Precision injection molding refers to an injection molding method in which precision is very high, it is difficult to achieve required injection using a general-purpose injection machine and a conventional injection process compared with conventional injection molding. With rapid development of polymer materials, engineering materials occupy a certain position in industrial production. Because of its light weight, resource saving and energy conservation, many industrial product components have been replaced by engineering plastic parts. Precision molded plastics are widely used in industrial products such as instrumentation, electrical and electronic, aerospace, communications, computers, automobiles, video recorders, and watches.
Plastic products should replace high-precision metal parts. Conventional injection molding products are difficult to handle. Because of higher requirements on dimensional accuracy, working stability and residual stress of precision molded plastics, concept of precision plastic injection molding has emerged.

Concept of precision injection molding includes two aspects: one refers to geometric precision, that is dimensional accuracy and shape accuracy of product; second refers to mechanical precision, which refers to requirements according to actual situation in addition to geometric accuracy such as surface smoothness, transparency, stiffness, mechanical strength, internal stress. Precision of plastic products cannot be simply equated with precision of metal parts replaced. This is because plastic products, raw materials, even molding methods have many essential differences from metal products. Precision of precision injection molding products is also improved with ordinary injection molding. Difference is that precision injection molding has its own precision standards.
Precision of precision molded plastics must be stipulated reasonably. High precision of products will lead to difficulties in manufacture of molds and equipment, increase cost; if accuracy regulations are too low, can’t meet requirements. At present, there is no uniform standard for size limit and precision grade of precision molded plastics in the world. German standards are shown in Table 1. It reflects actual situation of production of precision molded plastics, taking into account precision of plastic parts and production costs, and is more convenient to use.
Table 1 German DIN 16901 precision molded plastics dimensional tolerance mm

Precision injection molding requires products with high dimensional accuracy, low warpage, excellent transferability, and excellent optical properties. Injection molding includes plasticizing, injection, pressure holding, cooling and other processes, so there are many factors affecting quality and dimensional accuracy of precision molded plastics, such as material selection, injection mold design and manufacturing, injection machine, moulding process, operator level, and management of production, etc. Main factors are molding shrinkage, injection mold design and injection equipment.

 

Tolerances and geometrical accuracy required for precision molded plastics are not met by any molding material. For precision injection molding materials, high-molecular materials with high mechanical strength, dimensional stability, good creep resistance, and wide environmental applicability should be selected. In addition to requiring them to have good flow properties and formability, it is also required that articles formed therefrom have stability in shape and size.
Molding shrinkage is one of important reasons that affect precision of precision molded plastics. Due to shrinkage and expansion often occur in plastic moulding process, actual shrinkage rate of each batch of raw materials fluctuates within a certain range, so that in most cases, actual shrinkage rate of raw materials in moulding process is different from shrinkage ratio selected when designing mold. Even if same mold, shrinkage rate of melt flow direction is different from shrinkage rate perpendicular to flow direction of melt. In order to improve precision of plastic parts, raw material manufacturers should strive to reduce shrinkage range of various materials. Main types of shrinkage include heat shrinkage, phase change shrinkage, orientation shrinkage, and compression shrinkage.
Heat shrinkage refers to polymer melt at high temperatures and shrinkage of mold cavity during heating or cooling. It is thermophysical property inherent in molding materials. If mold temperature is too high, shrinkage of product will increase, so temperature of precision injection molding mold should not be too high.
Phase change shrinkage refers to shrinkage caused by a decrease in heat capacity when crystalline resin is crystallized during forming process. Mold temperature is high, crystallinity is high, shrinkage rate is large, product density is increased, coefficient of linear expansion is decreased, and shrinkage rate is lowered. Therefore, actual shrinkage rate is determined by combined effect of these two.
Orientation shrinkage means that molecular chain is oriented in flow direction when molding material is melt-filled, and macromolecule is re-curled upon cooling to restore shrinkage in orientation direction. Degree of molecular orientation is related to injection pressure, injection speed, resin temperature, and mold temperature, but mainly injection speed. The faster filling rate, the stronger molecular orientation.
Compression shrinkage refers to shrinkage caused by a significant change in specific heat capacity of a plastic under high pressure. After pressure is removed, there is a certain amount of recovery from specific heat capacity, called elastic recovery. When temperature is constant, pressure is increased, specific heat capacity of molded article is reduced, density is increased, expansion coefficient is decreased, and shrinkage rate is remarkably lowered. When article is removed from mold, volume of article is elastically restored to reduce shrinkage of article.

Ordinary injection molding involves four aspects of "material, art, machine, mold", and precision injection forming in precision injection molding mold is more important.
A prerequisite for ensuring accuracy of product is that shrinkage of raw material itself is small, precision that plastic part can ultimately achieve is also related to mold. If precision of mold is high enough and process conditions are also well controlled, then shrinkage of product can be neglected, precision of product will only be controlled by precision of mold, so as to ensure a high reproduction accuracy of product. Therefore, quality of injection mold design and manufacturing determines quality and yield of product.
Only by ensuring precision of mold can shrinkage of product be effectively reduced and precision of product can be improved. To this end, mould manufacturing tolerances have been developed accordingly. Formulation of mold tolerance should be suitable, because machining tolerance of mold is too high, which increases production cost and increases difficulty of mould manufacturing; while machining tolerance is too low, it is difficult to ensure precision of plastic part. It is usually specified that machining tolerance of mold is about 1/3 of tolerance value of corresponding plastic part.

