Plastisol casting process

M propellants Method of manufacture Murry-cast propellants (Plastisol propellants) flHlVw-extfusion process tfCtassical extrusion method J her energy smokeless propellants Btfc fences... 

Plastisol and organosol casting process involves melting the polymer or polymeric mixture by an action of the temperature and presence of either a plasticizer or solvent... 

FIGURE 2.43 Plastisol casting processes (a) dip casting (b) slush casting (c) rotational casting. 

Slush molding is a mnq>le variation of the casting process. The low-viscosity material, such as, plastisol is poured into a hot mold and after a thick casting is formed on the wall of the mold cavity, the excess material is poured out. In order to ensure uniform coating, the mold is often rotated. 

Following the gelation of the cast base plastisol, the white inner layer of the upright cone is cascaded down the preheated cone mold. Optimized gelation characteristics cause the inner layer of plastisol to gel at a consistent thickness from the top to the bottom of the mold. The plasticizer type and concentration significantly impact this gelation process, as well as end-use performance properties, such as modulus and low-temperature impact resistance. Preferred plasticizer systems ate GP plasticizers such as DINP, often optimized with partial substitution of a lower-molecular-weight phthalate such as DIHP. 

Plastisols allowing the use of particular liquid-state processing techniques casting, roto-moulding, dipping, coating... 

In the plastisol propellant process, it is essential that the resin particles not solvate too rapidly at processing temperature since a rapid increase in viscosity of the propellant mix interferes with the mixing and casting operation. There must be adequate pot life of the mixed propellant. The resin-plasticizer system itself is the dominating influence on pot life, and for this reason certain combinations cannot be used in the plastisol process. 

The upper limit depends quantitatively on the viscosity that can be processed through the casting fixtures in reasonable time. Too high a viscosity may also lead to problems under certain flow conditions as, for example, when propellant folds over on itself to form a void space which may remain as a defect in the cured grain. If the propellant grain is to be formed and cured by screw extrusion, however, somewhat higher viscosities can be handled. A viscosity of 1600 poise has been reported (9) for a PVC plastisol propellant processed this way. 

After mixing, the propellant is transferred into a casting pot, a container in which it may be held and from which it may be cast. The long pot life of mixed PVC plastisol propellant permits it to be held almost indefinitely to meet the convenience of the production schedule. The long pot life also makes possible the full characterization of a batch of mixed propellant, by extensive tests on batch samples, before the batch is committed to further processing. 

Continuous screw extrusion is another way to form PVC plastisol propellant into the desired size and cross-sectional shape. Simultaneous curing of the propellant is also accomplished. This process has been described fully by Rossen and Rumbel (9) and is only summarized here. Mixed propellant is fed from a casting pot into the feed end of a continuous worm-screw extruder which is fitted at the discharge end of the screw with a device for constricting propellant flow. The action of the... 

Plastics film and sheet can be made in a number of different ways (including casting film from solvent or plastisols, pressing, and skiving from material in block form) but in commerce the most important processes for making these intermediates from thermoplastics are calendering and extrusion. 

In a plastisol process the gelation of PVC paste, which is a suspension of PVC particles in a plasticizer such as tritolyl phosphate, involves diffusion of the plastidzer into the polymer mass, resulting in a rise of the paste viscosity. Diffusion processes are involved in the production of cellulose acetate film by casting from solution, as casting requires removal of the solvent. Diffusion also plays a part in plastic molding. For example, lubricants in plastics compositions are required to diffuse out of the compound during processing to provide lubrication at the interface of the compound and the mold. Incompatibility with the base polymer is therefore an important criterion in the choice of lubricants in such cases. 

PVC plastisols are suspensions of PVC resin in a hquid phase mainly constituted by the plasticizer and the stabilizer. Plastisols can be processed by different techniques, such as rotational casting, slush molding, dip molding, dip coating, or spray coating. In these pro-... 

Cast embossing n. A process of casting films against an embossed temporary carrier. Vinyl plastisols, organosols, solutions, or lattices are used as film formers, which may be backed up with layers of foam or fabric. The temporary carrier is often paper, embossed with the desired pattern and treated so as to be easily stripped from the fused film or laminate. 

Electroformed mold n. A mold made by electroplating a model, which is subsequently removed from the metal deposit. The deposit is sometimes reinforced with cast or sprayed metal backings to increase its strength and rigidity. Such molds are used in slush casting of vinyl plastisols and other forming processes done at low pressures. 

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