Hollow articles

In recent years rotational casting methods have made the slush moulding process virtually obsolete. In these processes an amount of material equal to the weight of the finished product is poured into a mould. The mould is then closed and rotated slowly about two axes so that the paste flows easily over the cavity walls in an oven at about 200-250°C. When the compound has gelled, the moulds are cooled and the moulding removed. Compared with the slush moulding process there is no wastage of material, little flash, and more even wall thickness. Completely enclosed hollow articles such as playballs are most conveniently made. 

Initially a molten tube of plastic called the Parison is extruded through an annular die. A mould then closes round the parison and a jet of gas inflates it to take up the shape of the mould. This is illustrated in Fig. 4.21(a). Although this process is principally used for the production of bottles (for washing-up liquid, disinfectant, soft drinks, etc.) it is not restricted to small hollow articles. Domestic cold water storage tanks, large storage drums and 2(X)... 

In Section 4.2.7 we considered the process of extrusion blow moulding which is used to produce hollow articles such as bottles. At that time it was mentioned that if molecular orientation can be introduced to the moulding then the properties are significantly improved. In recent years the process of injection blow moulding has been developed to achieve this objective. It is now very widely used for the manufacture of bottles for soft drinks. 

This is a method for making hollow articles using liquid plastics, particularly PVC plastisols. A shell-like mould is heated to a pre-determined temperature (typically 130°C for plastisols) and the liquid is then poured into the mould to completely fill it. A period of time is allowed to elapse until the required thickness of plastic gels. The excess liquid is then poured out and the plastic skin remaining in the mould is cured in an oven. The moulding is then taken from the mould. 

Paint traps should be avoided, e.g. close bends of 180 , and rolled and bent edges. Drainholes should be located low down in hollow articles. 

A process for making hollow mbber articles from a single sheet of calendered unvulcanised mbber. The surface of the sheet is semi-vulcanised and the application of a swelling agent causes the sheet to split thus forming the hollow article, the vulcanisation of which is completed thereafter. Homo polymer... 

A process for making hollow articles from latex. A heat-sensitised compounded latex is poured into a hollow non-porous mould which is then rotated about several axes until the latex has gelled on the surface of the mould. See Rotational Moulding. Similar to slip casting of ceramics. Ketones... 

A development of slush moulding used for the production of hollow articles from plastisols. The moulds are continuously rotated in two directions during the curing cycle. 

Blow molding is not the only process from which polymeric hollow articles can be made, bnt it does offer distinct advantages over other methods snch as injection molding... 

Blowing agent Compounding ingredient used to produce gas by chemical or thermal achon, or both, in manufacture of cellular or hollow articles. 

Rotational Molding. Hollow articles and laige, complex shapes are made by rotational molding, usually from polyethylene powder of relatively low viscosity (57—59). The resin is in the form of a fine powder. A measured quantity is placed inside an aluminum mold and the mold is heated in an oven and rotated at low speed. The resin sinters and fuses, coating the inside of the mold. The mold is then cooled by water spray and the part solidifies, duplicating the inside of the mold. 

Rotational casting is usually used in making hollow articles such as vessels, dummies, dolls, buoys, floate, etc. The weighed amount of plastisol is given into a metallic sealed mould which then is rotated in three mutually perpendicular directions upon heating in a chamber furnace. Upon gelatination the mould is cooled, stopped, opened, and the article is taken out. 

Polystyrene was the first synthetic polymer used for blow molding during World War II and polyethylene was the first material to be implemented in commercial applications. Until the late 1950s, the main application for blow molding was the manufacture of PE-LD articles such as squeeze bottles. Blow molding produces hollow articles that do not require a homogeneous thickness distribution. Today, PE-HD, PE-LD, PP, PET, and PVC are the most common materials used for blow molding. 

Blow molding — The process of forming hollow articles by expanding a hot plastic element against the internal surfaces of a mold. In its simplest form the process comprises extruding a tube (parison) downward between opened halves of a mold, closing the mold, and injecting air to expand the tube, which is pinched on the bottom. 

Blow mold usually consists of two halves, each containing cavities which, when the mold is closed, define the exterior shape of the BM (Chapter 17). Multiple cavity molds are used. Because the process produces a hollow article, there are no cores to define the inner shape. Mold details and actions will vary considerably according to the geometry of the product and the BM process in use. Even though the following review concentrates on EBM, the information can also be applied to IBM. The two halves that meet on a plane are known as the parting line. The plane is chosen so that neither cavity half presents an... 

Blow moulding is the most common process for making hollow articles such as bottles. There are two main types of blow moulding, injection blow moulding and extrusion blow moulding. 

With hollow articles, the value of depends on wall thickness and on the radius of curvature. For the case of tubes cooled from outside at a wall thickness to ID ratio of 0,1 to 1.0, < = —0.0054 to —0,0044 (external surface) and -0,0027 to 0.0023 for the internal surface. For container glass (current types of bottles and jars), the common value for both external and internal surface independent of size was established experimentally when d = Jidl) was substituted for the effective dimension, where dis diameter of the article and I is bottom thickness (Schill, 1977). 

Vlnoflex . [BASF BASF AG] PVC for extruskm, calaidering, and inj. molding (film, pipes, profiles, hollow articles, panels), rigid or flexible parts. 

Blowing of hollow articles such as bottles and containers can be carried out online with the extrusion process. 

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