This paper analyzes the key technologies of precision injection molding from the perspective of mold clamping structure of injection molding machines, and puts forward some important technical indicators of precision and ultra-precision injection molding through the successful application of four-cylinder straight-lock two-platen injection molding machines in disc injection molding machines, so as to provide reference for the formation of precision and ultra-precision injection molding technology concepts and the formulation of standards in the future.
Keywords: precision injection molding four-cylinder straight-lock two-platen disc injection molding machine standard
Precision injection molding technology is a comprehensive technology involving raw material properties, formulas, molding processes and equipment; Precision plastic products include DVD digital discs, DVD laser heads, digital camera parts, computer connectors, light guide plates, aspheric lenses and other precision products, such products are characterized by not only high dimensional accuracy requirements, but also the inherent quality and yield requirements of the products are also extremely high. The mold of the molded product is an important condition to determine whether the product can meet the dimensional tolerance required by the design, and the precision injection molding machine is the key equipment to ensure that the product is always formed within the required dimensional tolerance and ensure a very high yield.
Experiments show that the dimensional accuracy of the product is almost determined by the clamping device, and the requirements of precision injection molding technology for injection molding machines are mainly in three aspects:
1. The accuracy of opening and closing mold is high, which mainly refers to the repeatability of position, clamping force and balanced force of the template;
2. Sensitive and reliable low-pressure mold protection effect, (1) because the price of the mold required for precision molding is very expensive, sometimes the price is higher than that of the working machine; (2) The pace of production is accelerated, coupled with decentralized processing and assembly, in case of mold problems, it will cause heavy losses; (3) Many products have inserts, if the inserts are not placed properly and the low-pressure mold protection fails, the mold will be seriously damaged;
3. The mold opening force is large, and it is easy to adjust, and the opening and closing mold stroke requirements are long, because many molds are not only complex, but also have two or three parting surfaces, in addition to the large mold opening force requirements, the stroke of the mold opening is also long.
According to the above analysis, the performance of the clamping device is crucial for precision injection molding. From the perspective of transmission, the clamping device can generally be divided into toggle type (or machine hinge) and hydraulic type, and from the perspective of force, it can be roughly divided into three-plate type and two-platen injection molding machine.
Traditional injection molding machines are three-plate stressed (as shown in Figure 1), the mold is the "top" force mode, toggle rod type (machine hinge) and all-electric, single cylinder or multi-cylinder liquid-filled type, are all this force mode. The toggle rod type has its inherent limitations for precision injection molding: 1. Processing accuracy, assembly accuracy and mechanical wear will cause the clamping accuracy of the toggle rod type to decrease, and even the ambient temperature will affect the clamping accuracy, because the toggle clamping force is obtained by the tensile deformation of the tie rod, so the expansion and contraction of the tie rod caused by large temperature changes will seriously affect the clamping force; 2. Since the low-pressure slow section is just close to the toggle magnification sharp rise area, the low-pressure slow section is difficult to master, and it is neither sensitive nor difficult to adjust;
3. The toggle rod type mold opening force is small, and the opening and closing mold stroke is small.
The above three shortcomings seriously affect the application of toggle type in high-end machines, including forming precision products and complex products. For the single-cylinder multi-cylinder liquid-filled type, the tie rod force is still very long, the system stiffness is poor, the single-cylinder liquid-filled type also has the disadvantages of uneven clamping force, and the clamping cylinder is easy to leak.
Full hydraulic two-platen injection molding machine (as shown in Figure 2) has a fundamental difference from the above structure, its clamping force only acts between the moving and fixed templates, the bracket is not bearable, the mold is a "clamp" force mode, the two-plate type because the cylinder acting on the four corners of the template is connected, so the force acting on the four corners of the mold is almost equal, the force of the four corresponding angles of the fixed and moving template is the relationship between the force and the reaction force, so the force is completely equal, even if the mold is not parallel, It can also be automatically positioned so that the mold fits completely on the parting surface, and the force on the four corners and eight points of the mold is almost exactly equal, an advantage that other injection molding machines do not have. For forming precision products, the straight-locking two-platen clamping structure has the highest clamping accuracy, mainly because the mold is uniformly stressed and the clamping force repeatability is high. In the injection molding process, the differences between the two are as follows:
(1) It can be seen from Figure 3 that during the filling process, with the filling of the mold cavity, the cavity pressure velocity rises to the maximum value (point B), and the injection pressure also reaches the maximum value. Subsequently, the cavity pressure gradually decreases, the injection pressure is converted to packing, and the graded holding pressure is used to accommodate this change.
