Homemade muffle electric furnace (small)

Here I will describe the design of a small size budget electric muffle furnace. The furnace power is 500 W, the theoretical temperature is up to 800 degrees, but I did not heat it up there, because I have a more serious furnace for this. A feature of this design is the extreme simplicity and extremely low cost of components. Such a design can be made from improvised materials in just a few days, of which most of the time will be spent on drying the furnace muffle.
The top case of the furnace with an open door. The muffle itself is located in the center of the body. Thermal insulation of the door, as seen in the photo, using asbestos cardboard on studs. The window is covered with two layers of mica with some gap between the layers.
Complete muffle furnace. It consists of two hulls attached to each other. The muffle itself is located in the upper case, and the control unit is located in the lower case.

I immediately advise you to make an oven like mine in different cases. This will allow you not to bathe with cooling the control unit with various fans. The top case will heat up and create draft, which, combined with the perforations in the bottom case, will be enough to cool the temperature controller.

Muffle making.

The muffle can be made in many different ways. You can take the finished ceramic pipe. Best of all, mullite-silica MKR, you can use a pipe from an old rheostat, from a large fuse. If you prefer a rectangular chamber, then it is better to sculpt yourself. Since my site focuses on practical designs that I managed to make myself, here is the recipe for my muffle.

Kaolin (kaolin clay) - 1 part. Can be found near the porcelain factory. They are brought in wagons for the production of porcelain, faience, electrical ceramics. If not, any clay fatter will do.
Sand - 3 parts. Better quarry sand than river.
We mix all this thoroughly, shut it with water until the lump does not spread yet, but keeps its shape and leave it in a plastic bag for a couple of days. Then take out and mix again until smooth. Then we sculpt the muffle.
Retreat.
There are many things on the market today that weren't there until recently. Now I use such a binder for such work. Mortar Yekaterinburg LLC Pechnik and its characteristics. It should be borne in mind that this is a ready-made mortar, i.e., a filler is already present in it so that it does not lose volume during drying. Therefore, to add to it a large fraction, such as sand, is already in a smaller volume.

So, molding a muffle. A rectangular muffle is molded in a rectangular plywood or kragues box. A muffle with a flat bottom and an arched vault is molded in the same box. The box size is external dimension muffle plus 3-6% shrinkage. It is always molded from the inside of the box, since the muffle shrinks during drying and cracks are inevitable when molding from the outside. To prevent the mixture from sticking to the walls of the box, the walls are lined with polyethylene from the inside. If the mixture is semi-dry, then you can put paper. This saves drying time.
After the muffle is molded, it is left to dry for several days. When the walls of the muffle gain the necessary strength, turn over and remove the box from the muffle. Further, if the muffle is not strong enough for winding with a spiral, it is dried for several days on a battery or in an oven. Then slowly fired to 900 degrees. If you are stressed with firing, in extreme cases, you can leave a dry, unfired muffle. But the strength will not be the same.
If the muffle is strong enough, then it is wrapped in a spiral, coated and dried and fired as an assembly. It is preferable to do this as an assembly, since the coating will be better kept on a half-moist muffle. Carefully make sure that there are no voids inside the spiral, everything is filled with a coating. Otherwise, there will be local overheating of nichrome.

Calculation of the heater.

