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Injection Molding Machine

The Behind-the-Scenes 'Craftsman' of Plastic Products

2025-07-11

The Development History of Injection Molding Machines

The development history of injection molding machines is a history of continuous innovation and technological evolution. Its origin can be traced back to the mid-19th century, when it gradually developed on the basis of metal die-casting machines. In 1920, the technology of injection molding machines officially started the industrialization process. In 1926, Germany manufactured the first injection molding machine according to industrial standards. In 1932, the German Franz Braun factory successfully developed the world's first fully automatic plunger injection molding machine. In 1948, the screw plasticizing device was introduced into injection molding machines, and in 1965, the first reciprocating screw injection molding machine came out, which greatly improved the economy of plastic part molding. Since then, injection molding machines have continued to move forward on the road of technological innovation, constantly meeting the growing production needs.

The Working Principle of Injection Molding Machines

The working principle of an injection molding machine is quite similar to that of a syringe for infusion. It uses the thrust of a screw (or plunger) to inject molten (i.e., viscous flow state) plastic that has been plasticized into the closed cavity of the injection mold at high pressure and high speed. After cooling, solidification and shaping, plastic products that are almost consistent with the shape of the mold cavity are obtained. This process can be specifically divided into five steps: quantitative feeding, melting and plasticizing, pressure injection, mold filling and cooling, and mold opening and part taking. These steps are closely connected to form a cyclic production process.

Quantitative Feeding

Granular plastic raw materials are added to the hopper of the injection molding machine, and the device under the hopper will transport the plastic raw materials to the barrel according to the set amount.

Melting and Plasticizing

The heating ring outside the barrel heats the plastic raw materials. At the same time, the screw in the barrel rotates under the drive of the motor. The materials are pushed by the screw, transported forward along the screw groove and compacted. Under the dual action of the screw shear force and external heating, the plastic is gradually plasticized, melted and homogenized, and a storage space is formed at the head of the screw.

Pressure Injection

Under the thrust of the piston of the injection cylinder, the screw injects the molten plastic in the storage chamber into the mold cavity through the nozzle at high speed and high pressure.

Mold Filling and Cooling

After the plastic melt fills the mold cavity, it cools in the mold and gradually solidifies and forms. During the cooling process, the temperature control of the mold has an important impact on the quality and performance of plastic products.

Mold Opening and Part Taking

The mold is opened under the action of the clamping mechanism, and the ejection device ejects the shaped plastic products from the mold, completing an injection cycle.

The Basic Structure of Injection Molding Machines

Although injection molding machines come in various types and forms, their basic components are the same, mainly consisting of four parts: injection device, clamping device, driving device and control system.

Injection Device

The injection device can be called the "heart" of the injection molding machine, and its role is crucial. It can make the plastic uniformly heated, melted, plasticized and reach a flowing state; inject a certain amount of melt into the mold cavity under a certain pressure and speed; after the injection is completed, maintain pressure on the melt in the mold cavity and replenish material into the mold cavity.

The injection device is usually composed of a plasticizing device and a power transmission device. The plasticizing device is a key working part of the injection molding machine, responsible for homogenizing and plasticizing the plastic. There are two common types: plunger type and screw type. Taking the plasticizing device of a screw injection molding machine as an example, it is mainly composed of a feeding device, a barrel, a screw, a glue passing component, a nozzle, etc.; the power transmission device includes an injection cylinder, an injection seat moving cylinder and a screw driving device (melting motor).

Clamping Device

The role of the clamping device is also crucial, mainly reflected in three aspects: realizing the reliable opening and closing of the mold; ensuring sufficient locking force during injection and pressure keeping to prevent plastic parts from flashing; realizing the demoulding of plastic parts.

The clamping device is mainly composed of a clamping mechanism, a mold adjusting mechanism, a demoulding mechanism, front and rear fixed templates, a moving template, a clamping cylinder and a safety protection mechanism. Generally speaking, the clamping device connects the front and rear fixed templates through four tie rods to form an integral rigid frame. The moving template slides between the front and rear fixed templates, and the demoulding mechanism is located at the rear side of the moving template. When the mold is opened, the moving template can eject the plastic parts from the mold cavity through the demoulding mechanism in the mold. A mold adjusting mechanism is also provided on the moving template or fixed template, which can adjust the thickness of the mold to adapt to the needs of molds with different thicknesses.

Driving Device

The driving device is like the "power source" of the injection molding machine, mainly playing two roles: one is to provide the required power for the injection molding machine to perform actions according to the process requirements; the other is to meet the requirements of force and speed for moving parts during movement.

At present, most injection molding machines adopt hydraulic drive devices, which are usually composed of a main circuit that controls the pressure and flow of the system and branch circuits of each executive mechanism, mainly including various hydraulic components and hydraulic auxiliary components. The components of the circuit include pumps, filters, flow valves, pressure valves, directional valves, speed control valves, stroke valves, accumulators, indicating instruments, switching elements, etc. Among them, the oil pump and motor are the power sources of the injection molding machine, and various valves control the oil pressure and flow to meet the requirements of the injection molding process.

Control System

The control system is the "brain" of the injection molding machine. It controls various actions of the injection molding machine, enabling it to effectively control and adjust time, temperature, pressure, speed and various program action parameters according to the pre formulated program.

At present, relay control systems are widely used, and a few injection molding machines have adopted microcomputer control. This control system can perform action program control, temperature control, hydraulic pump motor control, etc. It is mainly composed of relays, electronic components, detection elements and automatic instruments, and generally has four control modes: manual, semi-automatic, fully automatic and adjustment. The control system is organically combined with the hydraulic system to accurately and stably control the process program of the injection molding machine.

