Higher mold temperatures result in better transferability of the product surface, especially for products with patterns on the molded surface, where the mold temperature should be appropriately increased.
Mold Temperature
Figure 2-10 shows the temperature distribution of the mold during injection molding. To ensure product quality, there is an optimal temperature setting for the mold. For example, when manufacturing ABS box-shaped products with high appearance requirements, the temperature of the outer surface side (fixed mold plate side) of the product in the mold cavity can be set at 50-65℃, while the temperature of the inner surface side (moving mold plate side) can be set about 10℃ lower than the outer surface side. At this temperature, the resulting product surface has no shrinkage marks and a good appearance. Furthermore, a higher mold temperature results in better transferability of the product surface, especially when molding products with patterns; in such cases, the mold temperature should be appropriately increased.
Figure 2-10 Temperature-time curves at different locations inside the mold
(a-Mold cavity surface; b-Cooling pipe wall; c-Cooling pipe outlet; d-Cooling pipe inlet)
For crystalline plastics, the crystallization rate is governed by the cooling rate. Increasing the mold temperature, due to the slower cooling, can increase the crystallinity, which is beneficial for improving the dimensional accuracy and mechanical properties of the finished product. Crystalline plastics such as nylon, polyoxymethylene, and PBT require higher mold temperatures for this reason.
Injection speed
Injection speed refers to the speed at which the screw advances and fills the mold cavity with molten plastic. It is generally expressed as the injection mass per unit time (g/s) or the screw speed (m/s). Injection speed, along with injection pressure, is one of the important conditions in injection molding. Different injection speeds can produce different effects. Figure 2-11 shows the material flow during low-speed and high-speed mold filling.
During low-speed injection, the melt flow rate is slow, and the melt gradually flows from the gate towards the far end of the cavity. The leading edge of the melt is spherical. The melt that enters the cavity first cools and its flow rate slows down. The portion near the cavity wall cools into a thin, highly elastic shell, while the portion farther from the cavity wall remains a viscous hot flow, and the leading edge of the melt remains spherical. After completely filling the cavity, the thickness of the cooled shell increases and it hardens. This slow filling process, due to the long time the melt takes to enter the cavity and the slow cooling, increases viscosity and flow resistance, requiring a higher injection pressure.
Injection volume
Injection volume refers to the total mass (g) of the product, including the main runner and branch runners. Theoretically, molding is possible if this value is less than the maximum injection volume (g) of the injection molding machine. However, generally, the injection volume should be less than 85% of the injection molding machine's rated injection volume. If the actual injection volume is too small, the plastic will undergo thermal decomposition due to excessive residence time in the barrel. To avoid this, the actual injection volume should be at least 30% of the injection molding machine's rated injection volume. Therefore, the injection volume is generally best set between 30% and 85% of the injection molding machine's rated injection volume.
Screw ejection position
Injection position is one of the most important parameters in injection molding. It is generally determined by the total weight of the plastic part and the sprue (residue). Sometimes, the injection position of the backfill stage needs to be rationally set based on the type of plastic used, mold structure, and product quality.
Most plastic products are injection molded using a three-stage or higher injection method. Key points of controller injection methods include setting different injection start positions, screw switching positions, holding pressure volume, remaining buffer amount, and pressure release amount, as shown in Figure 2-12.
(Figure 2-12 Screw ejection position)
Injection time
Injection time is the time pressure is applied to the screw, including the time required for plastic flow, mold filling, and holding pressure. Therefore, injection time, injection speed, and injection pressure are all important molding conditions. Finding the correct injection time can be done using two methods: appearance setting method and weight setting method.
Although the injection time is very short and has a small impact on the molding cycle, adjusting the injection time plays a significant role in pressure control of the gate, runner, and cavity. A reasonable injection time helps the melt achieve ideal filling and is crucial for improving the surface quality of the product and reducing dimensional tolerances. The injection time should be much shorter than the cooling time, approximately 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the total molding time of the plastic part, as shown in Figure 2-13.
(Figure 2-13 Proportion of injection time in the molding cycle: 1 - Start of injection cycle; 2 - Injection filling; 3 - Pressure holding switch; 4 - Cavity filling)
Cooldown time
The cooling process begins primarily at the start of injection molding, not after it is completed. The cooling time should be as short as possible while ensuring the part can be easily removed from the mold. Generally, the cooling time accounts for 70% to 80% of the cycle, as shown in Figure 2-14.
Figure 2-14 Cooling cycle time - Filling time: 4 - Holding time: " - Remaining cooling time: - Cooling time: 1, - Plasticizing time: " - Mold opening and closing time: 1 - Cycle time (1+.+)
Anti-drooling amount (screw loosening amount)
After the screw metering (pre-plasticizing) reaches its position, it retracts linearly a short distance, increasing the space for the melt in the metering chamber, reducing internal pressure, and preventing the melt from flowing out of the metering chamber (through the nozzle or gap). This retraction action is called anti-drooling, and the retraction distance is called the anti-drooling amount or anti-drooling stroke. Another purpose of anti-drooling is to reduce the pressure in the nozzle flow channel system and decrease internal stress when the nozzle is not retracting during pre-plasticizing, and to facilitate the removal of the main runner during mold opening. The anti-drooling amount depends on the viscosity of the plastic and the characteristics of the product. Excessive anti-drooling amount will cause air bubbles to be trapped in the melt in the metering chamber, severely affecting product quality. For high-viscosity materials, no anti-drooling amount is required (generally 2-3 mm).
Residual material
After the screw injection is complete, it's not desirable to inject all the melt from the screw head; some should be retained as a reserve. This serves two purposes: firstly, it prevents mechanical collisions between the screw head and the nozzle; secondly, this reserve allows for control over the repeatability of the injection volume, thus stabilizing the quality of the molded product (too little reserve won't provide adequate cushioning; too much will lead to excessive residue accumulation). A typical reserve is 5–10 mm.