There are a lot of materials about the calculation of the heater in the network. All of them have a different degree of scientific nature of this issue. For example, you can not only read various considerations, but also calculate the heater on the built-in calculator. Input data are furnace power, heater material, heater and product temperatures, design and placement of heaters. At the output, we obtain the diameter and length of the heater wire. But upon closer examination, it turns out that the diameter was chosen for reasons of saving the material of the wire and the operating conditions are close to ideal. In real life, it's usually the other way around. Usually a skein of old nichrome lies in the bins and its owner is tormented by the question of whether it can be used for the benefit of a person. And with the power of the furnace, too, solid questions.
Therefore, I will give my calculation method, albeit not as scientific, but based on my experience in manufacturing such devices.
So, the first thing to decide is the power of the furnace. Power directly depends on the size of the muffle and the lining used. You determine the size (volume) of the muffle yourself, depending on the size of the heated products.
For modern ovens with the use of fibrous heat insulators (MKRV, ShPV-350, etc.), the approximate power per liter of volume will be:
Furnace chamber volume (liters) Specific power (W/liter)
1-5 500-300
5-10 300-120
10-50 120-80
50-100 80-60
100-500 60-50
Suppose, for example, you have a chamber volume of 3 liters, so the furnace power will be 1200 watts. I have a muffle volume a little more than a liter, so let's take a heater power of 500 watts.
Next, we calculate the current through the heater :
I = P/U= 500/220 = 2.27 A
And the resistance value of the heater
R \u003d U / I \u003d 220 / 2.27 \u003d 97 ohms
Next, we climb into the bins and look at the diameter of the existing nichrome. I had nichrome with a diameter of 0.65 mm. Further, according to the table, we estimate whether our nichrome can withstand such a current.

Diameter (mm) 0.17 0.3 0.45 0.55 0.65 0.75 0.85
Permissible current (A) 1 2 3 4 5 6 7

As you can see, with a diameter of 0.65, the allowable current is 5 A, so it will withstand our 2.27 A with a large margin. In general, when making a heater, you need to take a thicker wire, because the thicker the wire, the longer the temperature and service life it can withstand.
Maximum operating temperatures of heating elements. Here:
GS 40 Nichrome
GS 23-5 Eurofechral
GS SY Superfechral
GS T Eurofechral

BUT! This is a double-edged sword. We cannot greatly thicken the wire diameter, because in order to obtain a design resistance of 97 Ohm, we will have to greatly increase the length of the wire, which may not be acceptable from design considerations.
According to the table, we determine the nominal resistance of 1 linear meter of wire. Here:
GS 40 Nichrome
GS 23-5 Eurofechral
GS SY Superfechral
GS T Eurofechral

So, from the table for a diameter of 0.65 mm we take (and confirm by subsequent measurement with the device), the nominal resistance is 3.2 ohm / meter. Therefore, the length of the wire will be:
L=R/3.2=97/3.2=30 meters
Here is the retribution for the excessive diameter of the wire with excessive footage. But it does not matter, because I will not wind this wire as it is, and there is a danger of not keeping track and allowing an interturn short circuit on our muffle. This wire must be wound on the rod. The tip of the wire, together with the rod, is clamped into the chuck of the drilling machine, at worst, the chuck hand drill. The wire is fed under slight tension.

When winding, the following recommendations must be observed. The diameter of the rod for winding wire with a diameter of up to 4.5 mm must be at least:
- for nichromes four times the diameter of the wire;
- for fechrals five times the diameter of the wire.
For all alloys with a diameter greater than 4.5 mm, at least six times the diameter of the wire.
There is another ambush when working with fehral. Fechral, unlike nichrome, becomes brittle after calcination, so it should not be turned over.
The finished spiral is evenly stretched to a length that is comfortable for winding the muffle. But no more, because it will be much more difficult to compress evenly. We wrap the muffle along the grooves and apply the coating, as in Fig. 4.
Next, we place our muffle in a metal case.

The main lining is made of blocks of lightweight fireclay bricks ShL-0.4. Brick is easily processed with the tool previously described. Pay attention to the hole in the rear block of the lightweight for the thermocouple and two holes for the nichrome leads.
Damaged during installation side wall muffle, but it's not scary, it will be restored by the same composition after installation.

