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Plastic molding is an engineering technology, which involves the transformation of plastic into materials and products with use value and can maintain the original performance, even beyond the original performance. There are many molding methods for plastics, among which injection molding is one of the most important. Injection molding can also be called injection molding, or injection molding for short. The injection molding process is a typical intermittent operation cycle process. Its basic process is: the granular polymer material is extruded and heated by the screw of the injection molding machine to become a melt that can flow in the molten state. Driven by the screw, the plastic melt enters the mold cavity through the nozzle of the injection molding machine, the main flow channel, the shunt channel and the gate of the mold, forming a certain shape and Process of dimension products. Injection cycle is mainly composed of closed mold, injection seat forward, injection, pressure maintaining, pre molding, cooling, mold opening, ejection products and other procedures.
I. Theoretical Basis of plastic forming
1. Heating and cooling of polymer
When the polymer is heated, the temperature difference that promotes the heat transfer efficiency cannot be raised too high. Because the heat transfer of the polymer is not good, the local temperature may be too high, which will cause degradation. When the polymer melt is cooled, the temperature difference between the cooling medium and the melt should not be too large, otherwise the internal stress will be produced because of the rapid cooling. Because the degradation rate of the cortex is much faster than that of the inner layer when the polymer melt cools rapidly, and the cortex is hard. When the inner layer gets further cooling, it will be in the state of stretching due to contraction, and at the same time, the cortex will be stressed. In this case, the physical properties of polymer products are lower than the expected values.
2. Crystallization of polymer
During the heating process of polymer, there are heating, cooling and pressure maintaining functions, which will strongly affect the morphology and final product quality of crystalline polymer, because the morphology of crystalline polymer is not only related to the molecular structure of polymer itself, but also closely related to the history of crystallization formation.
II. Influence of injection molding process on plastic properties
1. Injection rate
In the process of injection molding, the injection rate is one of the most important factors affecting the filling and final distribution of materials. Injection rate refers to the volume flow rate that can be achieved in unit time during injection, which is the ratio of injection volume and injection time. Injection rate has a great influence on plastic parts, and the determination of injection rate should be based on the structure, shape and size of different injection products, the properties of sprue system and plastic, and the relevant rheological data. In general, the injection rate must guarantee the shear rate and the shortest flow length allowed for the specified material in the rheological data, and the thermal effect caused by shearing must not exceed the allowable degree of the thermophysical properties and shear strength of the polymer. The injection rate mainly affects the flow behavior of the melt in the mold cavity. The injection rate directly affects the impact resistance of the injection parts.like baby wet wipes lid and wipes
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With the increase of injection rate, the filling pressure will be increased. With the increase of injection rate, the high temperature of melt can be maintained, the viscosity of melt will be reduced, and the resistance loss of flow passage will be reduced. In this way, the molten material can enter the mold cavity with high pressure, which can increase the flow length and make the quality of plastic parts uniform and dense, so that the strength of parts will increase with the increase of injection rate. Enlarge. However, too high mold filling rate will increase the pressure loss, which will cause the unstable flow of molten material and the occurrence of elastic turbulence, resulting in the phenomenon of overflow of molten material. In addition, in high-speed injection, the gas in the mold cavity is often too late to be discharged, mixed in the melt and seriously affect the quality of the plastic parts, so the strength is reduced.
2. Pressure holding time
Pressure holding time, that is, a period of time when the plastic in the mold is replenished after filling the mold cavity during injection molding. When the raw material is crystalline plastic, the control of holding time is very important. With the increase of holding time, the strength of simply supported beam increases. When the holding time reaches a certain value, the strength of the workpiece is relatively stable. Because the holding time is too short, the density and shrinkage of polymer melt will be inconsistent, resulting in the reduction of the strength of the parts.
