1. The injection molding process can be simply expressed as follows:
The last cycle is over - mold closing - filling - holding pressure - glue back - cooling - mold opening - demoulding - start the next cycle
In the filling and holding pressure drop section, the cavity pressure rises with time. After filling the cavity, the pressure will remain in a relatively static state to make up for the lack of glue due to shrinkage. In addition, this pressure can prevent injection due to The colloid backflow phenomenon caused by the reduction is the pressure-holding stage. After the pressure-holding is over, the pressure of the mold cavity gradually decreases, and it can theoretically drop to zero over time, but it is not zero in reality. Therefore, after demoulding, the internal memory of the product Stress, so some products need to be post-processed to remove residual stress. The so-called stress is the force that comes from the free movement of the Vogel chain or the chain segment, that is, bending deformation, stress cracking, shrinkage cavity, etc.
Second, the main parameters of the injection molding process
1. The temperature of the injection molding material and the melt temperature play a major role in the fluidity of the melt. Since the plastic has no specific melting point, the so-called melting point is a temperature range in a molten state. The structure and composition of the plastic molecular chain are different, so its The influence of fluidity is also different. Rigid molecular chains are more obviously affected by temperature, such as PC, PPS, etc., while the fluidity of flexible molecular chains such as PA, PP, PE, etc. is not obvious by changing the temperature, so it should be determined according to different materials. Adjust the reasonable injection temperature.
2. The injection speed is the speed (MM/S) of the melt in the barrel (also the propulsion speed of the screw). The injection speed determines the product appearance, size, shrinkage, flow distribution, etc. Generally, it is slow first - fast - —Later slow, that is, first use a higher speed to make the melt pass through the main channel, the runner, and the gate to achieve the purpose of balancing the injection, and then fill the entire cavity with a fast filling method, and then at a slower speed Supplement the lack of rubber caused by shrinkage and backflow until the gate freezes, which can overcome poor quality such as burning, air marks, and shrinkage.
3. Injection pressure is the resistance required by the melt to overcome the progress, which directly affects the size, weight and deformation of the product. Different plastic products require different injection pressures. For materials such as PA and PP, increasing the pressure will make them fluid Significantly improved, the injection pressure determines the density of the product, that is, the gloss of the appearance.
4. Mold temperature. Some plastic materials require higher mold temperature due to high crystallization temperature and slow crystallization speed. Some plastic materials require higher temperature or lower temperature due to size control and deformation, or the need for demoulding, such as PC. It is required to be above 60 degrees, and for PPS to achieve a better appearance and improve fluidity, the mold temperature sometimes needs to be above 160 degrees, so the mold temperature has an inestimable effect on improving the appearance, deformation, size, and plastic mold of the product.
3. Explanation of the meaning of injection molding professional parameters
1. Injection volume
Injection volume refers to the amount of melt injected into the mold by the screw of the injection molding machine during injection molding.
Injection volume = screw propulsion volume * ρ * C
ρ is the density of injection molding material
C is 0.85 for crystalline polymers and 0.93 for amorphous polymers
The injection molding machine cannot be used to process products that are less than 1/10 of the injection volume or exceed 70% of the injection volume
2. Metering stroke (pre-plastic stroke)
After each injection program is terminated, the screw is at the front end of the barrel. When the pre-plastic program arrives, the screw starts to rotate, and the material is sent to the head of the screw. The screw retreats under the reaction of the material until it touches the limit switch. This process is a metering process.
The size of the injection volume is related to the accuracy of the metering stroke. If it is too small, the injection volume will not be enough; if it is too large, the residual material after each injection at the front of the barrel will be too large, resulting in uneven melt temperature or overheating and decomposition.
After pre-molding, the melt in the metering practice has a temperature difference between the longitudinal temperature and the radial temperature, and the number of screw revolutions, the pre-molding back pressure and the temperature of the barrel will all have a greater impact on the melt temperature and temperature difference.
3. Anti-delay
The amount of anti-delay means that after the screw is metered in place, it retreats linearly for a distance, so that the specific volume of the metering chamber increases, the internal pressure drops, and the fluid is prevented from flowing out of the metering chamber.
Another purpose of anti-slip is to reduce the pressure of the nozzle flow channel system and internal stress when the injection nozzle is not backed up for pre-molding, and it is easy to pull out the material handle when the mold is opened. , For materials with high viscosity, there is no need to prevent delay.
The above parameters can be adjusted reasonably to obtain products that meet the quality requirements. For example, the size can be achieved through injection pressure, mold temperature, injection speed, and back pressure.
Four, how to adjust the injection molding process parameters
· Temperature control
Thermocouples are also widely used as sensors in temperature control systems. On the control instrument, the desired temperature is set, and the sensor's display is compared to the temperature produced at the set point. In the simplest system, when the temperature reaches a set point, it is turned off, and the power is turned back on when the temperature drops. This system is called on/off control because it is either on or off.
· temperature
Temperature measurement and control is very important in injection molding. While making these measurements is relatively simple, most injection molding machines don't have enough temperature sampling points or lines.
On most injection molding machines, temperature is sensed by thermocouples. A thermocouple basically consists of two different wires joined at the end. If one end is hotter than the other, a tiny electrical signal will be generated; the more heated, the stronger the signal.
· Melt temperature
Melt temperature is important, and the temperature of the shot pot used is only a guideline. Melt temperature can be measured at the nozzle or using the air jet method. The temperature setting of the injection cylinder depends on the melt temperature, screw speed, back pressure, injection volume and injection cycle.
If you do not have experience with a particular grade of plastic, start with the lowest setting. For ease of control, the firing cylinders are divided into zones, but not all are set to the same temperature. If operating for a long time or at high temperature, please set the temperature of the first zone to a lower value, which will prevent the plastic from melting and shunting prematurely. Before injection molding begins, make sure the hydraulic oil, hopper closure, mold, and injection cylinder are at the correct temperature.
· Injection pressure
This is the pressure that causes the plastic to flow and can be measured with a sensor on the nozzle or hydraulic line. It has no fixed value, and the more difficult it is to fill the mold, the higher the injection pressure, and the injection line pressure is directly related to the injection pressure.
