Abstract: Polymer precision injection molding technology, factors affecting precision injection molding, including precision molds, control accuracy of injection molding machines, precision injection process control and computer control technology. These factors are discussed and relevant solutions are given, and finally some advanced molding technologies are introduced.
Keywords: precision injection; injection molding machine, precision injection molding technology; precision injection mold
In recent years, the rapid development of electronics, communications, medical, automotive, and other industries on high precision plastic products, as well as high-performance requirements increasingly precision molding technology continues to progress, the emergence of new technologies. Precision injection molded products not only require high dimensional accuracy, low warpage, well written, but also to have excellent optical properties. Injection molding is one of the most important plastic molding methods, including plastic, injection molding, packaging, cooling, and some of the basic process, thus affecting the precision injection molding of many factors.
1, precision mold
To make fine products, precision molds are essential. Molds in general, the precision of temperature control, mold and precision mold design and manufacturing, the use of plastic shrinkage impact.
1.1 Precision mold temperature control
Generally precision injection molding parts, mold temperature control to improve productivity. However the mold temperature control precision injection molding has a huge impact, it affects the shrinkage, shape, crystallization of the part, such as stress, heat and cold circuit design, mold temperature distribution reasonable requirements, precise control accuracy, the best and Lengshuiji control mold temperature. Mold temperature control and cooling time on the product and its impact on the performance of the following factors:
(1) molten plastic injection mold and one or two cycles will be generated in the mold temperature difference fluctuations, so should minimize fluctuations in differential peaks.
(2) heat transfer mold heat through the specific dead in the transfer of the media, molding, media control is the key to its cooling water temperature difference between the general import and export is less than 1 ℃ application of theoretical calculations of the average temperature in order to ensure the stability of heat transfer. Media flow and heat transfer is connected, mold surface temperature can be used to control the media flow sensor to compensate for the changes in the mold temperature caused by changes in the ambient temperature.
(3) so that the stability of the mold heat absorption and propagation of the balance of the injection molding machine melt injection strictly maintain a stable temperature. Mold temperature around the mold has a great impact on the general will die in the melt convection heat transfer coefficient of change in the impact, so it is best to set the surface temperature sensor in the mold surface temperature, die at any time of observation, and keep it stable.
(4) changes in heat exchange efficiency, mold in the cooling water pipe and swimmers for a long time in the scale appeared, and then reduce the thermal conductivity of the interface should be timely removal of scale.
1.2 Precision mold design and manufacturing
Mold design is good or bad to ensure that the dimensional accuracy of the plastic parts of the premise, in order to improve the precision of precision injection molds, in the design of a computer-aided analysis is a necessary process, in particular, the pouring system and the mold temperature distribution of the flow analysis of the thermal regulation system. The application should be as close as possible to the simulation software used and to the actual loading conditions of the melt in mimicking the true analysis during pouring, cooling, and other non-steady state processes.
Mold manufacturing is the key to quality-assured dimensional accuracy of plastic parts, and precision molds are the main feature of the manufacturing and assembly business, in addition to polishing, without manual processing. Dead general processing and manual processing ratio of about 6:4 to 7:3, molds and processing precision ratio of 9:1 manual processing. Mold materials and heat treatment is in the molding process mold protection, mold temperature in the country, and even mold parts of the low-temperature tempering, high-temperature melt production and repeat and make the parts tempered hardness is low, should also be considered residual austenite volume expansion caused by. Therefore, the general tempering temperature should be at least twice as high as the molding process, mold temperature and low temperature treatment is carried out properly in order to eliminate residual austenite.
For molding precision parts of high-precision products, precision mold assembly accumulated in the direction of sub-surface accuracy as well as parallelism should reach the micron level. The mold should have sufficient strength, rigidity and wear resistance, under the pressure of injection molding deformation of hard wear. In order to achieve the above processing can be three-dimensional precision machining centers, CNC machine tools, CAD/CAM/CAE and other new technology applications.
1.3 precision mold design using shrinkage
Thermoplastic molding shrinkage of volatile, mold design, determine and control the product cavity size dimensional accuracy difficult. Therefore, in plastic mold design should understand the difference between the rate of shrinkage site of the part shape characteristics and shrinkage caused by, and then use the necessary compensation measures. For high-precision plastic parts can be in a simple design mold, injection molding to measure the actual shrinkage rate of the different parts, which can greatly improve the dimensional accuracy of the parts.
1.4, precision mold gate design
The gate type, size, location and number of all affected parts dimensional accuracy. Gates in the jet, but the poor results of feeding, thick-walled parts are not applicable. The flow distance of the melt, the location of the gate and the influence of the flow, the longer the process the greater the shrinkage. Multi-gate processes can be shortened, but improve weld marks. Gated parts should be selected based on actual experience with size, material and software to help accomplish the use.
