muffle furnace

Some models of muffle furnace can produce different amounts of thermal power. A conductor produces a certain amount of energy over its entire area. It is possible to determine what this indicator should be so that the surface does not lose its operational and mechanical properties. It is not recommended to exceed the permissible value, otherwise the spiral will quickly fail. To avoid this, it is necessary to make a miscalculation with a larger diameter wire or increase the length of the heating element.

The indicator is calculated βadd according to the formula: βeff × α , where βeff is the effective specific surface power, α is the efficiency coefficient thermal radiation spirals.

At the intersection of columns and rows, the value βeff is formed

As you can see, the design and operation of a muffle furnace is a complex system. You can learn more about the capabilities of the equipment from the specialists of the Labor company. Call us, we will select for you the optimal type of electric furnace for any purpose!