Structural Foam Injection Molding
Structural foam molding is a low-pressure injection molding process that uses a foaming agent mixed with plastic to produce nitrogen to create thicker plastic products. The process creates a cellular interior and strong outer skin during molding that prevents warping and shrinkage defects, making it an ideal manufacturing method for large, thick plastic parts.
From engineering design assistance, to complete project management, we can effectively manage your structural foam molding needs and provide complete custom molding solutions for your structural foam molded products.
  • Detailed DFM analysis
  • Mold making and prototyping
  • ISO 9001:2015, IATF 16949:2015 certificated
  • 24/7 engineering support
  • Material selection assistance
How does Structural Foam Injection Molding Work?
1. Mixing Materials & Foaming Agent
Mix the plastic resin and foaming agent evenly and add them to the barrel of the injection molding machine to prepare for injection molding.
2. Injection Molding
Like traditional injection molding, the mixed material is injected into the mold to evenly fill the mold. At the same time, the gas generated by the foaming agent (mainly nitrogen, a small amount of water and carbon dioxide) expands in the plastic mold and quickly "fills" the mold cavity. As the material pushes against the wall of the mold, it creates a thick skin while leaving the inside of the part with a cellular structure.
3. Cooling & Stripping
After waiting for the parts to cool and solidify, forming a solid plastic part with a cellular interior, the ejection system works to strip the product.
4. Pick Up & Packaging Parts
Manual or automatic robots pick up parts, trim the runner, assemble and package them, and put them into boxes.
Why Choose Attractive Plastics for Structural Foam Injection Molding
Rich Structural Foam Molding Experience
Scientific molding is the key to the success of injection molding, especially for plastic parts with multiple undercut, slider, insert molding, overmolding and other extreme complex structures feature and high surface requirement.
Quality Assurance
We rely on advanced measuring equipment, as well as a skilled team of quality engineers, technicians and inspectors. We will take the careful measurements necessary for each project to ensure that you receive the highest quality components.
Strong Engineering Team
Our engineering team with over 20 years of experience provides strong professional knowledge and technical support in precision insert molding tooling manufacturing. we will work closely with you to formulate a reliable mold design and machining plan.
We offer design for manufacturability analysis to ensure that your insert molding parts balance quality, efficiency and cost. Our advantages of automation and upfront mold design translate into more competitive pricing when running in high volumes, which also enables our customers to reach a larger market.
Structural Foam Injection Molding Materials
We offer a variety of materials for structural foam injection molding projects, depending on the geometry, complexity and requirements of your project. Most any thermoplastic material can be used in a structural foam application.
Rigid Plastics
  • Polypropylene (PP)
  • Polycarbonate (PC)
  • ABS
  • PMMA (Acrylic)
  • Nylon
  • PE
  • PBT
  • POM
  • SAN
  • PS
  • PC/ABS
  • PFA
  • PEEK
  • PPS
  • PEI
  • LCP
  • PPSU
  • PPA
  • PES
Foaming Agent
  • Expanded polypropylene (EPP)
  • Expanded polystyrene (EPS)
  • Expanded ABS
  • Expanded PC
  • Expanded NYLON
  • Foams with special properties
  • And More
Gallery of Structural Foam Injection Molding Parts
Dive into Attractive Plastics' extensive gallery that showcases some of our completed super thick wall plastic parts and take confidence that we can create your structural foam molding project according to your strict specifications.
Benefits of Structural Foam Injection Molding
Structural foam injection molding can make large plastic parts that can‘t be made by traditional high-pressure injection molding, and is an ideal choice for making thick-walled plastic parts. It has many advantages
Increased Mechanical Properties
The cellular structure of the foam core provides the injection molding part with greater strength adn rigidity, making it highly resistant to bending, impact and torsional forces.
Increased design flexibility
Cellular structures contain thicker walls, ribs and other complex features without sacrificing part weight. This flexibility allows designers to integrate many functional components directly into molded parts.
Reduced Cost
Foam molding parts have many small holes inside, helping customers save materials, reduce weight, improve efficiency, save costs, and improve competitiveness.
Improved Appearance
The nitrogen gas during molding offsets the internal stress, avoiding shrinkage, flow lines, warping and other molding defects, and improving the appearance of the parts.
Structural Foam Injection Molding Design Tips/Considerations
Design tips/considerations
Wall Thickness
An optimal wall thickness of 0.250 in. (6.35 mm) is recommended because thin walls can impede necessary chemical reactions. However, wall thicknesses as low as 0.180 in. (4.5 mm) and as high as 0.500 in. (12.7 mm) or greater are also available.
Bosses and Ribs
Structural foam ribs, bosses can be thicker and shorter, and are not limited by standard injection molding designs
Transition Areas
There is generally no sink mark in the transition area of structural foam molding, but try to maintain a uniform wall thickness in the design for optimal material flow. When the wall thickness needs to be changed, large radii and fillets should be used to make the transition.
Interior Stress
Structural foam molding has lower internal stresses than standard injection molding because the nitrogen used during molding partially offsets the internal stresses.
The pressure inside the mold of structural foam is small, so you can consider using aluminum molds to save costs.
Poor Appearance
The finished surface of structural foam parts is essentially rough. Try to consider using internal parts, or painting, or use rough texture on the surface of the product to cover up this defect.
High dimensional stability
The combination of low-pressure injection and expanded nitrogen ensures product dimensional stability with smaller internal stress.
Reviews from Our Customers
A customer’s words have a more substantial persuasion than a company’s claims. We have served hundreds of satisfied customers and here are some reviews they give us.
Through trialing advanced structural foam injection technologies, they developed an insulation core for our electric vehicle seating that is 30% lighter than alternatives while retaining enhanced energy absorption characteristics. Miniature cell formation assures consistent performance.
David Moon
Vice President
Precise control of foaming agent levels and ejection timing are critical for our high-volume automotive armrest, and their process knowledge delivered samples with an elegant finished surface and consistent dimensions. We look forward to a long production partnership maximizing efficiencies.
Carl Harris
Pruchasing Manager
Through microscopic analysis of molecular structure changes at high pressures, they optimized their specialized foam injection process to minimize our product's weight without sacrificing structural rigidity. Ultra-light housing prototypes have strengthened our competitive edge in the aviation industry.
Tony Jones
Attractive Plastics provides comprehensive services from mold design to production. Whether it is early design assistance or later mold production and injection molding production, they control every detail very carefully to minimize errors and defects. The products they offer are of excellent quality and competitive prices. A great partner for our business!
Ben Holder
We have been working with Attractive Plastics for over 5 years. Their design solutions are proven. And the production tolerances are strictly controlled. Their team is professional, experienced, responsive and ready to tackle any challenges faced during the molding process.
Anne Zaremba
Our Structural Foam Injection Molding Service for Various Industries
Our custom structural foam injection molding process is suitable for solve the various demands of industries, including the automotive, medical, electronic & electrical and consumer goods fields etc.
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Structural Foam Injection Molding FAQs

