The size of the machined gaps determines how easily the melt can flow from one melt channel to an adjacent flow channel and also sets the size of particle or contaminant that will be captured in the gaps between the tops of the ribs and the inner diameter of the machine nozzle. Thus the melt may continue to traverse from one grooved flow channel into adjacent grooved channels by flowing over the tops of the ribs. These gaps allow melt to roll over the thin walls that separate adjoining machined groove melt channels. This creates a predefined “gap” between the ribs or thin walls of steel that separate each of the machined groove melt channels and the inner bore of the machine nozzle. The walls or thin ribs of steel on either end of the machined grooves are machined down slightly. The melt travels to the end of the machined-groove melt channels where they are then trapped within the gap filter with no way to access the machined groove melt channels that exit the filter into the mold. Most molders will not utilize screen packs or will limit their utilization to injection molding presses and/or molds where this type of pressure drop is deemed absolutely necessary.Īt this point the melt can flow from the injection barrel into the gap filter through half of the machined-groove melt channels. This pressure drop is large enough to impact the processing window, and thus is unacceptable to many molders. This results in a noticeable and significant pressure drop as the melt traverses through the screen pack. Thus, by its very design, the screen pack reduces the flow-channel area by at least half. The first and foremost disadvantage is that to ensure the structural integrity of the metal disk against the injection molding pressure, the area of the solid metal will be greater than the aggregate area of the small-diameter holes contained within the metal disk. There are multiple disadvantages to screen packs, as well. It is also relatively easy to size the diameters of the holes in the metal disk. Particles or contaminants larger than the holes are captured and prevented from traveling into the mold.Īdvantages of a screen pack are its simple design and low cost. As the melt passes through the flow channel of the nozzle, it encounters the metal disk and is squeezed through the small-diameter holes. It consists of a metal disk perforated by a number of small holes, which is inserted along the inner diameter of the flow channel of the machine nozzle. The most common style is known as a screen pack. Nozzle filters come in a variety of styles. Nozzle filters capture contaminants in the melt stream to prevent such particles from entering the mold and the molded parts.
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