What is Structural Foam?

Structural foam is a low-pressure form of injection molding, which utilizes most thermoplastics, (including post-consumer regrind), to mold rigid product, which can have thicker walls and higher stiffness-to-weight ratios than standard injection molded product. 

In the structural foam process, a physical or chemical blowing agent is mixed in with the resin.  The resin is shot into the cavity, but not completely filled or packed out. The blowing agent expands to push the resin to the extremities of the cavities. As the part cools, the internal pressure of the foaming action takes up the internal shrinkage, and reduces sinks over ribs or heavy cross-sections.

What are the advantages of the Structural-Foam process?

There are many advantages of the structural foam process which combine to provide an economical advantage over other processes, and allow production of parts which can’t be matched by any other process. Following is a list of many of the advantages of the Structural-foam process:

  • Low cost tooling with the same molded-in detail and features possible in standard injection molding.
  • Multiple parts and multiple tooling can be run on a single machine.
  • Two different materials and or two different colors can be run at the same time.
  • Product can easily be gated at multiple points on the part, which can be strategically positioned to optimize filling, and processing.
  • The process utilizes low-pressure to mold the parts which results in low stress and warpage in the parts.
  • The low pressure part of the process allows molding of large parts with low machine tonnage.
  • The process allows a wide range of design flexibility.
  • Superior part and process repeatability are possible with the process.
  • High strength-to-weight ratio.
  • Reduced part weight while maintaining high stiffness-to-weight ratio.
  • Can mold medium wall thickness through very thick wall sections with minimal sink marks.
  • Reduced weight of 10 to 20% over solid plastic part.
  • Total shot weight of up to 200 pounds.

When should the structural foam process be considered for a part?

  • The structural foam process should be considered when a part of substance is desired, which will need to have structural integrity.
  • When small to large parts are required with medium to thick wall cross-sections.
  • When an increase in stiffness of 3 to 4 times the strength of parts of similar weight is desired.
  • If there is a need for varied wall thickness within a part.
  • If sink-free surfaces of ribbed parts are desired.
  • When multiple, “family” parts are to be made. The various parts can often be molded together in the same machine, which will reduce overall cost.

What does a part molded with the structural foam process look like?

A product molded utilizing the structural foam process will typically have a surface which appears to have swirls on it.  This is due to the internal “foaming action”, in which some of the small internal bubbles come to the surface of the part. A cross-section of a structural foam molded part will show a cellular internal core sandwiched between two solid wall sections.   

How much weight reduction can be expected by utilizing the structural foam process?

The amount a part is reduced in weight by utilizing the structural foam process is dependent on many variables including part configuration, wall thickness, material type, and processing. However, a reduction of 10% to 20% is common.

Why is aluminum tooling utilized in the structural foam process, and what is the life expectancy of an aluminum mold?

Since the injection pressures in the structural foam process are relatively low, the use of aluminum tooling is prevalent. Aluminum tooling provides superior cooling over steel, while the aluminum tooling can be built quicker and more cost effectively than a steel tool. As with any tool, the life of a tool depends on the molder and the care taken with the tool by the molder. An aluminum tool will last as long as a steel tool if it is taken care of. There are many examples of aluminum tooling running in excess of a million parts.