Polymer precision injection molding technology, the factors affecting precision injection molding, including precision mold, control accuracy of injection molding machine, precision injection process control and computer control technology. These factors are discussed and the corresponding solutions are given. Finally, some advanced molding technologies are introduced.
Keywords: precise injection; Injection molding machine, precision injection technology; precision injection mold
In recent years, with the rapid development of high-precision plastic products in electronics, communication, medical care, automobile and other industries, and the increasing demand for high performance, precision molding technology has been continuously improved and new technologies have emerged. Precision injection molding products require not only high dimensional accuracy, low warpage and good writing, but also excellent optical properties. Injection molding is the most important plastic molding method, including plastic, injection molding, packaging, cooling and other basic processes, so there are many factors affecting precision injection molding.
1, precision mould
Precision molds are very important for well-made products. General molds, precision of temperature control, design and manufacture of molds and precision molds, and use of plastic shrinkage.
1. 1 Temperature control of precision molds
General precision injection parts, mold temperature control to improve productivity. However, mold temperature control has a great influence on precision injection molding, which affects shrinkage, molding and crystallization. For example, stress and cold and hot circuit design require reasonable mold temperature distribution and accurate control accuracy. It is best to control mold temperature with a water cooler. The mold temperature control and cooling time affect the product and its performance as follows:
(1) The temperature difference of molten plastic injection mold and one or two cycles will fluctuate, so the differential peak of fluctuation should be minimized.
(2) The mold heat of heat transfer transfers the medium through the concrete mold, and the key is mold forming and medium control. Generally, the temperature difference between the inlet and outlet of cooling water is less than 65438 0℃, and the average temperature is calculated theoretically to ensure the stability of heat transfer. Media flow and heat transfer are connected, and the temperature of the mold surface can be used to control the media flow sensor to compensate for the change of mold temperature and environmental temperature.
(3) Balance the heat absorption and heat dissipation of the stable mold, and strictly maintain the stable temperature of melt injection of the injection molding machine. Generally, the mold temperature around the mold has a great influence on the change of melt convection heat transfer coefficient, so it is best to set a surface temperature sensor on the mold surface to observe and maintain stability at any time.
(4) Change the heat exchange efficiency, and mold will appear on a large scale in the cooling water pipe and companion pipe for a long time, and then remove the scale in time when the heat conduction interface decreases.
1.2 design and manufacture of precision mould
On the premise of ensuring the dimensional accuracy of plastic parts, it is necessary to design a computer-aided analysis, especially the flow analysis of gating system and the thermal adjustment system of mold temperature distribution, in order to improve the accuracy of precision injection mold. The application program should simulate the use of software as much as possible, and be close to the actual load conditions, and simulate the real analysis of melt during pouring, cooling and other unstable processes.
Mold manufacturing quality is the key to ensure the dimensional accuracy of plastic parts. Precision mould is the main feature of polishing manufacturing and assembly business, and there is no manual processing. The ratio of dead machining to manual machining is about 6:4 ~ 7:3, and the ratio of die to machining accuracy is 9: 1. Mold materials and heat treatment are the protection of the mold in the molding process. The low tempering hardness of parts is caused by the domestic mold temperature, even the low-temperature tempering of mold parts, the generation and repetition of high-temperature melt and the volume expansion of retained austenite should be considered. Therefore, generally speaking, the tempering temperature should be at least twice as high as that of molding. In order to eliminate residual austenite, mold temperature and low temperature treatment should be properly carried out.
For high-precision products forming precision parts, the assembly of precision molds should accumulate in the direction of surface accuracy, and the parallelism should reach micron level. The mold should have sufficient strength, rigidity and wear resistance, and bear the hard and wear-resistant pressure of injection molding deformation. In order to realize the above processing, new technologies such as three-dimensional precision machining center, numerical control machine tool, CAD/CAM/CAE can be applied.
1.3 design and shrinkage of precision mould
The fluctuation of molding shrinkage of thermoplastics, mold design, determination and control of the dimensional accuracy of product cavity are all difficult points. Therefore, in the design of plastic mold, it is necessary to understand the shape characteristics and shrinkage of parts at the differential shrinkage parts, and then adopt necessary compensation measures. For high-precision plastic parts, we can simply design the mold and measure the actual shrinkage of different parts through injection molding, which can greatly improve the dimensional accuracy of parts.
1.4, Gate Design of Precision Mould
Gate type, size, location and the number of dimensional accuracy of all affected parts. In the injection gate, but the feed with poor effect, the thick wall part is not suitable. The effects of melt flow distance, gate position and flow rate lead to greater shrinkage in a longer process. Multi-grid technology can shorten the welding process, but improve the welding trace. The size, material and software of the door assembly should be selected according to the purpose to help complete the practical experience.
