With the development of the Internet, IT digital products have entered our lives in large quantities. According to statistics, the total global market capacity of DVD-R in 2005 was 1.7 billion pieces, and it is expected to reach 3.5 billion pieces in 2006, and will continue to grow at a rate of 2.5~3 times in the future. As far as China is concerned, the market consumption potential is very huge. In recent years, VCD, DVD, PC and other related equipment have entered the market in large quantities, which has also driven the market demand for optical discs. Compared with regional markets such as Europe and the United States, China's potential demand is still quite huge.
Optical discs are also known as optical memory. The master disk is made of high-temperature resistant high-strength polycarbonate as the material, and is processed and manufactured on an ultra-precision injection molding machine with a mirror accuracy of 20~30 nm. On the master disk, there are countless signal recording grooves, these grooves are molded under the strong clamping force of the injection molding machine pressed out, the grooves are different according to the disc format, of which CD, VCD signal recording groove width is 0.56 microns, depth is 0.11 microns, line spacing is 1.5 microns; DVD signal recording groove width is 0.28 microns, depth is 0.11 microns, line spacing is 0.75 microns. The prefabricated grooves of the CDR (One-Time Burning of Read-Only Optical Discs) are coated with organic plastic, and the signal is permanently retained on the disc by the ablation of the laser signal of the burning laser head, and the signal can be picked up after reading. The signal groove of CDRW (can be erased and burned for many times) is relatively shallow only 40~50 nm, and the memory alloys Ag, In, Sb, Te, etc. are deposited in it, using the difference between the reflectivity of the alloy to the laser in the crystalline state and the amorphous state to erase the record and read the signal.
It is precisely because the depth of the signal groove of the disc is at the nanometer level, the accuracy requirements for the mold and the injection molding equipment are particularly high. In the international market, the injection molding machine of the optical disc production line is mainly concentrated in the production of KraussMaffei (Krauss-Maffei) in Germany, NEtSTAL in Switzerland, Sumitomo in Japan, TOOLEX in Sweden, especially NEtSTAL in Switzerland and Sumitomo in Japan are the most prestigious, representing the highest level of injection molding machines.
Technical difficulties
The disc injection molding machine is composed of clamping device, injection device, cooling system, hydraulic system and computer control system. The outer diameter of the disc master disc is divided into 80 mm and 120 mm, but the thickness is less than 1.2 mm, which means that the flow-to-runner ratio (the ratio of process depth to runner thickness) of the melt material of the master disk material in the mold cavity is large. Therefore, the quality of the plastic melt is not easy to be uniform in the process of mold filling. Moreover, the master disc is a circular disc, the main runner is set in the center of the disk, and after the injected plastic melt enters the mold cavity, it must be advanced quickly and evenly along the circumference of 360 degrees, along the radial orderly flow to the periphery of the circle. If the local flow rate is too fast in the process, channeling or auxiliary flow will lead to the orientation of local melt molecules and the formation of internal stresses; If the speed of the master disc is not uniform during cooling, local residual stress will also form; If the clamping platen is not parallel, it will not only make the quality of the master disc unstable, but more seriously, it may damage the mold. Therefore, in view of the characteristics of high precision, good stability performance, high production efficiency, high cleanliness and good optical performance of optical disc products, the requirements for injection molding machines are mainly as follows:
◆ The overall operation of the system is stable and reliable, the opening and closing of the mold is fast and stable, the starting and stopping positioning accuracy of the template is high, the parallelism error of the template is small, and the clamping force is accurate and stable.
◆ The plasticizing system has high efficiency, fast speed, good quality and accurate measurement.
◆ High injection pressure, fast injection speed, multi-stage adjustable. The holding pressure conversion point is accurate, the holding pressure is stable, and the multi-stage is adjustable.
◆ Accurate temperature control.
◆ The whole machine has low noise, clean and no oil pollution.
Solution comparison analysis
According to the information of the China International Optical Storage Equipment Exhibition and Academic Exchange Conference that ended in Shanghai in early September, the current international optical disc production line mostly adopts all-hydraulic or all-electric injection molding machines, and in recent years, electric-hydraulic composite injection molding machines have also been applied.
Now the classification of all-hydraulic injection molding machines and all-electric injection molding machines is compared.
