What’s the Difference Between Blow Molding and Injection Molding?

Injection blow molding and plastic injection molding have an important role in the custom manufacturing of reusable and disposable plastic parts that are used worldwide in the research and biomedical fields, and in medical diagnostic laboratories. OEMs can produce a wide variety of specimen containers, bottles, vials, beakers and the like, sterile and non-sterile. Because applications vary, materials range from custom formulated polymers to polyethylene, polycarbonate, polypropylene, and thermoplastics. Nevertheless, whether the manufacturer utilizes blow molding or injection molding depends on the product. Each method of molding uses a different process and, as a result, there are several ways in which they differ.

The Origins of Blow Molding

Blow molding originated and evolved from the age-old art of glassblowing. A patent for extruding a celluloid polymer by blowing, or pushing, air into a mold was issued in the 1880s. Blow molding is an extrusion method where molten plastic is pushed through a two-dimensional die opening into a mold cavity, inflating it in the cavity with compressed air until the molten form gets its desired shape. The part is then cooled before removing from the mold. In blow molding, the finished products produce linear shapes and have two-dimensional forms which are continuous in length.

What is Injection Molding?

Injection molding is a process that evolved out of and is based on the molten die-casting method. First developed in the 1930s by melting plastic and injecting it into a predesigned mold, its advantages include a minimal loss of scrap and finishing requirements, and near 100% recycling. The injection-molding machine consists of two essentials: the actual injection unit and the clamping unit. Unlike extrusion, injection molding also forms three-dimensional shapes.

Solid or Hollow Parts

The most obvious differences in determining which method of molding will be used are related to the kind of product that will be manufactured. In general, blow molding is used to make singular, hollow products like bottles and beakers. Injection molding makes solid parts, like plates and discs, or is used to produce solid parts or pieces for plastic products.

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Wall thickness of the product is also an important difference between the two methods. In blow molding, wall thickness varies depending on how much the plastic is stretched as it is blown. The wall thickness of a part made by injection molding is determined by the mold itself and the core relationship.

How the molds are made varies between the two, as well. In addition to making the mold, the type of plastic, the process temperature, velocity and pressure of the air blown and its speed are all key factors that have to be administered precisely in blow molding. Injection molding is all about the mold itself, approximately 90% of the process. Plastic resins are melted and injected with force into the mold to create the solid part or piece.

The Injection-Blow Molding Processes

The processes of molding between the two methods are different, too. Blow molding machines consist of three major parts: the extruder, accumulator die and product molds that differ in shapes and sizes. Blow molded containers require a parison, i.e., a plastic tube being heated and filled with air. The parison is inserted into the blow molding machine and the mold is clamped around the tube until the parison is formed in the shape of the part. During the injection molding process, melted material is injected into a mold, which is held under intense pressure. Once the material has solidified, it is ejected and the mold is filled again.

In the plastic manufacturing of parts, objects, and pieces, blow molding and injection molding are both common methods to produce the products used on a global market. While injection molding and blow molding may seem similar, there are major differences between the two methods that are ultimately determined by the manufacturer’s needs.