Precision injection molding technology research

Abstract: The first volume precision injection molding polymer materials technology, discusses the impact of precision injection molding of factors, including the accuracy of the mold, injection molding machine control accuracy, precision injection process control and other computer-controlled technology. These factors were analyzed and discussed, and gives the relevant solution, and finally the first volume a number of advanced molding technology.Keywords: Precision injection; injection molding machine; precision injection process; precision injection molds
In recent years, with the electronics, telecommunications, medical, automotive and other industries the rapid development of high precision plastic products, increasing demand for high-performance, precision molding technology continues to promote the advancement of new technologies continue to emerge. Requirements of precision injection molding products, not only has high dimensional accuracy, low warpage, excellent transcription, but also should have excellent optical properties [1]. Injection molding is the most important one of the methods of plastic molding, plastics, including plastic, injection, packing, cooling, and several basic processes, thereby affecting the precision injection molding of many factors.http://www.prmindustry.com/Abstract...Articles section 1 Mold
To create a precision product, precision mold is essential. The general accuracy of the mold by the mold temperature control, precision mold making and mold design for plastic shrinkage selection and other effects.
1.1 Precise mold temperature control
In general, precision parts injection molding, mold temperature control is mainly to advances in productivity. However, mold temperature control for precision injection molding of a great impact [2], it affects parts of the contraction, shape, crystallinity, stress, etc., so the design requirements of the mold when the temperature distribution of hot and cold circuit justice, precise control precision, the best use of mold temperature and the chiller control. Mold temperature control and cooling time on product performance and its influencing factors are as follows [3]:
(1) injection of plastic melt into the mold and the mold cycle time of both the mold resulting in fluctuations in temperature difference, so should try to reduce this volatility difference peak.
(2) mold heat transfer of heat through the mold and the mold concrete mold to pass out to the media, the mold is the same, so the control of the media is the key, generally the cooling water inlet temperature is less than 1 ℃, uniform application of theory to calculate the temperature [4], this can ensure the stability of heat transfer. Media flow and heat transfer are interrelated, the mold surface temperature sensor can be used to control the media flow to compensate for temperature changes due to environmental temperature changes caused by mold.
(3) steady state of the die mold absorbed and the heat balance, out of the melt injection molding machine to strictly maintain a stable temperature. Mold around the temperature of the mold had a great impact on the general media, the mold will affect the convection coefficient of variation, it is best to set the temperature sensor in the mold surface, mold surface temperature observed at any time and keep it stable.
(4) changes in heat exchange efficiency of the mold used for a long time after the cooling water pipe rust and scale will appear, then the thermal conductivity of the interface reduce the response time scale removal.
1.2 Design and manufacture of precision molds
The quality of the mold design is to ensure conditions of dimensional accuracy of plastic parts, precision injection mold for the accuracy of progress in the design process using computer-assisted analysis is necessary, especially for the casting mold system activity behavior and the heat distribution temperature control system the analysis. Should be possible to apply analysis simulation software and use close to the actual loading conditions to analyze the simulated melt in the mold pouring, cooling and other non-steady-state process.
Mold plastic parts manufacturing quality is to ensure that the key to dimensional accuracy, precision mold manufacturing features in addition to the main polishing and assembly operations, but are not hand-processing. General mold machining and manual machining ratio of about 6:4 ~ 7:3, while the precision mold machining and manual machining ratio of 9:1.
Mold materials and heat treatment is the precision mold protection in the molding process, mold high temperature state, so even low-temperature tempering of the mold parts, production parts will melt and temper repeatedly leaving the hardness decreased, while should also consider residual austenite caused by volume expansion. Therefore, the tempering temperature should be at least generally forming more than twice when the mold temperature, and appropriate low-temperature processing in order to eliminate residual austenite.
For high-precision products require precision molding parts, assembly total accuracy, mold-oriented parts of the sub-surface accuracy and parallelism should micron. Mold should have sufficient strength, stiffness and wear resistance, not easily deformed in the injection pressure, wear and tear. To achieve the above can be used for precision three-dimensional machining center, CNC machine tools and application of CAD / CAM / CAE and other new technologies.
1.3 Mold design, selection of material shrinkage
Thermoplastic molding shrinkage rate when volatile, to mold design, to determine the cavity size and control of product size precision difficult [5]. Therefore, in the mold should be designed to understand the characteristics of plastic shrinkage and appearance parts caused by the shrinkage of the part of the difference, and then adopt the necessary compensatory measures. High-precision plastic parts can be simple in design before making the mold, molding to measure the actual shrinkage of the parts, so that progress can be greatly dimensional accuracy of parts [6].
1.4 Precision mold gate design
Gate type, size, location and number of affected parts dimensional accuracy. Point gate jet force, but poor feeding, does not apply to the thick-walled parts. Location of the gate affect the melt flow and flow distance, the longer the process the greater the contraction. Multi-gate process can be shortened, but the weld increased. Gate design should be based on the size and the selected parts of the material and the use of analysis software to determine the ultimate means of practical experience.
