Add granular or powdered plastic into hopper of injection machine. Plastic is heated and melted in injection machine to keep it flowing, then injected into a closed mold under a certain pressure. After cooling and shaping, molten plastic solidifies into desired plastic part.

 

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. This is difficult to achieve by other plastic molding methods; secondly, injection molding is easy to achieve automation in production process, such as injection, demolding, and gate removal, and other operations can be automated. Therefore, injection molding has been widely used.

Molding cycle is short, 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 a wide range of applications.

Price of injection molding equipment is relatively high; structure of injection mold is complicated; production cost is high, production cycle is long, it is not suitable for production of single and small batches of plastic parts.

Except for a few thermoplastics (fluoroplastics), almost all thermoplastics can be produced by injection molding. Injection molding is not only used for molding of thermoplastics, but also has been successfully used for molding of thermoset plastics.
At present, its molded products account for 20-30% of all current plastic products. In order to further expand 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, injection compression molding of optically transparent plastic parts.

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 excessive moisture and volatiles in material to prevent defects or degradation on the surface of plastic part after molding, which will affect appearance and internal quality of plastic part. Material drying method: small batch production, using oven drying; mass production, using boiling drying or vacuum drying.
Barrel cleaning: Barrel needs to be cleaned when changing products, changing materials and colors.
Insert preheating: reduce temperature difference between material and insert, reduce shrinkage stress of plastic around insert, and ensure quality of plastic.
Selection of release agent: Commonly used release agents include zinc stearate, liquid paraffin and silicone oil.

Feeding: Add granular or powdered plastic into hopper of injection machine.
Plasticization: Through heating of heating device of injection machine, plastic raw material in screw is melted and becomes a plastic melt with good plasticity.
Mold filling: Plasticized plastic melt is pushed by plunger or screw of injection machine to enter, fill mold cavity through nozzle and pouring system of mold at a certain pressure and speed.
Pressure maintaining and feeding: after melt fills cavity, under plunger or screw of injection machine, melt still maintains pressure for feeding, so that melt in barrel continues to enter cavity to supplement shrinkage of plastic in cavity and prevent melt from flowing back.
Cooling after gate is frozen: After a period of time, molten plastic in cavity is solidified into a solid to ensure that plastic part has sufficient rigidity when it is demolded, it will not warp or deform.
Demoulding: Plastic part is cooled to a certain temperature, and ejection mechanism pushes plastic part out of mold.

Reasons and effects of post-processing:
Due to uneven plasticization or uneven crystallization, orientation and cooling of plastic in cavity; influence of metal inserts or improper secondary processing of plastic parts, some internal stresses inevitably exist in plastic parts, which will cause plastic parts to deform or crack during use. Therefore, we should try to eliminate them.
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, etc.) or hot air circulation oven for a period of time, and then slowly cooled to room temperature.
a) Temperature: 10°~15° higher than operating temperature or 10°~20° lower than heat distortion temperature.
b) Time: It is related to type of plastic and thickness of plastic part. Generally, it can be calculated at about half an hour per millimeter.
c) Function: Eliminate internal stress of plastic part, stabilize size of plastic part, increase crystallinity, stabilize crystalline structure, thereby improving its elastic modulus and hardness.
Humidity conditioning treatment: a post-treatment method that puts newly demolded plastic parts into a heating medium (such as boiling water, potassium acetate solution) to speed up moisture absorption balance. (Mainly used for plastics that are highly hygroscopic and easily oxidized, such as PA)
a) Temperature: 100~121℃ (upper limit is taken when heat distortion temperature is high, and lower limit is taken vice versa).
b) Time: Holding time is related to thickness of plastic part, usually 2~9h.
c) Purpose: Eliminate residual stress; make product reach moisture absorption balance as soon as possible to prevent dimensional changes during use.

