Taking plunger injection machine as an example, principle of injection molding is shown in Figure 4-1. First, granular or powdery plastic is sent from hopper of injection machine into barrel equipped with a heating device for heating, melting and plasticizing to make it a viscous liquid melt. Then, driven by high pressure of plunger of injection machine, it is injected into closed cavity with a lower temperature through nozzle at the front end of barrel at a high flow rate.   injected at a high flow rate through nozzle at the front end of barrel into a closed cavity with a lower temperature. After a period of pressure holding and cooling, mold opening and parting can release plastic part (plastic part) with a certain shape and size from cavity (closed space of molded plastic part), thus completing a molding cycle.

Injection molding has advantages of strong adaptability to plastic varieties, one-time molding of plastic parts with complex shapes, precise dimensions, metal or non-metal inserts, high production efficiency, and easy automation. It is an important method for thermoplastic molding.
Injection molding process has developed rapidly. In addition to thermoplastic injection molding, some thermosetting plastics can also be successfully used for injection molding, have characteristics of high efficiency and stable product quality; low anti-foaming plastics (density 0.2-0.9g/cm3 ) injection molding can produce plastic parts with excellent properties such as cushioning, sound insulation, and heat insulation; two-color or multi-color injection molding can produce plastic products with various colors and beautiful appearance.
In addition, application of hot runner injection molding process has obvious advantages in obtaining large plastic parts, reducing or eliminating condensation of gating system, etc. Injection molding is also an important process for obtaining hollow plastic product parisons.

Injection machine is key equipment for injection molding production. Injection molding system refers to injection system, clamping system and mold installed on injection machine that are directly used for molding actions in injection machine.

Classified according to size and specifications of injection machine

See section 4.1 for its working principle. This equipment (Fig. 4-1) has a simple structure. Heat required for melting plastic is mainly provided by heater 5 outside barrel 3 in the form of heat conduction. Due to poor thermal conductivity of plastic and thick wall of barrel, at the same time, its movement state in barrel is like laminar flow, thus forming a large temperature difference between outer layer of plastic (near inner wall of barrel) and inner layer, resulting in poor uniformity of plasticization. Although splitter cone 4 is installed in barrel to increase plasticizing effect, plasticizing uniformity is still poor, material retention is serious and pressure loss is large. Required injection pressure is about 2-3 times that of screw type, which is only suitable for molding of small parts.

 

Schematic diagram of horizontal screw injection machine
1-Mold closing hydraulic cylinder 2-Clamping mechanism 3-Moving mold fixed plate 4=Tie rod 5-Mould 6-Fixed mold fixed plate 7-Nozzle 8-Cylinder 9-Heater 10-Screw 11-Hopper 12-Hydraulic motor 13-injection hydraulic cylinder 14-sliding seat 15-oil cylinder 16-electric motor and hydraulic pump 17-body

Screw injection machine uses a heating cylinder and a screw to realize plasticization and injection of molding material, as shown in Figure 4-2. Its mold clamping, injection, pressure keeping, cooling and demoulding process are same as plunger injection machine, difference is that screw has two functions of plasticizing and injection. Rotation of screw produces strong mixing and shearing, uniform plasticization, less material retention, less pressure loss, less residual stress of molded plastic parts, and simple equipment structure. It is currently the most widely used machine.

 

Structural diagram of screw preplastic injection machine
1-Check valve 2-Premolded barrel 3-Injection plunger 4-Injection barrel

Screw pre-plasticizing injection machine uses one set of barrel and screw for plasticization, and another set of barrel and plunger for injection, as shown in Figure 4-3. This type of injection machine has uniform plasticization, accurate measurement, and small pressure loss. It is suitable for precision molding, but material retention is large and structure is complex.

Horizontal injection machine is the most basic and common form of injection machine products. Its structural feature is that injection system of molding material coincides with axis of clamping mechanism and is parallel to ground (Figure 4-2). Its advantage is that fuselage is low and stable, it is more convenient to add materials, operate and maintain, and product can automatically fall after being released from mold, which is easy to realize mechanization or automation, but there are disadvantages such as inconvenient disassembly and assembly of molds, troublesome installation of inserts, and large floor space.

Structural feature of this injection machine is that axis of injection system coincides with clamping mechanism and is perpendicular to ground, as shown in Figure 4-4a. This structure has advantages of small footprint, convenient disassembly and assembly of mold, easy installation of inserts, and material in hopper can enter barrel evenly. However, after product is released from mold, it needs to be taken out manually, which is difficult to realize mechanized or automated operation. In addition, there are disadvantages such as high fuselage, instability, inconvenient feeding, and certain requirements for height of factory building. At present, this type of injection molding machine is mainly used to produce multi-insert products below 60cm3, its structure is mostly plunger type, and plasticizing effect is not good.

Structural feature of this injection machine is that axes of clamping mechanism and injection system are perpendicular to each other. Use and installation characteristics are between previous two types of injection machines, especially suitable for production of plastic parts with asymmetrical shapes and molds with side gates. At present, right-angle injection machines that are widely used in China clamp mold along horizontal direction and inject along vertical direction, as shown in Figure 4-4b. Mold clamping mechanism adopts screw drive, which is widely used. Figure 4-4c is a right-angle injection machine that injects in horizontal direction and closes mold in vertical direction, which is rarely used.

 

Schematic diagram of vertical injection machine and right-angle injection
1-Body 2-Injection system 3-Clamping system
In addition, there are eccentric injection molding machines, double-barrel injection machines, two-color or multi-color injection machines, dual-mode system injection machines, etc.

