注塑工艺设计.上海.May 2010.pptVIP

* * Another process impact on injection pressure is the injection speed (or RAM speed) profile. If you push the plastic into the mold at a constant speed the pressure may spike suddenly as the plastic reaches areas of resistance such as the gate at the beginning of fill and the last places to fill at the end of injection. In order to minimize the pressure, an injection speed profile may be used to try and keep the melt front moving at a uniform speed as the melt front area changes in the runner system and part cavity. We will discuss this further in the next section. This is shown in Figure 4-1 in the Design Guide and a discussion is there as well. Assume you are injecting plastic into the end of this part at a constant ram speed. What happens as the melt reaches the core in the center of the part where the melt is forced to split? (the melt speed increases) The volume flow rate of the plastic has remained unchanged while the space to push that volume into has suddenly decreased (due to the core section in the middle). It’s similar to water flowing through a hose at a constant volume. As soon as you restrict the opening at the end of the hose, the water velocity increases. As melt front area decreases, the melt front velocity increases. As melt front area increases, the velocity decreases. So why does it matter if the melt front speeds up or slows down? (changes in velocity can affect orientation, which affects shrinkage for fiber filled resins, and it can also cause burning, differing surface finish, or jetting. * We use an injection speed profile to try and control the speed of the melt front. How many of you have seen this done? Can someone describe what would happen to the example part we just looked at if I used either of these two ram speed settings? (the constant speed would cause the melt front to speed up as discussed around the core; the profiled case would probably produce a more even melt front velocity because the injection slows down in