Blow molding is the process that is used to make items such as plastic bottles and other types of hollow plastic components. It is similar to glass blowing, but is used to mass produce items. There are three types of blow molding processes: extrusion, injection and stretch blow molding. All of these styles of blow molding are based on the same method of inflating an extruded or formed plastic tube against the sides of a shaped mold.
Different types of plastics such as PVC, PS, PP, LDPE and HDPE … can all be blow molded.
Extrusion blow molding is generally used for larger things such as bulk food containers, waste disposal containers, laundry bins and components that are greater than 12 ounces. During large scale extrusion blow molding the plastic tube is heated and softened and is then extruded through the die at a constant rate. It may take several molds to give the final product its shape and detail.
Injection blow molding is the least common method as it is used for accurate components that require precise wall thickness, a bottleneck finish and for cosmetic and pharmaceutical plastic bottles with a full weight of less than 12 ounces. The injection blow molding process is made up of several basic steps as the core pin is coated with hot plastic which is inflated in the mold, taken out and then cooled.
Stretch blow molding is employed for polypropylene, polyethylene, terephthalate and other crystallizable polymers that cannot be blown by any other means. With stretch blow molding, the plastic is heated to a temperature which won’t allow crystals to form in it. A hollow rod is then used to inflate and stretch the plastic to whatever size and shape you like. In these processes the plastics are stretched to harden them and to make them more resistant to deforming while under pressure. This makes the bottles ideal for carbonated drinks.
Important factors to be considered for blow molding are: polymer viscosity at shear rates of low and high, melt strength, uniform wall thickness without holes, strain recovery, crystallization rate, specific heat, thermal diffusion rate, thermal conductivity, crystallization kinetics on the core pin during blowing and cooling, etc.
Blow molding is a high-speed production process and can be used to produce components of high quality and accuracy.