First, the mold clamping part
Disc injection molding requires fast, smooth and fast pressure conversion for mold clamping, and high requirements for plate parallelism and start-stop position accuracy.
Although the full hydraulic injection molding machine has the problem of compressibility of hydraulic oil, the pressure of the hydraulic pressure is equal everywhere when it is stationary, so the force on the template and mold is quite uniform.
All-electric injection molding machines generally use servo motors instead of the original oil cylinders to push the toggle rod for opening and closing mold movement. Therefore, the problems of the original toggle structure continue to exist, such as high processing accuracy requirements, easy wear, and difficult mold adjustment. Due to the use of servo motors, the toggle can be stopped at different angles, which is conducive to the change of clamping force, and compression molding can be performed. Moreover, because there is no influence of oil pressure inertia, the noise is relatively low and the control accuracy is high. But all this is premised on the absence of wear and tear on the toggle and ball screws, the high precision of manufacturing and assembly, and the flat surface of the mold installation. Generally, the all-electric injection molding machine will be used for about a year, and the trouble caused by the wear of the elbow rod and ball screw will occur. At this time, not only the precision of the machine is reduced, but also an additional radial force appears on the ball screw, which accelerates the wear, further rapid reduction in precision, and brings problems such as reduced efficiency and increased noise. In addition, if the toggle bar is injected before it is fully straightened, the strong mold expansion force of the high-pressure molten plastic on the mold will act on the spindle of the ball screw and servo motor, and this force is large enough to cause damage to the ball screw and servo motor!
Second, the injection part
Optical discs are thin-walled products, and the depth of the information grooves on the discs is nanometer precision. In order to ensure the accuracy of the replication signal, the injection process is required to be carried out under the condition of high speed and high pressure, and the speed and pressure of the injection and pressure holding process are adjustable in multiple stages, and the holding pressure conversion point is accurate. Otherwise, defects such as signal loss, incomplete signal, and disc warping and silver grain may occur after the disc is manufactured. At the same time, in order to ensure that the cycle cycle of the whole machine is as short as possible, the plasticization speed of the barrel screw is very high, and the plasticization amount is required to be very accurate, and the plasticization speed and back pressure are also multi-stage adjustable to meet the good plasticizing effect in the case of high speed. Therefore, a special screw is used, which is also beneficial to ensure that there will be no defects such as bubbles, black spots and impurities during the molding process.
At present, the structure of the injection part of the full hydraulic injection molding machine is generally divided into two types: double cylinder injection and single cylinder injection. The double-cylinder injection device has the advantages of short axial length and simple connection between injection cylinder, screw and oil motor. However, this structure has a large injection load and is difficult to control precisely. The single-cylinder one-line injection device is conducive to precise control, but the structure is more complex and the manufacturing accuracy requirements are relatively high.
In all-electric injection molding machines, the injection system generally has two structures, one is the belt drive structure, and the other is the motor direct drive. Obviously, the accuracy will be affected to a certain extent after adding the belt drive, and although the structure is relatively simple without belt drive, it requires a lower motor speed, a large torque, and its overload protection ability is weak. However, when plasticizing, the servo motor is used to control the screw rotation, the accuracy of plasticization measurement will be relatively high, the speed is more stable, and the speed can be infinitely adjusted. At the time of injection, the servo motor drives the ball screw to drive the screw for injection, and the control accuracy is also very high, and the response is also very sensitive, which is conducive to multi-stage injection adjustment. However, due to the high load, high speed and frequent start and stop of the ball screw, the wear is also relatively large. The servo motor starts faster than the general hydraulic type during injection, but if the hydraulic system is equipped with an accumulator, it will start faster than the servo motor, and if the accumulator is used for rapid injection, its speed can also be faster than the injection speed of the servo motor. Of course, the use of servo motor for the control of injection pressure and speed is relatively accurate, but when entering the holding pressure, because the servo motor can not provide large torque in the state of stopping, so it can only rotate intermittently, and make up the pressure by measuring the pressure of the material in front of the screw. In this way, on the one hand, the ball screw will accelerate the wear when it is constantly started and stopped by the impact force, on the other hand, frequent start and stop of the motor will lead to heating, energy consumption will also become larger, and the pressure is not easy to stabilize.
