The injection molding machine barrel and screw make up the extrusion system. Like the screw, the barrel also works under high pressure, high temperature, severe wear and certain corrosion conditions. During the extrusion process, the barrel also has the function of transferring heat to the material or transferring heat away from the material. A heating and cooling system is also installed on the barrel, and the machine head is installed.
In addition, the feeding port should be opened on the barrel. The geometry of the feeding port and the selection of its position have a great influence on the feeding performance. The smoothness of the inner surface of the barrel, the grooves on the inner wall of the feeding section, etc., have a great impact on the extrusion process, and the above factors must be considered when designing or selecting the barrel.
1. Injection molding machine barrel structure
According to the overall structure of the plastic injection molder barrel, there are integral barrels and combined barrels.
(1) Injection molding machines’ overall barrel
It is processed on the overall blank. This structure is easy to ensure high manufacturing accuracy and assembly accuracy, and can also simplify the assembly work, facilitate the setting and assembly and disassembly of the heating and cooling system, and the heat distribution is relatively uniform along the axial direction. Naturally, this kind of barrel requires higher processing and manufacturing conditions. .
(2) Injection mold machines’ combination barrel
It means that a barrel is composed of several barrel sections. Experimental extruder and vented extruder multipurpose combination barrel. The former is to facilitate changing the length of the barrel to adapt to the injection molding screw with a different 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 acquisition and processing, which 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 barrel heating and increases heat loss. It is also inconvenient to set up and maintain the heating and cooling system.
(3) Plastic injection mold machines’ 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 ordinary carbon steel or cast steel. After the bushing is worn, it can be removed and replaced. The bushing and the barrel should be well matched to ensure that the heat conduction on the entire barrel wall is not affected; there can be no relative movement between the barrel and the bushing, and they must be easily removed. This requires choosing the appropriate fit accuracy.
(4) Injection molding machines’ IKV barrel
1). Longitudinal grooves are opened on the inner wall of the feeding section of the barrel
In order to improve the solid conveying rate, according to the theory of solid conveying, one method is to increase the friction coefficient of the surface of the barrel, and another method is to increase the area of the material at the feeding port passing 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 feeding section near the feeding port into a cone are the embodiment of these two methods.
According to relevant information, the specific structure of opening longitudinal grooves or processing tapers 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 lengthened to (6-10) D. The size of the taper depends on the diameter of the material particles and the diameter of the screw. When the diameter of the screw increases, the taper should be reduced (and the length of the feeding section should be correspondingly increased).
Longitudinal grooves can only be opened on the section of the barrel before the material is still solid or begins 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 slot 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.
2). Forced cooling feeding section of plastic molder’s 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, to avoid the appearance of the molten film, and to maintain the solid friction properties of the material.
After adopting the above method, the conveying efficiency is improved from 0.3 to 0.6. Moreover, the extrusion volume is less sensitive to changes in head pressure.
However, this system also has the following disadvantages: 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, there are damages with grooves The danger of the thin-walled barrel; the screw wear is larger; the extrusion performance is more dependent 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 of barrel
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 the material to be added to 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 set up a cooling system in this section. The shape of the feeding port (top view) is round, square or rectangular. Under normal circumstances, a rectangle is used, its 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) Injection molding machine barrel material
Just like the screw, in order to meet the working requirements of the barrel, it must be made of high-quality high-temperature, wear-resistant, corrosion-resistant and high-strength materials. These materials should also have good machinability and heat treatment properties. In addition to 45# steel, 40Cr, 38CrMoAL, the barrel can also be made of cast steel and ductile iron. The lined feed section 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 placed on the wear resistance 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, which is currently widely used abroad. This material has a low melting point, is hard, has good weldability with steel, has good machinability, good casting performance, and has no casting stress. Even if it is bent after casting, it will not fall off in scales.
It is applied to the barrel in this way: the powdered Xaloy alloy and the barrel are heated together at a high temperature. Due to its low melting point, it can be melted into a flowing state at about 1200 ° C. The barrel rotates at high speed, and the huge centrifugal force generated by the molten Xaloy makes it cast on the inner wall of the red-hot barrel with a thickness of about 2 mm. 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 its corrosion resistance is 12 times greater than that of nitrided steel.
(2) Determination of barrel wall thickness and strength calculation:
1). The decision of the wall thickness of plastic molder barrel
Barrels are rarely scrapped due to insufficient strength, mainly due to corrosion and wear. The decision of the barrel wall thickness, in addition to considering the strength, is more to consider the craftsmanship and thermal inertia of the barrel structure. The wall thickness determined by the latter two factors is often larger than the value calculated by the strength condition. Since there is no mature calculation method for calculating the wall thickness of the barrel according to the heat transfer characteristics of the barrel, most of the wall thicknesses are determined based on empirical statistical analogy, and then the strength is checked.
2). Injection mold machines` barrel strength calculation
The strength calculation of the barrel is carried out according to the thick-walled barrel. no further discussion here
Note: When the barrel is a brittle material, the first strength theory can be used for calculation.
The fourth strength theory is used to calculate when the barrel is a plastic material.
When there is a bushing in the barrel, it is equivalent to a press-fit 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.
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