Screw and Check Ring ....

• Feed Zone: The portion of the screw that picks up the material from the feed opening (base of the hopper) and begins to soften the material as it is being conveyed. The flight depths are constant.
•  Transition Zone (Compression Zone):  The portion of the screw where the root diameter increases gradually resulting in the decrease of the flight depth. This causes the softened plastic pellets to further melt and compress eliminating any of the air pockets.
•  Metering Zone: The depth of the flights in this section are minimum but constant. This helps the accurate control of the melt discharge for the molding shot. In other words, it meters the amount of melt. The flight depths are constant.

b. The Check Ring:
The check ring is used to prevent the plastic from being blown back during the injection phase of the molding cycle. It is similar to a non return valve.

c. What is Intensification ratio?:

In case of hydraulic machines, hydraulic pressure is applied to a ram behind the screw and this ram pushes the screw forward during injection. Consider that the hydraulic pressure applied is 1000 psi and the cross sectional area of the ram is 5 square inches. Therefore the total force applied is 5000 lbs and this is the force acting on the screw. If the cross sectional area of the screw is 2 square inches, then the force per unit area is pressure per square inch = 5000/2 = 2500 psi. Therefore the plastic that is in front of the screw is now subjected to 2500 psi. The hydraulic pressure of 1000 psi is now intensified to 2500 psi. The intensification ratio is 2500/1000 = 2.5

Another simpler way to look at this is by taking the ratio of the area of the ram to that of the screw. In this case, 5/2 = 2.5. This means any hydraulic pressure is now intensified by 2.5 times and is transferred to the screw. As a practical example, we apply a large force on the flat head of a nail (typically with a hammer), which is intensified at the tip of the nail, to drive it into the wall.

Intensification ratios usually range from about 6:1 to 23:1. For electric machines, the ratio is 1:1 since there is no hydraulic ram. When moving a mold from one press to another, the plastic pressures must be duplicated and the knowledge of actual plastic pressures is therefore important.