Successive extrusion

Shah RD, Kabadi M, Pope DG, Augsburger LL. Physico-mechanical characterization of the extrusion-spheronization process. Part II rheological determinants for successful extrusion and spheronization. Pharm Res 1995 12(4) 496-507. 

Figure 7.. Change in melt flow after six successive extrusions at 258°C. Melt flow, 230°C., 216 kg.

If billets are shaped correctly, the orientation of the billet may be changed between passes without a change in the die or tooling set. Appropriate shapes are elongated billets with square cross-section, or square, flat plates. By changing the orientation of the billet between successive extrusions, complex microstructures and textures can be developed. While numerous possibilities exist for combinations of passes, all combinations may be represented by four independent routes developed by Segal (1996 1998). These routes have been labeled as follows ... 

Journal of Vinyl and Additive Technology 3, No.l, March 1997, p.28-32 SUCCESSFUL EXTRUSION OF SMALL DIAMETER CPVC PIPE... 

A common problem of all ring die extruders in which material passes from the outside to the inside of a cylinder was and is that the extruded material must reliably discharge from the inside of the die without becoming entangled and/or stuck. To allow successful extrusion, the particulate mass must be moist and, to produce... 

Instead of a roller press a pellet mill may be used for densification. The rest of the system is essentially the same. However, for successful extrusion (pelleting) some lubricant (liquid) may have to be added. Although the temperature rise caused by friction in the die holes may often be sufficient to naturally dry-out the small amount of liquid, the resulting granules do still not exhibit the same uniform quality as those originating from sheets made with roller presses. 

This section aims at discussing the conditions and recommendations for successful extrusion of blends containing commodity resins as the major component. The conditions recommended by manufacturers are hsted in Table 10.19, while the recommended procedures and accepted practices are discussed below for each type of blend. 

While an experienced die designer may intuitively know where flow balancing is required within a profile, it is difficult to quantify this. This is compounded if a requirement comes along for a profile to be produced from a new material with which the designer has little experience. The ability to simulate the flow distribution based on the geometry and the flow characteristics of the polymer allows the designer to more accurately determine the flow channel details for successful extrusion. 

Successful extrusion of thermosetting materials has been achieved and put into commercial practice, but extruded thermosets represent only a tiny fraction of the amount of thermoplastics extruded annually. 

The solidification process of the extruded material also influences the filament diameter. Materials that solidify quickly would not lead the molten material to spread over i.e. the dimension of the deposited filament remains same as the diameter of the filament as extruded. The appropriate selection or setting of process parameters is crucial for successful extrusion of polymer scaffolds. Many different factors contribute to successful deposition. The extruded material must flow consistently through the nozzle when pressure is applied to avoid inadequate dispensing. However, it is also essential that the pressure should not set too... 

Hypothesis of isothermal incompressible steady laminar flow. The variation of the system physical quantities versus time can be ignored when arriving at a successive extrusion process. 

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