Hollow objects molding

HDPE is important for producing bottles and hollow objects by blow molding. Approximately 64% of all plastic bottles are made from HDPE. Injection molding is used to produce solid objects. Another important market for HDPE is irrigation pipes. Pipes made from HDPE... 

Pigment powders continue to be used in thick-walled articles, such as extruded sheets, or hollow objects, or in injection-molded products although color concentrates are beginning to be more important even in these areas. 

Molding employs a mobile prepolymer that may be thermoset or using a thermoplastic polymer. The polymer can be injection-molded (often for solid objects), blow-molded (for hollow objects such as bottles), rotation-molded, compression-molded, transfer-molded, or thermoformed. Casting is closely related to molding, except that the pressure is typically not used. 

The Hartford-Empire 28 is a press-and-blow machine used to make articles such as drinking glasses (tumblers). It uses paste molds and the ware is rotated to avoid the mold seams jars with screw threads cannot be produced. The product leaves the machine as an almost closed, hollow object and is finished by severing and fire-polishing with a bum-off machine. 

Rotational molding is used to make hollow objects. In rotational molding, a carefully measured amount of powdered polymer, typically polyethylene, is placed in a mold. The mold is then closed and placed in an oven where the mold turns about two axes as the polymer melts, as depicted in Fig. 3.73. 

Bottles and Other Hollow Objects—Blow Molding... 

The type of molding machine available has an important influence upon the blow opening system used. Some machines, for instance, use needle blowing exclusively. Hollow needle insertion at the mold parting line is considered when a very small opening is required that may even need closing or if the hollow object must be blown up on the side, e.g., on carousels with a series of molds. 

With rotational molding, a closed mold is rotated in two or more planes during fusion or solidification of the fluid polymer system. The action of the rotation deposits an even coating of polymer upon the inside of a "clam shell" type mold. After a predetermined time under heat, the molds are removed from the oven and cooled, and the finished product is removed whereupon the cycle is started again. Conventional gas fired ovens or hot salt bath sprays are used for heat transfer, and cooling is usually effected by spraying or immersion in cold water. This technique is well suited to molding completely hollow objects. 

Fluoropolymers can be fabricated into hollow objects by rotomolding. They are, however, hard to mold due to their relatively high melting point andmelt viscosity. The most frequently rotomolded fluoropolymers are PFA, FEP, PVDF, ETFE, and ECTFE. Fluoropolymers constitute about 1% of all rotomolded parts. 

Rotational Lining - Rotational lining is a process by which a hollow object is lined with a plastic. The surface of the part, contrary to rotational molding process, is prepared to adhere the liner to the mold wall. See also Rotational Molding. 

Catalysts of metal modified amorphous silica are made from the powders by compacting the powder under pressiure. The powder may be evenly flowed into a steel mold or die and compacted with a plunger. Hydrostatic compaction around a mandrel in a mbber mold which is squeezed by hydrostatic pressure can be employed for making cylindrical, hollow objects. Powders can be roll-formed into sheets or bars or can be extruded through a die to form rods. 

Slush casting is similar to slip casting (drain) of ceramics. The liquid is poured into a preheated hollow metal mold, which has the shape of the outside of the object to be made (Figure 2.43b). The plastisol in immediate contact with the walls of the hot mold solidifies. The thickness of the cast is governed by the time of stay in the mold. After the desired time of casting is finished, the excess liquid is poured out and the solidified plastisol with the mold is kept in an oven at 350°F-400°F (176°C—204°C). The mold is then opened to remove the plastic part, which now bears on its outer side the pattern of the inner side of the... 

In rotational casting a predetermined amount of liquid plastisol is placed in a heated, closed, two-piece mold. The liquid is uniformly distributed against the walls of the mold in a thin uniform layer (Figure 2.43c) by rotating the mold in two planes. The solidified plastisol in the mold is cured in an oven the mold is then opened, and the part is removed. The method is used to make completely enclosed hollow objects. Doll parts, plastic fruits, squeeze bulbs, toilet floats, etc. can be made by rotational casting of plastisols. 

Whenever hollow objects are desired, like bottles and containers, blow molding (a modification of injection molding) is used. In this process, an extmder... 

A molding process used to produce hollow objects in which a hollow tube (parison) is forced into a shape of the mold cavity using internal air pressure. The two primary types are injection blow molding and extrusion blow molding. Blow molding is a method of fabrication in which a warm plastic parison is placed between the two halves of a mold and forced to assume the shape of that mold cavity by use of air pressure introduced through the inside of the parison that forces the melt against the surface of the mold. 

Rotational molding is used to produce hollow objects. Rotational molding can produce large parts with uniform thickness at low costs compared with injection molding. It can be seen as a sequence of six different steps ... 

Rotational Molding. The process of rotational molding uses centrifugal force to distribute the plastic material within a spinning or rotating mold. The material may be injected as a liquid or added as a finely ground powder that is fused within the mold. The process is used exclusively for the production of hollow objects. 

In the first attempt over 100 years ago, to blow-mold hollow objects, two sheets of cellulose nitrate were clamped between two female mold halves. Steam injected between the sheets softened the material, sealed the edges, and expanded the heated sheets to form the inside shape of the two female mold halves. The high flammability of cellulose nitrate, however, limited the usefulness of this technique. 

Blow molding, until the last few years, was the main plastic process utilized to produce a hollow object. In the past few years, other plastic processes, such as rotational molding and twin sheet thermoforming, have evolved with technical achievements and plastic raw materials improvements to where today, they can compete with blow molding for many uses as toys, gasoline tanks, holding tanks, etc. 

The range of processes that may be used for fabricating a plastics product is determined by the scale of production, the cost of the machine and the mold, and the capabilities and limitations of the individual processes. For example, complex and precise shapes can be achieved by injection molding, hollow objects via blow molding and rotational molding, and continuous lengths by extrusion. 

Rotational molding is another polymer-processing operation that is used to produce hollow objects. It is a cyclic and semicontinuous process. 

---