2, the best precision injection molding process parameter setting
Select the best molding process parameters to reduce the shrinkage of plastics. Plastics are heat shrinkable plastics. The comprehensive reflection of elastic recovery, plastic deformation, shrinkage and aging after shrinkage is usually caused by the rearrangement of water-absorbing materials or molecular chains, which is embodied in the changes of linear shrinkage and * * * value with shrinkage volume. The fluctuation of shrinkage of thermoplastic injection products, especially crystalline plastic injection products, depends not only on the crystallinity of chemical structure, but also on the size and design of cooling process parameters (cooling rate, melt temperature, mold temperature and product thickness), which is an urgent need for the parameters concerned by injection molding law that affect plastic shrinkage. In some thickness differences, it is generally believed that there are two factors:
First of all, high-pressure melting causes slight deformation of the cavity mold, and the material with elastic modulus after expansion. Generally speaking, quality control and accuracy can achieve good dimensional accuracy, and the quality of precision parts can be accurately controlled at high temperature, low viscosity and small viscosity gradient of die melt and certain screw back pressure. But liquid crystal polymer (LCP) only needs a lower mold temperature, because it will cool and solidify in time under the condition of cold LCP.
The state of low melting point thermal ordered structure of LCP changed from liquid crystal to crystalline solid, and when it was completely cooled, the liquid-solid transition was almost completed. At the time of cold death, when a cavity, cavity materials and most of the doors have been solidified, it is difficult to add melting compression in stages to make the parts very close to the size of the deformed cavity.
Compared with crystalline plastics, amorphous plastics have lower shrinkage, and the mixture of polymer materials has lower shrinkage. At the same time, with the increase of elastomer content, the shrinkage of the material decreases. With the increase of plastic filling amount, the heat capacity decreases and the stiffness increases obviously. With the decrease or increase of shrinkage fiber filling amount, the shrinkage rate decreases. Crystal precision plastic products are also under pressure. In the process of accelerating pressure crystallization (P 1), holding pressure (P2) and feed pressure (P3) by injection pressure, the crystallization of P 1, P2 and P3 showed an upward trend. P 1, increase the crystallinity of crystallization pressure, and increase shrinkage to increase P2 and P3. On the one hand, the increase of shrinkage is due to the increase of crystallinity, on the other hand, more pressure P2 is added to melt, and the cavity has completely compacted the plastic; P3 feed increase, effectively preventing backflow is conducive to reducing shrinkage. Therefore, the shrinkage of plastic parts, together with the shrinkage of P 1 of P3 P2, first increases, then decreases, and then increases.
Precision plastic parts need not only stable dimensional tolerance, but also strict mechanical properties. These characteristics mainly depend on the cooling stage of the historical process of melt thermal action. Mold temperature and cooling time have a direct impact on product quality and productivity. It is found that the dimensional accuracy of the product has not been significantly improved due to the mold temperature and cooling time, but has played a prominent role in a certain period of time. In the general cooling process, there are two cooling at such a critical moment. First, although the product precision is high, it is not the most stable state, and the time and cooling time are long. However, in the second point, the product is the most stable state.
3. Adopt precision injection molding machine to replace traditional injection molding machine.
The injection power of precision injection molding machine is usually greater than. In addition to such injection pressure and injection speed requirements, power itself will also play a specific role in improving the accuracy of injected products. The control system of precision injection molding machine usually has high control accuracy and is a product it needs. High-precision injection process parameters ensure that the control has good accuracy and product precision, so as to avoid the fluctuation of process parameters. Therefore, the general injection, injection pressure, injection speed and pressure, back pressure and screw speed of precision injection molding machine are all controlled by multistage feedback. The precision injection molding of its modulus requires the system to have enough rigidity and accuracy, otherwise the product will reduce the model due to elastic deformation. The second mold of the system must be able to accurately control the size, otherwise too large or too small will adversely affect the accuracy of the product model. Therefore, in the design, the rigidity of the mold should be considered and used as a model system to accurately control the dimensional accuracy of products, especially flat and thin-walled products. When the die is large, the deflection of the guide post must be checked. The precision injection molding machine must also be able to accurately control the working temperature in the hydraulic circuit, so as to prevent the product from losing precision due to the changes of viscosity and flow rate, temperature and further fluctuation of injection process parameters caused by oil.
3. 1 Consistency of parts molding cycle time
There are three typical injection molding machines: manual, semi-automatic and fully automatic. Due to various factors, the first two molding cycle times of different models may be different, which will affect the temperature and materials will stay in Liaodong until they die, thus affecting the accuracy of parts. Automatic mode should be used as much as possible in precision molding.
