Two-plate and Three-plate Injection Molding Machines Performance Difference Analysis

With the development of material technology, the application proportion of plastic parts on the car body is increasing, such as the front / rear bumper, dashboard and door guard. Plastic parts also have a trend of getting bigger and heavier, which leads to the tonnage of the clamping force of the injection molding machine selected by the relevant injection molding manufacturers.For the injection molding machine of more than 2000 t, most of the foreign countries choose the two-plate injection molding machine (direct pressure mode clamping), while the vast majority of domestic manufacturers choose the three-plate injection molding machine (crankshaft mode clamping). In this paper, the differences between them in performance are compared, and some useful suggestions for the type selection of injection molding machine are put forward.

1 Two Board Injection Molding Machine

1.1 Design Principles

Plastic injection molding machine is referred to as injection molding machine, usually there are two kinds of screw type and plunger type. The screw injection molding machine with more than 2000t mainly includes injection device, clamping device, hydrostatic transmission system and electric control system.

• Injection device, mainly composed of screw, barrel, nozzle, hopper, metering device, transmission device, injection seat and mobile hydraulic cylinder.

2) Mould clamping device, mainly composed of front / rear fixed template, mobile template, Golin column (commonly known as tie rod), mould clamping hydraulic cylinder, mould shifting hydraulic cylinder, mould clamping mechanism, mould adjusting device, mould making device

Product ejection device (commonly known as neutron) and other components. (3) hydrostatic transmission system and electric control system. Two plate injection molding machine and three plate injection

The biggest difference in the press is the clamping device part. The clamping device of the second plate injection molding machine mainly includes moving template, static template, rotating nut system, hydraulic thimble system, rapid mold moving system and high pressure hydraulic cylinder clamping system (see Figure 1), clamping high pressure hydraulic cylinder to provide clamping force.

The second plate injection molding machine mainly adopts the direct pressure mode, that is, four high pressure clamping liquid pressure cylinders pull the Golin column at the same time, pressurize the mold and hold the pressure. Injection molding machine production plastic parts of the main process is: slow clamping – fast clamping – low pressure slow clamping – rotating nut locking – high pressure clamping hydraulic cylinder clamping – injection – holding pressure – holding pressure after release pressure – high pressure opening – nut opening – slow opening – high speed opening – low speed opening – product ejection – thimble return. Clamping process is relatively simple, clamping in place, rotating nut action, high pressure clamping hydraulic cylinder to provide clamping force. The mold opening process is opposite to the mold locking process. First, the high-pressure mold locking hydraulic cylinder is relieved, then the nut is rotated to open, and then the moving mold is returned.

1.2 Performance Features

1.2.1 Benefits

(1) The opening force required by the hydraulic cylinder is small. Generally, more than 2,000 tons of injection molding machine opening hydraulic cylinder needs to open the mold force of 110 t or less, which is half of the same tonnage of three-plate injection molding machines. The hydraulic cylinder with small mold opening force is easy to be processed, easy to maintain in the process of actual use, long service life, and not easy to be damaged.

(2) The clamping force can be set freely. Two-plate injection molding machine can change the clamping force by changing the number of hydraulic cylinders and hydraulic cylinder working force.

(3) mode-locking accuracy is relatively high. The error between the clamping force set by the two-plate injection molding machine and the actual clamping force is small, and the actual clamping force can be measured by measuring the hydraulic pressure of the hydraulic cylinder through the sensor built in the hydraulic cylinder. When the error between the set clamping force and the actual clamping force is large, the microcomputer automatically adjusts the actual clamping force by changing the working force of the hydraulic cylinder or the working number of the hydraulic pressure cylinder, and the error between the two is controlled within the allowable range. In the injection molding production process, the accuracy of the clamping force has a great impact on the products and equipment, mainly reflected in the following aspects: 1) The high-precision clamping force can make the dynamic mold and the static mold combine just right, The gas generated in the process of product forming can be discharged smoothly through the gap of the mold, which is conducive to the stability of the quality of the product. 2) Excessive clamping force will cause deformation of the mold, and the mold deformation will affect the flow direction of the plastic material in the mold cavity, thereby affecting the internal stress of the product; 3) If the clamping force is not accurately controlled, it is easy to cause damage to the mold, and it takes a lot of manpower and material resources to repair the damaged mold, and the service life of the mold is greatly shortened.

(4) the second board injection molding machine adopts direct pressure clamping, dynamic and static template flatness is easy to adjust, generally adjust to the flatness of the process requirements (0.1mm / m) need a short time.

1.2.2 Shortcomings

(1) The requirements for high-pressure die locking hydraulic cylinders are relatively high. Two plate injection molding machine through four high-pressure clamping hydraulic cylinder to achieve clamping, the work of high-pressure clamping hydraulic cylinder need to provide a lot of pressure, which requires the hydraulic cylinder to have a high accuracy; At the same time, the hydraulic oil used should also be kept clean and free of impurities; Also to avoid oil leakage in the production process hydraulic cylinder.

