Hollow objects

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. 

Large, hollow objects can be formed using the coiling method. Here clay is rolled into ropelike strands that are coiled one upon another and joined together. 

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. 

Injection blow moulding is used for the production of hollow objects in large quantities. The main applications are botdes, jars and other containers. The Injection blow moulding process produces bottles of superior visual and dimensional quality compared to extrusion blow moulding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. 

As already stated, there are many different methods of processing plastics, according to the material used and the desired finished product. Plastics are wonderful materials for shaping. They can be made into flat sheets, or they can be reinforced with fibres. They can be blow moulded and made into hollow objects such as drinks bottles, or they can be thermoformed for making food cartons. There is a variety of processes for the different synthetic plastics, but all of them start with the required chemicals in pellet, powder or liquid form. These are melted, mixed with additives, heated and shaped. 

For hollow objects like the fullerenes, a general distinction has to be made between outside and inside reactivity. Modifications to the outside are termed exohedral functionalization, and those to the inside are endohedral. Both variants are observed for the fullerenes. Classical fullerene chemistry deals with exohedral functionalization by one or more groups attached to the carbon atoms. Endohedral chemistry, on the other hand, studies compounds consisting of atoms or small molecules included in the cavity within the fullerene cage. The exohedral processes may further be divided into covalent and noncovalent interactions with the reaction partner. 

Being hollow objects, carbon nanotubes possess yet another parameter of strac-tural stability the tubes become increasingly softer the larger they get in diameter. The effect shows in the interaction with a substrate-wide tubes flatten and their contact area with the supporting material grows (Figure 3.47). The effect is less pronounced for MWNT as the van der Waals interactions between the constituent tubes provide additional stabilization of the cylindrical shape. 

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. 

Latin conno, and Greek Kdiwa, are presumably de rivatives from some Sanscrit root si irying a tubular or hollow object. See Howitzer Moutar. 

Hollow objects can be produced using twin-sheet thermoforming, in which two sheets of plastic are thermoformed and sealed together. This can be done sequentially, with one sheet formed first and then the other added and formed, or simultaneously, with both sheets formed and sealed together in a single step (Fig. 10.11). 

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. 

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. 

Testing of plastics articles - Determination of the effect of internal pressure on hollow objects by long-time test... 

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

---