What is the principle, characteristics, application and process of injection molding by injection molding machine?

Today let us talk about the principle, characteristics, application and process of injection molding by เครื่องฉีดขึ้นรูป.

1. The principle of การฉีดขึ้นรูป. Add granular or powdered plastic into the hopper of the injection machine. The plastic is heated and melted in the injection machine to keep it flowing, and then injected into a closed mold under a certain pressure. After cooling and shaping, it melts The plastic is solidified to become the required plastic part. ถึง

2. Characteristics of injection molding
Injection molding has a short production cycle and high productivity. Injection molding can produce plastic parts with complex shapes, high size requirements and various inserts, which is difficult to achieve by other plastic molding methods; secondly, injection molding is in the production process It is easy to realize automation, such as injection, demolding, gate removal and other operations can be automated, so injection molding has been widely used.

2.1 Advantages:
The molding cycle is short, the production efficiency is high, and it is easy to realize automation. It can form complex shapes, accurate dimensions, plastic parts with metal or non-metal inserts, stable product quality, and wide application range.

2.2 Disadvantages:
The price of injection molding equipment is relatively high; the structure of injection molds is complex; the production cost is high, the production cycle is long, and it is not suitable for the production of single-piece and small-batch plastic parts.
3. Application Except for a few thermoplastics (ฟลูออโรเรซิ่น), almost all thermoplastics can be used to produce plastic parts by injection molding. Injection molding is not only used for the molding of thermoplastics, but also has been successfully used for the molding of thermoset plastics. ในปัจจุบัน, its molded products account for 20-30% of all current plastic products. In order to further expand the scope of injection molded plastic parts, some special injection technologies have been developed specifically for molding plastic parts with special properties or special structural requirements. Such as precision injection of high-precision plastic parts, multi-color injection of composite color plastic parts, sandwich injection of sandwich plastic parts composed of different materials inside and outside, and injection compression molding of optically transparent plastic parts. ถึง

4. Injection molding process

4.1 Preparation before molding

Raw material appearance inspection and process performance measurement: including the inspection of plastic color, particle size and uniformity, fluidity (ดัชนีละลาย, viscosity), thermal stability and shrinkage rate. ถึง

Plastic preheating and drying: remove excessive moisture and volatiles in the material to prevent defects or degradation on the surface of the plastic part after molding, which will affect the appearance and internal quality of the plastic part. Material drying method: small batch production, using oven drying; mass production, using boiling drying or vacuum drying.
Barrel cleaning: The barrel needs to be cleaned when changing products, changing materials and colors. ถึง

Insert preheating: reduce the temperature difference between the material and the insert, reduce the shrinkage stress of the plastic around the insert, and ensure the quality of the plastic. ถึง

Selection of release agent: Commonly used release agents include zinc stearate, liquid paraffin and silicone oil. ถึง

4.2 Injection process

Feeding: Add granular or powdered plastic into the hopper of the injection machine. ถึง

Plasticization: Through the heating of the heating device of the injection machine, the plastic raw material in the screw is melted and becomes a plastic melt with good plasticity. ถึง

Mold filling: The plasticized plastic melt is pushed by the plunger or screw of the injection machine to enter and fill the mold cavity through the nozzle and the pouring system of the mold at a certain pressure and speed. ถึง

Pressure maintaining and feeding: after the melt fills the cavity, under the plunger or screw of the injection machine, the melt still maintains the pressure for feeding, so that the melt in the barrel continues to enter the cavity to supplement the plastic in the cavity Shrinkage is required and can prevent the melt from flowing back.
To Cooling after the gate is frozen: After a period of time, the molten plastic in the cavity is solidified into a solid to ensure that the plastic part has sufficient rigidity when it is demolded and does not cause warpage or deformation. ถึง

Demoulding: The plastic part is cooled to a certain temperature, and the ejection mechanism pushes the plastic part out of the mold. ถึง

