Low-Pressure Structural Foam Injection Molding FAQs

Plastic Structural foam molding is unique in the size, strength, and versatility it provides. The range of opportunities it brings designers and engineers is just as great.

Along with the Frequently Asked Questions about low-pressure plastic injection molding below, we’ve created a Low-Pressure Structural Foam Molding Design Guide to help you get the most out of this valuable process.

Part Questions

What is structural foam injection molding?

It is a low-pressure plastic injection molding process used as an alternative to vacuum forming, rotational molding, sheet metal, & traditional injection molding to manufacture large, ridged, & element resistant parts

What is injection molding & how is that different from structural foam molding?

Traditional injection molding uses large amounts of machine pressure to fill a mold with melted plastic. Structural foam is similar; however, it uses lower machine pressures and a foaming agent to fill a similar mold.

What materials do you mold?

Our most popular material is recycled high density polyethylene. Other popular materials are polypropylene, ABS, Styrene.

What are the differences between high-pressure & low-pressure molding?

The biggest difference between the two processes is the pressure at which the molten plastic is injected into the mold cavity. Low-pressure structural foam injection molding is different from traditional injection molding because it is used to produce larger, less dense parts with a thick wall section on smaller tonnage machines.

How thick of a part can you make?

Suggested nominal wall section is .250 inches. Parts can be made with thicker wall sections but this increases cycle time and adds to the cost.

How thin of a part can you make?

.180 inches is about as thin as you can make while still maintaining a cellular core.

How big of a part can you make?

Our largest machine can hold a tool up to 107 X 161 in size.

How small of a part can you make?

Our smallest machine is 350 tons.

Can you get a flame retardant material for the molding?

Yes, structural foam materials can include flame retardant properties that meet various flammability specifications.

Can structural foam & gas-assist parts be painted?

Yes, if your product requires a more attractive and finished appearance the structural foam components can be painted.

Painted finishes can also be applied to structural foam parts when a very high level of finish is desired. However, the cosmetic finish produced with the latest structural foam molding technology means that painting is not needed for many parts even those that are visible on the exterior of a finished product.

What color is the material?

Recycled material is black. Virgin material can be just about any color you wish.

Can you insert mold steel bars or other reinforcements into structural foam parts?

Yes, however the difference in thermal expansion between the insert and the structural foam plastic material can cause issues with the heating and cooling cycle.

Can the parts be printed with logos or identifying information?

Most often they are pad printed, painted or hot stamped.

Can SF parts be glued together?

In similar fashion as typical thermoplastics, structural foam parts made out of the correct materials can be glued together.

Can structural foam parts be screwed together?

Yes, you can drive screws directly into the structural foam panel or insert threaded fasteners.

What is the surface finish of structural foam parts?


In its most basic form, single-nozzle structural foam molded parts will have a visible swirl on the outer surface. This swirling is also known as silvering or streaking. Swirling is usually not an issue for parts that do not require a high cosmetic finish, for example on reservoir or tank units that will be buried underground.

When a product’s specifications call for a more attractive outer surface, techniques such as gas-assisted molding reduces and even eliminates swirling. These advanced techniques are outlined below.

Process Questions

What is gas assist and how is that different than structural foam?

Gas assist uses gas channels while structural foam molding uses a gas blowing agent. We sometimes will use both processes in the same part.

What is RIM molding and how is that different than SF molding?

RIM molding is a chemical reaction where two different materials are combined to create a third material after molding. RIM is a very low pressure molding operation and the tools are much simpler than those used for structural foam molding. Structural foam is thermoplastic before, during and after the molding process.

What is injection molding and how is that different than SF molding?

Structural foam molding is the cousin of injection molding. The major difference is how the molds are filled. Structural foam uses inert gas pressure to fill the tools while injection molding uses hydraulic or electric machine pressure.

What are the differences between high-pressure vs low-pressure molding?

High-pressure molding, typically referred to as injection molding, solely uses machine pressure to fill a mold. Low-pressure molding uses both machine pressure and a chemical blowing agent to fill and pack the mold into the desired product shape.

Low-pressure molding requires thicker wall sections in order to allow the foaming agent to expand. If the wall sections were too thin, the foaming agent wouldn’t have room to expand and the part would have solid wall sections like a high-pressure part. High-pressure molding uses thousands of pounds of pressure per square inch compared to low-pressure molding which only uses hundreds of pounds. The blowing agent works with the machine pressure to fill the mold.

The blowing agent aids the fill and pack of the tool cavity so a smaller tonnage machine is required. Smaller machines save on machine costs.

Similar but very different

Since structural foam molding is injection molding much of the process, technology, terminology and design considerations are the same as with conventional injection molding.

What are the difference in structural foam vs injection molded parts?

Other than the size and physical properties of the parts produced, here are a few of the notable differences between structural foam and conventional injection molding:

  • Lighter Parts
  • Stronger more impact-resistant parts
  • Better strength to weight ratio
  • Less molded-in part stress
  • Thicker walls
  • Wall thicknesses can vary
  • Taller features such as bosses and ribs can be used
  • Acoustic sound dampening properties
  • Design flexibility
  • Low-pressure allows the use of aluminum molds
  • Less demand on tools resulting in longer life compared to high-pressure tools

What are the benefits of structural foam molding?

The low pressure molding of thermoplastics allows for:

Large Structural Foam Piece
  • Larger part size – Shot sizes up to 200 lbs.
  • 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

Is your part a good fit for structural foam or gas-assist molding?

Ideal for large durable parts

Structural foam molding is mainly used to manufacture larger parts and components that are extremely strong and durable.