2 precision injection molding process parameters set the best
Select the best molding process parameters to reduce plastic shrinkage. Shrinkage characteristics of plastic is plastic, heat shrinkable, elastic recovery, plastic deformation, contraction after contraction and comprehensive reflection of the aging, often due to material absorption or rearrangement of molecular chains caused by the specific performance of linear shrinkage and volume shrinkage changes in property values ​​that often shrink. Thermoplastic injection molding shrinkage rate when volatile, especially for crystalline plastic injection molding products, more obvious, because crystallinity depends not only on chemical structure, but also by the cooling process parameters (cooling rate, melt temperature, mold temperature , product thickness) impact to the mold cavity size and design to determine the accuracy of product size control difficult, there is an urgent need to understand a variety of plastic injection molding process parameters on shrinkage of the influence of the [7]. Parts is generally believed that differences in wall thickness is caused by two factors:
First, the high-pressure melt slightly deformed due to the mold cavity; Second, when the mold after mold elastic expansion. In general, the quality control of precision can be a good dimensional accuracy, while at higher mold temperature in the melt of low viscosity, viscosity gradient so small, screw back pressure in some, the quality of parts accuracy can be precisely controlled. However, for liquid crystal polymer (LCP), it just needs a lower mold temperature, which is due to LCP cold amorphous after rapid cooling.
LCP has a low melting heat and the orderly structure of the state, so the liquid crystalline state to the transition between the crystalline solid small changes, when fully cooled, the liquid - solid transition is almost instantaneous. In the cooler mold, when filled cavity, the cavity in most of the material and the gate has solidified, the compression stage to complement the melt, and thus the size and undeformed parts of the cavity size is very close to [8].
Amorphous plastic shrinkage rate than the crystalline plastics, compounds, shrinkage lower than the pure polymer materials, and with the increase in the amount of elastomer, the material shrinkage decreased. Enhance or filled by plastic, the heat capacity decreases, the rigidity increases, a significant reduction in shrinkage, and higher fiber or filler content, the smaller the shrinkage [9]. Crystalline plastics precision is also affected by pressure. As the pressure-induced crystallization effect, the injection pressure (P1), packing pressure (P2) and the feeding pressure (P3) to accelerate the crystallization process, the degree of crystallinity with the P1, P2, P3 increase in the rise. For P1, the pressure-induced crystallization progress in the degree of crystallinity, the contraction rate increase; P2 and P3 increase, on the one hand to make progress due to shrinkage of crystallinity increases, the other hand, P2 can be pressed into the more elevated melt, the type cavity plastic is fully compacted; P3 increased to increase the fill quantity, and effectively prevent backflow, helps reduce shrinkage. Therefore, the plastic parts of the shrinkage is the result of two aspects, so the shrinkage with the P1, P2, P3 elevated first increases and then decreases [10].
Precision plastic parts requires not only a stable tolerance, but also require strict mechanical properties. These features depend on the melt in the cooling phase of thermal - mechanical action of the historical process. Mold temperature and cooling time on product quality and productivity have a direct impact. The study found that in a given mold temperature, the dimensional accuracy of products does not increase the cooling time of the extension of significant progress, and only after a certain period of time a significant role. In general, this cooling process has two critical cooling time, cooling time in the first point of course, the production of products and higher precision, but not the most stable state, while the second cooling time, cooling time is longer, But the state can make the most stable products [11, 12].
3 used instead of conventional precision injection molding machine injection molding machine
Precision injection molding machines are generally the larger injection power, so in addition to the injection pressure and injection rate to meet the requirements, the injection of power itself, but also from some of the products improved accuracy of role. Precision injection molding machine control system with generally high control accuracy, which is required for the preparation itself. High-precision control of injection process parameters to ensure good repeatability, precision products in order to avoid fluctuations due to changes in process parameters. Therefore, precision injection molding machine is generally the amount of injection, injection pressure, injection speed, holding pressure, back pressure and screw speed and other parameters of the multi-level feedback control. Precision requirements of the mold injection system has sufficient rigidity, accuracy or products will be due to the elastic deformation of clamping system to lower. Second clamping system clamping force must be able to precisely control the size, or too big or too small clamping force will have an adverse impact on precision products. So in design, mold stiffness should be taken into account, the system stiffness and clamping force to precisely control the size of the precision products, especially flat-panel thin-walled products. When the mold is larger, the deflection must be checked on the orientation columns [13]. Precision injection molding machine must be able to work on the hydraulic circuit for accurate temperature control to prevent the work due to temperature changes caused by the oil viscosity and flow rate changes, further leading to injection process parameters due to fluctuations in the accuracy of product failure.