I would like to warn you against some ambushes that may lie in wait for you in the manufacture of the lining.
First of all, I want to warn you if you are tempted to use asbestos. Yes, it melts at 1500 degrees, but at 800 degrees it loses chemically bound water and turns into powder. Therefore, products made from it, such as cardboard or cord, can work up to this temperature. In addition, Fechral must not come into contact with asbestos. I used it, since this furnace is sharpened to this temperature and I have nichrome.
Further, about the use of liquid glass as a binder. It can be used for molding muffles operating up to 1088 degrees, when this temperature is exceeded, the muffle will float. In addition, fechral also does not like contact with liquid glass.
Regarding the use of fibrous materials on a mineral (basalt) basis, I will repeat what I wrote on one of the forums. It's almost the same. Produced by blowing from the melt. Hold temperature well. But they have a binder that will not withstand 250 degrees. But on the Internet, cunning sellers quote the fire resistance of the fiber itself. Formally, they are right. But the fact that after the first calcination the binder will burn out and they will crumble in a bunch is not written. There are varieties with a refractory binder, but there is very little information. Only indirect signs - for example, intended for baths and fireplaces. And again, the fire resistance of the fiber itself is carried out. And is it worth repeating that the fechral does not like them either. So if there is an opportunity to fly, it is better to use already proven ones. And of the ones I have tested, mullite-silica felts are most suitable, for example, MKRVKh-250 (1300 gr).
By the way, production of Cerablanket, Cerachem Blanket, Cerachrom Blanket ceramic blankets has been launched in Sukhoi Log. I dealt with the first of them, withstands the direct flame of the burner. The last two are even more fireproof. But I haven't tried them myself.
Descriptions of furnaces are walking around the network, which are tearing everything apart from each other, in which fireclay clay appears as a muffle material. Ordinary clay has a high shrinkage and is used as a binder. Chamotte is nothing more than fired clay. Chamotte is not molded, it is used as a filler and requires a binder, for example, ordinary unbaked clay. Therefore, what is meant by the expression fireclay clay is not at all clear.

Control block.

Since I promised a description of the most budgetary, the simplest oven, then the temperature controller will be appropriate. A good inexpensive regulator Sh-4501, which can be bought at a price of 1 to 2 thousand rubles. The cheapest and most angry regulator. Is issued with ranges of measurement and regulation of temperatures from 0-200 to 0-1600 degrees. As a measuring element of thermocouple XK, XA and PP.
Technical description and operating instructions for regulating millivoltmeter Sh4501. Read at your leisure.
Front panel of the control unit. This version of the regulator for the range from 0 to 800 degrees, XA thermocouple.
Below, from right to left, the switch of the control unit, the neon lamp TLO (orange) indicating the supply of voltage to the load, the lamp TLZ (green) indicating the load is off, and the red lamp indicating a broken thermocouple.

Connections on the back side of Ш4501. For the slow-witted, the wiring diagram is once again shown on the plastic cap. Please pay attention - the compensation wire must go to the very terminal block with the compensation coil.
Such fittings for indicator lamps are no longer available, so I recommend using modern XB2-EV161 types. They come in red, yellow, green, white and blue. Scheme electric block management. If you do not find a sufficiently powerful toggle switch for turning on the control unit, then place it after the contacts of the PE23 relay. The relay comes with the Sh4501 device. The power of the relay contacts is 500 VA in the AC circuit. The diagram does not show - I have 3 groups of contacts in parallel, so the switched power is up to 1500 VA. It has been corrected in the diagram - the TLZ lamp is suitable for normally closed contacts, TLO for normally open contacts.

Implementation of mounting the control unit in this box. The regulator is pushed in front of the skis. Connector is connected (on the right). The relay is attached to the rear cover from the inside.

Complete oven. Back view. As you can see, the thermocouple wires and heater leads are simply air-cooled, no frills. The heater wires are connected through a terminal block, preferably with a ceramic base. I recommend using a ceramic core from an outlet or a ceramic lamp holder.
The thermocouple leads are also through the terminal block. A segment of the compensating wire corresponding to the graduation is connected to the same contacts of the terminal block. If it is a regular wire, then the device will lie by the value of the temperature difference between this terminal block and the back panel of the Ш4501 with a measuring coil. Outside, on the back cover, there is an overhead socket for connecting the load, and on the back cover of the muffle box there is a terminal block for connecting a thermocouple. This allows you to use this control unit not only with this muffle, but also to control the temperature in your other devices. It is enough to screw a thermocouple of such a graduation to the terminal block and insert the plug into the socket.

