What are the principles, characteristics, applications and processes of injection molding?

1. Injection molding principle

Granular or powdery plastic is added to the hopper of the injection machine. The plastic is heated and melted in the injection machine to keep it flowing. Then it is injected into the closed mold under a certain pressure. After cooling and shaping, the molten plastic solidifies into Required plastic parts.

2. Injection molding characteristics

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 are difficult to achieve with other plastic molding methods. Secondly, injection molding is very important in the production process. It is easy to realize automation. Operation processes such as injection, demoulding, and gate removal can all be automated, so injection molding has been widely used.

2.1 Advantages:

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

2.2 Disadvantages:

The price of injection molding equipment is high; the injection mold structure 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 (fluoroplastics), almost all thermoplastics can be produced by injection molding. Injection molding is not only used for the molding of thermoplastics, but has also been successfully used for the molding of thermosetting plastics.

At present, 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 made of different materials inside and outside, injection compression molding of optically transparent plastic parts, etc.

4. Injection molding process

4.1 Preparation before molding

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

Plastic preheating and drying: remove excess moisture and volatile matter in the material to prevent surface defects or degradation of the plastic parts after molding, which will affect the appearance and intrinsic quality of the plastic parts. Material drying method: For small batch production, oven drying is used; for large batch production, boiling drying or vacuum drying is used.

Barrel cleaning: The barrel needs to be cleaned when changing products, changing raw materials and colors.

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 parts.

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

4.2 Injection process

Adding materials: Add granular or powdered plastic to the hopper of the injection machine.

Plasticizing: Heating by the heating device of the injection machine melts the plastic raw material in the screw and becomes a plastic melt with good plasticity.

Mold filling: The plasticized plastic melt, driven by the plunger or screw of the injection machine, enters and fills the mold cavity through the nozzle and the pouring system of the mold at a certain pressure and speed.

Pressure-maintaining feeding: After the melt fills the mold cavity, the melt still maintains pressure for feeding under the push of the plunger or screw of the injection machine, so that the melt in the barrel continues to enter the mold cavity to supplement the plastic in the cavity. Shrinkage is required and prevents melt backflow.

Cooling after the gate freezes: After a period of time, the molten plastic in the cavity solidifies into a solid, ensuring that the plastic part has sufficient stiffness when demoulding and will not warp or deform.

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

4.3 Post-processing of plastic parts

Reasons and functions of post-processing:

Due to uneven plasticization or uneven crystallization, orientation and cooling of plastic in the mold cavity; or due to the influence of metal inserts or improper secondary processing of plastic parts, there will inevitably be some internal stress inside the plastic part. , resulting in deformation or cracking of plastic parts during use, therefore, efforts should be made to eliminate them.

Annealing treatment: A heat treatment process in which plastic parts are left to stand for a period of time in a fixed-temperature heated liquid medium (such as hot water, hot oil, liquid paraffin, etc.) or a hot air circulation oven, 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: Depending on the type of plastic and the thickness of the plastic part, it can generally be calculated as about half an hour per millimeter.

c) Function: Eliminate the internal stress of plastic parts, stabilize the size of plastic parts, increase crystallinity, stabilize the crystal structure, thereby increasing its elastic modulus and hardness.

Humidity conditioning treatment: A post-processing method that places the newly demolded plastic parts into a heating medium (such as boiling water, potassium acetate solution) to speed up the moisture absorption equilibrium. (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 deformation temperature is high, otherwise the lower limit is taken).

b) Time: The holding time is related to the thickness of the plastic part, usually 2 to 9 hours.

c) Purpose: eliminate residual stress; make the product reach moisture absorption balance as soon as possible to prevent dimensional changes during use.

5. Process parameters of injection molding service

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 to 20C higher than that of the screw barrel.

Plastic characteristics: Heat-sensitive plastics such as polyformaldehyde, polyvinyl fluoride, etc. must strictly control the maximum temperature of the barrel and the residence time in the barrel; thermoplastics with increased glass fiber must appropriately increase the barrel temperature due to poor fluidity. In order to prevent early hardening of the melt in the barrel of thermosetting plastics, the barrel temperature tends to take a small value.

