Important knowledge of injection molding machine barrels

The barrel and the screw make up the extrusion system. Like the screw, the barrel works under high pressure, high temperature, severe wear and tear, and certain corrosion conditions. During the extrusion process, the barrel also has the role of transferring heat to and from the material. The barrel also has to set up a heating and cooling system and install the head.

In addition, the barrel has to be opened with a filling port. And the geometry of the feeding port and its location is selected to have a great impact on the feeding performance. The inner surface finish of the barrel and the grooves on the inner wall of the charging section have a great influence on the extrusion process, so the above factors should be taken into consideration when designing or selecting the barrel.

I. Barrel structure

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

(A), The whole barrel

It is processed on the whole blank. This structure is easy to ensure high manufacturing accuracy and assembly accuracy, and can also simplify the assembly work, easy to set up and disassemble the heating and cooling system, and the heat is distributed more evenly along the axial direction, naturally, this kind of barrel requires higher processing and manufacturing conditions.

(B), Combined barrel

It means that a barrel is combined with several barrel sections. Experimental extruders and exhaust extruders use a combination of barrels. The former is to facilitate the change of barrel length to adapt to the screw, not to the aspect ratio, the latter is to set the exhaust section. In a sense, the use of combination barrels is beneficial to small and medium-sized plants as it facilitates local material extraction and processing. But in fact, the combination barrel requires high processing accuracy. Each barrel section of the combination barrel is mostly connected together with flange bolts. This destroys the uniformity of barrel heating and increases heat loss. It is also not convenient to set and maintain the heating and cooling system.

(C) Bimetallic barrel

In order to meet the material requirements of the barrel, but also to save valuable materials, many barrels in the general carbon steel or cast steel base body is set inside an alloy steel bushing. The bushings can be removed and replaced after wear, bushings, and barrels to match well, to ensure that the entire barrel wall heat transfer is not affected; barrels and bushings can not have relative movement between, but also to be able to easily removed, which is to choose the appropriate fit accuracy.


(D) IKV barrel

1) Longitudinal grooves are opened on the inner wall of the barrel charging section

In order to improve the solids conveying rate, solids conveying theory knows that one way is to increase the friction coefficient of the barrel surface, and another way is to increase the material at the feeding port through the area of the cross-section perpendicular to the axis of the screw. The longitudinal groove in the inner wall of the barrel feeding section and the conical shape of the barrel inner wall near the feeding port is the concretization of these two methods.

According to the relevant information, the specific structure of longitudinal grooves or machined taper at the barrel charging section is as follows.

In general, the length of the taper can be taken (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 is determined by the diameter of the material particles and the diameter of the screw. When the diameter of the screw increases, the taper is reduced (and the length of the charging section is increased accordingly).

The longitudinal groove can only be made in the section of the barrel before the material is still solid or starts to melt. The grooves are about (3-5)D long and tapered.

The number of grooves is related to the screw diameter and, according to IKV, corresponds to about one-tenth of the screw diameter (cm). Too many grooves can lead to material backflow and reduce the conveying capacity. The shape of the grooves can be rectangular, triangular, or other shapes. The width and depth of the groove with rectangular cross-section are related to the diameter of the screw.

2), Forced cooling of the charging section barrel

In order to improve the solids conveying capacity, there is another method. It is to cool the filling section barrel, the purpose is to keep the temperature of the material being conveyed below the softening point or melting point, to avoid the emergence of molten film, in order to maintain the solid friction nature of the material.

With the above method, the conveying efficiency is increased from 0.3 to 0.6. And the extrusion volume is less sensitive to the change of head pressure.

However, this system also has the following disadvantages: strong cooling can cause significant energy losses; the risk of damage to thin-walled barrels with grooves due to the extremely high pressures (some as high as 800-1500 kg/cm²) that can be generated at the end of the barrel charging section; greater screw wear; and greater dependence of extrusion performance on the raw material. In addition, the use of this structure on small extruders is limited.

(E) The shape and location of the filling port

The shape of the filling mouth and its opening position on the barrel has a great impact on the feeding performance. The design should also take into account whether the charging port is suitable for setting the charging device, whether it is good for cleaning, and whether it is convenient for setting the cooling system in this section. The shape of the charging port (top view) is round, square, or rectangular. In general, more rectangular, its long side parallel to the axis of the barrel, the length of about 1.5-2 times the diameter of the screw.
 

Ⅱ, barrel material, and strength calculation

(a), 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, high-strength materials. These materials should also have good machinability and heat treatment properties. The barrel can be made of cast steel and ductile iron, in addition to 45 steel, 40Cr, 38CrMoAL. The charging 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 made for the abrasion and corrosion resistance of the barrel. Xaloy alloy developed by the United States, then and other countries is a novel wear-resistant and corrosion-resistant material, which is now widely used abroad. This material has a low melting point, is hard, has good fusion with steel, good machinability, good casting performance, and no casting stress, even after casting by bending, will not become scaly off.

Will it be applied to the barrel is the use of such a method: at high temperature will be this powder Xaloy alloy and barrel together with the heating, because of its low melting point, about 1200 ℃ can be melted into a flow state, then make the barrel high-speed rotation, molten Xaloy generated by the huge centrifugal force will make it cast in the red hot barrel wall, its thickness is about 2 mm, after cooling with the method of diffraction grinding to grind away About 0.20 mm can meet the general requirements of the barrel.

According to reports, Belgium produced a Xaloy alloy, hardness values up to Rc58-64, at 482 ℃, no significant decline in hardness, corrosion resistance than nitriding steel 12 times greater.

(B), The decision of the barrel wall thickness and strength calculation.

1, The decision of the barrel wall thickness

The barrel is rarely scrapped due to insufficient strength, mainly due to corrosion and wear. Barrel wall thickness decision, in addition, to considering the strength, more is to consider the barrel structure of the process and thermal inertia. The wall thickness determined by the latter two factors is often greater than the value calculated by the strength condition. Since there is no mature calculation method to calculate the wall thickness of the barrel according to the heat transfer characteristics of the barrel, most of the current decisions are based on empirical statistical analogy, and then strength checks are performed.

2、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 is available for calculation.

When the barrel is plastic material, use the fourth strength theory to calculate.

When there is bushing in the barrel, it is equivalent to the press-fit connection in the "mechanical arts", the stress state of the bushing and the barrel are more complicated, and the strength calculation is also more complicated.