Research progress of the hottest precision injecti

Research progress of precision injection molding technology

Abstract: This paper introduces the precision injection molding technology of polymer materials, and discusses the factors affecting precision injection molding, including mold accuracy, injection molding machine control accuracy, precision injection process control and some other computer control technologies. These factors are analyzed and discussed respectively, and the relevant solutions are given. Finally, some advanced molding technologies are introduced

key words: precision injection; Injection molding machine; Precision injection process; In recent years, with the rapid development of electronics, telecommunications, medical, automotive and other industries, the requirements for high precision and high performance of plastic products are increasing day by day, which promotes the continuous progress of precision molding technology and the emergence of new technologies. Precision injection molding requires that products not only have high dimensional accuracy, low warpage deformation, excellent transcribing, but also have excellent optical properties [1]. Injection molding is one of the most important plastic molding methods, including plastic plasticization, injection, pressure maintaining, cooling and other basic processes, so there are many factors affecting precision injection molding

1 precision mold

to manufacture precision products, precision mold is essential. The accuracy of the mold is generally affected by the temperature control of the mold, the precision manufacturing of the mold and the selection of plastic shrinkage in the mold design

1. 1 precise control of mold temperature

in the injection molding of general precision parts, the purpose of controlling mold temperature is mainly to improve productivity. However, mold temperature control has a great impact on precision injection molding [2], which affects the shrinkage, shape, crystallization, internal stress, etc. Therefore, the design of mold cold and hot circuit requires reasonable temperature distribution and accurate control accuracy, and it is best to use mold temperature machine and water cooler to control. The influence of mold temperature control and cooling time on product performance and its influencing factors are as follows [3]:

(1) plastic melt injection and mold opening cycle time will produce temperature difference fluctuations in the mold, so this fluctuation difference peak should be minimized

(2) heat transfer in the mold. The heat of the mold is transmitted through the mold and the media in the mold. The mold is constant, so the control of the media is the key. Generally, the temperature difference between the inlet and outlet of the cooling water is less than 1 ℃, and the average temperature theory is applied to calculate [4], which can ensure the stability of heat transfer. The media flow and heat transfer are interrelated. The mold surface temperature sensor can be used to control the media flow to compensate for the mold temperature changes caused by environmental temperature changes

(3) the stable state of the mold is to keep the heat absorbed and emitted by the mold in balance, and the melt injected by the injection molding machine must strictly maintain a stable temperature. The temperature around the mold also has a great impact on the mold, which generally affects the change of media convection coefficient in the mold. Therefore, it is best to set a temperature sensor on the mold surface to observe the mold surface temperature at any time and keep it stable

(4) change of heat exchange efficiency after the mold is used for a long time, rust spots and scale will appear in the cooling water pipe. At this time, the thermal conductivity of the interface will be reduced, and the scale should be removed in time

1. 2 design and manufacture of precision molds

the quality of mold design is the premise to ensure the dimensional accuracy of plastic parts. In order to improve the accuracy of precision injection mold, it is necessary to use computer-aided analysis in the design process, especially for the flow behavior of gating system and the analysis of heat distribution of mold temperature regulation system. The analysis and simulation software should be applied as much as possible and the near actual loading conditions should be used to analyze and simulate the unsteady processes such as pouring and cooling of the melt in the mold

the manufacturing quality of molds is the key to ensure the dimensional accuracy of plastic parts. The main manufacturing characteristics of precision molds are that there is no manual processing except polishing and assembly operations. Generally, the ratio of mechanical processing and manual processing of molds is about 6:4 ~ 7:3, while the ratio of mechanical processing and manual processing of precision molds is 9:1

mold materials and heat treatment are the guarantee of mold accuracy. During the molding process, the mold is in a high-temperature state. Therefore, even if the mold parts are tempered at low temperature, the high-temperature melt will repeatedly temper the parts during production, which will reduce the hardness. At the same time, it should also be considered that the contribution rate of bulk aluminum ore and its concentrate caused by residual austenite to the increase of import is 168%. Therefore, generally, the tempering temperature should be at least twice the mold temperature during forming processing, and low temperature treatment should be carried out appropriately to eliminate residual austenite

for high-precision products, it is required that the accuracy of mold forming parts, the cumulative accuracy of assembly parts to occupy market share, the centering accuracy of mold guide parts and the parallelism of parting surface should reach the micron level. The mold should have sufficient strength, rigidity and wear resistance, and it is not easy to deform and wear under the injection pressure. To achieve the above machining accuracy, new technologies such as three-dimensional machining centers, CNC machine tools and the application of CAD/cam/cae can be adopted

1. 3 selection of material shrinkage in precision mold design

the shrinkage of thermoplastic products fluctuates greatly during molding, which brings difficulties to mold design, determination of cavity size and control of product size accuracy [5]. Therefore, in the mold design, we should understand the shrinkage characteristics of plastic and the difference of shrinkage rate of various parts caused by the shape of the workpiece, and then take necessary compensation measures. For high-precision plastic parts, simple molds can be made before design to measure the actual shrinkage of each part during molding, which can greatly improve the dimensional accuracy of the parts [6]

