10-21 FORMING OPERATIONS

10-21 FORMING OPERATIONS

In most forming operations the polymer is subjected to intensive shearing and stressing, usually at molten or nearly molten states. This results in the deformation and orientation of the polymer chains, imparting to the polymer a specific molecular configuration and morphology of the final product. This is particularly strongly marked during spinning, drawing, blowing, and rolling processes, which produce fibers and films. Most forming operations, especially molding, extrusion, and casting, involve the melting or softening of the polymer by heating it to a temperature at which it will flow usually through a narrow nozzle under pressure to fill the cavity of the mold. Under such conditions, most polymeric resins are highly viscous materials having

4 8 4 viscosities between 10 9 and 10 Pa.s (10 and 10 P), depending on the temperature and pressure. Furthermore, the presence of fillers and other additives may considerably affect the rheological

characteristics of the polymer melts. Usually, polymer melts behave as pseudoplastic materials whose viscosity de creases rapidly with increasing shearing rate. Furthermore, they exhibit viscoelastic behavior that becomes more and more pronounced on gradual cooling of the polymer during molding or extrusion operations. During the polymer flow, the whole polymer chains cannot slide over one another completely, but movement occurs by segmental motion. The movement of the entire chain length is restricted because of numerous entanglements between the chains.

Molding. Molding is our most widely used forming operation; it involves injection molding, blow molding, compression molding, and transfer molding (see Fig.10-8). Many of the problems that arise during molding, especially injection molding. are caused by rapid changes in the volume and the density of the polymer with pressure and temperature. The increase in density with decreasing temperatures at constant pressure results in considerable shrinkage of the polymer which, in some cases. may result in cracks and imperfections in the molded article. Furthermore, this also gives rise to strains within the body that will adversely affect the Molding. Molding is our most widely used forming operation; it involves injection molding, blow molding, compression molding, and transfer molding (see Fig.10-8). Many of the problems that arise during molding, especially injection molding. are caused by rapid changes in the volume and the density of the polymer with pressure and temperature. The increase in density with decreasing temperatures at constant pressure results in considerable shrinkage of the polymer which, in some cases. may result in cracks and imperfections in the molded article. Furthermore, this also gives rise to strains within the body that will adversely affect the

FIGURE 10-8 Diagram of a conventional plunger injection-molding machine. (From Textbook of Polymer Science, 2nd edition, F. W. Billmeyer. Jr., p. 493. Fig. 17-2. Copyright by John Wiley & Sons, Inc., New York, 1971. Reprinted by permission.)

When the molding pressure is released, the polymer will tend to expand according to the relation

For all substances the volume decreases with increasing pressure; hence the compressibility factor is always a positive quantity.

However, the effect of molding pressure on reducing thermal shrinkage is much less in crystalline polymers than in amorphous polymers. This is because crystallization occurring on cooling from the molten state results in much denser material than the amorphous one, causing considerable volume changes. Thus even very high injection pressure cannot prevent excessive shrinkage of the crystalline polymers. The increasing pressure increases the packing density of the polymer chains and favors crystallization at temperatures higher than those for lower molding pressures (Fig. 10-9).

FIGURE 10-9 Diagram of a compression-molding press and mold. (From Textbook of Polymer Science, 2nd edition, F. W. Billmeyer, Jr., p. 492, Fig. 17-1. Copyright O by John Wiley & Sons, Inc., New York, 1971. Reprinted by permission.)

Another cause of strains in the molded article is the orientation of the polymer molecules during its flow into the mold and through the orifice. On cooling in the cold mold this orientation of the chains becomes frozen; furthermore, a nonuniform cooling of the polymer within the mold also contributes to frozen strains. For example, in thick molding, a stiff outer skin may be formed, Another cause of strains in the molded article is the orientation of the polymer molecules during its flow into the mold and through the orifice. On cooling in the cold mold this orientation of the chains becomes frozen; furthermore, a nonuniform cooling of the polymer within the mold also contributes to frozen strains. For example, in thick molding, a stiff outer skin may be formed,

Extrusion. in the extrusion process, the polymer is continuously forced along a screw through a region of high temperature and pressure where it is melted and compacted and then forced through a die, shaped to give the final article (see Fig. 10-10). At the entry to the die the polymer melt is subjected to rapidly changing deformation rates. The energy imparted to the material during the deformation is not only used up to cause the polymer to flow, but some part of it is stored as the elastic energy. This causes normal stresses in the material, which will subsequently relax during the flow of the polymer through a die.