 

Barrel temperature should be between viscous flow temperature (or melting point) and thermal decomposition temperature. Temperature of plunger barrel is 10-20°C higher than temperature of screw barrel.
Plastic characteristics: Heat-sensitive plastics such as polyoxymethylene, polyvinyl fluoride, etc. must strictly control maximum temperature of barrel and residence time in barrel; thermoplastic plastic with glass fiber should increase barrel temperature due to poor fluidity. In order to prevent melt from curing early in barrel, temperature of barrel tends to take a small value.
Plastic parts and mold structure: For thin-walled parts, barrel temperature is higher than thick-walled parts; for parts with complex shapes or with inserts, barrel temperature should also be higher.
Temperature distribution of barrel generally follows principle of high front and low back, that is, temperature of rear section of barrel (feeding port) is the lowest, and nozzle temperature is the highest.
For screw injection machine, in order to prevent thermal degradation of plastic due to shear friction heat between screw and melt, melt and melt, melt and barrel, temperature of front section of barrel can be slightly lower than middle section. To judge whether temperature of barrel is appropriate, air injection method can be used to observe or directly observe quality of plastic parts.
During air injection, if material flow is uniform, smooth, bubble-free, and uniform in color, material temperature is appropriate; if material flow is rough, silver wire or discoloration, it means that material temperature is inappropriate.

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

Mold temperature is determined by characteristics of plastic, size and structure of plastic part, performance requirements and other process conditions. Mold temperature ↑, fluidity ↑, density and crystallinity ↑, shrinkage rate and productivity ↓.
Mold temperature is usually controlled by a constant temperature cooling medium; there is also a way to maintain a certain temperature by injecting molten material into 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. Mold is heated to keep mold at a constant temperature. But no matter what, for plastic melt, it is a cooling process.

(1) Plasticizing pressure (back pressure): refers to pressure of melt on the top of screw when screw retreats when screw injection machine is used.
Plasticizing pressure increases, temperature of melt and its uniformity are improved, color materials are mixed uniformly, gas in the melt is discharged. However, plasticization rate is reduced and molding cycle is prolonged.
In general operation, under premise of ensuring quality of plastic parts, plasticizing pressure should be as low as possible, generally about 6MPa, and usually less than 20MPa.
(2) Injection pressure: refers to pressure exerted by plunger or top of screw on plastic melt.
Function: During injection, it overcomes flow resistance in process of melt flow and mold filling, so that melt has a certain filling rate; when pressure is maintained, melt is compacted and prevents backflow.   
Size: It depends on type of injection machine, type of plastic, mold structure, mold temperature, wall thickness of plastic part, structure and size of gating system.
Under normal circumstances: injection pressure of high-viscosity plastic> low-viscosity plastic; thin-walled, large area, and complex shape plastic parts have high injection pressure; simple mold structure, larger gate size, lower injection pressure; plunger injection machine injection pressure> screw injection machine; barrel temperature and mold temperature are high, and injection pressure is low.

Time required to complete an injection molding process is called injection molding cycle.

According to use requirements of plastic parts and process characteristics of plastic, correct selection of molding method, determination of molding process and molding process conditions, rational design of plastic mold and selection of molding equipment, etc., ensure smooth progress of molding process to make plastic parts meet requirements. This series of work is usually called formulation of plastic parts process regulations.
It is a guiding technical document in plastic molding production and an important basis for organizing production. It runs through all stages of production process and must be strictly implemented.

 

Shape and structure of plastic part determine structure of mold, and it has a great influence on whether plastic part can be formed smoothly and quality after forming.
In order to ensure quality of plastic parts, usually following points need to be paid attention to:

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

(1) Whether structure of plastic part meets requirements of molding processability
(2) Dimensions, tolerances and technical standards of plastic parts

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

Appropriate process conditions should be selected for qualified plastic parts formed by various molding methods. There are many factors that affect plastic molding process, there are many process conditions that need to be controlled, relationship between process conditions is very close. Therefore, it is necessary to make a comprehensive analysis based on characteristics and actual conditions of plastics, initially select more reasonable process conditions, then gradually correct process conditions according to actual conditions of molding of plastic parts and inspection results of plastic parts during trial mold process.

When molding method is determined, appropriate molding equipment must be selected, relevant process and installation parameters of 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, specify specifications and technical parameters of equipment used in accordance with process.

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