How is precision injection molding machine defined?
Precision injection molding machine refers to the molding machinery and equipment suitable for the production of precision plastic products. For a precision injection molding machine, how to measure or judge?
Generally, there are two indicators of precision injection molding machine: one is the repeated deviation of product size; the other is the repeated weight deviation of product. The former is difficult to compare due to the different size and thickness of the products, while the latter represents the comprehensive level of the injection molding machine.
In general, the weight repetition error of common injection molding machine is about 1%, the better machine can reach 0.8%, the precision machine is less than 0.5%, and the ultra precision machine is less than 0.3%. As mentioned before, precision injection molding machine requires that the dimensional accuracy of products is generally within 0.01-0.001mm.
Many precision injection molding machines also require:
① High injection pressure, high injection speed;
② The clamping system has enough rigidity and clamping accuracy. The so-called clamping accuracy refers to the uniformity, adjustability, stability and repeatability of the clamping force, as well as the high opening and closing position accuracy;
③ The pressure, flow, temperature and measurement can be controlled to the corresponding accuracy, and multi-stage injection can be used to ensure the reproduction of molding process and repeated accuracy of products.
What are the requirements of precision injection molding for injection molding machine?
1. Characteristics of technical parameters:
① Injection pressure:
Ordinary injection molding machine: 147-177mpa; precision injection molding machine: 216-243 MPa; ultra-high pressure injection molding machine: 243mpa or above, generally between 243-392 MPa.
The advantages of high-pressure molding of precision injection molding machine are as follows:
A. Improve the precision and quality of precision products.
When the injection pressure reaches 392 MPa, the molding shrinkage is almost zero. At this time, the accuracy of the product is only affected by the mold control or the environment. The experimental results show that the mechanical strength of the parts can be increased by 3-33% when the injection pressure is from 98-392 MPa.
B. It can reduce the wall thickness of precision products and increase the molding length.
Taking PC as an example, the injection pressure of ordinary machine is 177 MPa, and the products with 0.8 mm wall thickness can be formed, while the products with 0.45 mm thickness can be formed when the injection pressure of precision machine is 392 MPa. High pressure injection machine can obtain products with larger flow length ratio.
C. Increasing the injection pressure can fully muddy the injection rate.
In order to achieve the rated injection rate, there are two ways: one is to increase the maximum injection pressure of the system; the other is to modify the injection device or injection system parameters, including screw parameters;
② Injection speed:
The injection speed of precision injection machine is high. The injection speed of precision hydraulic injection molding machine is required to be more than 200 mm / s, and that of all electric injection molding machine is required to be more than 300 mm / s.
2. Features of precision injection machine in control:
① It is necessary to adopt multi-level injection feedback control because of the high requirement for the repeatability (reproducibility) of injection molding parameters.
1. Multi level position control;
2. Multi level speed control;
3. Multistage pressure maintaining control;
4. Multistage back pressure control;
5. Multi stage screw speed control.
The accuracy of the displacement sensor shall be at least 0.1mm, which can strictly control the metering stroke, injection stroke and the thickness of the residual material pad (injection monitoring point). Ensure the accuracy of each injection and improve the molding accuracy of products.
② Accurate temperature control of barrel and nozzle
When the temperature control system heats up, the overshoot and temperature fluctuation should be small. The precision injection molding adopts PID proportional, integral and differential temperature control, which makes the temperature accuracy between ± 1 ℃, the temperature control accuracy of ultra precision injection molding machine reaches ± 0.1 ℃, and the temperature deviation can be stably controlled between ± 0.2 ℃.
③ Temperature control of hydraulic oil for injection molding machine
The change of oil temperature of injection molding machine results in the fluctuation of injection pressure. It is necessary to adopt cooling device for hydraulic oil, and it is better to keep the oil temperature at 50-55 ℃.
④ Control of mold temperature requirements
Mold temperature has a considerable impact on the later thermal shrinkage of the parts, as well as on the apparent quality and crystallinity of the parts, and also on the mechanical properties of the parts. If the cooling time is the same, the thickness of products with low mold cavity temperature is larger than that of products with high temperature.
3. Hydraulic system of precision injection molding machine
① The oil system needs to use proportional pressure valve, proportional flow valve or closed-loop variable pump system.
The hydraulic system of injection molding machine needs to adopt proportional pressure valve and proportional flow valve to realize different working pressure and flow of the system. Whether it is the quantitative pump hydraulic system or the variable pump hydraulic system, there are proportional valves to control the working pressure and flow of the oil pump output, and realize the proportional linear adjustment.
② In the direct press type die closing mechanism, the oil circuit of the die closing part and the oil circuit of the injection part are separated.
This requirement is for the direct pressure injection molding machine, because the direct pressure injection molding machine starts to inject the melt and fill the mold cavity at the moment when the mold is closed and under high pressure. At the same time, there must be oil filling cylinder to form high pressure.
Otherwise, the middle plate will retreat due to filling pressure and die expanding force, which is not allowed to happen. This requires the separation of the mold closing oil circuit and the injection oil circuit, otherwise the problem of oil pump overload will occur due to the oil filling the mold closing oil cylinder and the injection oil cylinder at the same time, and will affect the injection speed and system stability.
③ Because of the high speed of precision injection molding machine, the response speed of hydraulic system must be emphasized.
Precision injection molding is mostly completed under the condition of high injection pressure and high injection speed (connector or mobile phone button). The filling time is very short, generally only a few seconds, which requires the system to have high response performance, otherwise the response will be sluggish, resulting in system output