When this relaxation occurs after the material has left the die, the swelling of the extrudate occurs and surface irregularities result. This phenomenon, known as Tordello’s effect or melt fracture, is due to the recovery of the elastic shear strain in the exit region. To avoid this swelling, the relaxation time of the extruded polymer should be less than its residence time during the flow in the die. This can be controlled by decreacing the rate of flow through a die, increasing the length of a die, or increasing the polymer temperature in the die.

Rolling and Drawing. During drawing or rolling operations, the resultant deformation consists of (1) the instantaneous elastic deformation caused by the distortions of the bond angle and bond stretching; (2) the molecular alignment, which involves mobilization of whole chains or their segments or plastic deformation in the crystallites: and (3) the nonrecoverable viscous flow.

The orientation can be predominantly uniaxial or biaxial but, in general, the orientation appears to be a combination of the two. In uniaxial orientation a polymer is stretched in one direction. For example, during the drawing of fibers, the polymer chains align in the direction of stretching.

Orientation can be accomplished by cold drawing and hot drawing. Cold drawing of the polymers is carried out below its annealing temperature. Cold drawing, as compared to hot drawing, requires considerable expenditure of energy to produce the desired deformation; it results in residual or internal stresses, which may be subsequently removed by annealing.

Some fiber-forming polymers can be cold drawn in the form of ribbons or fibers to many times their original length. The process is quite similar to stretching the rubber (with nylon a 400% extension is possible before breaking occurs). In a molecular sense, drawing is a flow process, similar to rubber-elastic stretching, in which a parallel orientation of the polymer chains and the chain segments occurs until the chains are so interlocked (because of crystallization) that a further gliding past each other becomes impossible. However, even before drawing, there is some crystallization. Hot drawing refers to the deformation of crystalline or semicrystalline polymers at temperatures between the annealing temperature and the melting point as, for example, in Some fiber-forming polymers can be cold drawn in the form of ribbons or fibers to many times their original length. The process is quite similar to stretching the rubber (with nylon a 400% extension is possible before breaking occurs). In a molecular sense, drawing is a flow process, similar to rubber-elastic stretching, in which a parallel orientation of the polymer chains and the chain segments occurs until the chains are so interlocked (because of crystallization) that a further gliding past each other becomes impossible. However, even before drawing, there is some crystallization. Hot drawing refers to the deformation of crystalline or semicrystalline polymers at temperatures between the annealing temperature and the melting point as, for example, in

Hot drawing imparts to the polymer a high degree of crystal orientation, and it is a preferred method for producing commercial fibers and films because high deformation rates could be obtained. Rolling through compression rollers is similar in some respects to the hot and cold drawing operations. If the polymer is rolled below its annealing range or if it is deformed very rapidly, residual stresses will

be present. Orientation of the amorphous polymer is carried out above its glass transition temperatures

where polymer chains have sufficient mobility to disentangle and align themselves in a preferred direction. Below T g in its glass state, the polymer fractures in a brittle manner during orientation.

Strengthening Effect. The strengthening of the polymer through the predeformation is phenomenologically identical to the strain hardening observed and utilized in metals. The strain

at which hardening sets in depends on the actual polymer. This occurs only when a substantial amount of orientation hardening has taken place and. consequently, at characteristic necking strains. This indicates that such differences are due to some structural features of the polymer. For example, necking strains are 0.4 to 1.5 for PVC, 2.24 for polycarbonate, 3.5 for polystyrene,

2.8 to 3.5 for nylon, and 5 to 10 for polyethylene.