I、Types of plastic molding:

A. injection molding

The plastic material is first heated and melted in the heated barrel of the injection molding machine, and then the reciprocating screw pushes the melt into the mold cavity of the closed mold. It not only can produce high precision and high quality products with high productivity, but also can process many kinds of plastics, large output (about 1/3 of the total amount of plastics) and wide application, therefore, injection molding is one of the important molding methods in plastic processing.

B. Extrusion molding

Extrusion is a method of continuous molding of plastics in a flowing state through an extruder by heating and pressurizing. It is generally used for sheet. Pipe. Monofilament. Flat wire. Film. Wire and cable wrapping, etc. Wide range of applications. High output. Therefore, it is one of the important molding methods in plastic processing.

C. foam molding

It is the method of adding appropriate foaming agent to the foaming material to produce porous or bubble products. The foamed products have small relative density, high specific strength, low raw material consumption and sound insulation, heat insulation, etc. The foaming materials are PVC, PE and PS. Products include: films, sheets, pipes, and profiles, etc. Foaming can be divided into chemical foaming and physical foaming.

D. Blow Molding

Blow (expanded film) molding (or hollow blow molding) is a molding method in which hot thermoplastic blanks or sheets in a closed mold are blown into hollow products with the help of fluid (compressed air) pressure. Plastic containers produced by this method. Such as a variety of bottles, square, round or flat barrels, gasoline tanks, etc. have been widely used, newly developed a variety of industrial parts and daily-use products, such as double-walled box-shaped products, L-ring-shaped large round barrels. Palletizing board. Surfboard. Seat backs and desks, as well as the front damper for automobiles. Belt cover. Instrument panel. Air conditioning ventilation ducts, etc., have been applied in practice, and the processed materials have developed from being daily-use plastics to engineering plastics. Now blow molding method has become one of the important molding methods in plastic processing.

However, the basic steps of the blow molding process are

Melt the material.

Forming the molten resin into a tube or blanks.

Blow molding the hollow blanks into the mold to melt and seal.

Blowing the blanks in the mold.

Cooling the blow molded product.

Remove the product from the mold.

Trimming.

E. Injection Blow Molding

Injection blow molding is a blow molding method. The plastic is first made into a bottomed blank by injection molding, and then it is transferred to the blow molding mold and blown into a hollow product. This method can be produced for daily products. Cosmetic products. Pharmaceuticals. Packaging containers for food, etc. But its volume should not exceed 1 L. Commonly used plastics are polyethylene. Polystyrene and polyvinyl chloride, etc.

F. extrusion blow molding

Extrusion blow molding is a blow molding method. It is different from injection blow molding. It is manufactured by extrusion method for the shape blank.

G .Stretch blow molding

Stretch blow molding is a blow molding method. It is made by extrusion. Injection molding and other methods are used to make the blanks. Then the billet is heated to stretching temperature. It is stretched longitudinally by mechanical force from inside (e.g. mandrel) or outside (e.g. jig). At the same time or later by compressed air blowing and transverse stretching.

2、 the injection molding principle and injection molding process introduction

Injection molding (or injection molding) is a method in which the plastic is first heated and melted in the heated barrel of the injection molding machine, and then the plunger or reciprocating screw pushes and squeezes the melt into the cavity of the closed mold. It not only can produce high precision and high quality products with high productivity, but also can process a wide variety of plastics and a wide range of uses, so injection molding is one of the important molding methods in plastic processing.

A.The basic function of injection molding machine

Injection molding is realized by injection molding machine. The basic functions of an injection molding machine are: 1. to heat the plastic and bring it to a molten state; 2. to apply high pressure to the melt so that it is injected and fills the mold cavity.

B. Injection molding process/equipment

The injection molding operation of thermoplastics is generally composed of plasticizing. Mold filling. Compaction and cooling. The equipment used is composed of injection molding machine. The equipment used is composed of injection molding machine, injection mold and auxiliary equipment (such as material drying, etc.).

