Blow molding Hollow part processes

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. 

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. 

Blow molding is a process for converting thermoplastics into hollow objects. Like injection molding, the process is discontinuous or batchwise in nature, involving a sequence of operations that culminates in the production of a molding. This sequence or cycle is repeated automatically or semi-automatically to produce a stream of molded parts. 

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... 

The blow molding process consists of a sequence of steps leading to the production of a hollow tube or parison from a molten thermoplastic resin. This is then entrapped between the two halves of a mold of the desired shape. Air, usually at about 100 psi, is blown into the soft parison, expanding it against the contours of the cold mold cavity. The part is cooled and removed from the mold, and where necessary the excess plastic material or flash accompanying the molded part is trimmed and reclaimed for reuse. 

When not being used in conjunction with another process, like thermoforming or blow molding, profile extrusions are the variety most often used by product designers. They can be either solid or hollow and, for the most part, are made of thermoplastic materials (although thermoset materials can be extruded with special equipment). Therefore, the effect of nonuniform cooling is precisely the same for extrusion as it is for the molding processes sink and distortion. 

Extrusion blow molding uses a section of hot extruded tubular material called a parison. This is extruded into an open mold, and compressed air or steam forces the walls of the parison to the sides of a cold mold. The process is commonly used to make bottles, industrial containers, medical items, technical parts, and toys. Blow molding has a low tool and die cost, and parts can be made very rapidly and in one piece. It also can be used to produce relatively complex shapes, although limited to hollow or tubular parts. The wall thickness is difQcult to control. Continuous tubing and film can be made in a manner similar to blow molding. 

Blow molding is another important process for making plastics articles, particularly hollow container shapes such as bottles. In this process a tubular element of hot plastics is captivated in a mold which has the external shape of the part desired. Air is introduced into the... 

Many product designs are inherently limited by the economics of the process that must be used to make them. For example, TSs cannot be blow molded, and to date they have limited extrusion possibilities. Many hollow parts, particularly very large ones, may be produced more economically by the rotational process than by blow molding. The need for a low quantity of parts may eliminate certain molding processes and indicate the use of casting or others. The extrusion process has fewer problems with TPs than does injection... 

Thick-sheet thermoforming can be used to produce hollow parts that are more difficult to fabricate using a blow molding process. In this method, referred to as twin-sheet thermoforming, two sheets are fed to the open mold so that the sheets remain separated from each other with a blow-pin placed between the two sheets. The mold is closed and the polyethylene is forced into the mold above and below the sheets with a combination of air pressure introduced through the blow-pin between the sheets, while at the same time drawing a vacuum between the sheets and the molding wall to remove air from the mold above and below the two sheets. 

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