(2) Because the four-cylinder direct lock injection molding machine its clamping pressure can increase 1.5 times with the increase of injection pressure, when the injection is over, the clamping pressure decreases, and maintains a lower pressure during the cooling process, see Figure 4, from Figure 5 we can also see the comparison of the traditional toggle type and the four-cylinder direct lock clamping pressure, because the mold and template only bear the maximum clamping force in the time area with the largest expansion force, in the whole process, the four-cylinder straight-lock two-plate clamping force and the expansion force change synchronously, maintain a balanced state, avoid the toggle type mold is subjected to high pressure for a long time, the amount of elastic deformation produced by the mold is very small, which is conducive to improving the accuracy of the product。 Moreover, it also reduces the fatigue damage caused by high pressure of molds, templates and other stressed parts for a long time.
The above advantages make the full hydraulic four-cylinder straight-lock two-platen injection molding machine in precision injection molding has excellent performance, the structure can even be applied to digital disc injection molding machine, Guangdong Hongli Machinery Co., Ltd. is using this structure in 2003 successfully developed China's first digital disc injection molding machine, the production of CD, VCD, DVD disc all meet international standards. It can be seen that even in the most demanding disc injection molding technology, the all-hydraulic four-cylinder straight-lock two-platen type is still competent, and its performance is naturally self-evident for general precision products. Precision injection molding technology in China's plastic machine industry in the early stage of development, it is believed that in a number of years will have great development, therefore, in the case of today's high-end market of Chinese plastic machinery is almost monopolized by foreign enterprises, in the case that precision molding technology has not yet formed a unified standard, the concept of precision injection molding technology and some reference standards are proposed, which will have important reference value for the development of precision injection molding technology in China in the future. Here, I would like to make some observations.
Combined with the long-term practical experience of precision injection molding technology, I believe that the general precision injection molding machine has two indicators: one is the repeat error of the product size, and the other is the repeated weight error of the product. The former due to the size and product thickness is difficult to compare, and the latter represents the comprehensive level of injection molding machines, the general ordinary injection molding machine weight repeat error in about 1%, better machines can reach 0.8%, less than 0.5% for precision machines, less than 0.3% for ultra-precision machines, according to previous reports, the international highest level is less than 0.15%, 2004 Germany Cologne Plastic Machinery Expo, Germany Albemarle company announced that its latest injection molding machine weight repeatability can reach 0.07%. Precision injection molding machines require that the dimensional accuracy of products is generally within 0.01-0.001mm, and many precision injection molding machines also require injection molding machines to have high injection pressure and high injection speed; The mold clamping system is required to have sufficient rigidity and high enough clamping accuracy, the so-called clamping accuracy refers to the uniformity, adjustability, stability and repeatability of the clamping force, and the high position accuracy of the opening and closing mold; It is required that the pressure, flow, temperature, measurement, etc. can be accurately controlled to the corresponding accuracy, and multi-stage or stepless injection is used to ensure the reproduction conditions of the molding process and the repeatability of the product.
The specific requirements for precision injection molding machines are:
(1) Injection pressure≥ 250MPa;
(2) Injection speed≥ 300mm/s;
(3) The dimensional accuracy of the product is 0.001mm; (4) The standard deviation coefficient (rate of change) of the quality of the product is ≤ 0.1%;
(5) Opening and closing position accuracy: opening ≤ 0.03mm, closing ≤ 0.01mm;
(6) Injection position accuracy (holding pressure end point) ≤ 0.03mm;
(7) The force balance of the tie rod ≤ 1%;
(8) Premolding position accuracy≤ 0.03mm;
(9) Balance degree of fixed and moving template; A: 0.03mm when the clamping force is zero≤, B: 0.005mm when the clamping force is maximum≤0.005mm;
(10) The temperature control accuracy of the barrel and screw ≤± 0.5 °C.
