About the principle of injection molding and the injection molding process

Injection (molding) plastic (or injection molding) is a method in which plastic is first melted in a heating cylinder of an Injection molding machine, and then extruded by a plunger or a reciprocating screw to a cavity of a closed mold. . It not only produces high-precision, high-quality products at high productivity, but also has a wide variety of plastics and applications. Therefore, injection molding is one of the important molding methods in plastic processing.

Basic functions of the injection molding machine

Injection molding is achieved by injection molding machines.

The basic functions of the injection molding machine are:

1. heating the plastic to a molten state;

2. Apply a high pressure to the melt to cause it to exit and fill the cavity.

Injection process / equipment

Injection molding of thermoplastics is generally done by mastication and filling. The equipment used for compaction and cooling is composed of injection molding machine, injection mold and auxiliary equipment (such as material drying).

Injection device

The injection device mainly realizes mastication and metering in the process of the injection molding machine. Injection and pressure-preserving and other functions. The screw type injection device is used most, and it is formed by unifying the screw mastication and the injection plunger into one screw.

In essence, it should be referred to as a coaxial reciprocating plunger injection device. When it is working, the plastic in the hopper falls into the heating cylinder by its own weight. When the screw rotates, the plastic moves forward along the screw groove. At this time, the material is heated by the external heater of the heating cylinder, and the inside is also sheared. The heat generated by the cutting rises and the temperature rises to a molten state.

With the storage of the material at the front end of the heating cylinder, the reaction force (back pressure) generated by these materials pushes the screw backwards, and the limit switch is used to limit the amount of retreat. When retracting to a certain position, the screw stops rotating, thereby determining (Measuring) The amount of injection once.

After the material in the mold is cooled, once the product is taken out, the mold is closed again and the injection process is started. At this time, the hydraulic cylinder (injection cylinder) of the injection device applies a force to the screw, and under high pressure, the screw becomes a shot rod, and the front end thereof is The melt is injected into the mold from the nozzle.

The screw injection device is composed of a screw, a barrel, a nozzle and a driving device. The screw for injection is generally divided into three parts: feeding, compression, and metering, the compression ratio is 2~3, and the aspect ratio is 16~18.

When the melt is ejected from the nozzle, a part of the melt will flow back to the rear through the screw groove of the screw due to the reaction force on the pressurized melt being afraid of the reaction force. To prevent this, a check valve is attached to the end of the screw. For hard polyvinyl chloride, a conical screw head is used.

The barrel is part of the loading screw and it is made of heat resistant. Made of high pressure resistant steel. An array of electric heating rings is installed on the periphery of the barrel to heat the contents of the barrel. The temperature is controlled by a thermocouple to give the plastic a suitable temperature.

The nozzle is the transition between the barrel and the mold, which is fitted with a separate heating coil because it is an important part of the plastic melt directly. In general, injection molding uses open nozzles. For low viscosity polyamines, needle valve nozzles are used.

The rotation of the drive screw can be achieved by an electric motor or a hydraulic motor, and the reciprocating motion of the screw is achieved by means of hydraulic pressure.

The parameters of the injection molding machine are characterized by the injection device: the injection amount refers to the maximum amount of injection molding machine injected into the mold each time, which can be expressed by the mass of the injected polystyrene melt, or by the volume of the injected melt;

The injection pressure refers to the pressure applied to the cross section of the barrel at the time of injection; the injection speed refers to the moving speed of the screw at the time of injection.

Molding device

In addition to completing the opening and closing action of the mold, the main function of the mold clamping device is to withstand a high pressure of the melt injected into the mold with sufficient force to lock the mold and prevent it from opening.

Whether the mold clamping mechanism is mechanical or hydraulic or hydraulic mechanical, the mold opening and closing should be flexible, punctual, fast and safe.

From the technical requirements, the opening and closing molds must have a buffering effect. The running speed of the template should be slower and slower when clamping the mold, and slower and slower when opening the mold. In order to prevent damage to the mold and parts.

The force applied to the mold during the forming process to keep the mold closed is called the clamping force, and its value should be greater than the product of the cavity pressure and the projected area of ​​the part (including the split runner). The average pressure in the cavity is generally between 20 and 45 MPa.

Since the clamping force force line reflects the size of the molding product of the injection molding machine, the maximum clamping force of the injection molding machine is used to indicate the specifications of the injection molding machine, but there is also a general proportional relationship between the clamping force and the injection amount.

However, the mold clamping force representation does not directly reflect the volume of the injected product, and it is not convenient to use. Many manufacturers in the world use the clamping force/equivalent injection volume to indicate the specifications of the injection molding machine, for the injection volume, for different machines. There is a common comparison standard, the theoretical injection volume when the injection pressure is repeated 100Mpa, that is, the equivalent injection volume = theoretical injection volume * rated injection pressure / 100Mpa.

Control System

The hydraulic control system of the injection molding machine is mainly divided into a conventional hydraulic control system, a servo control system and a proportional control system.

Due to the complexity of the hydraulic system, the proportional valve oil passage system is taken as an example to illustrate the outline. The characteristics of this system are: there are control flow and pressure fear proportional elements in the oil circuit system (electromagnetic proportional flow valve or electromagnetic proportional flow reversing valve, electromagnetic proportional pressure valve).

The proportional flow of the given electric power and the proportional force of the magnetic force are used to control the opening amount of the valve core or the spring force of the valve core to control the flow rate or pressure of the system, thereby achieving the injection speed, the screw speed, the opening and closing speed and the injection pressure. Holding pressure. Screw torque. Injection seat thrust ejection force. The mold protection pressure is single-stage, multi-level or stepless.