2, the best precision injection molding process parameter settings
Select the best molding process parameters to reduce plastic shrinkage. Plastic is heat shrinkable plastic, shrinkage of the elastic recovery, plastic deformation, shrinkage and aging of the comprehensive reflection of the shrinkage, usually caused by water-absorbing materials or molecular chain rearrangement, specifically expressed as linear shrinkage and the value of *** with the shrinkage volume shrinkage rate changes expressed. Thermoplastic injection molding products shrinkage fluctuations, especially crystalline plastic injection molding products more obvious, not only depends on the degree of crystallinity due to the chemical structure, but also the cooling process parameters (cooling rate, melt temperature, mold temperature, product thickness) of the size of the mold cavity and the design of the mold cavity to determine the dimensional accuracy of controlling the product and the difficulty of controlling the impact of the injection molding of plastic shrinkage and the impact of the law, which is the impact of the injection molding of plastic shrinkage attention! Parameters are urgently needed. Differences in part thickness are generally believed to have two factors:
First, high pressure melting causes slight cavity mold deformation when the material expands after the modulus of elasticity. In general, quality control and precision can be very good dimensional accuracy, and at higher temperatures, mold melt viscosity is low, the viscosity of a smaller gradient in a certain screw back pressure, the quality of precision parts can be precisely controlled. But in the case of liquid crystal polymers (LCP), it just requires a lower mold temperature, which is because the cold LCP will cool after shaping in time.
LCP's low-melting-point, thermally ordered structure changes between the liquid-crystalline state to a crystalline solid, with smaller changes, and when completely cooled, the liquid-solid transition is almost complete instantaneously. In the cold death, when a cavity, the cavity material and most of the door has been cured, it is very difficult to add the stage of melting compression, and thus partially and deformed cavity dimensions are very close to the size.
Plastics amorphous than crystalline plastics have a low shrinkage and low shrinkage mixtures of polymeric materials, at the same time the elastomer content increases, the shrinkage of the material decreases. Increase or plastic filling, heat capacity decreases stiffness increases significantly, reducing shrinkage fiber filler content or higher, the smaller the shrinkage. Crystal precision plastic products are also subjected to pressure. In the role of injection pressure pressure crystallization (P1), holding pressure (P2) and feed pressure (P3) to accelerate the crystallization process, P1, P2, P3 crystallization, increasing in the upward trend. P1, increase the crystallinity of the crystallization pressure to increase the shrinkage rise P2 and P3, on the one hand, because of crystallinity increase shrinkage increases, on the other hand, increase the pressure for more P2 melting, the cavity has been fully compacted! Plastic; P3 feed increase increase, effectively prevent reflux is conducive to reducing shrinkage. Therefore, the result of the shrinkage of plastic parts, with P1, P2, P3 shrinkage, first increase and then decrease, increase.
Precision plastic parts not only need a stable dimensional tolerance, but also need strict mechanical properties. These characteristics depend largely on the cooling phase of the melt's thermal-mechanical action during the history of the process. Mold temperature and cooling time have a direct impact on product quality and productivity. It was found that due to the mold temperature and cooling time, the dimensional accuracy of the products is not extended was significantly improved and a prominent role in a certain period of time after. In the general cooling process at such a critical point in time there are two cooling, cooling time at the first point although the production of products with high precision, but it is not the most stable state at the time and cooling time is longer, the second point, however, the product makes the most stable state.
3, the use of precision injection molding machine to replace the traditional injection molding machine
Precision injection molding machine is usually greater than the injection power, in addition to such injection pressure and injection according to the speed requirements, the power itself will be a specific role in the injection molded products to improve the accuracy. Precision injection molding machine control systems usually have high control accuracy and are themselves required by the product. The high precision can be injected into the process parameters to ensure that the control has good accuracy, precision products, in order to avoid changes due to fluctuations in process parameters. Therefore, precision injection molding machine general injection, injection pressure, injection speed and pressure, back pressure and screw speed and other process parameters of multi-level feedback control. Its modulus of precision injection molding requires the system to have sufficient stiffness, accuracy or product will be reduced for the elastic deformation model. The second mold of the system must be able to accurately control the size, or too large or too small precision product model will have adverse effects. Therefore, in the design, the rigidity of the mold should be considered, as well as the stiffness, as a model system, in order to accurately control the dimensional accuracy of the product, especially flat thin-walled products. When the mold is larger, the deflection of the guide column must be checked. Precision injection molding machine must also be able to work in the hydraulic circuit working temperature is accurately controlled to prevent the oil caused by changes in viscosity and flow, temperature changes, and further fluctuations in the injection molding process parameters and make the resulting products will lose their accuracy.
3.1 Piece molding cycle time consistency
Generally a typical injection molding machine has three models: manual, semi-automatic and fully automatic. Due to a variety of factors, the first two molding cycle time of each model may be different, which will affect the dead in Liaodong stay in the temperature and material, thus affecting the accuracy of the part, in precision molding should try to use the automatic mode.