Key research contents of precision injection molding technology:
(1) Clamping force: In the process of precision injection molding, high injection pressure (>250MPa) is generally required, otherwise it is difficult to form, but we should note that the overall size and thickness of precision injection molding products are relatively small under normal circumstances, so the clamping force is not very demanding for all products, on the contrary, if the clamping force is too large, or the four corners are unevenly stressed, or the clamping force is unstable, the mold compression deformation is inconsistent and uneven, and eventually it will affect the molding quality of the product. According to the process requirements of injection molding products, reasonable, convenient and accurate adjustment of clamping force, to overcome the difference in length and size caused by the length and size of the clamping parts of the toggle rod and electric injection molding machine due to the manufacturing accuracy affecting the force of the connecting rod and the long-term force deformation of the stressed element (such as toggle rod, tie rod, etc.) and the congenital problem of unstable clamping force caused by room temperature, mold temperature changes, etc., therefore, how to give full play to the advantages of high precision of the all-hydraulic mold clamping structure is an important topic for precision injection molding machines to study in the future.
(2) Plasticization quality: the uniformity of plastic plasticization not only affects the molding quality of plastic parts, but also affects the size of the resistance of molten plastic through the gate, in order to obtain uniform plasticization, the design of a special screw and the use of special plasticizing technology is essential. In addition, the temperature of the barrel should also be accurately controlled, now the screw, barrel temperature more use PID control (proportional, differential, integral), the accuracy can be controlled at ±0.5 °C ~ ± l °C, basically can meet the requirements of precision injection molding, if the use of FUZZY control method, it is more suitable for precision injection molding.
(3) Measurement accuracy: to maintain high stability of plastic products, each time injected into the mold cavity of the hot melt plastic must be equal, that is to say, each time the screw reset accuracy requirements are consistent, but in fact the position of the screw reset is impossible to be completely consistent, the larger the diameter of the screw, the greater the system inertia, the more difficult the position accuracy to control, generally should control the screw reset accuracy within 0.3mm, preferably within 0.2mm. /
(4) injection speed: injection speed is an important indicator of precision injection molding machine, because hot melt plastic from the beginning of injection into the mold cavity to the completion of the filling between a period of time difference, during this time, hot melt plastic will begin to cool, in order to make the gate and mold cavity material evenly cooled, should take high-speed injection, to improve the injection speed, must be configured with one or several accumulators that can quickly release energy to shorten the time difference. Increase the original injection speed by more than 3 times to 300~800mm/s, which is very beneficial for precision injection molding.
(5) Injection pressure: precision injection molding requires a large injection pressure to improve the compactness of plastic parts, reduce shrinkage deformation, and ensure the accuracy of parts, but it is not the greater the injection pressure, the better, the pressure will produce flashing, under the optimal injection pressure, the injection cycle time change should not exceed 0.05 seconds.
(6) Pressure holding: pressure holding has a great impact on precision plastic parts, to be precise, pressure holding can better compensate for shrinkage, reduce the deformation of plastic parts, control the accuracy of plastic parts, the stability of holding pressure determines the molding accuracy of plastic parts, and the end position of the screw is unchanged is the determining factor that determines the pressure holding effect.
(7) Mold temperature: For precision injection molding, the temperature change of the mold cavity surface should be controlled within ±1 °C, which must be controlled by PID, but for ultra-precision injection molding, this is far from enough, and the more advanced method is to use FUZZF control, that is, fuzzy control. With the increasing progress of science and technology, plastic products, especially precision plastic products will have a broader space for development, China's plastic machine industry started late, the foundation is weak, and Germany, Japan and other developed countries have a certain gap, but according to today's development momentum, it is estimated that the time to catch up with the world's advanced will not be too long, therefore, the precision plastic machine belonging to high-end products to form a unified understanding as soon as possible, early formulation of state-owned standards, the future development of the industry will have an important impact.
