Blow Molding

Table of Contents

Blow molding is a process to create a hollow object by inserting pressurized air into the heated plastic structure into its mold. It's mostly used in manufacturing bottles and other hollow symmetrical objects.

There are 3 common types of blow molding:

  1. Injection Stretch Blow Molding
  2. Extrusion Blow Molding
  3. Injection Blow Molding

Injection Stretch Blow Molding

This technique increases axial stretch, which makes blow molding easier and reduces energy consumption. The volume of products that can be processed by injection stretch blow molding is larger than that of injection blow molding. This method is used mostly in forming bottles and containers with a volume between 0.2~20L. 

The working process is as follows:

  1. First comes the injection of the parison (rod shaped plastic raw material), the principle is the same as an ordinary injection molding workflow;

  2. Then transfer the parison to be heated and follow a temperature adjustment process to make the parison soft;

  3. Transfer to the stretch and blow station and close the mold. The push rod in the core stretches the parison in the axial direction while blowing air into it to make the parison close to the mold wall and then cool down the material

  4. Demold and take out

Extrusion Blow Molding

Extrusion Blow Molding can be applied in a wide range of processes, from small products to large containers and auto parts, aerospace, chemical products, etc. The process of extrusion blow molding is as follows:

  1. First melt and mix the polymer, and the melted material enters the machine head to become the tube condition parison;

  2. After the parison reaches the predetermined length, the blow mold is closed and the parison is sandwiched between the two mold halves;

  3. Blow air into the parison to make it close to the mold cavity;

  4. Cool the material;

  5. Open the mold and remove the formed object.

Injection Blow Molding

Injection Blow Molding is a molding method that combines injection molding and blow molding. It is currently mainly used in beverage bottles and medicine bottles that require high blowing accuracy and some small structural parts like grooves and ridges. The working process is as follows:

  • Use the normal injection method to inject the parison first

  • The mandrel (think about it as a popsicle stick that keeps the parison in position) and parison are moved to the blow molding station.

  • The mandrel sandwiches the parison between the blow mold sections and the mold is closed. Then, compressed air is blown into the parison through the middle of the mandrel, inflated to make it form around the mold wall, and cooled down.

  • After demolding, the mandrel is transferred to the injection station for another run.

Blow molding offers lighter products with a large volume and thin wall thickness. In injection molding, the product is filled with plastic, while blow molding ensures the structure is hollow inside. Injection molding requires the plastic to first be melted into liquid form while blow molding deals with flexible heated plastic (before its melting point).