3.1 parts molding cycle time consistency
Generally typical injection molding machine has three modes: manual, semi-automatic. Due to various factors, the first two modes of each molding cycle time is different, this will affect the mold material in the barrel temperature and residence time, thereby affecting the accuracy of parts, so precision forming in the automatic mode should be used [14].
3.2 precision injection molding machine temperature control and a new screw design
Injection molding machine cylinder thermostat cycles on and off of the barrel leading to the melt density and viscosity changes, which affect part quality and dimensional accuracy of cyclical fluctuations; injection molding machine nozzle close to the mold cavity Therefore, the temperature of the molded parts nozzle also has a significant impact. Modern injection molding machine equipped with a special process control software to suppress temperature fluctuations, which is proportional integral derivative (P ID) control. Temperature difference from the barrel at least galvanic point of view, optimizing P ID parameter can completely eliminate the temperature fluctuations [15].
Obtained high quality and stability to the plastic parts, plastics injection molding machine unit is very important. Plasticizing unit for an important criterion for judging are: injection volume, plasticizing rate, injection rate, the polymer residence time in the plasticizing unit.
As the quality of plastic parts for dimensional accuracy of the error has a significant impact and should therefore be precise control of injection molding machine injection volume. Progress measurement precision injection molding machines the most effective way is to use technology to achieve the smallest screw diameter, especially for the light parts even more so. As the screw metering section of the relative length and overall length of the smaller screw, so the material stays in the plasticizing unit time becomes shorter. Screw threads are also similar to widen, to avoid the material stays longer, screw stable operation. Spiral groove width to depth ratio correspondingly smaller, which produce a lot of plastic parts for the small-diameter screw is particularly effective. Melt homogeneity is not due to the small compression ratio decreases, which is due to the very shallow spiral groove caused by very strong shear rate caused. Feed section is the design of the difficulty, it must ensure that all aggregates can be fed into to the uniform. Taking into account the need for shorter cycle, plastics rate must be large enough so the feed section design must effectively address this contradiction. In addition, to screw through the two-stage injection to achieve precise control of injection amount error, which requires the measurement of the melt through the ball check valve to the piston by the injection molding machine injection molding machine [16].
Previously by the microprocessor-controlled injection molding process can not be obtained by injection precision voltage comparator has been successfully resolved. Voltage comparator promised from the transmitter and other sensitive devices coming very accurate voltage signal when the set point and actual values ​​identities coincide with a signal sequence immediately sent to the control of the microprocessor, so the cycle by a sequence of running the program time time caused by the asynchronous transfer process through a direct signal to eliminate [17], much progress in the control accuracy.
4 new precision forming technology
4.1 Application in the mold injection compression molding (ICM) techniques
ICM technology is mold under the conditions of open intervals injection began, after a certain amount injected into the mold cavity began to melt close to the implementation of compression, injection mold completely closed end, and then packing, cooling until out the product. Injection compression molding is compacted by compression behavior products, making products with a uniform surface pressure distribution, so that compaction of the product high dimensional accuracy and stability, and deformation. It is in the mold on the case will melt injection into the cavity and, therefore, a large channel activity, the required injection pressure is very low, and reduce or eliminate the resin caused by the packing of molecular orientation and internal stress products, to progress products dimensional stability. ICM technology, flexibility and control are more general injection mold greater progress. Therefore, the use of this technology can produce more accurate parts, especially high-precision cylindrical parts [18-21].
4.2 high-speed injection molding
High-speed injection molding method of melt filling speed faster than the traditional 10 to 100 times, so that melt in the mold cavity to produce high shear activity, viscosity decreased filling fast, slow down hardening of the plastic surface, which can progress limit the thickness of thin-walled molded products, inhibit excessive molding pressure, mold and because low activity, stress reduction products. For thin-walled precision products, you can use the injection screw forward to absorb the energy caused by the melt, the screw stops moving full speed through the expansion of the melt cavity to achieve.
4.3 No packing injection molding
No packing injection molding technology refers to the plastic melt at high speed, high pressure filling into the mold, and then closed the valve at the nozzle to the cavity of the plastic melt automatically compensate for the contraction of various parts of products, such products can significantly reduce the warpage. But this method requires prior estimation of the contraction to add some packing, and therefore need to have a higher injection pressure cavity, the clamping force required is also high.
4.4 Other intelligent control technology
Precision injection molding processing conditions during the course of the implementation of continuous monitoring and precise control is very important. With the development of computer technology, computerized injection molding has been widely used. Including statistical process control (SPC) techniques, P ID technology, fuzzy logic control (FCC) method, the central network control (NNC) method and the reverse process models based on size of the central network control [22].
5 Conclusion
Plastic products for high-precision, high-performance requirements of the growing precision injection molding technology is the driving force, people understand the principles of precision injection molding precision injection molding is the deepening of technological advancement. With new materials, new processes and new equipment have emerged, especially in the plastic processing machine widely used for precision injection technology to create a good condition. For as long as the fair use of these technologies, we will be able to produce sophisticated products.

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              Peter Huang