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To the average person, creating a plastic part seems like a simple task. But to those in the industry, we know it’s so much more than mere liquid plastic and molds. The manufacturing process is important and can change the result if chosen incorrectly.

Two widely employed techniques, blow molding and structural foam molding, each possess distinct characteristics and applications. This blog compares and contrasts these two processes, providing an inside view of their strengths and limitations.

Blow Molding

Blow molding is a versatile manufacturing process used for creating hollow plastic parts. It involves several key steps:

  1. Raw material in granular form is heated to melt the granules and disperse any colorant added to produce a specific color of plastic.
  2. A molten plastic tube is extruded from the machine head, cut with a hot blade, and then captured by a water-cooled mold.
  3. A blowing tube is inserted into the mold, allowing pressurized air to blow into the plastic, forcing it to take the shape of the mold.
  4. Once cooled and solidified, the mold opens, and the shaped plastic is ejected. From there, the plastic travels to remove any excess from the design.
  5. In some processes, this excess is then conveyed to the beginning of the process to be ground and reused, wasting almost nothing.

Structural Foam Molding

Structural foam molding is a low-pressure injection molding method, initially created as an alternative to traditional injection molding. Low pressure molding particularly suits the production of large, rigid plastic parts. The structural foam molding process involves the following steps:

  1. Melted plastic, such as polypropylene or ABS, is mixed with nitrogen or a foaming agent in an injection molding machine.
  2. The materials are then mixed and injected into the mold. The gas/ plastic mixture expands to fill the mold, creating a core inside the product. This plastic production process requires less machine pressure for this step because the gas or blowing agent helps to fill the mold.
  3. As the part cools, the foamed core, often called a cellular core, begins to harden, creating a ridged center. The outer walls, sometimes called ‘skin,’ produce a solid & smooth surface so that the inner core is not visible.
  4. Once the structural foam part is completely cooled, the mold cavity and core separate to release a heavy-duty plastic part.


When comparing the two manufacturing options, you need to look at the design opportunities for the parts produced by each. Blow molding is more economical for producing hollow parts, while structural foam molding is much better for thick, durable pieces.

Regarding material usage, blow molding uses less plastic, as most of the part is hollow, only requiring plastic to construct the walls. Low-pressure molding uses much more plastic, with walls, on average, being a quarter of an inch thick, plus a cellular core at the center. However, this means very strong parts that last.

Additionally, both processes utilize a closed-loop system. Manufacturers usually recycle excess plastic from production and use it in other parts instead of throwing it away.

One caveat of blow molding, however, is that the empty cavity at the center can make the plastic part weaker than a solid part. Finished parts must also go through a post-processing system depending on which blow molding process where excess plastic is cut off which can be automated.

Structural foam molding adds more material for a stronger and longer-lasting part with a core and outer layer. This makes the low-pressure injection process perfect for large, heavier final products.

Part Size

Structural foam molding excels in producing large, rigid plastic parts, which can weigh up to 325 lbs. In contrast, blow molding is better suited for high-quality, smaller products with hollow interiors.

Surface Finish

Blow molding typically yields products with a smoother surface, resulting in a more polished and aesthetically pleasing appearance. The process creates parts that are seamless and airtight.
Structural foam molding creates a swirled surface. This surface has a swirling or silvering effect. The effect is because of the cellular core. Some manufacturers prefer this effect for certain uses.


Blow-molded products are generally simpler in design. Commonly found examples include packaging equipment, like a plastic toolbox, or automotive parts, like plastic air-intake components. The blow-molded parts are often sleek, with no perforation.

Blow-molded products are generally simpler in design and are often sleek, with no perforation

Structural foam molding is ideal for creating complex shapes and products that require intricate details and structural support. One example of this is the grid pattern found on plastic pallets.

Structural foam molding is ideal for creating complex shapes and strong products

Both blow molding and structural foam molding offer unique advantages and are ideal in their applicable scenarios. The decision between the two ultimately hinges on what you want for an end product. What are the specific requirements of the product you’re manufacturing?

Once you have this information, it will help you decide which process is best for you. They’ll walk you through the process and help you decide your best path forward. Contact us today.

Macay Hunter

Macay Hunter

An expert in plastics manufacturing, Macay works with engineers and supply chain managers during the planning, design and implementation stages of their molded plastic productions.