Brief introduction of plastic molding types and in

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Briefly describe the types of plastic molding and the principle of injection molding

1 Types of plastic molding:

A injection molding: it is a method in which plastic materials are heated and melted in the heating barrel of the injection molding machine, and then the melt is pushed into the mold cavity of the closed mold by the reciprocating screw. It can not only produce high-precision and high-quality products under high productivity, but also has many kinds of plastics that can be processed, large output (about 1/3 of the total plastic) and wide applications. Therefore, injection molding is one of the important molding methods in plastic processing

b extrusion molding: extrusion is a method in which plastic is continuously molded through the die in a flowing state by heating and pressurizing in the extruder. It is generally used for plates. Pipe. Monofilament. Flat wire. film. The coating of wires and cables is formed, which is widely used. High output. Therefore, it is one of the important molding methods in plastic processing

c foaming molding: it refers to the addition method of adding appropriate foaming agent into foaming materials to produce porous or foam products. Foaming products have relatively low density, high specific strength, less amount of raw materials, sound insulation, heat insulation and other potential points. Foaming materials include PVC, PE and PS. Products include: film, plate, pipe, and profile, etc. Foaming can be divided into chemical foaming and physical foaming

e blow molding: Blow (film expansion) molding (or hollow blow molding) refers to a molding method that uses the pressure of fluid (compressed air) to blow the hot thermoplastic parison or sheet in the closed mold into hollow products. Plastic containers produced by this method. Such as all kinds of bottles, square, round or flat barrels, gasoline tanks, etc. have been widely used. Newly developed industrial parts and daily-use products, such as double wall box products, L-ring large barrels. Palletizing plate. Surf board. Seat backs and desks, as well as front baffles for cars. Belt cover. Dashboard. Air conditioning and ventilation pipes have been applied in practice, and the processed materials are now developing from daily-use plastics to engineering plastics. Now blow molding has become one of the important molding methods in plastic processing. But the basic steps of blow molding process are: 1 Molten material. 2. Form the molten resin into a tube or parison. 3. Melt and seal the hollow parison in the blow mold. 4. Blow the parison in the mold. 5. Cool blow molded products. 6. Take the product out of the mold. 7. Trimming

f injection blow molding: injection blow molding is a blow molding method. First, the plastic is made into a bottomed parison by injection molding, and then it is moved to a blow mold to blow into a hollow product. This method can be used to produce daily necessities. Cosmetics. Medicine. Packaging containers for food, etc. However, its volume should not exceed 1L. Commonly used plastics are polyethylene. Polystyrene and polyvinyl chloride, etc

g extrusion blow molding: extrusion blow molding is a blow molding method. Different from injection blow molding. Its parison is made by extrusion

h stretch blow molding: stretch blow molding is a blow molding method. It is extruded. The parison is made by injection molding and other methods. The parison is then heated to the drawing temperature. Longitudinal stretching is carried out under the action of internal (such as mandrel) or external (such as fixture) mechanical force. At the same time or later, it is expanded by compressed air and stretched horizontally

2. Injection molding principle and injection process introduction

injection molding (injection molding) is a method in which plastic is first heated and melted in the heating barrel of the injection molding machine, and then the melt is pushed into the cavity of the closed mold by the plunger or reciprocating screw. It can not only produce high-precision and high-quality products under high productivity, but also have many kinds of plastics that can be processed and widely used. Therefore, injection molding is one of the important molding methods in plastic processing

a basic functions of injection molding machine:

injection molding is realized by injection molding machine. The basic functions of injection molding machine are: 1. Heat the plastic to a molten state; 2。 High pressure is applied to the melt to make it shoot out and fill the mold cavity

b injection molding process/equipment

the injection molding operation of thermoplastic is generally by plastic refining. Mold filling. Composed of compaction and cooling. The equipment used is an injection molding machine. It is composed of injection mold and auxiliary equipment (such as material drying, etc.)

