Injection pressure
◎Injection Molding Process Parameters(part 2)
Injection pressure is used to overcome the resistance of the melt during its flow. This resistance necessitates the injection pressure from the injection molding machine. As shown in Figure 2-1, the pressure is highest at the injection nozzle during injection molding to overcome the flow resistance throughout the melt's path. Subsequently, the injection pressure gradually decreases towards the front of the melt as the flow length increases. If the mold cavity has good venting, the final pressure at the front of the melt is atmospheric pressure.
Figure 2-2 shows the distribution of melt pressure along its flow path. As the flow length increases, the resistance that needs to be overcome along the way also increases, and the injection pressure also increases accordingly. In order to maintain a constant pressure gradient to ensure uniform melt filling speed, the injection pressure must be increased accordingly with the change of flow length, and therefore the pressure at the melt inlet must be increased accordingly.
Holding pressure
Near the end of the injection process, the injection pressure switches to the holding pressure, entering the holding phase. During the holding phase, the injection molding machine feeds material into the cavity from the nozzle to fill the volume vacated by the shrinkage of the part. If the cavity is filled without holding pressure, the part will shrink by approximately 25%, especially at the ribs where excessive shrinkage will leave shrinkage marks. The holding pressure is generally about 85% of the maximum filling pressure, but this should be determined based on the specific circumstances.
Figure 2-3 shows the pressure holding process control: 1 indicates the start of injection; 2 indicates the melt enters the cavity; 3 indicates a pressure holding switch has occurred during filling; 4 indicates the cavity is full; 5 indicates the filling process has entered the shrinkage compensation stage; 6 indicates shrinkage compensation has ended and cooling has begun. The post-filling stage includes two processes: pressure holding and cooling.
(Figure 2-3 Pressure Holding Process Control)
Experiments show that both excessively long and short holding times are detrimental to molding. Excessive holding time leads to uneven pressure distribution, increased internal stress in the molded part, and increased susceptibility to deformation, potentially causing stress cracking. Conversely, insufficient holding time results in inadequate pressure retention, severe volume shrinkage, and poor surface quality.
The holding pressure curve consists of two parts: a constant pressure holding period of approximately 2-3 seconds, called the constant holding pressure curve; and a gradually decreasing pressure release period of approximately 1 second, called the delayed holding pressure curve. The delayed holding pressure curve has a significant impact on the molded part. A longer constant holding pressure curve results in less volume shrinkage, and vice versa. Similarly, a steeper slope and shorter delayed holding pressure curve result in greater volume shrinkage, and vice versa. A segmented and elongated delayed holding pressure curve results in less volume shrinkage, and vice versa.
During the filling process of molten plastic, when the cavity is almost full, the screw's movement switches from flow rate control to pressure control. This transition point is called the holding pressure switching control point. Holding pressure switching is crucial for controlling the molding process. Before the holding pressure switching point, the melt's forward speed and pressure are high; after the switching point, the screw's pressure pushing the melt forward is lower. If holding pressure switching is not performed, the pressure will be very high when the cavity is full of melt, causing a sharp increase in injection pressure, requiring greater clamping force, and even leading to defects such as flash (excess material). Holding pressure switching in injection molding machines is generally performed according to the injection position; that is, when the screw reaches a certain position, holding pressure switching occurs. The position, timing, and pressure of holding pressure switching are shown in Figure 2-4.
Screw back pressure
During the melting and plasticizing process of plastic, the melt continuously moves towards the front end of the barrel (metering chamber), and the amount increases, gradually forming a pressure that pushes the screw backward. To prevent the screw from retracting too quickly and to ensure uniform compaction of the melt, a pressure in the opposite direction needs to be provided to the screw. This pressure in the opposite direction that prevents the screw from retracting is called back pressure, as shown in Figure 2-6.
Back pressure, also known as plasticizing pressure, is controlled by adjusting the return oil throttle valve of the injection cylinder. A back pressure valve is installed at the rear of the injection cylinder to adjust the oil discharge speed of the injection cylinder during screw rotation and retraction, maintaining a certain pressure in the cylinder. The screw retraction speed (resistance) of the all-electric motor is controlled by an AC servo valve.
Properly adjusting the back pressure has significant benefits for injection molding quality. In injection molding, appropriately adjusting the back pressure can achieve the following benefits:
① It can compact the melt in the barrel, increasing density and improving the stability of injection volume, product weight, and dimensions.
② It can "squeeze out" gas from the melt, reducing surface gas bubbles and internal air bubbles, improving gloss uniformity.
③ It slows down the screw retraction speed, allowing the melt in the barrel to fully plasticize, increasing the mixing uniformity of color powder/masterbatch with the melt, and preventing color mixing in the product.
④ Appropriately increasing the back pressure can improve surface shrinkage and perimeter flow of the product.
⑤ It can increase the temperature of the melt, improve the plasticizing quality of the melt, improve the fluidity of the melt during mold filling, and eliminate cold glue marks on the surface of the product.
clamping force
Clamping force is set to resist the expansion force of molten plastic on the mold, and its magnitude is determined by specific factors such as injection pressure. However, in reality, after the molten plastic is ejected from the injection molding machine's barrel nozzle, it passes through the main runner, branch runners, and gates of the mold before entering the mold cavity, resulting in significant pressure loss along the way. Figure 2-7(a) shows the change in injection pressure throughout the process from the barrel to entering the mold. As can be seen from the pressure change in Figure 2-7(b), the pressure drops to only 20% of the initial injection pressure at the end of the mold cavity.
Barrel temperature
The melt temperature must be controlled within a certain range. If the temperature is too low, poor plasticization of the melt will affect the quality of the molded parts and increase the difficulty of the process; if the temperature is too high, the raw materials are prone to decomposition. In actual injection molding, the melt temperature is often higher than the barrel temperature. The difference is related to the injection rate and the properties of the material, and can reach up to 30°C. This is because the melt is subjected to shear as it passes through the gate, generating a lot of heat, as shown in Figure 2-8.
(1 - Barrel heating begins; 2 - Screw plasticizing begins; 3 - Melt reaches the end of the runner; 4 - Melt passes through the gate; 5 - Filling ends)
Barrel temperature is a crucial factor affecting injection pressure. Injection molding machine barrels typically have 5 or 6 heating zones, and each material has its suitable molding temperature. Specific molding temperatures can be found in the data provided by the supplier. Table 2-3 lists the molding temperatures for commonly used plastics..