A little about a homemade thermocouple. For the final budget of our furnace, I used a self-made thermocouple of XA graduation. I prefer homemade thermocouples, not because of greed, but simply because they have less inertia compared to factory ones. Although there is a risk of burning the input circuits of the regulator. I will not dwell on the manufacture of such a thermocouple in detail, because this process is well covered in the literature (Bastanov. 300 practical advice) and on the Internet.

The material was the conductors from the compensating wire of XA graduation. The ends are welded with a tungsten electrode in an argon atmosphere. If you weld it weakly, then as it is described in books in graphite with borax using a powerful transformer. Then the thermocouple is thrust into a ceramic two-channel MKR tube. Here you, sorry, will have to fork out.

Assembled heating chamber. The wall is stuccoed, the cracks are smeared. Putty is then applied around the mouth of the muffle with some excess. Then it is covered with polyethylene and the lid is closed. The relief of the lid is imprinted on the putty. The polyethylene is removed and the whole economy is dried. The gaps between the cover and the chamber are minimal.

Muffle assembled. After laying the spiral, it is coated with the same composition that the muffle consists of. Fasten the ends of the spiral with a loop of glass tape with mica. Do not forget to put a mortgage rod under the spiral. When the muffle dries, the rod is removed and a hole for the thermocouple remains.

Muffle without binding. Pay attention to the grooves on the corners of the muffle. They are so that the spiral does not move during coating. Bottom groove for thermocouple. The thermocouple must be in close proximity to the coil.

For the vast majority of people, muffle furnaces are completely useless. At the same time, furnaces are necessary for those whose profession or hobby is related to the manufacture jewelry, firing ceramic products or metal smelting. Moreover, muffle furnaces are used in the production 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 the school courses of physics, in particular, thermodynamics, then it is quite possible to build such a structure with your own hands.

According to the design features of the furnace 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 only the first option is possible at home.

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

gas;
electric.

The operation of gas appliances is cheaper, but their independent production prohibited by law. Electric ones are more expensive, but they have significant advantage– the possibility of precise temperature control.

Important! A home-made muffle furnace can be given any shape and dimensions, it can be made in a style that would fit general interior at home.

What will be required in the work

The most convenient to use is vertical design. For its manufacture you will need:

grinder, circles for metal;
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 goggles.

Basic structural elements

Important! The muffle furnace consumes a lot of electricity. For example, a device capable of heating up to 1000ᵒС needs 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

From steel sheet a rectangle of the appropriate size is cut out, bent into a radius, and the seam is welded with welding machine. The resulting cylinder is covered 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. From the inside, the cylinder is laid out with basalt wool.

Stage two. Thermal insulation is being constructed, for which, as mentioned earlier, only fireclay bricks need to be taken. 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.

Bricks are laid out in a row, while marks are made on each of them, according to which they will be cut. Next, cutting takes place directly, after which the bricks must be of such a shape that they can be connected into one hollow pipe. In order to simplify the procedure, the bricks can be numbered. At the end of the cutting, the pipe is assembled, pulled together with wire, and the symmetry of the shape is checked. If necessary, adjustments are made.

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

Stage three. Nichrome wire it is twisted into a ø6 mm spiral by winding it around the base (used electrode, pencil, etc.). Then the brick is 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. It is not allowed to touch the turns, because this can 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 out of ceramic tiles, with made polypropylene and wire channels. These channels will greatly simplify repair work in future.

Stage six. The finished structure is placed in a steel case. In this case, one brick is placed on the bottom of the body, previously coated with refractory glue. To output ceramic channels, holes are made in the body in the appropriate places.

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

Stage seven. For the manufacture of the cover, the same sheet steel is used. A circle is cut out according to the size of the structure, for more reliable fixation, fireclay bricks (1 pc.) Are attached to it from above. Then handles, a canopy and a metal latch are welded. The edges of the lid that come into contact with the walls of the furnace are covered with a layer of fire-resistant silicone. Silicone is applied only to a previously degreased surface.