The Basic Classification of Injection Molding Machines

There are various classification methods for injection molding machines, and the common ones are classified by machine arrangement (shape characteristics) and by plastic plasticizing method in the barrel.

Classification by Machine Arrangement

Horizontal Injection Molding Machine: The axes of the injection device and the clamping device are arranged on the same horizontal line. Its advantages are low machine body, convenient operation and maintenance; low center of gravity of the machine, good installation stability; after the plastic parts are ejected, they can automatically fall by their own weight, which is easy to realize automatic operation. The disadvantages are that mold installation and part placement are more troublesome, and the floor area is larger. Horizontal injection molding machines are suitable for large, medium and small injection molding machines, and are widely used in large and medium-sized injection molding machines at home and abroad.

Vertical Injection Molding Machine: The axes of the injection device and the clamping device are arranged on the same vertical line. The advantages are small floor area, convenient mold assembly and disassembly and insert placement. The disadvantages are that plastic parts often need to be taken out manually after ejection, which is not easy to realize automation; the machine is tall, the center of gravity of the machine is high, the stability is poor, and maintenance and feeding are also inconvenient. Vertical injection molding machines are mostly small injection molding machines with an injection volume of less than 60cm³.

Angular Injection Molding Machine: The axes of the injection device and the clamping device are arranged perpendicular to each other, and the axis of the injection device and the parting surface of the mold are in the same plane. Its advantages are simple structure. When injection molding, the melt enters the cavity from the side of the mold, which is especially suitable for processing products where the central part is not allowed to have gate marks. The disadvantage is that the mold opening and closing mechanism is purely mechanical transmission, which cannot accurately and reliably inject, maintain pressure and clamping force, and the mold is subject to large impact and vibration.

Multi-Mold Turntable Injection Molding Machine: This is a special injection molding machine with multi-station operation. It is characterized in that the injection system is fixed, the clamping mechanism adopts a turntable structure, or the clamping mechanism with multiple sets of molds is fixed, and the injection system moves or swings along the track. This injection molding machine gives full play to the plasticizing capacity of the injection device, can shorten the production cycle, improve the machine production capacity, and is especially suitable for mass production with long cooling and setting time or more auxiliary time for placing inserts. However, its clamping system is large and complex, and the clamping force is often small, which is suitable for the production of plastic shoes and other products that need reaction, foaming or vulcanization after injection.

Classification by Plastic Plasticizing Method in the Barrel

Plunger Injection Molding Machine: It uses a plunger to push the material forward, and injects it into the mold through a spreader and a nozzle. Its plasticizing system is composed of a feeding device, a barrel, a plunger, a spreader and a nozzle. The injection molding machine only relies on the heater on the outer wall of the barrel to heat the raw materials. The melt lacks mixing, with large temperature difference and uneven distribution.

Screw Preplasticizing Plunger Injection Molding Machine: The plasticizing device and the injection device are separated. The plasticizing parts are the screw and the barrel, and the injection is completed by the plunger. The rubber material is first uniformly plasticized by the screw in the plasticizing barrel, then enters the injection barrel through the one-way valve, and finally is pushed into the mold cavity by the plunger.

Mobile Screw Injection Molding Machine: The plasticizing parts are the screw and the barrel. The rubber material is softened not only by the heating of the heating barrel, but also by the shear heat and friction heat generated by the rotation of the screw. The injection process is completed by the screw. During the plasticizing process, the rotation of the screw makes the plastic melt produce various forms of movement, with good mixing effect, uniform temperature, large plasticizing capacity, good quality and wide application.

The Application Fields of Injection Molding Machines

Injection molding machines have been widely used in various fields due to their strong molding capacity.

National Defense Field

It is used to manufacture some high-precision and high-performance plastic parts, such as missile shells and radar covers. These plastic products have the characteristics of light weight, high strength and corrosion resistance, which can meet the special needs of national defense equipment.

Mechanical and Electrical Field

It produces various electrical enclosures and internal components, such as TV enclosures, washing machine liners, computer cases, etc., which not only ensure the appearance quality of the products, but also meet their requirements for dimensional accuracy and performance.

Automobile Industry

Many parts of automobiles are produced by injection molding machines, such as lamp housings, interior parts, bumpers, grilles, etc. Injection molded auto parts have good dimensional stability and surface quality, which can improve the overall performance and beauty of automobiles.

Transportation Field

In rail transit, it can manufacture train seats, armrests, interior decoration parts, etc.; in the aviation field, it is used to produce aircraft interior parts, avionics equipment housings, etc.

Building Materials Field

It produces plastic pipes, pipe fittings, door and window profiles and other building materials. These plastic products have the advantages of corrosion resistance, good insulation and convenient installation, and are widely used in the construction industry.

Packaging Field

It manufactures various plastic packaging containers, such as plastic bottles, plastic boxes, plastic packaging bags, etc. Injection molded packaging containers can be designed into various shapes and sizes according to the needs of different products, with good sealing and moisture resistance.

Agricultural Field

It produces plastic pipes and plastic greenhouse films for agricultural irrigation. These plastic products can improve agricultural production efficiency and promote the development of agricultural modernization.

Cultural, Educational and Health Field

In the cultural and educational aspect, it manufactures stationery, toys, etc.; in the health field, it produces disposable syringes, infusion tubes, surgical instrument handles and other medical devices. Injection molded products are non-toxic, hygienic and accurate in size, which can ensure people's health and safety.

Daily Life Field

Plastic products produced by injection molding machines are everywhere in our daily life, such as disposable water cups, plastic stools, hangers, trash cans, etc., bringing great convenience to our daily life.

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