Barrel For Injection Molding Machine: Characteristics, Materials, And Strength Calculation

The barrel and screw constitute the extrusion system. Like screw, barrel works under high pressure, high temperature, severe wear and corrosion. In the extrusion process, the barrel can transfer the heat to or from the material. The heating and cooling system shall be set on the barrel and the machine head shall be installed.

In addition, the feeding port should be opened on the barrel. The geometry and location of the feeding port have great influence on the feeding performance. The smooth finish of the inner surface of the barrel and the groove in the inner wall of the feeding section have a great influence on the extrusion process. The above factors should be considered when designing or selecting the barrel. In this article, we discuss the structure of injection molding machine barrel, material and strength calculation of barrel for injection molding machine. 

 
Injection Molding Barrel Structure

According to the overall structure of the barrel, there are integral barrel and combined barrel.

1. Integral Barrel

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

 

2. Combined Barrel

It means that a barrel is composed of several barrel sections. The experimental extruder and the exhaust extruder use the combined barrel. The former is for the convenience of changing the length of the barrel to adapt to the screw with no aspect ratio, and the latter is for setting the exhaust section.

In a certain sense, the use of composite barrel is conducive to local materials and processing, and is beneficial to small and medium-sized factories. But in fact, the machining accuracy of combined barrel is very high. Each section of the combined barrel is connected with flange bolts. In this way, the uniformity of barrel heating is destroyed and the heat loss is increased. It is not convenient for the setting and maintenance of heating and cooling system.

 

3. Bimetal Barrel

In order to not only meet the material requirements of the barrel for injection molding, but also save valuable materials, many barrels are inlaid with an alloy steel bushing in the matrix of general carbon steel or cast steel. After the Bush is worn out, it can be removed and replaced. The Bush should be well matched with the barrel to ensure that the heat conduction on the whole barrel wall is not affected; There is no relative movement between the barrel and the bushing, and it can be easily removed, so it is necessary to select the appropriate matching precision, which is adopted by some factories.

 

4. IKV Barrel

1) Longitudinal groove is set on the inner wall of feeding section of barrel

In order to improve the solid conveying rate, according to the solid conveying theory, one method is to increase the friction coefficient of the material cylinder surface, and another method is to increase the area of the material passing through the cross section perpendicular to the screw axis at the feeding port. The two methods are to set up longitudinal groove on the inner wall of the feeding section and to make the inner wall of the feeding section near the feeding mouth into a cone.

 

According to the relevant information, the specific structure of opening longitudinal groove or machining taper at the feeding section of barrel is as follows:

 

– In general, the length of taper can be (3-5) d (D is the inner diameter of barrel). When processing powder, the taper can be extended to (6-10) d. The size of taper depends on the diameter of material particles and screw diameter. When the screw diameter increases, the taper will decrease (and the length of the feeding section will increase accordingly).

– The longitudinal groove can only be opened on the barrel before the material is still solid or begins to melt. The length of the groove is about (3-5) d with taper.

– The number of grooves is related to the screw diameter. According to IKV, it is equivalent to about one tenth of the screw diameter (CM). If there are too many grooves, the material will flow back and the conveying capacity will be reduced. The shape of the slot can be rectangular, triangular, or other. The width and depth of the groove with rectangular cross section are related to the screw diameter.

 

2) Forced cooling feeding section barrel

In order to improve the solid conveying capacity, there is another method. The purpose is to keep the temperature of the transported material below the softening point or melting point, avoid the appearance of molten film, and maintain the solid friction property of the material.

 

The conveying efficiency is increased from 0.3 to 0.6 by using the above method, and the extrusion quantity is less sensitive to the change of die pressure.

 

But 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 is a risk of damage to the thin-walled barrel with grooves; The wear of screw is large; The extrusion properties depend on the raw materials. In addition, this structure is limited in small extruders.

 

5. Shape And Position Of 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 be able to make the materials freely and efficiently into the barrel without bridging. The design should also consider whether the feeding port is suitable for setting the feeding device, whether it is conducive to cleaning, and whether it is convenient for setting the cooling system in this section. The shape of the feeding port (top view) is round, square and rectangular. In general, rectangle is often used. Its long side is parallel to the axis of the barrel, and its length is about 1.5-2 times of the screw diameter.

 

Material And Strength Calculation Of Barrel

1. Barrel Material

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 45 steel, 40Cr and 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 the extrusion of glass fiber reinforced plastics and plastics containing inorganic fillers, higher requirements are put forward for the wear resistance and corrosion resistance of the barrel. Xaloy alloy, developed in America and Belgium, is a new kind of wear-resistant and corrosion-resistant material, which has been widely used abroad. This material has low melting point, hard, good fusibility with steel, good machinability, good casting performance, and no casting stress. Even if it is bent after casting, it will not flake off.

 

When it is applied to the barrel, this kind of powder like Xaloy alloy is heated together with the barrel at high temperature. Due to its low melting point, it can be melted into a flowing state at about 1200 ℃. At this time, the barrel rotates at high speed, and the huge centrifugal force produced by the molten Xaloy makes it cast on the inner wall of the red hot barrel, with a thickness of about 2 mm, After cooling, grind about 0.20 mm by grinding, which can meet the requirements of general barrel.

 

It is reported that the hardness of a kind of Xaloy alloy produced in Belgium can reach rc58-64. At 482 ℃, the hardness does not decrease obviously, and its corrosion resistance is 12 times higher than that of nitriding steel.

 

2. Determination Of Wall Thickness And Strength Calculation Of Barrel

1) Determination of barrel wall thickness

The barrel is rarely scrapped due to insufficient strength, mainly due to corrosion and wear. In addition to the strength, the technological and thermal inertia of the barrel structure are more important to determine the wall thickness of the barrel. The wall thickness determined by the latter two factors is often greater than the value calculated according to the strength condition. Because there is no mature calculation method to calculate the wall thickness of the cylinder according to the heat transfer characteristics of the cylinder, at present, the wall thickness is mostly determined according to the empirical statistical analogy, and then the strength is checked.

 

2) Strength calculation

The strength of the cylinder is calculated according to the thick wall cylinder. 

 

Note: 

– When the barrel is a brittle material, it can be calculated by the first strength theory.

– When the barrel is a plastic material, the fourth strength theory is used for calculation.

– When there is a bushing in the barrel, it is equivalent to the press fit connection in the interference fit of “mechanical parts”. At this time, the stress state of the bushing and barrel is more complex, and the strength calculation is also more complex.