Injection Blow Molding Basics

Injection blow molding is a manufacturing process that combines the accuracy of injection molding with the minimal waste and rapid processing of blow molding. Injection blow molding can be used with plastics and is generally used to make hollow shapes such as bottles. The manufacturing process is more expensive than other blow molding techniques, so larger containers are generally fabricated using extrusion blow molding. The final products of injection blow molding have no seams, and can be glass clear.

Injection Blow Molding Process

The injection blow molding process has three phases: Injection of the material into the preform mold; blowing of the preform, or parison, into the product mold; and ejection of the product from the machine. A commonly used machine configuration is a three-stage blow molding machine, which rotates through 120 inches for each phase of the process. In the injection phase, molten plastic or glass is injected into the preform mold over a core pin, which forms the interior of the parison and finishes the interior of the neck. At this point, the neck of the container is fully formed. The parison is removed from the mold and inserted into the blow mold. The parison is inflated inside the blow mold by pressurized air. After a cooling period, the product is ejected from the machine, tested for leaks, and moved to another station for labeling or finishing.

Designing for the Injection Blow Molding Process

Designing products to be fabricated using injection blow molding includes selecting a thermoplastic resin for the product application, designing the mold geometry to take into account material shrinkage during cooling, calculating the right amount of material to be injected and choosing the right pressure to inflate the parison in the mold. Tolerances in the core rod and preform mold design are critical so that the core rod sits exactly in the center of the mold and the resulting parison has constant wall thickness prior to blowing.

Products Fabricated Using the Injection Blow Molding Process

Injection blow molding is used to create hollow products such as bottles, and is generally only used for small containers such as medicine bottles or single-serve beverage containers. The injection phase of the process can create a neck with very fine tolerances, with designs such as screw threads or threads for childproof caps. Unlike standard injection molding, undercuts can be incorporated into an injection blow molded design for some products, such as mascara bottles.