Stage seven. Trial run. For complete drying of the structure, it is installed in a warm, well-ventilated place. It is forbidden to use any heating appliances, since this can lead to cracking of the masonry. After drying, the spirals are connected to the machine with a stabilizer, the heating power and operating temperature are adjusted.

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

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

Muffle Furnace for Ceramics

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

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

Even in daylight, you will notice that shortly after switching on, the sides of the pot will glow with a dark red light. From this point on, the firing countdown begins, from five to twelve hours, depending on what is fired. If overshoot is observed, the power is slightly reduced.

Faience barrel stove

You can also make a larger oven 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 with metal corners. Then, grooves for the spiral (no more than three or four) are made in the upper part of the barrel - the heating element of the required power will be laid in them.

Stage three. It remains only 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 covered 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, until they light up (even without the outer shell), the ceramic product will be already burnt.

Safety

The muffle furnace should only be operated by persons who are familiar with the safety precautions for the operation of electrical appliances.
You can not 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 and 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 ones, are still not very common among home craftsmen.

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

Video - Muffle Furnace

The muffle furnace is primarily used for heat treatment of various things. It produces melting, hardening and firing of metal, glass, wax and even ceramic materials. Due to the versatility of such a unit, a fairly wide temperature range up to 1000º C is provided.

If you need to have such a unit in your arsenal, it is quite possible to make it yourself, since in stores the prices for muffle oven devices high enough. Self assembly such a unit is fast enough, especially if you prepare the right materials and tools.

Muffle furnace - main varieties

According to the type of elements used to heat their muffle furnaces classified into the following types:

working on electricity;
using gas for heating.

I would like to note right away that it is impossible to build a gas-powered muffle furnace with your own hands. First of all, because such home experiments are punishable by law. But the electric analogue of the furnace is available for homemade production to everyone who wishes. Wherein by its design such devices are divided into the following types:

horizontal muffle furnace as in the photo;
vertical or pot firing device;
bell-type furnace, an example of which is shown in the photo;
tubular system.

I would like to note that the construction of a muffle furnace is suitable for do-it-yourself construction, in which the heat treatment of materials takes place in the air. In turn, devices operating in a vacuum are manufactured only in the factory.

Before constructing such a device, it is necessary to understand the principle of its operation. First, you need to decide on what fuel the muffle furnace will operate: coal, electricity, gas or wood. At the same time, it is easiest to do it yourself electrical device, consisting of a chamber for heating and high-quality thermal insulation.

Heat in such a furnace is accumulated due to refractory bricks, and heat treatment of materials takes place in the main chamber - the muffle. For the manufacture of a furnace at home, any ready-made refractory form or one made with fireclay bricks or heat-resistant clay is suitable as a muffle.

It is very important to understand that the temperature at which the chamber of the muffle furnace begins to melt must be higher than that of the materials processed in the unit. At the same time, you can do it yourself build a small oven for melting and firing various materials in small quantities.

Structural components of the muffle furnace

For the case of such a device, an old one is suitable gas stove, more precisely, her oven or its electric counterpart. In order for the old oven to be suitable for further use, all parts made of plastic are removed from it. If there was no ready-made design for the body of the house, then it is welded from metal sheets 3 mm thick.

Of no less importance is heat-insulating part muffle furnace, the quality of which determines the efficiency and heat loss of such a device. As an internal thermal insulation layer, red heat-resistant brick is used, which can withstand high temperatures up to several thousand degrees.

For additional thermal insulation that reduces heat loss along the outer layer, perlite or basalt wool. It is not recommended to use asbestos materials, since carcinogenic substances are released during their heating.

Spirals are used as heating elements inside such a device, for the manufacture of which they take nichrome or fechral wire 1 mm thick. At the same time, nichrome material, or rather, nickel and chromium wire, is quite plastic and not subject to corrosion.

Features and types of muffles

For the manufacture of the muffle can be used different materials, while most often use ceramic or ceramic fiber.