c injection device:

the injection device mainly realizes plastic refining in the process of injection molding machine. Measurement. Injection, pressure maintaining and feeding. Screw type injection device is most used. It is formed by unifying screw molding and injection plunger into one screw. In essence, it should be called coaxial reciprocating multiple rod injection device. When it is working, the plastic in the hopper falls into the heating barrel by its own weight. Through the rotation of the screw, the plastic moves forward along the screw groove. At this time, the material is heated by the heater outside the heating barrel, while there is heat generated by shearing inside, and the temperature rises to become a molten state. With the storage of materials at the front end of the heating barrel, the reaction force (back pressure) generated by these materials pushes the screw backward, and the limit switch is used to limit its retreat amount. When it retreats to a certain position, the screw stops rotating, so as to determine (measure) the injection amount at one time

after the material with the later occurrence of in mold cracking is cooled, once the product is taken out, the mold is closed again and the injection process is entered. At this time, the hydraulic cylinder (injection cylinder) of the injection device applies force to the screw. Under high pressure, the screw becomes a ejector rod, and the melt at the front end of the screw is injected into the mold from the nozzle

screw injection device is composed of screw, barrel, nozzle and driving device. The screw for injection is generally divided into three sections: feeding, compression and metering. The compression ratio is 2~3 and the length diameter ratio is 16~18. When the melt is ejected from the nozzle, because the injection force on the pressurized melt is afraid of reaction, part of the melt will flow back to the rear through the screw groove of the screw. To prevent this phenomenon, install a check valve at the end of the screw. For UPVC, conical screw heads are used

the barrel is the part containing the screw, which is made of heat-resistant. Made of high pressure resistant steel. An array of electric heating rings are installed on the periphery of the barrel to heat the materials in the barrel, and the temperature is controlled by thermocouples to make the plastic have a suitable temperature

the nozzle is the transition part connecting the barrel and the mold, which is equipped with an independent heating ring, because it is an important part that directly affects the melting state of plastic. Generally, the open nozzle is used for injection molding. For low viscosity polyimide, the small flat at the front represents amine. Needle valve nozzle is used

motor or hydraulic motor can be used to drive the rotation of the screw, and the reciprocating motion of the screw is realized by means of hydraulic pressure

the parameters that characterize the injection molding machine through the injection device are: the injection volume refers to the maximum amount injected into the mold by the injection molding machine each time, which can be expressed by the quality of the injected polystyrene melt or by the volume of the injected melt; Injection pressure refers to the pressure exerted on the cross section of the barrel during injection; Injection speed refers to the moving speed of the screw during injection

d clamping device

in addition to completing the opening and closing action of the mold, its main task is to resist the high pressure of the melt injected into the mold with sufficient force to lock the mold. Don't let it open

whether the mold closing mechanism is mechanical or hydraulic or hydraulic mechanical, it should ensure that the mold opening and closing is flexible, punctual, rapid and safe. In terms of process requirements, the opening and closing of the mold must have a buffer effect. The running speed of the template should be fast first and then slow when closing the mold, and slow first 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 clamping force, which should be greater than the product of the mold cavity pressure and the projected area of the workpiece (including the shunt channel). The average pressure in the mold cavity is generally between 20~45mpa

since the careful line of clamping force reflects the size of the molded product area of the injection molding machine, the maximum clamping force of the injection molding machine is often used to represent the specification of the injection molding machine, but there is also a roughly proportional relationship between the clamping force and the injection volume. However, the expression of clamping force cannot directly reflect the volume of injection products, which makes it inconvenient to use according to Yang Jun. Many manufacturers in the world should use clamping force/equivalent injection volume to express the specification of injection molding machine. For injection volume, in order to have a common comparison standard for different machines, it is specially stipulated that the theoretical injection volume when the injection pressure is more than 100MPa, that is, 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 taken as an example to illustrate the outline. The characteristics of this system are: in the oil circuit system, there are flow control and pressure proportional elements (electromagnetic proportional flow valve or electromagnetic proportional flow reversing valve, electromagnetic proportional pressure valve)

through the proportional action of the external given electrical simulation signal and magnetic force, the opening amount of the valve core or the spring force of the valve core can be controlled to control the system flow or pressure, so as to achieve the injection speed, screw speed, opening and closing speed and injection pressure. Holding pressure. Screw torque. Injection seat thrust. Jacking force. Mold protection pressure is controlled by single-stage, multi-stage or stepless control

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