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Let FCS Tell You the Requirements of Precision Injection Molding on an Injection Molding Machine

What is the definition of a precision injection molding machine?
Precision injection molding machine refers to molding machinery and equipment suitable for molding and producing precision plastic products. How to measure or judge a precision injection molding machine?

Usually, there are two indicators for a precision injection molding machine: one is the repeated size deviation of the product; the other is the repeated weight deviation of the product. The former indicator is difficult to be compared due to the difference in size and thickness of products; while the latter indicator represents the comprehensive level of the injection molding machine.

The repeated weight deviation of an ordinary injection molding machine is approximately 1%, while that of a better machine can reach 0.8%, while that of a precision machine is less than 0.5%, and that of an ultra-precision machine is less than 0.3%. As mentioned above, as a precision injection molding machine, the size accuracy of products is required to be generally within 0.01 mm to 0.001 mm.

Many precision injections also require the injection molding machine to have the following characteristics:
①    High injection pressure and high injection speed
②    The clamping system has enough rigidity and mode-locking accuracy. The so-called mode-locking accuracy refers to the high uniformity, adjustability, stability, and repeatability of clamping force, and the high precision of mold-opening and mold-locking position.
③    Pressure, flow rate, temperature, and metering can be controlled to the corresponding precision, and multistage injection can be used to ensure the reproduction of the molding process and the repeated precision of products.



What requirements does precision injection molding have for an injection molding machine?
1. Technical features:
① Injection pressure 
Ordinary injection molding machine: 147-177 Mpa; precision injection molding machine: 216-243 Mpa; ultra-high-pressure injection molding machine: above 243 MPa, usually 243-392 MPa. 

The advantages of high-pressure molding by precision injection molding machine are as follows:
A. Improving the precision and quality of precision products. 
The injection pressure has the most obvious influence on the molding shrinkage of the product. When the injection pressure reaches 392 MPa, the molding shrinkage of the product is almost zero. At this time, the precision of the product is only affected by the mold control or environment. Experiments show that the mechanical strength of the product can be increased by 3-33% when the injection pressure is from 98 MPa to 392 MPa.

B. Reducing the wall thickness of precision products and increasing the molding length. 
Taking PC as an example, the injection pressure of an ordinary machine is 177 Mpa, and the products with a wall thickness of 0.8 mm can be molded, while the injection pressure of a precision machine is 392 MPa, and the products with a wall thickness above 0.45 mm can be molded. An ultra-high-pressure injection molding machine can produce products with a larger flow length ratio.

C. Giving full play to the effect of injection rate by increasing injection pressure. 
In order to achieve the rated injection rate, there are two methods: First, increasing the maximum injection pressure of the system; second, reforming the parameters of injection device or injection system, including screw parameters;

② Glue injection speed: 
High injection speed is required for precision injection molding machines. The injection speed of a precision hydraulic injection molding machine is required to be over 200mm/s, and that of an all-electric injection molding machine can be over 300mm/.

2.  Control features of precision injection molding machine:
①Since a high repeated accuracy (reproducibility) of injection molding parameters is required, multistage injection feedback control should be adopted. 
1. Multistage position control 
2. Multistage speed control 
3. Multistage holding pressure control
4. Multistage back pressure control 
5. Multistage screw speed control

The accuracy of the displacement sensor should be at least 0.1 mm, so that the metering stroke, injection stroke, and the thickness of the residual cushion (injection monitoring point) can be strictly controlled. Thus, the injection quantity can be ensured accurately every time to improve the molding accuracy of products. 

②The temperature control of the barrel and nozzle should be precise.
The overshoot and fluctuation of temperature should be small when the temperature control system is heated up. PID proportional, integral, and differential temperature control are used for precision injection to control the temperature accuracy to fluctuate within ±1°C. The temperature control accuracy of ultra-precision injection molding machine is within ±0.1°C when the temperature deviation is stably controlled within ±0.2°C. 

③The temperature of the hydraulic oil of the injection molding machine needs to be controlled.
The change of the oil temperature of the injection molding machine leads to the fluctuation of injection pressure, so a cooling device must be used for the hydraulic oil, and the oil temperature should be stabilized at 50-55°C. 

④The temperature of the mold needs to be controlled.
The temperature of the mold has a great influence on the later thermal shrinkage of the product, and it also affects the apparent quality and crystallinity of the product, as well as the mechanical properties of the product. If the cooling time is the same, the thickness of the product molded in a mold cavity with a low temperature is larger than that of the product molded in a mold cavity with a high temperature.

3.  Hydraulic system of precision injection molding machine
①    For the oil circuit system, a proportional pressure valve, proportional flow valve, or closed-loop variable pump system is required. 
For the hydraulic system of the injection molding machine, the proportional pressure valve and the proportional flow valve are required to realize different working pressures and flows. Proportional valves exist in both fixed pump hydraulic systems and variable pump hydraulic systems to control the working pressure and flow output by the oil pump and realize proportional linear adjustment.

