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Low Pressure Structural Foam Molding


What is Structural Foam Molding?


Structural Foam Molding is, in essence, a low pressure injection molding process. It is used to process thermoplastics, such as high density polyethylene (HDPE), in a low pressure environment. Instead of using high pressures to force the molten polymer to fill up the cavity of the mold, structural foam molding relies on the foaming action caused by an inert gas distributed in the plastic material to facilitate the flow. Alternatively, foaming can also be created by the gases released by the decomposition of a chemical blowing agent added to the resin.

Due to the lack of high pressures during the molding process, structural foam parts generally have thicker wall sections. Also, structural foams have a significant density reduction, sometimes as large as 40%, from its base material. They typically exhibit excellent strength-to-weight ratio, improved thermal and acoustic insulation properties, but suffer from lower tensile strengths.

Compared with injection molding, structural foam molding can use cheaper and lighter molds (aluminum instead of tool steel), and it can be used to produce larger parts. The downside is lower production speed. The setup cost of structural foam molding is also lower, making it a viable alternative to injection molding for low-volume applications.

In additional to HDPE, other polymers commonly used in structural foam molding are:

Other than needing thicker wall sections, the design considerations of structural foam molding is very similar to those of injection molding.


Pros and Cons of Structural Foam Molding



  • Lower setup costs than injection molding
  • Allows the use of lighter, less expensive (aluminum) molds
  • Allows larger product sizes than injection molding
  • Lower internal stresses than injection and compression molding


  • Higher setup costs than compression molding
  • Lower production rate than injection molding


DeKalb Molded Plastics Co., of Butler, Indiana, which does both structural foam and injection molding, was one of three finalists for Plastics News’  Molder of the Year Award.  Winners were announced March 5 at the Plastics News Executive Forum in Tampa, Fla. The finalists and award winners were honored at a reception, followed by a best practices panel discussion.  DeKalb Plastics was honored as Employee Relations winner for 2012.  This award is especially meaningful, as DeKalb has made a concerted effort to employ veterans.

See more on DeKalb Molded Products at,AAAAADr




  • A low pressure process where a nitrogen gas is injected into the plastic part in certain areas to hollow out the part. The nitrogen gas does not mix with the resin, but instead forms continuous channel through the hotter less viscous sections of the melt stream. The gas maintains internal pressure through out the cooling cycle.



  • Reduced part weight
  • Capable of molding large, complex parts
  • Low cavity pressure allows lower cost tooling
  • Multiple molds can be run at the same time
  • Molded parts up to .500 inches thick & 150 Lbs
  • Excellent substrate for high quality painted finish
  • High dimensional stability
  • Virtually any thermoplastic can be foamed


Low pressure structural foam parts
Low pressure structural foam molding Gas-assist molding Quality System Registered to ISO-9001-2008 Filler/reinforced parts High density polyethylene, glass fiber reinforcement, polypropylene, carbon fiber, nylon, glass bead filling, Noryl (PPO), rubber, polycarbonate, (PC), acrylonitrile butadiene styrene (ABS)  polycarbonate/acrylonitrile butadiene styrene (PC/ABS)flame retardant polystyrene (FRPS), general purpose polystyrene (GPPS), high impact polystyrene (HIPS), acrylic.