3.2 precision injection molding machine screw temperature control and new design
Injection molding machine barrel automatic thermostat on the open habit leads to the Liaodong cycle, the density and viscosity of the molten material changes, which in turn affects the injection molding machine nozzle with the mold so the quality and cyclical fluctuations in the dimensional accuracy of the nozzle parts, molding the temperature of the parts also has a significant impact. Modern injection molding machines are equipped with a special process control software to control temperature fluctuations, which is proportional integral differential (PID) control. Optimization of the P ID parameters completely eliminates temperature fluctuations, at least from the point of view of the barrel thermocouple.
To the quality and stability of plastic parts, plastic injection molding machine units are very important. The plastic unit is an important criterion for judging: the amount of plastic injected, the injection rate, the rate of polymer while in the plastic unit.
As a very important dimensional accuracy error affects the quality of plastic parts, it should be controlled by the injection molding machine precision injection. The most effective way to improve the measurement of the precision injection molding machine is to utilize technology to achieve the smallest screw diameter, especially for light parts. The measurement of the relative screw length and the smaller the total length of the screw, the shorter the time in the plastic material unit is. Threads similar to those of the expanding material can be avoided by staying longer and enabling the screw to run steadily. Small diameter screws are particularly effective in producing large quantities of engineering plastic parts while looking at relatively small deep groove width ratios. The homogeneity of the melt and the not inconsiderable reduction of the compression ratio, which is caused by the loss of very shallow grooves results in a very strong shear rate. Feeding difficulties are designed in such a way that it must be ensured that all input sets can be homogenized. Considering the need for shorter cycles, the plastic rate must also be large enough that the contradiction must be effectively resolved in the feed design. In addition, if a two-stage injection screw is used to achieve precise control of the injection error, this needs to be measured by the detected spherical valve melt injection molding machine in Detroit injection molding machine.
The injection molding process controlled by a microprocessor cannot be successfully solved by injecting a precision voltage comparator. The voltage comparator allows the transmitter and other sensitive voltage signals to be passed very accurately, and when the set point occurs immediately after the real-time signals are sent to the order of the microprocessor-controlled real-time value by running the program asynchronously from time to time through the signal processing to eliminate the direct transmission cycle order, which greatly improves the accuracy of the control.
4, precision molding technology
4.1 Application of injection compression molding (ICM) technology in the mold
ICM technology under the conditions of the injection mold begins to open a certain distance by means of injection to a certain amount, the mold cavity begins to melt when the closing pressure, the termination of the injection mold is completely closed, and then packaged and cooled until the removal of the product. By injection compression molding compaction of compressed products, the pressure on the surface of the product is evenly distributed, compaction and other products such as high dimensional accuracy, good stability and small deformation. It is in the case of mold opening, the melt enters the cavity, therefore, moving channels, for low-pressure injection molding, and reduces or eliminates the pressure caused by the stress and product molecular orientation of the resin, thus improving the dimensional stability of the product. ictm technology and flexible control capabilities than injection molds, greatly improved. As a result, more precision parts, especially high-precision cylindrical parts, can be produced using this technology.
4.2 High-speed injection molding
High-speed injection molding method fills the melt 10 to 100 times faster than the traditional speed, the melt in the mold cavity to produce high shear flow, reduce viscosity, injection speed, reduce plastic surface hardening, thus improving film formation of the product wall thickness limitations, inhibit excessive molding pressure, as well as due to the low voltage of the moving mode of the product to reduce pressure. Thin-walled precision products, we can absorb energy from the melt after the screw, by expanding the high-speed movement to stop melting full cavity realized by using the injection screw.
4.3 Pressureless injection molding
Pressureless injection molding technology refers to the plastic melt at high speed, high pressure filling into the mold, and then close the nozzle needle melts the plastic mold cavity of the product automatically compensates for the different parts of the contract, this type of product can greatly reduce warpage. However, this method requires prior estimation of the increase in packaging shrinkage, so the value of the higher injection mold cavity pressure is needed, we need clamping force is also very high.
4.4 Other intelligent control technology
Precision injection molding processing conditions of continuous monitoring and implementation of accurate control process is very important. With the development of computer technology, computer injection molding has been widely used. Among them are Statistical Process Control (SPC), P ID technology, Fuzzy Logic Control (FCC), Network Control Center (NNC) methods and network-based size control reverse processing models.
5. Conclusion
The high-performance requirements for high-precision plastic products, as well as more and more precision injection molding technology, is the driving force for progress, and the deepening of the understanding of the principles of precision injection molding people, precision injection molding technology, is the basis for progress. With the emergence of new materials, new processes and new equipment, especially in the plastics processing computer precision injection molding technology widely used to create good conditions. As long as the rational use of these technologies, we will be able to produce complex products.