Injection molding cycle time

Polycarbonate can be readily injection molded. Polycarbonates typically require only a short injection molding cycle time, because the polymer flows into the mold easily and solidifies rapidly. We injection mold polycarbonate to produce a wide variety of commercial goods, including compact disks, jewel cases, aircraft windows, kitchen utensils, and clear refrigerators drawers. Polycarbonates are also found in a wide range of disposable medical devices, such as the flow locks on intravenous tubes and the hard, disposable components of dialysis machines. Impact resistant polycarbonate is used to manufacture sports and other safety helmets. Glass fiber reinforced polycarbonate is used in the housings for power tools. 

Figure 2. Reduction of the injection molding cycle time since 1994.

Currently, use of PLA for injection molded articles is limited for commercial applications because PLA has a slow crystallization rate when compared with many other thermoplastics as well as standard injection molding cycle times. The overall crystallization rate and final crystallinity of PLA are controlled by the addition of physical nucleating agents as well as optimization of injection molding processing conditions [4]. 

Typical injection molding stock temperatures for Vamac compounds at the nozzle should be in the range of 70°C-85°C. The cold runner systems should be set in the same range of 70°C-85°C. Scorch can be a serious problem if the temperature exceeds 100°C-110°C. Mold cavity temperatures are typically 175°C-190°C for injection molding cycle times of 1-3 min. 

The injection molding cycle is depicting in Figure 7.74. We can begin the cycle at any point we wish, but let us start at the point the screw moves forward and fills the mold with polymer melt. The screw moves forward and fills the mold with melt and maintains the injected melt under pressure, during what is called the hold time. To ensure that polymer does not flow backward, a check valve is attached to the end... 

Figure 3.41 presents the sequence of events during the injection molding cycle. The figure shows that the cycle time is dominated by the cooling of the part inside the mold cavity. The total cycle time can be calculated using... 

This problem was approached from the material side, through a search for suitable internal releases, and through the development of improved external mold release compounds. From the equipment side, the development of automatic molds was required if the RIM process was to compete with classical injection molding with respect to mold cycle times and efficient production. 

Izod impact strength increases with increasing injection speed and lower melt temperatures. Since the fracture plane is normal to the direction of orientation in the Izod sample, the increased toughness with slower injection speed can be explained by the reinforcing effect of the oriented polymer on the surface. This effect is less noticeable at very high melt temperatures, where the surface can actually be annealed by the temperature of the core and subsequent extended molding cycle time required by the hot melt temperature. 

But there are even more tasks for the mold. It must close tightly during the injection phase so that the melt flows only into the cavity and not into the parting plane. At the same time, however, it must be possible to open the mold so that the molded part can be removed from the cavity at the end of the injection molding cycle. 

The various times making-up an injection molding cycle are shown with the example of a CD ... 

As soon as the set cooling time in the injection molding cycle has expired-in other words, once the injection-molded article has solidifed within the mold-the mold opens in the parting plane. The mold halves separate far enough to allow the article to be ejected. 

As mentioned previously, the molding process is a cyclic one that involves variation of both temperature and pressure with respect to time. Figure 8-15 shows a schematic of pressure with time for the injection-molding cycle. 

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