(2) The two-plate injection molding machine achieves clamping by pulling four Gorin columns by four high-pressure hydraulic cylinders. At a certain moment, the pulling force of the four high-pressure hydraulic cylinders cannot be guaranteed to be exactly the same, which will lead to a slight imbalance in the process of operation of the template. [3] 。

In view of the shortcomings of the two-plate injection molding machine, a “two-plate and a half injection molding machine” has appeared in the market in recent years, which has successfully solved the above problems.

2. Two and a half injection molding machine

The biggest difference between two and a half injection molding machine and two and a half injection molding machine is: two and a half injection molding machine

More than two plate injection molding machine template – hydraulic cylinder support plate (see Figure 2).

Six small hydraulic cylinders are evenly distributed on the hydraulic cylinder support plate, and the clamping force provided by the six small hydraulic cylinders shares the clamping force provided by the four high pressure clamping cylinders. The action process of the two-plate and half-injection molding machine is similar to the two-plate injection molding machine, but the difference is that after clamping, in addition to the high-pressure hydraulic cylinder to provide clamping force, six annular hydraulic cylinders also provide clamping force. The high pressure hydraulic cylinder and the annular hydraulic cylinder act on the moving template at the same time, and pass the pressure to the mold through the moving template, and complete the clamping process. Because the pressure is transmitted through the same plate, the parallel force of the mold is ensured, which effectively avoids the tiny non-parallel situation of the mold caused by the clamping of only four high pressure clamping hydraulic cylinders. [4] 。

The appearance of the two-plate semi-injection molding machine enables the clamping force to reach more than 6 000 t. From the perspective of actual use, the quality of injection molded products is stable, the probability of mold damage is small, and the problem of frequent mold repair is solved.

3 Three Board Injection Molding Machine

3.1 Design Principles

Three plate injection molding machine clamping device mainly has moving template, static template, crankshaft support plate, crankshaft clamping system, hydraulic thimble system, rapid mold moving system and high pressure hydraulic cylinder mold opening system (see Figure 3).

As can be seen from Figure 3, the main components of the clamping system of the three-plate injection molding machine are curved shafts, and the crankshaft is mainly designed according to the lever principle. The mould closing process is: the mould opening hydraulic cylinder pushes the crankshaft labor-saving arm to drive the moving template forward, and the bending crankshaft working arm is extended, when the crankshaft working arm is in a parallel position, the mould is locked. The opening process is opposite to the closing process [5] 。

3.2 Performance Features

3.2.1 Advantages

1) Compared with the same tonnage of two-plate injection molding machine, the equipment cost is low.

(2) the hydraulic cylinder, connector, valve, servomotor and oil feed pump in the system are common commodities in the market, so it is convenient to maintain the machine.

3.2.2 Shortcomings

(1) It is difficult to change the clamping force on the same injection molding machine. The clamping force of the three-plate injection molding machine is provided by the crankshaft, so in the design process of the clamping system, the length of the crankshaft and the maximum distance between the templates should be determined according to the clamping force. Once the crankshaft length and the maximum distance between the templates are determined, the clamping force is fixed. If you want to change the clamping force of the same injection molding machine, it is more difficult to change the length or structure of the crankshaft.

(2) It is difficult to accurately measure the actual clamping force. At present, there is no better method to accurately measure the actual clamping force of crankshaft clamping mode. Generally, it is calculated by the pressure and lever principle of the opening hydraulic cylinder, which often results in a large error between the actual clamping force and the designed clamping force. If the actual clamping force is too small, the product is prone to “flash” (the joint of the plastic product has unnecessary edges, affecting the appearance and quality of the product); If the actual clamping force is larger than the design clamping force, it often causes deformation of the mold, because the deformation is small, the human eye is difficult to find, it will accelerate the damage of the mold.

(3) When opening the mold, the opening force provided by the opening hydraulic cylinder is large. Taking the 3,200 t left and right plastic injection molding machine as an example, the mold opening force provided by the open mold cylinder is 200 ~ 230 t. Therefore, it is required that the machining accuracy of the hydraulic cylinder is higher, and the daily maintenance cost is slightly larger than that of the hydraulic cylinder.

4.Development Trend of  Large Tonnage Injection Molding Machines

In foreign countries, more than 1 200 t of plastic injection molding machine is mostly used in the second board plastic injection machine. In the country

In the early days, Gran, Baoyuan, Haitian, Zhende and Donghua and other manufacturers have produced two plate injection molding machines, but because the technology is not too mature, it is more difficult to promote in the market. After more than 20 years of development, the two-plate injection molding machine technology has gradually matured, especially the recently developed “two-plate and a half” machine, which has successfully solved the problem of uneven template balance during operation, and has gradually been accepted by users. [6] At present, the domestic market of large tonnage injection molding machine mainly to three-plate injection molding machines. Because the three-plate injection molding machine technology is very mature, the improvement space is very small, and the two-plate injection molding machine has many superior performance and very large room for improvement, it is expected that the two-plate injection molding machine will gradually become a trend in the domestic injection molding machine market.