1. 4 Design of precision mold gate

the type, size, position and quantity of gate all affect the dimensional accuracy of the workpiece. The injection force of the point gate is large, but the feeding effect is poor, which is not suitable for thick wall parts. The gate position affects the melt flow direction and the distance of the process. The longer the process, the greater the shrinkage. Multiple gates can shorten the process, but the weld lines increase. The design of the gate should be finally determined by using the analysis software according to the size of the parts and the selected materials, and with the help of practical experience

2 setting of the best process parameters of precision injection molding

selecting the best molding process parameters can reduce the shrinkage of plastic products. The shrinkage characteristic of plastics refers to the comprehensive reflection of thermal shrinkage, elastic recovery, plastic deformation, post shrinkage and aging shrinkage of plastics, which is usually caused by water absorption or molecular chain rearrangement of materials, and is specifically manifested in the changes of linear shrinkage and volume shrinkage, which are commonly expressed by shrinkage characteristic values. The shrinkage rate of thermoplastic injection molded products fluctuates greatly, especially for crystalline plastic injection molded products. Because the crystallinity not only depends on the chemical structure, but also is affected by the cooling parameters (cooling rate, melt temperature, mold temperature, product thickness) in the processing process, it is difficult to determine the cavity size and control the dimensional accuracy of products in the mold design, Therefore, it is urgent to understand the influence of injection molding process parameters on the shrinkage of various plastics [7]. The difference of the wall thickness of the workpiece is generally considered to be caused by two factors:

first, the slight deformation of the mold cavity caused by the high-pressure melt; The second is the elastic expansion of the material after the mold is opened. Generally speaking, the quality accuracy can well control the dimensional accuracy, while the viscosity of the melt is low at higher mold temperature, so the viscosity gradient is small. Under a certain screw back pressure, the quality accuracy of the workpiece can be accurately controlled. However, for liquid crystal polymer (LCP), it just needs a lower mold temperature, because LCP will quickly cool and set when it is cooled

lcp has low melting heat and ordered structural state, so there is a small change between liquid crystalline state and solid crystalline state. When fully cooled, the liquid-solid transition is almost completed in an instant. In colder molds, when the cavity is full, most of the materials and gates in the cavity have solidified, so it is difficult to supplement the melt in the compression stage, so the size of the workpiece is very close to the size of the undeformed cavity [8]

the shrinkage of amorphous plastics is lower than that of crystalline plastics, and the shrinkage of blends is lower than that of pure polymer materials. At the same time, the shrinkage of materials decreases with the increase of the amount of elastomer. After the plastics are reinforced or filled, the heat capacity decreases, the rigidity increases, and the shrinkage rate decreases significantly. The higher the content of fiber or filler, the smaller the shrinkage rate [9]. The accuracy of crystalline plastic products is also affected by pressure. Due to the effect of pressure induced crystallization, injection pressure (P1), holding pressure (P2) and feeding pressure (P3) can accelerate the crystallization process, so the crystallinity increases with the increase of P1, P2 and P3. For P1, its pressure induced crystallization improves the crystallinity and increases the shrinkage; The increase of P2 and P3, on the one hand, increases the shrinkage due to the increase of crystallinity, on the other hand, the increase of P2 can press more melts, so that the plastic in the mold cavity is fully compacted; P3 high energy increases the feeding amount and effectively prevents backflow, which is conducive to reducing the shrinkage. Therefore, the shrinkage of plastic parts is the result of two aspects, so the shrinkage first increases and then decreases with the increase of P1, P2 and P3 [10]

precision plastic parts require not only stable dimensional tolerance, but also strict mechanical properties. These characteristics mainly depend on the historical process of thermal mechanical action of the melt in the cooling stage. Mold temperature and cooling time have a direct impact on product quality and productivity. It is found that under a given mold temperature, the dimensional accuracy of the product does not increase significantly with the extension of the cooling time, but only after a certain period of time. In the general cooling process, there are two such key cooling time points. Although the accuracy of the products produced at the first cooling time point is also high, it is not the most stable state, while the cooling time at the second cooling time point is relatively long, but it can make the state of the products the most stable [11, 12]

3 replace conventional injection molding machines with precision injection molding machines

precision injection molding machines generally use large injection power, which can not only meet the requirements of injection pressure and injection rate, but also improve the accuracy of products. The control system of precision injection molding machine generally has high control accuracy, which is required by the product itself. The high control accuracy can ensure the good repetition accuracy of various injection process parameters, so as to avoid the change of product accuracy due to the fluctuation of process parameters. Therefore, precision injection molding machines generally adopt multi-level feedback control for process parameters such as injection volume, injection pressure, injection rate, holding pressure, back pressure and screw speed. Precision injection molding requires the clamping system to have sufficient stiffness, otherwise the product accuracy will be reduced due to the elastic deformation of the clamping system. Secondly, the clamping force of the clamping system must be accurately controlled, otherwise too large or too small clamping force will have a negative impact on the accuracy of the product. Therefore, in the design, we should comprehensively consider the mold stiffness, system stiffness and clamping force to accurately control the accuracy of products, especially thin-walled flat products. When the die area is large, the deflection of the guide column must be checked [13]. The precision injection molding machine must also be able to accurately control the working temperature in the hydraulic circuit, so as to prevent the viscosity and flow of the working oil from changing due to temperature changes, which further leads to the fluctuation of injection process parameters and the loss of due accuracy of the products