In general, all-electric injection molding machines and all-hydraulic injection molding machines have their own advantages and disadvantages, and the injection molding machines manufactured by combining the two are not satisfactory. Therefore, the international high-precision injection molding machine is still developing in the direction of all-electric and all-hydraulic. The following will be compared and discussed from 10 aspects:
1) Energy saving
All-electric injection molding machines have certain advantages over all-hydraulic injection molding machines in energy saving, but they are by no means as high as the energy-saving efficiency advertised by some manufacturers. Because the largest energy consumption of injection molding machines is in the heating process, injection process and plasticization process, the action of hydraulic injection molding machines is carried out sequentially, while electric injection molding machines must be started frequently; Moreover, the energy-saving effect of producing different products is very different, and if the production of thin-walled products is fully electric, it may not save so much. And the energy saving of many manufacturers is compared with the single-cylinder hydraulic type, if compared with other structures of the full hydraulic type or toggle type, the energy-saving effect is not so obvious. At the same time, if the all-hydraulic injection molding machine adopts multi-pump, variable pump hydraulic system, applies frequency converter to control the motor speed, adopts load-reactive injection system and other schemes, its energy-saving effect can also be comparable to that of all-electric type.
2) Noise reduction
One of the slogans of all-electric injection moulding machines is tranquility, which is true if the elbow rod and ball screw are not worn and the assembly accuracy is high. However, once wear occurs or the assembly accuracy is not high, there will be a lot of vibration and noise during injection and mold opening, and once it occurs, all this will accelerate its further deterioration.
3) Accuracy
Although the all-hydraulic type has problems such as the compressibility of hydraulic oil, because the pressure of the hydraulic pressure is equal everywhere when the hydraulic pressure is stationary, the accuracy of the two-plate hydraulic clamping mechanism is very high. If the injection and plasticizing system adopts servo valve control, its control accuracy will also be very high, all-electric mold clamping device due to the wear problem of elbow rod and ball screw, so mechanical accuracy and wear will inevitably affect the quality of the product, so its clamping accuracy is not as high as the full hydraulic type. This aspect has also been verified in the processing of optical discs, all-electric type can only produce DVD5, VCD, CD, etc., DVD9 and above, CD-R, DVD-R, DVD-R, etc. are difficult to apply electric injection molding machine processing. Moreover, the accuracy of the all-electric model continues to decline faster with the increase of use.
4) Price and cost aspect
The price of the all-electric type is generally more than 30% higher than the all-hydraulic type, and in the case of comprehensive performance is not particularly high than that of the all-hydraulic type, such a high price, whether the market accepts it is also worth considering.
5) Service life problem
The all-electric type is mainly the wear problem of the elbow rod and ball screw just mentioned, and this problem will accelerate the deterioration once it occurs. The main problem of the all-hydraulic type is the aging of hydraulic seals.
6) Maintenance cost problem
The quality of the servo motor should be credible, the maintenance rate is low, but if there is a failure, the maintenance cost will be high, and the maintenance of the ball screw and toggle bar is also a very difficult thing.
7) Usage fees
The all-electric version eliminates the need for hydraulic oil or coolers, which not only saves costs but also reduces pollution, with exceptions. Because with the increase of use time, some of the problems caused by wear and tear will follow, exposed.
8) Functional expansion
Since the all-electric type does not have the convenience of hydraulic type for increasing core pulling or reaming, it is difficult for users to choose when purchasing. If the machine is not equipped with these functions at the factory, it will be especially difficult to add these units later.
9) Overload protection function
The all-electric type relies entirely on mechanical transmission, which is both its advantage and its disadvantage. The advantage is high precision, the disadvantage is that once the machine fails, such as the screw is stuck, the servo motor may heat up and burn out due to stalling. In this regard, the all-hydraulic type is much better, because the all-hydraulic injection molding machine is equipped with a discharge protection device, once the system because of the fault oil pressure is high to the set value so that there may be danger, the protection device automatic discharge so that the system pressure will not rise again, to ensure that the equipment will not be damaged.
10) Environmental adaptability
Due to the high environmental requirements of all-electric injection molding machines, if there is more dust in the environment, it will accelerate the wear of servo motors and ball screws. Of course, the full hydraulic injection molding machine also has high requirements in this regard, because dust, water vapor, etc. in the air may affect the use effect and quality of hydraulic oil and lubrication device.