Plastic parts and mold structure: The barrel temperature of thin-walled parts is higher than that of thick-walled parts; the barrel temperature of parts with complex shapes or with inserts should also be higher.

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

For screw injection machines, in order to prevent thermal degradation of plastic due to 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 made slightly lower than the middle section. To determine whether the temperature of the barrel is appropriate, you can use the air injection method to observe or directly observe the quality of the plastic parts.

When injecting into the air, if the material flow is uniform, smooth, bubble-free, and uniform in color, it means the material temperature is appropriate; if the material flow is rough, silvery or discolored, it means the material temperature is inappropriate.

b) Nozzle temperature

Generally, it is slightly lower than the maximum temperature of the barrel to prevent the molten material from salivating at the nozzle. But it cannot be too low, otherwise the molten material will prematurely solidify at the nozzle and block the nozzle, or premature solidification material will be injected into the mold cavity and affect the quality of the plastic part.

c) Mold temperature

The temperature of the mold depends on 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 and productivity .

The mold temperature is usually controlled by the introduction of a constant-temperature cooling medium; it is also possible 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. The mold is heated to maintain a constant temperature of the mold. But no matter what, for plastic melt, it is a cooling process.

5.2 Pressure

(1) Plasticizing pressure (back pressure): refers to the pressure on the melt at the top of the screw when the screw rotates and retreats when using a screw injection machine.

The plasticizing pressure increases, the temperature and uniformity of the melt increase, the colorants are mixed evenly and the gas in the melt is discharged. However, the plasticizing rate is reduced and the molding cycle is prolonged.

In general operation, on the premise of ensuring the quality of plastic parts, the plasticizing pressure should be as low as possible, usually around 6MPa, and usually rarely exceeds 20MPa.

(2) Injection pressure: refers to the pressure exerted by the top of the plunger or screw on the plastic melt.

Function: During injection, it overcomes the flow resistance during the melt flow and mold filling process, so that the melt has a certain mold filling rate; when maintaining pressure, it compacts the melt and prevents backflow.

Size: Depends on the type of injection machine, type of plastic, mold structure, mold temperature, wall thickness of the plastic part and the structure and size of the pouring system, etc.

In general: the injection pressure of plastics with high viscosity > plastics with low viscosity; the injection pressure of thin-walled, large-area, complex-shaped plastic parts is high; the mold structure is simple, the gate size is large, and the injection pressure is low; plunger injection machine Injection pressure > screw injection machine; the barrel temperature and mold temperature are high, and the injection pressure is low.

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, correctly select the molding method, determine the molding process and molding process conditions, rationally design the plastic mold and the selection of molding equipment, etc., to ensure the smooth progress of the molding process and ensure that the plastic parts meet the requirements. This series of work is usually called the development of process procedures for 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 determines the structure of the mold, and has a great impact on whether the plastic part can be formed smoothly and the quality after molding.

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

6.1.1 Analysis of plastics

(1) Analysis of plastic performance

(2) Analysis of plastic process performance

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

(1) Whether the plastic part structure meets the molding process requirements

(2) Plastic part dimensions, tolerances and technical standards

6.2 Determination of plastic part molding method and process flow

Based on the characteristics of plastics, the requirements of plastic parts, the structure, size, production batch, usage conditions and molding equipment of plastic parts, a series of practical molding solutions are proposed. Through comparative analysis of various plans, the best molding method for plastic parts is determined based on the actual production conditions on site. After the plastic part molding method 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. Therefore, it is necessary to conduct a comprehensive analysis based on the characteristics of the plastic and the actual situation, select the more reasonable process conditions, and then test them. During the molding process, the process conditions are gradually corrected based on the actual molding conditions of the plastic parts and the inspection results of the plastic parts.

6.4 Selection of equipment and tools

After the molding method is determined, 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 molding equipment, other processes also need to select corresponding equipment, and indicate the specifications, models and technical parameters of the equipment used according to the process.

6.5 Formulation of process documents

The preparation of process documents is to summarize the contents and parameters of the above-mentioned process procedures and determine them in the form of appropriate process documents as the basis for production preparation and production process. The plastic parts process card is the most important process document in production.