Designing for Structural Foam Molding

When working with Low-Pressure Structural Foam Molding, you need to consider wall thickness, draft angles, bosses and ribs, transition sections and material flow, and assembly and secondary operations

Designing for Structural Foam Molding

Many of the design considerations for low-pressure structural foam molding are the same as with high-pressure injection molding. Transition sections, tolerances, draft angles, etc. However, specifications of the two different processes do vary and structural foam molding offers designers unique opportunities.

A few basics specifications and considerations are below. As always, it is advantageous for the designer and manufacturer to work with their molding company as early in the planning and design stages as possible. The engineers and design professionals at the molding company will offer invaluable insight and information.

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Wall Thickness

Thicker walls and parts can be molded with structural foam compared to standard injection molding. Advanced technologies make it possible to mold the larger and thicker parts with no sick marks or warpage.

Wall thickness as low as 0.180” (4.5 mm) and up to 0.500” (12.7 mm) or thicker can be used. Wall sections of 0.250” (6.35 mm) are often the optimal thickness.

Structural foam molding wall thickness diagram

Draft Angles

Draft angles are necessary in structural foam molding. However, because of the lower pressures used, smaller draft angles can often be used with structural foam compared to standard injection molding.

The thinner the structural foam wall thickness, the larger the draft angles required. This is because higher cavity pressures make the part more difficult to release from the mold.

Structural foam molding draft angles diagram

Bosses and Ribs

Bosses and ribs are commonly used features in structural foam molding. Taller and thicker bosses and ribs can be produced when gas-assist molding is used.

Bosses are frequently used to attach and insert fasteners when multiple structural foam molded parts are being assembled. Molded-in bosses, mounting pads, standoffs and retainers can often be used in place of metal brackets.

Ribs can be used to increase a structural foam part rigidity and loadbearing capability without increasing the wall thickness. The use of ribs instead of increasing wall thickness can reduce cycle time and part costs.

Structural foam ribs can be thicker and shorter compared to ribs on standard injection molded parts.

Adding gas-assist to the molding process can often eliminate sink marks around ribs.

Transition Sections and Material Flow

With structural foam molding transitions from thick to thin wall sections are routinely produced without sink marks. However, uniform wall thicknesses should be maintained in the design for optimal material flow.

When varying wall thicknesses are needed, transitions from thick to thin should be tapered with generous radii and fillets. It is often better to gate the part in the thinner section and have the material flow into the thicker section.

There is a higher resistance to material flow when molding walls less than 0.250” (6.35 mm) in thickness. This can be compensated with increased injection pressure. Wall sections greater than 0.250” (6.35 mm) have less opposition to flow.

Assembly and Secondary Operations

One of the main advantages of structural foam molding is that the large machines, low pressures, and capabilities such as multi-nozzle molding can allow for what used to require multiple parts to be designed and molded in a single part. This can reduce assembly work and cost.

When assembly is required, some of the methods that can be used include:

  • Self-Tapping Fasteners
  • Press-In Inserts
  • Snap Fits
  • Drilling and Tapping

Want ALL the details about designing for structural foam molding?

Download this handy design guide that covers the low-pressure foam molding process, its uses, and many benefits.

Learn about the various molding processes. The guide includes tips, illustrations and specifications to refer to when planning and designing for structural foam molding.


What’s included in this guide?

Designers and engineers are often not as familiar with structural foam molding as they are with standard plastic injection molding. Miles Products created this guide to highlight the many opportunities structural foam molding offers manufacturers.

Specifications and tips for reference while planning to work with structural foam molding, including:

  • Wall thickness
  • Material flow
  • Transition sections
  • Radii and fillets
  • Tolerances
  • Draft angles
  • Bosses and ribs
  • …many more design tips!
Sample pages from the guide:

Details about other options/variations within the low-pressure foam plastic molding process:

  • Multi-nozzle technology
    • The introduction of multiple nozzles allows for the production of larger and more complex parts, up to 72″x72″x24: in size. This technique is also used to mold multiple parts simultaneously.
  • Gas-assist molding
    • Also known as structural web, gas-assist molding gives designers more flexibility during production of large plastic parts. The process allows the molding of thinner walls and the use of features such as taller ribs and bosses.
  • Additional information
    • Assembly and secondary operations are highlighted. These include fasteners and inserts commonly used, typical snap-fit joints and specs, painting and finishing techniques.
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