With development of material technology, application proportion of plastic parts on automobile bodies is increasing, such as automobile front/rear bumpers, instrument panels and door guards. Plastic parts also tend to be bigger and heavier. This has led to larger and larger clamping force of injection molding machine selected by related injection molding manufacturers. For injection molding machines over 2 000 t, most foreign manufacturers use two-platen injection molding machines (direct pressure mode clamping), while most domestic manufacturers use three-platen injection molding machines (crankshaft mode clamping).

 

Plastic injection molding machines are referred to as injection molding machines for short, and there are usually two types: screw type and plunger type. Screw-type injection molding machines of more than 2 000 tons mainly include injection device, mold clamping device, hydraulic transmission system and electrical control system.
Injection device is mainly composed of screw, barrel, nozzle, hopper, metering device, transmission device, injection seat and mobile hydraulic cylinder.
Mold clamping device is mainly composed of front/rear fixed mold plate, movable mold plate, Collin column (commonly known as tie rod), mold clamping hydraulic cylinder, mold shifting hydraulic cylinder, mold clamping mechanism, mold adjustment device, product ejection device (commonly called neutron), etc.
Hydraulic drive system and electrical control system. The biggest difference between two-platen injection molding machine and three-platen injection molding machine is part of clamping device. Mold clamping device of two-platen injection molding machine mainly includes a movable template, a static template, a rotating nut system, a hydraulic thimble system, a rapid mold shifting system, and a high-pressure hydraulic cylinder clamping system (see Figure 1). Clamping high-pressure hydraulic cylinder provides clamping force.

 

Two-platen injection molding machine mainly works in direct pressure mode, that is, four high-pressure mold clamping hydraulic cylinders simultaneously pull Collin column to pressurize and maintain pressure of mold. Main process of two-platen injection molding machine to produce plastic parts is: slow mold clamping-fast mold clamping-low pressure and slow mold clamping-rotating nut locking-high pressure clamping hydraulic cylinder mold clamping-injection-holding pressure-pressure release after holding pressure —high pressure mold opening—nut opening—slow mold opening—high speed mold opening—slow mold opening—product ejection—thimble retreat. Clamping process is relatively simple. After clamping is in place, nut is rotated and high-pressure clamping hydraulic cylinder provides clamping force. Mold opening process is opposite to mold clamping process. First, high pressure mold clamping hydraulic cylinder releases pressure, then nut is turned to open, and movable mold is retracted.

Mold opening hydraulic cylinder requires less mold opening force. Generally, opening force required for opening hydraulic cylinder of an injection molding machine of more than 2 000 t is below 110 t, which is half of opening force of a three-platen injection molding machine of same tonnage. Hydraulic cylinder with small mold opening force is easy to be processed, easy to maintain during actual use, has a long service life, and is not easy to be damaged.
Clamping force can be set freely. Two-platen injection molding machine can change clamping force by changing number of hydraulic cylinders and working force of hydraulic cylinders.
Clamping accuracy is relatively high. Error between clamping force set by two-platen injection molding machine and actual clamping force is small, actual clamping force can be measured by measuring oil pressure of hydraulic cylinder by sensor built in hydraulic cylinder.
Two-platen injection molding machine adopts direct pressure clamping. Parallelism of dynamic and static templates is easy to adjust. Generally, it takes a short time to adjust to parallelism required by process (0.1 mm/m).

Requirements for high-pressure mold clamping hydraulic cylinders are relatively high. Two-platen injection molding machine uses 4 high-pressure clamping hydraulic cylinders to achieve mold clamping. High-pressure clamping hydraulic cylinder needs to provide a lot of pressure when working, which requires hydraulic cylinder to have high accuracy; at the same time, hydraulic oil used must be maintained It is clean and free of impurities; it is also necessary to avoid oil leakage in hydraulic cylinder during production process.
Two-platen injection molding machine uses 4 high-pressure hydraulic cylinders to pull 4 collimator columns to achieve mold clamping. At a certain moment, pulling force of 4 high-pressure hydraulic cylinders cannot be guaranteed to be completely consistent, which will cause a slight imbalance of mold plate during operation. .

Mold clamping devices of three-platen injection molding machine mainly include a movable template, a static mold plate, a crankshaft support plate, a crankshaft clamping system, a hydraulic ejector system, a rapid mold shifting system, and a high-pressure hydraulic cylinder mold opening system (see Figure 3).
It can be seen from figure below that main component of clamping system of three-platen injection molding machine is crankshaft, which is mainly designed based on principle of leverage. Mold clamping process is: mold opening hydraulic cylinder pushes crankshaft labor-saving arm to drive movable mold plate forward, bent crankshaft working arm extends at the same time. When crankshaft working arm is in parallel position, mold is locked. Mold opening process is opposite to mold clamping process.

 

Compared with two-platen injection molding machine of same tonnage, equipment cost is low.
Hydraulic cylinders, joints, valves, servo motors, and fuel supply pumps in system are all common commodities in the market, it is more convenient to maintain machine.

It is difficult to change clamping force on same injection molding machine. Clamping force of three-platen injection molding machine is provided by crankshaft. Therefore, in design process of clamping system, length of crankshaft and maximum distance between mold plates must be determined according to clamping force. Once crankshaft length and maximum distance between mold plate are determined, clamping force is also fixed. If you want to change clamping force of same injection molding machine, you need to change length or structure of crankshaft, which is more difficult.
It is difficult to accurately determine actual clamping force. At present, there is no better method to accurately measure actual clamping force of crankshaft type clamping method. It is generally calculated by pressure of mold opening hydraulic cylinder and principle of leverage, which often causes a large error between actual clamping force and design clamping force. If actual clamping force is too small, product is prone to "flash" (unnecessary edges at joint of plastic products affect appearance and quality of product); if actual clamping force is greater than designed clamping force, it will often cause molds deformation. Due to small deformation, it is difficult for human eye to detect, which will accelerate damage of mold.
When mold is opened, mold opening force provided by mold opening hydraulic cylinder is relatively large. Take an injection molding machine of about 3 200 t as an example, mold opening force provided by mold opening hydraulic cylinder is 200~230t. Therefore, machining accuracy of mold opening hydraulic cylinder is required to be high, and daily maintenance cost is slightly higher than that of ordinary hydraulic cylinder.

In foreign countries, two-platen injection molding machines are mostly used for injection molding machines over 1 200 t. In China, in the early stage, manufacturers of Grand, Baoyuan, Haitian, Zhende, and Donghua all produced two-platen injection molding machines, but because technology is not yet mature, it is more difficult to promote them in the market. After more than 20 years of development, technology of two-platen injection molding machine has gradually matured, especially recently developed "two-platen and a half" machine (as shown in figure below), which successfully solved problem of imbalance of mold plate during operation and was gradually accepted by users. At present, large-tonnage injection molding machines in domestic market are mainly three-platen injection molding machines. As three-platen injection molding machine technology has been very mature, there is little room for improvement, two-platen injection molding machine has many superior performance and very large room for improvement, it is expected that two-platen injection molding machine will gradually become a trend in domestic injection molding machine market.