Plasticating plasticating how the plastic granules

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Unformatted text preview: de the screw channel when screw is rotated. The screw has three sections. FEED ZONE, COMPRESSION ZONE and METERING ZONE. In the compression zone the material is gradually compressed. It therefore rubs against the barrel wall. This sets up shearing forces on the material. Plastic material under shear changes its viscosity. This is SHEAR THINING characteristic of plastics. Melt is then homogenised in metering zone. At the tip of the screw, nonreturn valve is fitted. It allows the melt to flow ahead through this valve while screw is rotated. Non return valve does not allow the melt to slip back through it to the screw channel. Feed Feed Zone The Feed zone flights are long and designed to move the material along the barrel as quickly as possible. As they move along, the granules are heated by friction (from the movement of the screw inside the barrel and from the movement of the granules themselves) and by the inside of the barrel against which the granules are forced by movement of the flight Compression Compression Zone In the Compression zone the flights become shorter and the plastic granules are further heated and compressed, removing any air pockets. The plastic is melted by now and becomes thoroughly mixed or homogenized by the continuing movement of the screw. Metering Metering zone The Metering zone contains the shortest flights, which are designed to pump the plastic or melt, through the injection/extrusion die. Screw Screw The characteristics of the screw are crucial indetermining the type and size of product that can be produced on an Injection Moulding Machine. One of the most important factors is the ratio between the length and the diameter, or the L/D ratio. 22:1 or higher (meaning that the screw is 22 times the length of the diameter) improves the mix of the melt and the quality of the product. Generally speaking for high quality products such as engineering components, a L/D ratio of 22:1 or higher is needed. 20:1 is suitable for medium quality products like garden furniture and 18:1 would typically be...
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