At present, specifications of injection machine are usually characterized by injection capacity (ie injection capacity) and clamping force.
Theoretical volume of plastic that can be injected by plunger or screw in barrel of injection machine in one injection stroke is called injection capacity, which is main parameter that characterizes capacity of injection machine.
According to international standards, nominal injection volume in specifications of injection molding machine needs to be expressed by value under constant pressure conditions (injection pressure is 100MPa). During working process of injection machine, force to lock and close cavity to prevent melt from overflowing is called clamping force. Size of clamping force is related to molding area allowed by injection machine.
Model compilation and labeling method of domestic SZ series plastic injection machine is shown in Figure 4-5.
 

Injection system is part of injection machine that is in direct contact with molding material and melt. Its main function is to make solid molding material evenly plasticized into a melt, inject melt into mold cavity with sufficient pressure and speed. Injection system includes a feeding device, a barrel, a plunger and a splitter cone (plunger injection machine), a screw (screw injection machine) and a nozzle.

Also known as a hopper, usually an inverted conical or square conical metal container, installed at a higher part of injection machine and connected to barrel.

That is, plasticizing chamber of molding material, which is mainly used to heat and melt material.

Plunger and splitter cone are injection and plasticizing parts installed in barrel of plunger injection machine. Plunger makes a reciprocating linear motion in barrel, pushes and compresses plastic melt and injects it into mold through nozzle. Diverter cone is a diverter part installed in the center of front end of barrel, as shown in Figure 4-6.
Its working principle is: when molten material passes through splitter cone, it is split into thin layers and produces a convergent flow, thereby shortening heat transfer distance between barrel and material, improving heat transfer effect; Accelerated movement occurs in the gap of barrel, and shearing action is enhanced, thereby generating a certain amount of frictional heat. The two aspects work together to improve plasticizing and balancing effect of material.

 

Divider cone installation structure diagram
1-nozzle 2-split cone 3-barrel
After splitter cone is set, plasticizing time of material can be shortened, flow performance of plastic melt and molding quality of plastic parts can be improved, and production efficiency can be improved at the same time.

It is an important part of screw injection machine. Through rotation and axial movement of screw in barrel, plasticization and injection of molding material are realized.

Installed at the front of barrel, nozzle hole inside is channel connecting barrel and mold, which guides plastic melt from barrel into mold.

Function of mold clamping system is to realize opening and closing action of mold, to lock mold during injection, and to push out plastic parts in mold when mold is opened.
The most basic requirement of injection molding for mold clamping system is to provide a reliable clamping force for mold when clamping mold, so as to prevent mold from expanding along parting surface under pressure of plastic melt, causing edge separation of product and affecting accuracy.
At present, structure of mold clamping system used on injection machine can be roughly divided into two categories: hydraulic type and hydraulic-mechanical type.

Generally, large-diameter pistons and hydraulic cylinders are used to open and close mold. Advantage is that movement mechanism is simple, it can adapt to closing height of different molds, maximum pressure can be reached and maintained at any time; disadvantage is that hydraulic cylinder is bulky when clamping force is large, energy consumption is large, and rigidity of system is poor.

Toggle mechanism is driven by hydraulic pressure to open and close mold, with self-locking function (Fig. 4-2). Clamping rigidity is high, and product is not easy to flash; opening and closing time of mold is relatively short, and it has a buffer effect, which can obtain a better mold clamping movement mode; but structure is complex, mechanical parts are easy to wear, and frequent maintenance is required.

Production process of injection molding is shown in Figure 4-7, which includes: preparation before molding, injection process, post-processing of plastic parts, etc., which are discussed in following.

In order to make injection process go smoothly and ensure quality of plastic parts, some necessary preparations are generally required before injection. For example, inspect and measure appearance of raw materials (such as color, particle size and uniformity), process performance (fluidity, thermal stability, shrinkage), and judge whether performance of raw materials conforms to required parameters;
Whether properties of raw materials are consistent with required parameters; dye raw materials; test water content, and fully dry some hygroscopic plastics (such as polyamide, ABS, etc.) to prevent silver wires, streaks, bubbles and degradation on the surface of product and other defects; before forming different types of plastics or when decomposition is found in plastics, barrel should be cleaned;
When molding plastic parts with inserts, inserts should be preheated to prevent plastic around inserts from cracking due to stress; for plastic parts that are difficult to demould, a good release agent should be selected. Due to different types and shapes of injection materials, structure of plastic parts, presence or absence of inserts, and requirements for use, preparations for various plastic parts before molding are not exactly same.

Injection process generally includes several steps of feeding, plasticizing, injection filling and demoulding.

Since injection molding is an intermittent production process, quantitative feeding is required to ensure stable operation, uniform plasticization of plastics, and finally obtain good plastic parts. Too much feeding, too long heating time, etc. will easily cause thermal degradation of material, and increase power loss of injection machine; too little feeding, lack of pressure transmission medium in barrel, pressure of plastic melt in cavity will decrease, and it is difficult to feed (ie supplementary pressure), it is easy to cause defects such as shrinkage, depression, and voids in plastic parts.

Process of heating plastic in barrel to make it change from a solid state (granule or powder) to a viscous fluid state with good plasticity is called plasticization. Main factors that determine quality of plasticization are heating and shearing of material.
Requirements for plastic plasticization are: plastic melt must be fully plasticized before entering cavity. It can provide enough molten plastic to ensure that plasticization of plastic is accelerated, and at the same time, it also strengthens mixing effect of plastic, making it extend to level of polymer molecules (not just static melting), which makes temperature distribution of plastic melt, material composition and molecular morphology have changed and tend to be more uniform. Therefore, plasticizing effect of screw injection machine is much better than that of plunger injection machine.

Injection filling refers to process of using a plunger or screw to push plasticized plastic melt to quickly fill mold cavity, make melt cool and solidify under pressure. Injection process can be divided into several stages such as mold filling, pressure holding, backflow and cooling after gate is frozen.