Barrel and screw constitute extrusion system. Like screw, barrel also works under high pressure, high temperature, severe wear, and certain corrosion conditions.
During extrusion process, barrel also has function of transferring heat to or away from material. A heating and cooling system is also provided on barrel, and machine head is installed. In addition, a feeding port should be opened on barrel. Geometry of feeding port and selection of its position have a great influence on feeding performance.
Smoothness of inner surface of barrel and opening of groove on inner wall of feeding section have a great influence on extrusion process. Above factors must be considered when designing or selecting barrel.

 

In terms of the overall structure of barrel, there are integral barrels and combined barrels.

It is processed on the whole base. This structure is easy to ensure high manufacturing accuracy and assembly accuracy. It can also simplify assembly work, facilitate installation, assembly and disassembly of heating and cooling system, heat is relatively evenly distributed along axial direction. Naturally, this kind of barrel requires higher processing and manufacturing conditions. .

It means that a barrel is composed of several barrel sections. Experimental extruder and vented extruder are multi-purpose combined barrels. The former is to facilitate changing length of barrel to adapt to screw that does not have a length-to-diameter ratio, and the latter is to set up an exhaust section. In a certain sense, use of combined barrels is conducive to local material and processing, is beneficial to small and medium-sized factories. But in fact, combined barrel requires high machining accuracy. Each barrel section of combined barrel is connected together with flange bolts. This destroys uniformity of barrel heating and increases heat loss. It is not convenient to set up and maintain heating and cooling system.

In order to not only meet material requirements of barrel, but also save valuable materials, many barrels are inlaid with an alloy steel bushing inside base of general carbon steel or cast steel. After bushing is worn out, it can be removed for replacement. Bushing and barrel must be well matched to ensure that heat conduction on the entire barrel wall is not affected; there is no relative movement between barrel and bushing, and it must be easily removed. It is necessary to select appropriate fit precision, and some factories use fit.

 

In order to improve solids conveying rate, it is known from solids conveying theory that one method is to increase friction coefficient of barrel surface, and another method is to increase area of material at feeding port through cross section perpendicular to screw axis. Opening a longitudinal groove on inner wall of feeding section of barrel and making inner wall of a section of barrel near feeding port into a cone are embodiments of these two methods.
According to relevant information, specific structure of opening a longitudinal groove or processing a taper at feeding section of barrel is as follows:
Under normal circumstances, length of taper can be (3 ~ 5) D (D is inner diameter of barrel). When processing powder, taper can be extended to (6-10) D. Size of taper depends on diameter of material particles and diameter of screw. When screw diameter increases, taper should be reduced (at the same time length of feeding section should be increased accordingly).
Longitudinal groove can only be opened on section of barrel before material is still solid or begins to melt. Groove length is about (3-5) D, with a taper.
Number of grooves is related to screw diameter. According to IKV, it is equivalent to about one-tenth of screw diameter (cm). Too many grooves will cause material to flow back and reduce conveying capacity. Shape of groove can be rectangular, triangular, or other shapes. Width and depth of groove with a rectangular cross section are related to diameter of screw.

In order to increase solids conveyance, there is another method. It is to cool feeding section barrel, purpose is to keep temperature of conveyed material below softening point or melting point, avoid appearance of melt film, and maintain solid friction properties of material.
After adopting above method, conveying efficiency is increased from 0.3 to 0.6. Moreover, extrusion volume is less sensitive to changes in die pressure.
However, this system also has following disadvantages: strong cooling will cause significant energy loss; due to extremely high pressure (some as high as 800-1500 kg/cm2) at the end of feeding section of barrel, there is a risk of damaging thin-walled barrel with grooves; screw wear is greater; extrusion performance is more dependent on raw materials. In addition, use of this structure on small extruders is limited.

 

Shape of feeding port and its opening position on barrel have a great influence on feeding performance. Feeding port should enable materials to be fed into barrel freely and efficiently without bridging. Design should also consider whether feeding port is suitable for setting up a feeding device, whether it is conducive to cleaning, and whether it is convenient to install a cooling system in this section. Shape of feeding port (top view) is round, square, and rectangular. Generally, rectangular ones are used, long side is parallel to axis of barrel, and length is about 1.5-2 times diameter of screw.

Just like screw, in order to meet working requirements of barrel, it must be made of high-quality materials with high temperature resistance, wear resistance, corrosion resistance and high strength. These materials should also have good machinability and heat treatment properties. In addition to use of 45 steel, 40Cr, 38CrMoAL, barrel can also be made of cast steel and ductile iron. Feeding section with bushing can be made of high-quality cast iron.
In recent years. With development of high-speed extrusion and engineering plastics, especially when extruding glass fiber reinforced plastics and plastics containing inorganic fillers, higher requirements are put forward for abrasion and corrosion resistance of barrel. Xaloy alloy developed by United States, Belgium and other countries is a novel wear-resistant and corrosion-resistant material, is currently widely used abroad. This material has a low melting point, is hard, has good weldability with steel, has good machining performance, good casting performance, and has no casting stress. Even if it is bent after casting, it will not fall off in a scaly shape.
It is applied to barrel by heating powdered Xaloy alloy and barrel together at high temperature. Because of its low melting point, it can be melted into a fluid state at about 1200℃. Barrel rotates at a high speed, huge centrifugal force generated by molten Xaloy makes it cast on inner wall of red hot barrel. Its thickness is about 2 mm. After cooling, it is grind away about 0.20 mm by method of grinding, which can meet requirements of general barrels.
According to reports, a Xaloy alloy produced in Belgium has a hardness value of Rc58-64. At 482℃, hardness does not decrease significantly, corrosion resistance is 12 times greater than that of nitrided steel.

Barrel is rarely scrapped due to insufficient strength, mainly due to corrosion and wear. Determination of barrel wall thickness, in addition to considering strength, is more to consider processability and thermal inertia of barrel structure. Wall thickness determined according to the latter two factors is often greater than value calculated according to strength conditions. Since there is no mature calculation method for calculating wall thickness of barrel according to heat transfer characteristics of barrel, currently most of wall thickness is determined based on empirical statistics and analogy, and then strength is checked.

Strength calculation of barrel is carried out according to thick-walled barrel. No more discussion here.
Note: When barrel is a brittle material, the first strength theory can be used for calculation.
When barrel is a plastic material, the fourth strength theory is used for calculation.
When barrel is inlaid with a bushing, it is equivalent to compression connection in interference fit of "mechanical parts". At this time, stress state of bushing and barrel is more complicated, and strength calculation is also more complicated.