Blow molding is a process that uses heated liquid plastic to create hollow objects, like bottles or other packaging. Pressure forces the material into a mold cavity, which gives the object its hollow shape. Drug Plastics uses four distinct types of blow molding: Extrusion Blow Molding (EBM), Compression Blow Molding (CBM), Injection Blow Molding (IBM), and Injection Stretch Blow Molding (ISBM).
In general, the size and geometry of the bottle, along with the type of plastic resin being used, determine which manufacturing process is best. We use our knowledge acquired from over 50 years of bottle making to select the best molding process for every product we manufacture. Drug Plastics is committed to deliver perfect bottles each and every time.
We believe it’s important for our customers to understand the primary differences between molding processes. Why? Because understanding how we manufacture our products will help you make more informed decisions about your plastic packaging. How do the types of blow molding processes differ from one another? What criteria do we use to select the best process for your bottle? Read further or watch how the blow molding processes vary. No matter which manufacturing process is used, each one begins the same way – with pellets of plastic resin.
Extrusion Blow Molding
In the Extrusion Blow Molding (EBM) process, the heated resin is extruded to form a parison. The parison is a vertical tube-like piece of plastic through which compressed air can pass. A two-part mold closes on the parison from both sides. Once the mold is sealed, a blow pin is inserted into the neck area of the bottle and air is blown into it. The air pressure causes the parison to conform to the shape of the mold, including the threads in the neck portion of the mold.
The mold is then cooled, and the bottom of the parison is trimmed. The mold opens and the bottle is placed on a conveyor belt, the top (typically dome-shaped) is trimmed, leaving the finished bottle. We use the EBM process to manufacture large-size, High Density Polyethylene (HDPE), Low Density Polyethylene (LDPE), and Polypropylene (PP) bottles with large necks.
Compression Blow Molding
Compression Blow Molding (CBM) is a specialized form of extrusion molding. In CBM, the blow pin not only blows air into the parison so the plastic conforms to the shape of the bottle, but it also uses physical pressure to force the plastic into the neck area of the mold. This results in increased thread definition and precise bore tolerance.
We use CBM when producing bottles with medium to large blow-up ratios. The blow-up ratio is the comparison between the diameter of the neck of the bottle and the diameter of the body of the bottle. Bottles with large blowup ratios typically have large bodies and relatively small necks. The CBM process can be used to manufacture bottles that are made with HDPE, LDPE, and PP resin, and can achieve very tight critical neck dimensions.
The EBM and CBM processes also allow the weight of the bottle to be adjusted. This is done by controlling the amount of plastic in the parison. This is ideal when the contents of the filled bottle require a thicker wall, or as a conscious effort to conserve raw materials by reducing the amount of plastic. The EBM and CBM processes are also used when a bottle has multiple plastic layers (outside and inside); and when a transparent window stripe is needed to allow for determining the amount of product remaining in the bottle.