3.2 Screw Temperature Control and New Design of Precision Injection Molding Machine
The habit of automatic constant temperature opening of injection molding machine barrel leads to Liaodong circulation, and the density and viscosity of molten materials change, which further affects the quality of injection molding machine nozzle and mold and the dimensional accuracy of nozzle parts with periodic fluctuations, and also has a significant impact on the temperature of molded parts. Modern injection molding machines are equipped with special process control software to control temperature fluctuation, which is proportional integral differential (PID) control. At least from the point of view of barrel thermocouple, the optimization of P ID parameters can completely eliminate temperature fluctuation.
The unit of plastic injection molding machine is very important to the quality and stability of plastic parts. Plastic unit is an important criterion to judge: plastic injection quantity, injection rate, injection rate, and polymer in plastic unit at the same time.
As a very important factor affecting the quality of plastic parts, dimensional accuracy error should be the precision injection control of injection molding machine. The most effective way to improve the metering accuracy of injection molding machine is to realize the minimum screw diameter by technology, especially for light parts. Compared with the measurement of screw length and the decrease of total screw length, the time in plastic material unit is getting shorter and shorter. Thread is similar to enlarged material, which can avoid longer residence time and make the screw run stably. You see, the aspect ratio of deep groove is small, and the small diameter screw for producing a large number of engineering plastic parts is particularly effective. The uniformity of melt and the reduction of compression ratio are not small, which is due to the very shallow groove loss leading to very strong shear rate. The design with difficult feeding must ensure that all input sets are consistent. Considering the short period, the plasticity rate must be large enough, and this contradiction must be effectively solved in material design. In addition, if two-stage injection screw is used to accurately control the injection error, it is necessary to measure the ball valve melt injection molding machine on Detroit injection molding machine.
The injection molding process controlled by microprocessor cannot be successfully solved by injecting precision voltage comparator. The voltage comparator allows the transmitter and other sensitive and very accurate voltage signals to pass through, and immediately sends the real value of the real-time signal controlled by the microprocessor to the command after the set value appears. The control accuracy is greatly improved by running the program asynchronously and eliminating the direct transmission periodic instructions from time to time through signal processing.
4. Precision forming technology
4. Application of1in ICM technology
Under the condition of ICM technology, the injection mold is opened for a certain distance. When a certain amount is injected, the mold cavity begins to melt and the injection mold is completely closed. Then, it is packaged and cooled until the product is taken out. Through injection compression molding and compaction of compressed products, the surface pressure distribution of products is uniform, and the compacted products have high dimensional accuracy, good stability and small deformation. When the mold is opened, the melt enters the cavity, so the runner is moved for low-pressure injection molding, which reduces or eliminates the stress caused by product stress and resin molecular orientation, and improves the dimensional stability of the product. Compared with injection mold, ICM technology and flexible control ability are greatly improved. Therefore, more precision parts, especially high-precision cylindrical parts, can be produced by using this technology.
4.2 High-speed injection molding
The high-speed injection molding method fills the melting speed 65,438+00 to 65,438+000 times higher than the traditional method, and the melt produces high shear flow in the mold cavity, which reduces the viscosity, injection speed and plastic surface hardening, thus improving the wall thickness limit of the products formed by the film, restraining the excessive molding pressure and reducing the pressure caused by the low-pressure moving mold products. In thin-walled precision products, we can absorb energy from the melt behind the screw and stop the melt from filling the cavity through high-speed expansion to realize the injection use of the screw.
4.3 Pressureless Injection Molding
Pressureless injection molding technology refers to the contract of injecting plastic melt into the mold at high speed and high pressure, then closing the nozzle needle to melt the cavity of the plastic mold and automatically compensating different parts. This product can greatly reduce warping. However, this method needs to estimate the increase of packaging shrinkage in advance, so the higher pressure value of injection mold cavity requires us to need higher clamping force.
4.4 Other intelligent control technologies
It is very important to continuously monitor the processing conditions of precision injection molding and implement accurate control. With the development of computer technology, computer injection molding has been widely used. Among them, statistical process control (SPC), P ID technology, fuzzy logic control (FCC), network control center (NNC) method and reverse machining model of size control based on network.
5。 conclusion
The high performance requirements for high-precision plastic products and more and more precision injection molding technologies are the driving force for progress. A deep understanding of the principle and technology of precision injection molding is the basis of progress. With the emergence of new materials, new processes and new equipment, especially the wide application of precision injection molding technology in plastic processing computers, good conditions have been created. As long as we make rational use of these technologies, we can produce complex products.