Hollow part processes

Hollow part processes. Hollow products can be made either by assembling parts made by most of the processes (even extrusions can be capped) or by one of the hollow part processes. Which is most cost effective varies considerably according to the application and to the state of the art of the techniques being applied. The savings associated with molding the part in one piece may be offset by the use of... 

For thermosets, there really is only one hollow part process—filament winding. That process is very limited in shape and structure such that, except for pressure applications, most thermoset hollow products are assembled. 

There are three hollow part processes for thermoplastics and they do have a pecking order of sorts. When proceeding with the development of thermoplastic hollow shapes which do not lend themselves well to assembly, the following sequence should be used, starting with the lowest volume requirements ... 

For thermosets, the hollow part processes are filament winding and pultrusion. These processes are limited in shape and structure. There are three hollow part processes for thermoplastics (1) rotational molding, (2) twin sheet thermoforming, and (3) blow molding. 

Rotational molding creates a wide variety of plastic products that cannot be made effectively, efficiently, or economically by other means. What sets this method apart from others is that it can create thin-walled, hollow parts that exhibit no weld lines or scarring from ejector pins and from the process itself. It also has the advantage of having little scrap and minimal molded-in stresses, due to the low pressure and low shear rate characteristics of the process. Finally, it can be used to make parts that are very large which would be impossible to manufacture by other methods. 

Composite processing can obtain nearly all part shapes including plane or warped surfaces profiles hollow parts bulky parts sheets, slabs and plates parts with inserts, etc. 

Hoechst have developed a process that sucks the parison onto the mould walls in blow moulding equipment, thereby producing lightweight hollow parts with good insulation properties. The process was developed for foamed PP air ducts for cars. A description is given in the article. HOECHST AG... 

Basic tooling construction. In general, the open-mold processes can be expected to have lower tooling costs than any of the closed-mold processes for parts of equivalent complexity because only one side of a mold has to be built. Other types of molding require both a male and female half (core and cavity) or two female halves (split cavity for hollow parts) however, open molds require either a male or a female section. This is illustrated in Fig. 8.8, which depicts a female mold which forms the outside of the part as in Fig. 8.8a and a male mold which forms the inside of the part as in Fig. 8.86. 

Extrusion blow molding is the process employed in the production of large containers such as drums, as well as complex automotive parts. This process is widely used to produce containers of various sizes and shapes. It is also adapted to make irregular, complex, hollow parts, such as those supplied to the automobiles, office automation equipment, and pharmaceutical sectors. In this process, the extrudates swell [5], as well as sag or draw down during parison extrusion stage. 

Rotational molding is an economical process used to produce medium-to large-sized hollow parts from polymeric materials. It is a shear- and pressure-free process with processability of limited polymeric materials. Viscosity, surface tension, and thermal properties of polymer influence the sintering behaviors. Changes in economy and competition with other process methods shift from these techniques to rotational molding. The type of materials could be used in the process is mainly limited to poly-ethylenes [1]. 

Rotational Molding. The process of rotational molding is ideal for creating hollow parts. Polyethylene is the most common polymer used in rotational molding. Powdered plastic is introduced into a clamshell mold. Color and additives, usually a micro-pulverized blend, are added and the mold is closed. The mold is then rotated. The powder is distributed to the mold s surface by the centrifugal force. The mold is heated and a melt skin forms on the mold s surface. Spinning continues as the mold is allowed to cool. The mold is then opened, the part removed, and the process starts again. 

Rotational molding, also called rotomolding or rotocast, is a thermoplastic process used for producing hollow parts by placing powder or liquid resin into a hollow mold and then rotating the tool biaxially in an oven until the resin melts and coats the inside of the mold cavity. Then, the tool is cooled and the part is removed from the mold. Rotational molding can be summarized via six steps ... 

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