The method of thermoplastic injection molding is to melt the plastic material and then inject it into the film cavity. Once the molten plastic enters the mold, it is shaped into a certain shape in accordance with the mold cavity.
The resulting shape is often the final product, and no further processing is required before installation or use as the final product. Many details, such as raised parts. Ribs and threads can be formed in a single step of injection molding.
An injection molding machine has two basic components: an injection device and a mold clamping device for melting and feeding plastic into a mold. The function of the mold clamping device is: (1) make the mold close under the condition of bearing the injection pressure; (2) take out the product.
The injection device melts the plastic before it is injected into the mold, and then controls the pressure and speed to inject the melt into the mold. There are two designs of injection devices currently used: screw preplasticizers or two-stage devices, and reciprocating screws. The screw pre-plasticizer uses the pre-plasticized screw (first stage) to send molten plastic to the injection rod (second stage).
The advantages of screw preplasticizers are constant melt mass, high pressure and high speed, and precise injection volume control (using mechanical thrust devices at both ends of the piston stroke). These strengths are exactly what is needed for transparent, thin-walled products and high production rates. Disadvantages include uneven residence time (causing material degradation), higher equipment costs and maintenance costs.
The most common reciprocating screw injection device does not require a plunger to melt and inject the plastic. The powdery or granular plastic in the hopper is melted and sent to the front end of the screw through the rotating screw. The plastic fluid flows through the front of the screw and accumulates in front of the screw. The accumulation of molten plastic in front of the screw pushes the screw to the rear of the injection device. The rotation of the screw, the accumulation of the molten material and the movement to the rear continue until a certain injection volume is formed. In the next equipment working cycle, the screw tip is closed to prevent the material from returning along the screw. The function of the screw tip and the feeding screw is like the injection plunger, which presses the plastic into the mold.
Advantages of reciprocating screws include reduced plastic residence time, self-cleaning screws and screw tips. These advantages are key when processing heat-sensitive materials and when the colored raw materials or resin varieties are changed, and the screw and barrel are cleaned.
The currently widely used clamping device designs include: toggle lever clamping device, hydraulic clamping device and hydraulic-mechanical clamping device. The toggle lever clamping device is suitable for small tonnage equipment in view of its low cost during manufacture. Its features include high mechanical benefits of latching operations, built-in mold clamping slow-down devices, slow mold damage, and fast mold clamping operations.
The mold clamping cylinder pushes the horizontal top plate forward, extending the elbow and moving the pressure plate forward. When the clamping device is closed, the mechanical benefits are reduced and the platen is moved quickly. When the pressure plate reaches the closed position of the mold, the elbow is changed from high speed to low mechanical benefits to low speed to high mechanical benefits. Low speed is the key to protect the mold, and high mechanical benefits are needed to form a large tonnage. Once the ribs are fully extended, hydraulic pressure is no longer necessary to maintain the tonnage. In order to open the mold clamping device, hydraulic pressure is applied to the opposite side of the mold clamping plunger, and in order to prevent the molded product from being damaged, the mold is opened slowly. Through the movement of the entire elbow joint device and the movement of the platen device along the tie rod (moved to the mold closing position before the elbow joint device is fully extended), the clamping device is adjusted to adapt to different mold heights. The advantages of the toggle lever clamping device include: fast clamping operation, reduced energy consumption and lower equipment costs. The disadvantage is that it is more complicated than the hydraulic mold clamping mold, and the connecting pins and bushings need to be repaired frequently. However, the development of elbow design has reduced the maintenance of the elbow clamping device. These developments include oil-free bushings, which has greatly reduced mandatory lubrication.
One of the developments is that all motors use the existing precision ball screw machine technology and advanced AC servo motors to replace hydraulic power units. These motors provide only the power required to perform the machine's functions, and they greatly reduce the total energy consumption of each product.
Hydraulic clamping device is widely used in 150-1000t equipment, and most of them are used in 250-700t equipment. With the help of the booster tube (or external oil cylinder) to close the mold quickly, the result of a large amount of oil on a small area is very fast. The predetermined fluid is pumped from the high reservoir to the rear of the main piston by gravity. Before the two mold halves are in contact, the mold should be in a low-speed and low-pressure protection state, which prevents damage caused by foreign objects, flashover, or products not taken out in the previous cycle. When the mold is closed, the pre-filling fluid closes the outlet to the pre-reservoir. The tonnage clamping force is generated at the rear of the main piston. After this injection cycle, the prefilled fluid is opened, prompting the mold clamping device to gradually open the mold flap. Within a short distance, the clamping device accelerates to a fast opening speed.
Hydraulic clamping device provides flexibility for installation and operation of the equipment. Since tons of clamping force can be generated at any part of the clamping stroke, as long as the position is adjusted to the position corresponding to the contact of the two half halves by the control of the equipment, the mold can be connected. The hydraulic mechanical clamping device combines the functions of both mechanical and hydraulic to move the clamping device and cause a ton-level clamping force. The hydraulic machinery design is limited by the flow rate of the pre-filling room, which is used to make the mold clamping device, all from about 1000t to larger. The speed of the clamping device is limited by the fluid flow controlled by the pre-filled fluid. The hydraulic mechanical design includes the following units: the hydraulic cylinder used to move the movable platen to the two halves of the mold, which are close to each other; (2) a mechanical lock plate that prevents the large tonnage clamping force from moving backward; 3) Short stroke hydraulic cylinder used to move the last distance of the mold to the closed state and generate the clamping force.
The machine control part coordinates all the functions of the machine. It has progressed to adopt multi-computer control system. In order to match the new control device, the hydraulic part of the machine has also been improved. Proportional valves with servo control and corresponding amplification devices increase flexibility and accuracy, while reducing machine function response time. Microcomputer control system and servo proportional hydraulic device provide dynamic response to complete a true closed loop system. Closed-loop systems adjust the machine to compensate for changes in oil temperature, raw material viscosity, and machine variables. High-level controls are also present on auxiliary equipment. (Dryer, cooling device and mold temperature control device), and make all machinery and equipment be adjusted and monitored by CRT and LCD. Various machines connected to the host computer provide overall workshop monitoring and production scheduling, and SPC provides real-time monitoring at the machine or host computer.
Important factors in plastics processing include: temperature, consistency, colorant distribution, and melt density. Both the conductive heat generated by the barrel temperature and the mechanical heat generated by the screw rotation help process high-quality melts. The most common case is that most of the energy used to melt the plastic is obtained by turning the screw.
As the screw rotates, mixing occurs between the threads, and the surface of the plastic pellets is melt-plasticized. As the material advances along the screw, mixing and shearing are repeated until the plastic is completely melted.