What is structural foam injection molding?

Structural foam injection molding is an injection molding process for producing strong, lightweight plastic parts with an interior foam structure. A gas is injected into the mold along with the polymer melt, resulting in an inner foamed core with a solid plastic skin. This provides weight reduction along with added strength and rigidity.

This molding process allows manufacturers to produce very large and thick plastic parts. Although it has some similarities to standard injection molding, this process requires much less pressure. As a result, parts may have thicker wall thicknesses while being lightweight and stiff.

How does structural foam molding work?

During the injection molding process, the nitrogen produced by the chemical reaction of the foaming agent causes a foam core to be formed inside the plastic, resulting in a lightweight and high-strength structural foam injection molded part. Here is a brief overview of how structural foam injection molding works:

  1. Mix the foaming agent and plastic particles evenly before injecting into the mold.
  2. Inject the mixed material of foaming agent and plastic into the mold under low pressure. The foaming agent absorbs heat and undergoes a chemical reaction to  produce nitrogen, carbon dioxide and a small amount of water.
  3. The expanding gas forms bubbles and cells within the core of the plastic part. The plastic near the mold walls will solidify first, creating an outer solid skin.
  4. Wait for the plastic part to cool and solidify to form a plastic part that is dense, foam-free and solid on the outside and honeycomb-shaped inside.
  5. The ejection system works to push the plastic parts out of the mold to complete the next complete production cycle.

What are the advantages and disadvantages of structural foam molding?

Structural foam molding provides an economical means of increasing strength, reducing weight, and improving dimensional stability of injection molded plastic parts. The process adds performance benefits without using expensive alloying or reinforcements.

Structural foam molding advantages:

  • Lightweighting – Foamed core results in density reductions of 10-25%, reducing material usage and costs.
  • Increased strength & rigidity – The foamed core provides reinforcement, improving load-bearing capacity.
  • Improved dimensional stability – The foam core minimizes shrinkage and warpage.
  • Complex geometries – Foaming allows for molded-in ribs, bosses, and stiffeners.
  • Lower cycle times – The foam reduces material mass, enabling faster cooling.
  • Cost effectiveness – Foaming reduces material costs and total part cost.
  • Structural foam molding reduces shrinkage, warping and internal stress.

The main challenges relate to higher equipment requirements, limitations on resin selection, increased process variability, and additional downstream operations. Proper design and process controls can minimize many of these disadvantages.

Structural foam molding disadvantages:

  • Higher tooling costs – Molds require more complex hot runner systems and venting.
  • Limited material choice – The plastic must have low gas permeability to restrict foam expansion.
  • Difficult color matching – Variations in foam density can affect color consistency.
  • Part weight variability – The foaming process is difficult to control precisely.
  • Reduced aesthetic appeal – The foamed layer lacks an attractive surface finish.It is suitable for interior plastic parts
  • Secondary operations needed – May require degating, drilling, finishing, plating, paint
  • Recycling challenges – Contamination and mixing of foamed and solid layers can occur.
  • Tolerances harder to maintain – Foaming can cause distortions if not properly controlled.

What are the applications of structural foam injection molding?

The lightweight strength and rigidity provided by structural foam molding make it ideal for large, flat, and thin-walled components across a broad range of industries. It allows downgauging and weight reduction without compromising performance.
Here are some of the main applications for structural foam injection molding:

  • Automotive – Bumpers, door panels, instrument panels, seat backs, wheel wells, trunk liners
  • Construction – Doors, windows, exterior trim, panels, decorative moldings
  • Furniture – Chair seats, office furniture, healthcare furniture, institutional casegoods
  • Recreational products – Playground equipment, kayaks, athletic helmets, floatation devices
  • Consumer goods – Luggage, tool handles, large storage containers
  • Packaging – Protective carrying cases, industrial crates and pallets
  • Appliances – Washer/dryer drums, refrigerator liners, air conditioner panels
  • Electronics – TV/monitor frames, battery cases, transformer covers
  • Medical – Imaging table components, stretcher parts, encapsulations

How thick is typical structural foam?

Typical structural foam molded parts range in thickness from 4 mm to 10 mm. We can make it thicker, but usually increase the strength by adding ribs to get more ideal strength and effect, while saving material and cost.

Can you paint the structural foam molding plastic products?

Yes, we can. generally foam molding plastic parts don’t have a very good appearance. Painting the outer surface is one of the common methods. We can paint plastic parts made of most materials. Visit our post-molding operations service help you learn more other secondary molding process.

Structural Foam Injection Molding Resources
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