4.3 Post-processing of plastic parts

Reasons and effects of post-processing:
Due to uneven plasticization or uneven crystallization, orientation and cooling of the plastic in the cavity; or due to the influence of metal inserts or improper secondary processing of the plastic parts, some internal stresses inevitably exist in the plastic parts , Resulting in deformation or cracking of plastic parts during use, so we should try to eliminate them. ถึง

Annealing treatment: a heat treatment process in which plastic parts are placed in a constant temperature heating liquid medium (such as hot water, hot oil, liquid paraffin, ฯลฯ) or hot air circulation oven for a period of time, and then slowly cooled to room temperature.
a) Temperature: 10°~15° higher than the operating temperature or 10°~20° lower than the heat distortion temperature.
b) Time: โดยทั่วไป, it can be calculated at about half an hour per millimeter related to the type of plastic and the thickness of the plastic part. ถึง
c) Function: Eliminate the internal stress of the plastic part, stabilize the size of the plastic part, increase the crystallinity, stabilize the crystalline structure, thereby improving its elastic modulus and hardness.Humidity conditioning treatment: a post-treatment method that puts the newly demolded plastic parts into a heating medium (such as boiling water, potassium acetate solution) to speed up the moisture absorption balance. (Mainly used for plastics that are highly hygroscopic and easily oxidized, such as PA)

a) Temperature: 100~121℃ (the upper limit is taken when the heat distortion temperature is high, and the lower limit is taken vice versa).
b) Time: The holding time is related to the thickness of the plastic part, usually 2~9h. ถึง
c) Purpose: Eliminate residual stress; make the product reach moisture absorption balance as soon as possible to prevent dimensional changes during use. ถึง

5. Process parameters of injection molding

5.1 Temperature

a) Barrel temperature

The barrel temperature should be between the viscous flow temperature (or melting point) and the thermal decomposition temperature. The temperature of the plunger barrel is 10-20°C higher than the temperature of the screw barrel. ถึง

Plastic characteristics: heat-sensitive plastics such as polyoxymethylene, polyvinyl fluoride, ฯลฯ. must strictly control the maximum temperature of the barrel and the residence time in the barrel; the thermoplastic plastic with glass fiber should increase the barrel temperature due to poor fluidity. In order to prevent the melt from being hardened early in the barrel, the temperature of the barrel tends to take a small value.
Plastic parts and mold structure: For thin-walled parts, the barrel temperature is higher than thick-walled parts; for parts with complex shapes or with inserts, the barrel temperature should also be higher. ถึง

The temperature distribution of the barrel generally follows the principle of high front and low back, นั่นคือ, the temperature of the rear section of the barrel (feeding port) is the lowest and the nozzle temperature is the highest.

For the screw injection machine, in order to prevent thermal degradation of the plastic due to the shear friction heat between the screw and the melt, the melt and the melt, and the melt and the barrel, the temperature of the front section of the barrel can be slightly lower than the middle section. To judge whether the temperature of the barrel is appropriate, the air injection method can be used to observe or directly observe the quality of the plastic parts.

During air injection, if the material flow is uniform, smooth, bubble-free, and uniform in color, the material temperature is appropriate; if the material flow is rough, silver wire or discoloration, it means that the material temperature is inappropriate. ถึง

b) Nozzle temperature

โดยทั่วไป, it is slightly lower than the maximum temperature of the barrel to prevent the molten material from drooling at the nozzle. But it should not be too low, otherwise the melt will have premature solidification at the nozzle and block the nozzle, or premature solidification will be injected into the mold cavity and affect the quality of the plastic parts. ถึง

c) ในกรณีการเติมแม่พิมพ์ที่น่าพอใจ

The mold temperature is determined by the characteristics of the plastic, the size and structure of the plastic part, performance requirements and other process conditions. Mold temperature ↑, fluidity ↑, density and crystallinity ↑, shrinkage rate and productivity ↓.
The mold temperature is usually controlled by a constant temperature cooling medium; there is also a way to maintain a certain temperature by injecting the molten material into the mold to achieve a balance between natural heating and natural heat dissipation; in special cases, resistance wires and resistance heating rods can also be used to maintain a certain temperature. The mold is heated to keep the mold at a constant temperature. But no matter what, for the plastic melt, it is a cooling process. ถึง