Assembly material and tools

In order to assemble a ceramic kiln with your own hands, you will need Availability the following materials and equipment:

electric grinder;
fireclay red brick;
electric welding;
finished case made of metal or sheet steel;
metal corners;
heat-insulating non-combustible materials;
finished heating coil or nichrome wire.

First you need to decide on the body of the device, the dimensions of which will determine the size of the main chamber of the furnace. This is true for an old oven or bucket. If the body is welded from sheet metal, then its dimensions are adjusted after the construction of the muffle.

Also, the working chamber for firing is made fireclay or ceramic pipe with medium diameter. Therefore, the process of assembling a muffle furnace with your own hands has certain nuances.

A muffle made of fireclay bricks, as in the photo, is connected into a circle. To do this, the brick is cut with a grinder until the necessary bevels are formed. After that, a groove is cut in each brick to accommodate the spiral. At the same time, on all bricks, it is cut horizontally, and on one with a slope, which ensures the displacement of the spiral by a level lower.
If the body is made from an oven, which is lined with refractory bricks, the masonry is made with a mortar based on clay, fireclay powder and sand. In this case, the masonry seams should not exceed 5 mm. After the masonry dries, grooves are made in it for laying the spiral. I would like to note that the width of the groove should correspond to the thickness of the spiral, and the distance between adjacent elements should be 20 mm.

Heating coil manufacturing and assembly

Nichrome wire 1 mm thick is wound on a steel rod with a smooth base. In this case, the diameter of the spiral must not exceed 6 mm. It is also important to ensure that adjacent turns do not touch during winding. The finished spiral is placed in grooves cut in fireclay bricks.

I would like to note that in order to prevent the spiral from falling out during heating, it is putty thin layer heat-resistant clay or attached in several places with wire. If a ceramic pipe is used as a muffle, then the spiral is wound around the outside.

To complete the assembly of the muffle furnace for firing with your own hands, it remains do some simple work that are within the reach of everyone.

A brick or ceramic working chamber is placed in a steel case at the bottom, which is laid with heat-insulating material.
On the outside, the bottom of the furnace is reinforced with steel corners. This is due to the fact that the weight of the brick is quite large. Between the walls of the muffle and the main body there should be a gap of up to 40 mm, in which the insulation material is placed.
The furnace cover is made of 2 layers of steel and thermal insulation material inside.

For convenience, it is desirable to fix the handle for opening the main chamber of the muffle furnace.

How to properly dry the finished muffle furnace

Dry the finished muffle furnace with your own hands need to gradually in a well ventilated place.

The drying time of the structure is from 5 days to several weeks.
In the case of intensive forced drying, the clay used in the manufacture of the furnace will crack, and such a device will not be used.
To check the quality of drying, the muffle furnace is turned on to a minimum and if there is no evaporation of moisture, then the device is completely dry.

It is important to remember that it is imperative to follow all safety regulations when manufacturing and using such a high temperature device.

Mini muffle furnace

For miniature work as a finished muffle furnace, as in the photo, use small power resistor up to 150 W. The diameter of the 150 W resistance is approximately 3.8 cm. The size is approximate, since the diameter of the electroporcelain tube is determined by each factory individually.

The resistance value of a wire resistor of 150 W at a voltage of 220 volts is 350 ohms. At the same time, contact clamps made of brass are considered weak points for using resistance as a muffle, which require additional heat removal, for which copper strips of 2 mm thickness are used.

After attaching the heat sink, which serves as legs, to the terminals of the wire resistor connect the wires, supply, power, and heat sinks are insulated with several layers of non-combustible, insulating material, which is subsequently impregnated with heat-resistant varnish.

On the last step a shell made of the thinnest sheet steel is put on the muffle. At the edges, the shell is bent inward at a right angle so as not to damage the insulation on the legs. At the same time, a complete flanging inward is not needed, since the insulating material will be retained in the shell in a quality manner. On this mini-muffle furnace is ready.

A muffle furnace is a device for heating various parts and materials to the required temperature. A muffle is a closed chamber for placing material. The muffle protects the material from the effects of fuel combustion products. It can be made from various materials- ceramics, MKRV, steel, brick.

