Program control of injection speed
The program control of the injection speed is to divide the injection stroke of the screw into 3 to 4 stages, and use the appropriate injection speed in each stage.
For example: slow down the injection speed when the molten plastic first passes through the gate, use high-speed injection during the mold filling process, and slow down the speed at the end of the mold filling. Using this method can prevent overflow, eliminate flow marks and reduce residual stress on the product.
When filling the mold at low speed, the flow rate is stable, the size of the product is relatively stable, the fluctuation is small, the internal stress of the product is low, and the stress in each direction inside and outside the product tends to be consistent (for example, a polycarbonate part is immersed in carbon tetrachloride and used for high-speed injection molding) The parts have a tendency to crack, but low-speed ones do not crack).
Under relatively slow mold filling conditions, the temperature difference of the material flow, especially the large temperature difference between the material before and after the gate, helps to avoid the occurrence of shrinkage cavities and dents.
However, due to the long filling time, it is easy for the parts to appear delamination and poor welding marks, which not only affects the appearance, but also greatly reduces the mechanical strength. During high-speed injection, the material flow speed is fast.
When the high-speed mold filling is smooth, the melt fills the cavity quickly, the material temperature drops less, and the viscosity drops less. Lower injection pressure can be used, which is a kind of hot material filling. modal potential.
High-speed mold filling can improve the gloss and smoothness of the parts, eliminate seam lines and delamination, reduce shrinkage and dents, make the color uniform, and ensure the fullness of larger parts of the parts.
However, it is easy for the product to become fat and blistered or yellow, or even burn and scorch, or cause difficulty in demoulding, or uneven mold filling. For high-viscosity plastics, it may cause melt rupture and cause cloud spots on the surface of the part.
High-speed and high-pressure injection may be considered in the following situations
- The plastic has high viscosity and fast cooling speed. Long-process parts are made with low pressure and slow speed and cannot completely fill all corners of the cavity;
- For parts with too thin wall thickness, the molten material is easy to condense and stay when it reaches the thin wall. A high-speed injection must be used so that the molten material can enter the cavity immediately before a large amount of energy is consumed;
- Plastics reinforced with glass fiber, or plastics containing a large amount of filler materials, have poor fluidity. In order to obtain parts with a smooth and uniform surface, high-speed and high-pressure injection must be used. For high-end precision products, thick-walled parts, parts with large changes in wall thickness, and parts with thick flanges and ribs, it is best to use multi-stage injection, such as level two, level three, level four or even level five.
Program control of injection pressure
The control of injection pressure is usually divided into the control of primary injection pressure, secondary injection pressure (holding pressure) or three or more injection pressures. Whether the timing of pressure switching is appropriate is very important to prevent excessive pressure in the mold, overflow or lack of material, etc.
The specific volume of the molded product depends on the melt pressure and temperature when the gate is closed during the pressure holding stage. If the pressure and temperature are consistent each time when switching from pressure holding to product cooling stage, the specific volume of the product will not change.
At a constant molding temperature, the most important parameter that determines the size of the product is the holding pressure, and the most important variables that affect the dimensional tolerance of the product are the holding pressure and temperature.
For example: after the mold filling is completed, the holding pressure decreases immediately. When the surface layer forms a certain thickness, the holding pressure rises again. In this way, large thick-walled products can be formed with low clamping force, eliminating pits and flash.
The holding pressure and speed are usually 50%~65% of the maximum pressure and speed when the plastic fills the mold cavity, that is, the holding pressure is about 0.6~0.8MPa lower than the injection pressure.
Since the holding pressure is lower than the injection pressure, the load on the oil pump is low during the considerable holding time, the service life of the solid oil pump is extended, and the power consumption of the oil pump motor is also reduced.
Three-level pressure injection can make the parts smoothly filled without welding lines, dents, flash and warping deformation. It is beneficial for the molding of thin-walled parts, multi-head small parts, long-process large parts, and even the molding of parts with uneven cavity configuration and loose mold clamping.
Program control of plastic filling amount injected into mold cavity
A certain amount of measurement is pre-adjusted so that a small amount of melt (buffer amount) remains at the end of the screw near the end of the injection stroke.
Further injection pressure (secondary or third injection pressure) is applied according to the filling situation in the mold. Add a little more melt. In this way, the product can be prevented from sinking or the shrinkage of the product can be adjusted.
Program control of screw back pressure and speed
High back pressure can cause strong shearing of the melt, and low rotation speed can also cause the plastic to be plasticized in the barrel for a long time. Therefore, simultaneous programming control of back pressure and rotational speed is currently used more frequently.
For example: during the entire screw metering stroke, first use high speed and low back pressure, then switch to lower speed and higher back pressure, then switch to high back pressure and low speed, and finally plasticize under low back pressure and low speed.
In this way, the pressure of the melt at the front of the screw is mostly released, reducing the rotational inertia of the screw, thereby improving the accuracy of screw metering.
Excessive back pressure often causes the discoloration of the colorant to increase; the mechanical wear of the barrel screw of the pre-molding mechanism increases; the pre-molding cycle is extended and the production efficiency decreases;
the nozzle is prone to drooling and the amount of recycled material increases; even if a self-locking type is used Nozzles, if the back pressure is higher than the designed spring locking pressure, will also cause fatigue damage. Therefore, the back pressure must be adjusted appropriately.
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