 

Precision injection molding process features high injection pressure, fast injection speed and high temperature control accuracy. In order to meet these process conditions, there are also high requirements for performance of injection molding machines. Precision injection molding equipment has its own unique features in terms of technical parameters, control accuracy, hydraulic system and structure.
Injection device is a key component of precision injection molding, and its main function is to provide plasticized uniform molten material for precision injection molding. At the same time, it is required that plasticizing part has strong plasticizing ability, high degree of homogenization, high injection speed, high injection pressure, large driving torque of screw, and stepless speed regulation. Precision injection molding machines generally require following conditions:
1 Technical parameters require high injection power, pressure and fast injection speed;
2 Control accuracy is high. Adopt multi-level feedback control, including position, speed, pressure, back pressure and screw speed multi-stage control to ensure accurate control of injection speed, injection pressure, holding pressure, back pressure, screw speed and other process parameters.
Molding conditions are stable, repeatability is high, and switch can be accurately performed at a determined stroke position. Precision control also includes PID control of barrel and nozzle temperature. Working oil temperature must be controlled by a closed-loop device with heating and cooling, precise control of mold temperature and clamping force, control of ambient temperature;
3 Hydraulic system requires fast reaction speed and high sensitivity of hydraulic components;
4 Precision injection molding machine structure requires high rigidity of clamping system, high efficiency of clamping mechanism, and strong plasticizing ability of plasticized parts.

Injection process is more complicated than other moulding processes due to its unsteady state. So far, a mathematical model that can describe whole process has not been found. Main process parameters such as pressure, temperature, speed and time are slightly changed, and quality of product changes accordingly. In order to resist influence of various internal and external disturbance factors and maintain set process parameters, precision injection requires injection machine to have good comprehensive control performance.
By strictly controlling parameters such as pressure, temperature, speed and time, it is ensured that a certain amount of uniform temperature is provided during plasticization. When filling mold, there is a stable flow state suitable for characteristics of cavity, and there is a suitable and stable state during pressure-holding cooling, thereby obtaining stable and high-quality precision products.
Choosing the best moulding process parameters can reduce shrinkage of plastic products. Shrinkage property of plastic refers to comprehensive reflection of heat shrinkage, elastic recovery, plastic deformation, post-shrinkage and aging shrinkage of plastics, usually caused by water absorption or molecular chain rearrangement of materials. Specific performance is change of linear shrinkage rate and volume shrinkage rate, which is usually expressed by shrinkage characteristic value.
Shrinkage rate of thermoplastic injection molded products fluctuates greatly, especially for crystalline plastic injection molded products, because crystallinity depends not only on chemical structure, but also affected by cooling parameters during processing (cooling rate, melt temperature, mold temperature), which makes it difficult to determine cavity size and control dimensional accuracy of mold.Therefore, it is urgent to understand influence of injection molding process parameters on shrinkage rate of various plastics.
Difference in wall thickness of part is generally considered to be caused by two factors: one is slight deformation of mold cavity caused by high pressure melt; the other is elastic expansion of material after mold is opened. In general, quality accuracy can control dimensional accuracy well, while viscosity of melt is lower at higher mold temperatures, so viscosity gradient is smaller. Under a certain screw back pressure, quality accuracy of workpiece can be accurately controlled.
For liquid crystal polymers (LCP), however, it requires a lower mold temperature because LCP cools rapidly after cooling. LCP has a low heat of fusion and an ordered structural state, so there is a small change between liquid crystalline state and solid crystalline state transition. When fully cooled, liquid-solid transition is almost instantaneous. In colder molds, when cavity is full, most of material and gates in cavity have solidified, so it is difficult to replenish melt during compression stage, size of part is very close to size of undeformed cavity.

 

As precision requirements for precision injection molding become higher and higher, quality management becomes more and more difficult. In order to maintain stable precision injection molding, on one hand, automatic monitoring system and automatic waste screening system are equipped on injection molding machine. It is found that actual operating parameters of machine exceed set value, injection molding machine will be displayed on the screen, and operator will be notified in time to to adjust immediately. If not processed in time, machine will automatically stop. At the same time, automated waste screening system automatically separates parts that do not meet process conditions, such as ARBURG's automatic flap structure.
Some companies are equipped with an automatic inspection system, which places parts on a precision balance (with an accuracy of ±0.11mg) by a robot, observes change of weight of part, judges qualified or not of part by comparing measured weight value with weight value of qualified part. Qualified parts are sent to packaging area, and unqualified parts are separated. This is one of the most advanced control methods in the world. However, with this method, it is not that manual inspection is not required. In order to ensure good quality of parts, operator should regularly inspect plastic parts produced, find abnormalities, and solve them in time. Plastic parts produced in this way will have a reliable quality guarantee.