Orifice die

Fig. 8-33 Example of changes in a PVC profile shape from the die orifice to the product where no dimensions remained the same.

In the extrusion process, high pressures are applied to a cast billet of aluminum, forcing the metal to flow through a die orifice. The resulting product is an elongated shape or tube of uniform cross-sectional area. 

The extrusion process requires the use of a lubricant to prevent adhesion of the aluminum to the die and ingot container walls. In hot extrusion, limited amounts of lubricant are applied to the ram and die face or to the billet ends. For cold extrusion, the container walls, billet surfaces, and die orifice must be lubricated with a thin film of viscous or solid lubricant. The lubricant most commonly used in extrusion is graphite in an oil or water base. A less common technique, spraying liquid nitrogen on the billet prior to extrusion, is also used. The nitrogen vaporizes during the extrusion process and acts as a lubricant. 

Usually when making pipe and tubing the extrudate is inflated to the dimensions of a sizing die in such instances it must be taken that on emerging from the die the swell will be in line with the rate of shear, that the thicknesses of sections will be reduced in proportion to the amount of inflation, and with ratio of sizing die to the external diameter of the die orifice. 

Extrusion Stiff plastic particle mix is pushed through a die orifice to form a continuous compact which may be cut to desired length. Vacuum auger, screw extruder, plunger press, piston extrusion apparatus... 

Melt fracture is the occurrence of distorted extrudate coming from the extruder. It is caused by flow disturbance at the point where flow cross-sectional area is rapidly reduced from the large diameter of the melt feed to the much smaller cross-sectional area of the die orifice. Here again it is aggravated by melt elasticity. 

The bubble diameter is normally always much greater than the die diameter. This bubble diameter divided by the die orifice diameter is called the blow-up ratio (BUR). The BUR is usually 1.5 to 4.0, depending on the plastic being processed and the thickness required. The bubble diameter must not be confused with the width of the flattened double layer of film between the nip rolls. The width of this double layer is 1.57 times the bubble diameter and is called the blown-film width (BFW). 

Figure 5.8 Example neck-in and beading that occurs between die orifice and chill roll...

As explained in Chapter 17 die configurations meet certain melt flow requirements such a drawdown ratio and draw ratio balance. Draw-down ratio (DDR) in a circular die, such as a wire die, is the ratio of the cross sectional area of the die orifice/opening to the final extruded shape. Another guide for setting uniformity and best repeatable references is the draw ratio balance (DRB) that aids in determining the minimum and maximum values that can be used for different plastics. 

The drawdown ratio (DDR) in a wire die or a circular die, is the ratio of the cross sectional area of the die orifice/opening to the final extruded shape [Figure 17.13(b)]. 

Orifice Jlanies can be cliaracterized as either prcmi.xed or diffusion flmnes. In a premixed flame, die air for combustion is already nii.xed widi the fuel gas before it leaves the orifice or pipe. In a diffusion flame, fiiel e.xidng die orifice is pure and the air needed for combustion diffuses into die fuel gas from the surroundings. Orifice flames can also be cliaracterized by die flame Reynolds number. Tlie flame lengdi of a diffusion flame can be calculated by Jost s equadon. ... 

Knowledge of flow of the melt at different temperatures and shear rates is required in the fabrication processing of the resins. Melt viscosity of these polymers is a function of shear rate. For example, pol5mier viscosity changes as extrusion rate is varied. In addition to melt viscosity, melt flow rate (MFR), also known as melt flow index (MFI), is a customary flow characteristic of resins. MFR is defined as the mass of molten polymer (in grams) that flows through the die/orifice of a rheometer in ten minutes and is inversely proportional to the melt... 

Die Land - The part of the die (orifice) that is downstream from the convergent section where both cross-sectional area and shape are constant. 

Extrusion - Process for converting an ingot or billet into lengths of uniform cross section by forcing material to flow plastically through a die orifice a product form produced by this process. Many variations of this process are used widely in working metals and processing plastics. 

Like the extrusion of metals, the extrusion of plastics involves the continuous forming of a shape by forcing softened plastic material through a die orifice that has approximately the geometric profile of the cross-section of the work. The extruded form is subsequently hardened by cooling. With the continuous extrusion process, such products as rods, tubes, and shapes of uniform cross-section can be economically produced. Extrusion to obtain a sleeve of the correct proportion almost always precedes the basic process of blow molding. 

FIGURE 2.31 Relationships between extruder die orifice and extruded section. 

FIGURE 3.32 Schematic representation of polymer flow through a die orifice. 

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