5.2 Pressure

(1) Plasticizing pressure (back pressure): refers to the pressure of the melt on the top of the screw when the screw retreats when the screw injection machine is used. ถึง

The plasticizing pressure increases, the temperature of the melt and its uniformity are improved, the color materials are mixed uniformly, and the gas in the melt is discharged. But the plasticization rate is reduced, and the molding cycle is prolonged. ถึง

In general operation, under the premise of ensuring the quality of plastic parts, the plasticizing pressure should be as low as possible, generally about 6MPa, and usually rarely more than 20MPa
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(2) แรงดันฉีด: refers to the pressure exerted by the plunger or the top of the screw on the plastic melt. ถึง

Function: Overcome the flow resistance of the melt flow during the filling process during injection, so that the melt has a certain filling rate; when the pressure is maintained, the melt is compacted and prevented from flowing backward. ถึง

Size: It depends on the type of injection machine, the type of plastic, the mold structure, the mold temperature, the wall thickness of the plastic part, and the structure and size of the pouring system. Under normal circumstances: injection pressure of high-viscosity plastic> low-viscosity plastic; high injection pressure for thin-walled, large-area, and complex-shaped plastic parts; simple mold structure, larger gate size, and lower injection pressure; plunger injection machine Injection pressure> screw injection machine; the barrel temperature and mold temperature are high, and the injection pressure is low. ถึง

5.3 Time

The time required to complete an injection molding process is called the injection molding cycle.

6. Formulation of plastic molding process regulations According to the use requirements of plastic parts and the process characteristics of plastics, the correct molding method is selected, the molding process and molding process conditions are determined, and the selection of plastic molds and molding equipment are rationally designed to ensure the smooth molding process Carrying out this series of work for the plastic parts to meet the requirements is usually called the formulation of the process specifications for the plastic parts.
It is a guiding technical document in plastic molding production and an important basis for organizing production. It runs through all stages of the production process and must be strictly implemented.6.1 Analysis of plastic parts

The shape and structure of the plastic part determine the structure of the mold, and it has a great influence on whether the plastic part can be formed smoothly and the quality after forming.

In order to ensure the quality of plastic parts, usually the following points need to be paid attention to:

6.1.1 Analysis of plastics

(1) Analysis of the performance of plastics
(2) Analysis of plastic process performance

6.1.2 Analysis of the structure, dimensions, tolerances, and technical standards of plastic parts

(1) Does the structure of the plastic part meet the requirements of molding processability?
(2) Dimensions, tolerances and technical standards of plastic parts

6.2 Determination of the molding method and process flow of plastic parts

According to the characteristics of the plastic, the requirements of the plastic parts, and the structure, ขนาด, production batch, use conditions and molding equipment of the plastic parts, a series of feasible molding programs are proposed. Through the comparative analysis of each plan, the best molding method of plastic parts is determined according to the actual production conditions on site. After the molding method of plastic parts is determined, the process flow should be determined.
6.3 Determination of molding process conditions

Appropriate process conditions should be selected for qualified plastic parts formed by various molding methods. There are many factors that affect the plastic molding process, and there are many process conditions that need to be controlled, and the relationship between the process conditions is very close. ดังนั้น, a comprehensive analysis must be made according to the characteristics and actual conditions of the plastic, and the more reasonable process conditions must be selected initially, and then tested During the molding process, the process conditions are gradually revised according to the actual situation of the molding of the plastic parts and the inspection results of the plastic parts. ถึง

6.4 Selection of equipment and tools

When the molding method is determined, the appropriate molding equipment must be selected, and the relevant process and installation parameters of the equipment and mold must be checked. Different molding methods use different molding equipment. In addition to the molding equipment, other processes also need to select the corresponding equipment, and indicate the specifications and technical parameters of the equipment used in accordance with the process. ถึง

6.5 Formulation of process documents

The preparation of process documents is to summarize the content and parameters of the above process regulations and determine them in the form of appropriate process documents as the basis for production preparation and production process. Plastic parts process card is the most important process document in production.