C. injection device

The injection device is mainly used in the injection molding machine process to achieve plasticizing. Metering. Injection and pressure-holding shrinkage and other functions. Screw injection device is the most used, it is the unification of the screw plasticizing and injection plunger into a screw made. In essence, it should be called a coaxial reciprocating rod injection device. When it is working, the plastic in the hopper falls into the heated barrel by its own weight, and through the rotation of the screw, the plastic moves forward along the screw groove, at which time the material is heated by the external heater of the heated barrel, and at the same time there is internal heat generated by shearing, and the temperature rises to a molten state. With the storage of material at the front of the heated barrel, the reaction force (back pressure) generated by these materials pushes the screw backward, and the limit switch is used to limit the amount of backwardness, and when the backwardness reaches a certain position, the screw stops rotating, thus determining (metering) the amount of injection at one time.

Once the material in the mold cools and the product is removed, the mold is closed again and the injection process begins. At this time, the hydraulic cylinder (injection cylinder) of the injection device applies force to the screw, which becomes the injection rod under high pressure and injects the melt at its front end from the nozzle into the mold.

The screw injection unit is composed of a screw, a barrel, a nozzle and a drive unit. The screw for injection is generally divided into three sections: charging, compression, and metering, with a compression ratio of 2 to 3 and a length-to-diameter ratio of 16 to 18. When the melt is injected from the nozzle, a portion of the melt will flow backward through the screw groove of the screw to the rear due to the fear of reaction force of the injection force on the pressurized melt. To prevent this phenomenon, a non-return valve is installed at the end of the screw. For rigid PVC, a conical screw head is used.

The barrel is the part that holds the screw, and it is made of heat-resistant. It is made of heat-resistant and high-pressure resistant steel. Several sets of electric heating rings are installed on the periphery of the barrel to heat the material inside the barrel, and thermocouples are used to control the temperature so that the plastic has the right temperature.

The nozzle is the transition part that connects the barrel and the mold, which is equipped with independent heating rings, because it is an important part that directly affects the molten plastic. General injection molding more open nozzle for low-viscosity poly you first amine. The needle valve nozzle is used.

Drive the rotation of the screw can be used electric motor or hydraulic motor, the reciprocating motion of the screw is achieved by hydraulic pressure.

Injection device to characterize the injection molding machine parameters are: injection volume is the injection machine each time the maximum amount of injection into the mold, the mass of injected polystyrene melt can be expressed, or the volume of injected melt; injection pressure is the pressure applied to the barrel cross-section during injection; injection speed refers to the speed of the screw movement during injection.

D. Clamping device

In addition to completing the opening and closing action of the mold, the main task of the mold closing device is to resist the high pressure of the melt injected into the mold with sufficient force to lock the mold. It does not allow it to open.

The mold closing mechanism, whether mechanical or hydraulic or hydro-mechanical, should ensure that the mold opening and closing is flexible, punctual, rapid and safe. From the process requirements, open and close the mold should have a buffer effect, the running speed of the template should be fast when closing the mold, and then slow when opening the mold. In order to prevent damage to the mold and parts.

The force applied to the mold to keep the mold closed during the molding process is called the closing force, and its value should be greater than the product of the cavity pressure and the projected area of the part (including the manifold). The average pressure in the mold cavity is generally between 20~45Mpa.

Since the clamping force reflects the size of the injection molding machine, the maximum clamping force of the injection molding machine is often used to indicate the specification of the injection molding machine, but there is also a rough proportional relationship between the clamping force and the injection volume. However, the clamping force method does not directly reflect the size of the injected product volume, and it is not convenient to use. For the injection volume, in order to have a common comparison standard for different machines, we specify the theoretical injection volume when the injection pressure is 100Mpa, i.e. equivalent injection volume = theoretical injection volume * rated injection pressure / 100Mpa.

E control system

The hydraulic control system of injection molding machine is mainly divided into conventional hydraulic control system, servo control system and proportional control system. Due to the complexity of the hydraulic system, the proportional valve oil circuit system is used here as an example to illustrate the stems. This system is characterized by: in the oil circuit system there are control flow of and pressure fear proportional elements (electromagnetic proportional flow valve or electromagnetic proportional flow reversing valve, electromagnetic proportional pressure valve).

Through the simulation signal of the outside given electricity and the proportional role of magnetic force to control the amount of spool opening or spool spring force on the system flow or pressure control, so as to achieve the injection speed, screw speed, opening and closing speed and injection pressure. Holding pressure. Screw torque. Injection seat thrust. Ejecting force. Mold protection pressure is controlled in single stage, multi-stage or stepless control.