The barrel and screw constitute the extrusion system. Like the screw, the barrel also works under high pressure, high temperature, severe wear, and certain corrosion conditions. In the extrusion process, the barrel also has the function of transferring heat to or away from the material. A heating and cooling system is also provided on the barrel, and the machine head is installed.
In addition, a feeding port should be opened on the barrel. The geometry of the feeding port and its location have a great influence on the feeding performance. The smoothness of the inner surface of the barrel and the opening of grooves on the inner wall of the feeding section have a great impact on the extrusion process. The above factors must be considered when designing or selecting the barrel.
1. Barrel structure
In terms of the overall structure of the barrel, there are integral barrel and combined barrel.
(1) The overall barrel
It is processed on the whole blank. This structure is easy to ensure high manufacturing precision and assembly accuracy, and can also simplify the assembly work, facilitate the installation and assembly and disassembly of the heating and cooling system, and the heat is relatively evenly distributed along the axial direction. Naturally, this kind of barrel requires higher processing and manufacturing conditions. .
(2) Combination barrel
It means that a barrel is composed of several barrel sections. Experimental extruder and vented extruder multi-purpose combined barrel. The former is to facilitate changing the length of the barrel to adapt to the screw with no aspect ratio, and the latter is to set the exhaust section. In a certain sense, the use of combined barrels is conducive to local material and processing, and is beneficial to small and medium-sized factories. But in fact, the combined barrel requires high machining accuracy. Each barrel section of the combined barrel is connected together with flange bolts. This destroys the uniformity of the barrel heating and increases the heat loss. It is not convenient to set up and maintain the heating and cooling system.
(3) Bimetallic barrel
In order to not only meet the material requirements of the barrel, but also save valuable materials, many barrels are inlaid with an alloy steel bushing inside the base of general carbon steel or cast steel. After the bushing is worn out, it can be removed and replaced. The bushing and the barrel must be well matched to ensure that the heat conduction on the entire barrel wall is not affected; the barrel and the bushing must not be able to move relative to each other, and they must be easily removed. It is necessary to choose a suitable fit precision, and some factories use fit.
(4) IKV barrel
1. A longitudinal groove is set on the inner wall of the feeding section of the barrel
In order to improve the solids conveying rate, it is known from the solids conveying theory that one method is to increase the friction coefficient of the barrel surface, and another method is to increase the area of ??the material at the feeding port through the cross section perpendicular to the screw axis. Opening a longitudinal groove on the inner wall of the feeding section of the barrel and making the inner wall of a section of the barrel near the feeding port into a cone are the embodiments of these two methods.
According to relevant information, the specific structure of opening a longitudinal groove or processing a taper at the feeding section of the barrel is as follows:
Under normal circumstances, the length of the taper can be (3 ~ 5) D (D is the inner diameter of the barrel). When processing powder, the taper can be extended to (6-10) D. The size of the taper depends on the diameter of the material particles and the screw diameter. When the screw diameter increases, the taper should be reduced (at the same time the length of the feeding section should be increased accordingly).
The longitudinal groove can only be opened on the section of the barrel before the material is still solid or starts to melt. The groove length is about (3-5) D, with a taper.
The number of grooves is related to the diameter of the screw. According to IKV, it is equivalent to about one-tenth of the diameter of the screw (cm). Too many grooves will cause the material to flow back and reduce the conveying capacity. The shape of the groove can be rectangular, triangular, or other shapes. The width and depth of the groove with a rectangular cross-section are related to the screw diameter, and the specific values ??can be found in Table 3-7.
2. Forced cooling feeding section barrel
In order to increase the solids delivery, there is another method. It is to cool the feeding section barrel, the purpose is to keep the temperature of the conveyed material below the softening point or melting point, avoid the appearance of melt film, and maintain the solid friction properties of the material.
After adopting the above method, the conveying efficiency is increased from 0.3 to 0.6. Moreover, the extrusion volume is less sensitive to changes in die pressure.
However, this system also has the following shortcomings: strong cooling will cause significant energy loss; due to the extremely high pressure (some as high as 800-1500 kg/cm2) at the end of the feeding section of the barrel, damage and groove The danger of the thin-walled barrel; the screw wears a lot; the extrusion performance depends on the raw material. In addition, the use of this structure on small extruders is limited.
(5) The shape and position of the feeding port
The shape of the feeding port and its opening position on the barrel have a great influence on the feeding performance. The feeding port should enable materials to be fed into the barrel freely and efficiently without bridging. The design should also consider whether the feeding port is suitable for setting a feeding device, whether it is conducive to cleaning, and whether it is convenient to install a cooling system in this section. The shape of the feeding port (top view) is round, square, and rectangular. Generally, rectangular ones are used, the long side is parallel to the axis of the barrel, and the length is about 1.5-2 times the diameter of the screw.
2. Barrel material and strength calculation
(1) Material of barrel
Just like the screw, in order to meet the working requirements of the 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 the use of 45 steel, 40Cr, 38CrMoAL, the barrel can also be made of cast steel and ductile iron. The feeding section with bushing can be made of high-quality cast iron.
In recent years. With the 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 the abrasion and corrosion resistance of the barrel. Xaloy alloy developed by the United States, Belgium and other countries is a novel wear-resistant and corrosion-resistant material, and 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 scale.
It is applied to the barrel by heating the powdered Xaloy alloy and the barrel together at high temperature. Due to its low melting point, it can be melted into a fluid state at about 1200°C. The barrel rotates at a high speed, and the huge centrifugal force generated by the molten Xaloy makes it cast on the inner wall of the red hot barrel. Its thickness is about 2 mm. After cooling, it is grind down by the method of grinding about 0.20 mm, which is sufficient. General barrel requirements.
According to reports, a Xaloy alloy produced in Belgium has a hardness value of Rc58-64. At 482°C, the hardness does not decrease significantly, and the corrosion resistance is 12 times greater than that of nitrided steel.
(2) The determination of barrel wall thickness and strength calculation:
1. Determination of the wall thickness of the barrel
The barrel is rarely scrapped due to insufficient strength, mainly due to corrosion and wear. The determination of the barrel wall thickness, in addition to considering the strength, is more to consider the process and thermal inertia of the barrel structure. The wall thickness determined by the latter two factors is often greater than the value calculated according to the strength conditions. Since there is no mature calculation method for calculating the wall thickness of the barrel according to the heat transfer characteristics of the barrel, currently most of the wall thicknesses are determined based on empirical statistics and analogy, and then the strength is checked.
2. Strength calculation
The strength calculation of the barrel is carried out according to the thick-walled barrel. No more discussion here
Note: When the barrel is a brittle material, the first strength theory can be used for calculation.
When the barrel is a plastic material, the fourth strength theory is used for calculation.
When the barrel is inlaid with a bushing, it is equivalent to the compression connection in the interference fit of "mechanical parts". At this time, the stress state of the bushing and the barrel is more complicated, and the strength calculation is also more complicated.More about:screw barrel for Injection molding machine