Structural Foam Molding

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Structural foam molding is a low-pressure plastic injection molding process that offers unique qualities and opportunities compared to conventional injection molding by injecting plastic & an inert gas or foaming agent into a mold cavity

What is Structural Foam Molding?

Structural foam molding is a low-pressure plastic injection molding process that offers unique qualities and opportunities compared to conventional injection molding by injecting plastic & an inert gas or foaming agent into a mold cavity. Modern structural foam molding technology is very advanced. However, some product designers and engineers are less familiar with the capabilities of structural foam than conventional plastic injection molding.

We have created this page to help demystify and explain the process which produces very robust ridged parts and not a soft foam product as the name could suggest. The benefits of structural foam are also outlined to help companies decide if structural foam molding is a good fit for their products.


Medium to Large-Sized Parts | Ideal for Many Applications

Structural foam product parts and components are produced by using low-pressure injection molding. Rather than relying solely on machine pressure like high-pressure/traditional injection molding, low-pressure structural foam molding introduces a chemical blowing agent into the plastic melt which helps fill a mold. This blowing agent, along with possible use of gas-assist, aids in the fill and pack out of the mold.

The process produces a cellular ‘foam’ core with solid outer surfaces known as the skin. The unique properties of finished structural foam molded material make it more rigid because of the solid outer skin. The cellular core adds to the strength and impact resistance. Plus, the density reduction of the core means parts are 10% to 15% lighter in weight compared to solid injection molded plastic.

Structural Foam Molding Specialties

Process & Technology

Our process is cutting edge and our technology is the very best.

Learn more about low pressure injection molding

Design Considerations

We know what you need to consider when designing parts for optimal production.

Learn how to design for structural foam injection molding

Multi-Nozzle Technology

Higher capacity means more money saved and more products made.

Learn more about the multi-nozzle foam molding process

The Low Pressure Injection Molding of Thermoplastics Allows For:

  • Larger part size – Shot sizes up to 400 pounds.
  • Durability, strength, and manufacturing efficiency
  • Dimensional accuracy over the entire production run
  • Weight reduction up to 15% (by volume) compared to standard injection molding
  • Superior impact resistance
  • Parts that are impervious to the elements
  • Color consistency and complete range of colors
  • Two different materials can be molded simultaneously
  • Ability to mold two colors simultaneously
  • Consistent surface finish
  • Parts are recyclable and returnable for supply chain cost effectiveness
  • Aluminum molds are less costly than steel

How does the low-pressure structural foam injection molding process work?

The process begins by mixing a melted plastic resin with a chemically inactive gas or foaming agent in the barrel of an injection molding machine.

The combined materials are then injected into the mold where the gas expands & ‘packs out’ the mold cavity. This reaction allows for low-pressure injection molding to make larger parts without causing inherent stresses commonly found in high-pressure injection molding.

As the part cools, the foamed core, often referred to as a cellular core, begins to harden creating a ridged center. The outer walls, sometimes called ‘skin’, makes a solid & smooth surface so that the inner core is not visible.

Once the structural foam part is completed cooled, the mold cavity & core separate to release a heavy-duty plastic part.

Why use plastic structural foam molding?

Structural foam molding is best fit for projects that:

  • Have multiple parts to an assembly or kit. For example, a utility cart, deck box, or utility housing.
  • Requires performance to be durable & weather and elements. For example, a plastic pallet, truck bed storage box, or sprinkler valve box.
  • Part or parts are heavy with total weight from 8 pounds to 400 pounds.
  • Design requires more features than vacuum forming, rotational molding, machining, or blow molding.
  • Require dimensional reliability that is repeatable across thousands of parts
  • Greater strength to weight ratio & impact resistance when compared to other processes
  • Want a reduction in molding cost by using smaller machines to mold larger parts
  • Want to use aluminum molds that can require minimal maintenance when built right & maintained properly across the life of the project
  • Want to use less material for larger parts
  • Want to decrease the visibility of sink or read-through from thick sections or ribs
  • Require faster cycle times than rotational molding or vacuum forming since there is usually no post mold machining needed
  • Want to establish effective barriers to entry against competitors due to the capital requirements for tooling
  • Aluminum molds are used & tool life meets or exceeds life of steel molds.

Things to consider when exploring plastic structural foam molding:

  • Low pressure structural foam injection molding allows for thicker wall sections compared to high pressure injection molding. It is recommended to not have wall sections thicker than .250”
  • Structural foam molding creates a ‘swirling’ or ‘silvering’ effect on parts.
  • Structural foam injection molding uses aluminum.
  • Structural foam molded parts do not have any truly representative ways to be prototyped.
  • Structural foam injection molding is a good candidate for large parts or multiple parts manufactured via injection molding, blow molding, roto molding, vacuum forming, thermoforming, wood, concrete, fiberglass, or sheet metal.
  • Structural foam molding is best utilized with cycle times around two to three minutes
  • Plastic structural foam molding, especially multi-nozzle molding, allows for multiple parts in assembly to be made in one mold or machine at once eliminating separate assembled parts / kits to be finalized in a separate factory location.

What’s the process to begin working with low-pressure plastic structural foam molder:

  • A 3D CAD model will need to be made for your part(s). If you don’t have one yet that’s no problem, we might be able to help.
  • Identify the project scope requirements like:
    1. What material to use for the parts
    2. Color & finish options
    3. Project timeline
    4. Number of pieces needed annually
    5. Secondary operations that may be needed
    6. Packaging requirements
  • Using the project scope, submit your file to understand what the mold & parts will cost in production
  • If you’d like to proceed, submit your initial payment for tooling.
  • Once the tool builder is involved, your project design will get a thorough review for possible design changes that may be needed as the tool is designed.
  • Once the mold build is complete, we can start molding parts.

Don’t sweat if you don’t have all the answers to the items above, contact us today and we can help talk about it further

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