What types of ovens are there?

There are several main parameters by which muffle furnaces differ:

heating method;
furnace design;
temperature range;
protective environment.

According to the method of heating, they distinguish:

gas muffle furnace;
electric muffle furnace.

The method of loading the material determines the design of the device. Furnaces available:

with vertical loading;
with horizontal loading;
bell-shaped (with a separation from the floor);
tubular (for heating thermocouples).

According to the heating range, there are furnaces with heating:

to moderate temperatures (from 100 to 500°C);
to medium temperatures: (from 400 to 900°C);
to high temperatures (from 900 to 1400°C);
to ultrahigh temperatures (up to 2000°C).

Heating of the material in the muffle can occur in various environments:

air;
vacuum;
various gases.

The heating element in furnaces is open and closed. The number of heating elements depends on the dimensions of the device.

The advantages of open elements are a high heating rate, no problems with replacement in case of failure. But they corrode more strongly and are affected by harmful substances produced during heating.
Closed ones are built into the muffle. They heat up more slowly. In the event of a breakdown, the entire chamber must be replaced. But on the other hand, they are not affected by harmful substances and they last longer.

How is the temperature controlled?

To regulate the temperature, special thermostats for the muffle furnace are installed. They can be of two types:

analog;
digital.

Analog has not been practically used in recent years due to high errors. They are a rotary switch with marked risks and a temperature scale. Turning the knob sets the required heating temperature.

Currently, digital thermostats are more commonly used. They can be:

with one display, which displays the set or actual temperature;
with two displays, which show both the set and the actual temperature;
with programmer to select different operating modes.

The purchase of ovens with a programmer is necessary for a long heating process with a change in temperature regimes without human intervention. But most of the time this is not necessary. Therefore, ovens with programmers are used much less frequently.

Applications and furnace selection

The scope of application of muffle furnaces is quite diverse. They are used for firing ceramic products, casting molds, in the manufacture of jewelry and wax products, and for melting non-ferrous metals.

When choosing a furnace, it is necessary to take into account many factors - the dimensions of the workpieces for heating, temperature conditions, purpose of application.

For training, a furnace with a small muffle size (100x100x100) is suitable, high temperature heating. Such furnaces are called "mini-muffle furnaces".

For jewelry workshops, a professional oven with a programmer is needed. For heating precious metals, the accuracy of adjustment is very important and several modes of operation are needed. In the jewelry business, enameling muffle furnaces are used to apply enamel to precious metals.

In dentistry, in the manufacture of prostheses, a furnace with small dimensions and high temperature is needed.

In industry, furnaces with large dimensions (at least 200x200x200) and ultra-high temperatures - up to 2000 degrees are used. Accordingly, the price of such a device is much higher.

The muffle laboratory furnace is used for carrying out laboratory analyses, heating samples and materials, and carrying out tests. Dimensions and temperatures depend on the tests being carried out. For such furnaces, a wide range of parameter adjustment is important.

Muffle furnace SNOL (SNOL) is a laboratory resistance furnace, chamber with a rectangular muffle for heating in an air environment with normal pressure.

Electric chamber furnaces of the SNOL type for testing were called " Muffle Furnaces EKPS".

Electric furnaces are produced with chambers made of ceramic or heat-resistant fibrous material. Ceramics is much more reliable, more durable than fiber, does not require large energy consumption, and allows maintaining temperature uniformity. But ceramics take a long time to cool and heat up. Therefore, it is not possible to quickly change the processing mode.

Fiber also has its advantages. In them heating elements are pressed into the fiber and their radiation does not affect the heated material. But they have much more disadvantages: high energy costs, the release of harmful gases, brittleness. To remove harmful substances in the furnaces, ventilation is provided.

Additional equipment

For the convenience of working with the oven, a number of additional components are provided:

SiC - plate (to protect the muffle);
heat resistant gloves;
forceps;
if you have a programmer, you need to purchase software;
set of spare parts (heaters).

These components must be purchased separately.