② In the direct-pressure clamping mechanism, the oil circuit of the clamping part is separated from the oil circuit of the injection part. This requirement is for the direct-pressure injection molding machine because the direct-pressure injection molding machine starts to inject melt and fill the mold cavity immediately after the clamping starts at high pressure. At the same time, the clamping cylinder must be filled with oil to form a high pressure for clamping. Otherwise, the middle plate will retreat due to the filling pressure and expansion force, which is not allowed to happen. Therefore, the oil circuit of the clamping part is required to be separated from that of the injection part. Otherwise, the oil pump overload problem will occur because the oil fills the oil cylinder for clamping and the oil cylinder for injection at the same time, which will affect the injection speed and system stability.

③ Because of the high speed of the precision injection molding machine, the response speed of the hydraulic system must be emphasized. 
The precision injection is mostly completed under the condition of high injection pressure and high injection speed (connectors or buttons of mobile phones), and the filling time is very short, generally only a few tenths of a second. Therefore, the system is required to have high response performance. Otherwise, there will be a response delay phenomenon, which will eventually lead to a large deviation between the requirements of the system output signal and the input signal. Since the molding process is difficult to be debugged, the size or weight stability of products will be directly affected in production. 

The high responsiveness of the injection molding machine depends on the scanning time of the controller, the response time of the oil pump, and the response time of the oil valve. The hydraulic system with a servo valve has considerable advantages in response and repeated accuracy. 

Variable pump injection molding machine has high responsiveness because the pressure and flow output of oil pump is adjusted by the swashplate swing angle of oil pump, and the repeated accuracy of oil pump can reach below 0.2%.

④ The hydraulic system of the precision injection molding machine should fully embody the integration project of machine-electricity-liquid-instrument. 
At present, all-computer control is widely used in injection molding machines, with gradually improved intelligent functions, such as "man-machine dialogue", fault self-diagnosis, molding state monitoring, and SPC quality monitoring. The hydraulic system of an injection molding machine is the power component for each action of the injection molding machine, the controller is the control center of the hydraulic system, and multiple instruments are the auxiliary monitoring parts for the running state of the machine. In fact, an injection molding machine is an intelligent body organically combined with machines, electricity, liquids, and instruments.

4. Structural features of precision injection molding machine
(1)    Because of the high injection pressure of the precision injection molding machine, high rigidity is required for the clamping system, and the parallelism of moving and fixed templates should be controlled within the range of 0.08-0.10mm.
(2)    The low-pressure mold needs to be protected and the precision of the clamping force needs to be controlled because the clamping force will affect the degree of deformation of the mold and ultimately the size tolerance of the product.
(3)    The mold-opening and mold-closing speed should be fast, generally about 60mm/s.
(4)    Plasticizing parts such as screw, screw head, check ring, and charging barrel should be designed into structural forms with strong plasticizing ability, good homogenization degree, and high injection efficiency; the driving torque of the screw should be large, and it can be changed infinitely.


Manifestations of precise injection achieved by injection molding machine
Precision injection molding machine refers to molding machinery and equipment suitable for molding and producing precision plastic products.

As mentioned earlier, injection products have been applied in different fields and are widely used to replace metal parts with high precision. As a result, strict requirements are put forward on the size accuracy, quality accuracy, apparent quality, and mechanical properties of injection products. At the same time, higher requirements are put forward on the technological factors that affect the quality of injection products.

The ideal state of product control by injection molding machine is to directly control the variables of product size, quality, apparent quality, and mechanical properties for feedback control. However, at present, these non-electrical signals cannot be directly measured and converted into electrical signals. Therefore, only the controllable variables of the injection molding machine that affect the above product quality can be controlled, mainly including: 
 (1) For the temperature of each section of screw melting barrel, an aperiodic signal, generally, at least 4 sections are selected for heating; 
 (2) For the temperature of hydraulic oil, an aperiodic signal, a conventional thermocouple is used for detection 
 (3) For pre-molding displacement of the screw, a displacement electronic ruler is used to detect periodic signals, thereby leading out speed signals 
 (4) For the pressure of the injection system, a strain sensor is installed on the oil cylinder for injection to detect periodic signals and measure injection pressure, holding pressure and back pressure signals 
 (5) For the screw speed, a speed sensor is used for detection, and the back pressure is kept constant 
 (6) For the screw retreat stop position, which is used for measuring injection quantity, an electronic ruler is used for detection 
 (7) For the injection pressure, at least four levels of curves are set and feedback signals are detected for feedback control 
 (8) For the injection speed, at least four levels of curves are set and feedback signals are detected to feedback displacement variables and conversion speed for feedback control.

Further reading:Tips For You To Choose A Proper Injection Molding Machine
 
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