3. Barrel temperature
Increasing the barrel temperature is conducive to improving the filling status and the transfer of melt in the cavity, reducing the orientation, and improving the comprehensive properties of the products, but too high temperature is not desirable. When the barrel temperature is close to injection molding
At the upper limit temperature, on the one hand, it is easy to produce more gases, which will cause bubbles, discoloration, scorching, etc. in the plastic, on the other hand, it will cause flash due to the low viscosity; on the other hand, it will degrade the plastic if the temperature is too high. Therefore, the barrel temperature should be well controlled. 4. Mold temperature
Mold temperature refers to the surface temperature of the mold cavity contacting with the product during the injection process. When the plastic melt is injected into the mold cavity, it will release a lot of heat and solidify. At this time, the mold cavity needs to be maintained at a proper temperature. At this temperature, it will be most conducive to the molding of plastic parts, with the highest molding efficiency and the smallest internal stress and warpage. Mold temperature is the main factor to control the cooling and finalization of products, which directly affects the flow behavior of the die, the cooling speed of the plastic parts and the properties after forming. If the mold temperature is selected reasonably, the internal performance and appearance quality of the plastic parts can be effectively improved.
In the injection molding process, the mold temperature is controlled by the cooling medium. The lower the mold temperature is, the faster the cooling speed is, the faster the melt temperature is reduced, the higher the melt viscosity, the difference in shrinkage caused by uneven cooling, and even the insufficient filling of the mold will lead to the reduction of the strength of the parts. With the increase of mold temperature, the melt fluidity increases, the required filling pressure decreases, and the surface quality of parts improves. For crystalline plastics, because higher temperature is conducive to crystallization, increasing mold temperature can improve the density or structural crystallinity of products. At higher mold temperature, the relaxation process of polymer macromolecules is faster, and the molecular orientation and internal stress will be reduced. Therefore, increasing the mold temperature within a certain range will increase the strength of the parts, but the mold temperature should not be too high, otherwise it will cause ejection difficulties. At the same time, too high mold temperature will increase the cooling time, which will reduce the productivity of products.like plastic medical products.
5. Injection pressure
The pressure is related to many features of the plastic part. Increasing the injection pressure helps to fill the mold, increase the density of the part, reduce the shrinkage of the part, and improve the stability of the size. However, the internal stress and orientation of the plastic parts also increase, which makes it difficult to demould the parts, damages the surface of the products and may cause flash. Generally higher injection pressure is beneficial to the comprehensive performance of the product
6. Pressure maintaining pressure
The pressure maintaining pressure is the pressure when replenishing the material in the mold cavity after injection to prevent the material from backflow or cooling defects. In the process of pressure maintaining, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume that is vacated due to the shrinkage of the part. At the end of the injection molding process, the screw stops rotating and only pushes forward. At this time, the injection molding enters the pressure maintaining stage. The function of the holding stage is to continuously apply pressure, compact the melt and increase the density of the plastic to
Compensate for the shrinkage of the plastic. In the process of pressure maintaining, because the cavity has been filled with plastic and the back pressure is high, in the process of pressure maintaining and compaction, the screw of injection molding machine can only move forward slightly slowly, and the flow speed of plastic is also relatively slow. At this time, the flow is called pressure maintaining flow. In the pressure maintaining stage, the cooling and solidification of the plastic by the mold wall are accelerated, and the viscosity of the melt is also increased rapidly, so the resistance in the mold cavity is very large. In the later stage of pressure maintaining, the density of material continues to increase, and the plastic part also gradually forms. The pressure maintaining stage should continue until the gate solidifies and seals. At this time, the cavity pressure in the pressure maintaining stage reaches the highest value.
When the pressure is lower or higher than the optimum holding pressure, the strength will decrease. Because the pressure maintaining pressure is too small and the feeding effect is small, the inner part of the injection molded part will form bubbles due to shrinkage, and the surface of the plastic part will also form dents; if the pressure maintaining pressure is too large, it is easy to generate large internal stress around the gate, which will lead to the reduction of the strength of the part.
7. Plasticizing pressure
Plasticizing pressure refers to the pressure on the melt at the top of the screw when the screw retreats, also known as back pressure. Generally speaking, with the increase of back pressure, the screw's backward speed decreases, the melt's receiving pressure increases, the shear effect is strengthened and the plasticizing effect is improved. However, when the back pressure is increased, on the one hand, the reverse flow and leakage flow of the melt at the edge of the spiral groove reduce the amount of plasticization, resulting in the shortage of materials in the metering section; on the other hand, it will make the shearing too high and the shearing stress too large, which will degrade the materials, generate bubbles or scorch, and affect the quality of the parts.
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