Typical part sizes range from around 12”x12”x12”, about the size of a kitchen toaster, to a maximum size of 6’x6’x3’ or larger depending on the part geometry. Flat panel-like parts can be molded as large as 13”x6’ or larger in dimensions.

The composition of the finished molded plastic makes is very strong, rigid, impact-resistant and impervious to extreme weather conditions.

What resins are commonly used in structural foam molding?

Acrylonitrile Butadiene Styrene (ABS)

ABS is an opaque thermoplastic and amorphous polymer. It performs very well across the board including tensile strength, stiffness, impact resistance, compressive strength and combustibility.

Its high material ratings make it higher in cost compared to the other materials commonly used in structural foam molding.

High Impact Polystyrene (HIPS)

HIPS is hard and rigid with highly rated tensile strength, stiffness and compressive strength. It does not rate as highly for impact resistance compared to other materials. This makes it better suited for products such as tanks and enclosures which don’t need to stand up to large amounts of impact.

High-Density Polyethylene (HDPE)

HDPE is a lower cost material. However, it rates highly for impact resistance, weather resistance, mold and mildew resistance which makes it a good choice for many types of structural foam molded products such as outdoor consumer products.

Polypropylene (PP)

PP has very good overall property ratings. It scores slightly lower for impact resistance and compressive strength but is a very cost-effective material.

Polypropylene is commonly used for structural foam molded crates and boxes. It also holds up well against cleaning agents which makes it a good option for medical equipment. It is also used for products that require a higher flammability rating.

Structural foam molding resin comparison chart.

What colors can be structural foam molded?

Colored Resin Material

Black resin is most commonly used for structural foam molding. One reason is that regrind resin material is often used when suitable. We molded over 40 million pounds of recycled material in 2019.

Structural foam is used for many outdoor and garden products. Green or brown materials are commonly used for these types of products.

Other colors of resins are available and can be used. However, some colors, such as blue, can be affected by prolonged exposure to sunlight.

Our teams will advise on the best material for the project during the planning phase.

What size machines do you use for structural foam molding?

Modern low-pressure structural foam molding machines are highly advanced. The presses feature technology and controls that allow the production of very large parts with superior strength and surface cosmetics that rival high-pressure injection molding.

Most of the latest machines have multi-nozzle and gas-assist molding capabilities.

A range of press sizes mean the optimum tonnage machine can be used for each specific product.

Milacron is one of the largest press makers. They manufacture low-pressure presses that range from 500 ton to 6,750 ton.

These are the specifications of a few of the machines in their range. More press specs can be seen on our facilities page.

500 ton

Platen Size 98”x89” (2489 mm x 2261 mm)

Maximum Shot Size: 150 lbs. (68 kg)

1000 Ton

Platen Size 103”x186” (2616 mm x 4724 mm)

Maximum Shot Size: 200 lbs. (91 kg)

2500 Ton

Platen Size 110”x200” (2794 mm x 5080 mm)

Maximum Shot Size: 300 lbs. (136 kg)

What are other names of structural foam molding?

Structural foam molding can also be referred to as:

  • Structural Web
  • Low-Pressure (Injection) Molding
  • Cellular Plastics
  • Gas-Assisted Molding
  • Multi-Nozzle Molding
  • Foamed Plastic

What processes does structural foam molding get confused with?

With ‘structural’ and ‘foam’ in its name, it is not surprising that other manufacturing processes are sometimes confused with structural foam molding.

These processes are not structural foam molding:

Urethane Foam Forming

This is a foam material fabrication process that uses 3-D CNC Cutting, Die-Cutting, Heat Thermoforming and other techniques used to manufacture seat cushions, foam packaging inserts, safety foam materials and other items made of soft foam.

Architectural Foam

This is a fabrication process that is used to make decorative architectural pieces used in the home building industry. Expanded polystyrene (EPS) or molded urethane are often used to make items such as balusters,, columns, crown moldings and other architectural products.

NOTE, structural foam molding can be and is used to manufacture architectural pieces such as crown moldings. Structural foam would be used when a much more robust product and/or larger quantities needed.

Can you use both multi-nozzle & gas-assist at the same time?

Structural Foam Casing Cover

The advantages of gas assist and structural web:

  • Thinner walls and hollow parts for large structural components
  • Smoother surface finish that rivals high-pressure injection molding
  • Consistent surface color and finish – no swirls
  • Low pressure molding advantages
  • Aluminum molds are less costly than steel
  • Maximum part size with minimum clamp tonnage

Is thermoforming the same as vacuum forming?

Thermoforming involves shaping a plastic sheet by heating it to a high temperature and forming it over a mold. Vacuum forming, a subtype of thermoforming, employs a vacuum force to draw the heated plastic into the contours of the mold.

Tooling Questions

The tools are made from what materials?

Because structural foam is a lower pressure process we make tools out of aluminum.

How long does it take to build the tools?

Generally 12-16 weeks after the CAD models are complete and the PO has been issued.

How long does a structural foam tool last?

A well built structural foam tool will produce millions of parts.

What are the tooling requirements for structural foam molding vs traditional/high pressure injection molding?

Aluminum Molds

The low pressures used in structural foam molding mean that aluminum tools can often be used. This saves money on tooling costs.

With the very large-sized molds often needed for structural foam molding, the cost savings or aluminum compared to steel molds is also significant.

Low-pressure molding is also less stressful on the tool itself compared to high-pressure molding. This extends the lifetime of the tool and saves money.

Two-Plate Molds

Structural foam molding is done using tools that are similar in design to high-pressure molding.

Two-plate molds with two halves and a core plate are commonly used.

Other than generally being much larger in size, there are other differences between structural foam and conventional injection molding tools that must be designed and built into the tool.

The most common differences are:

No Runner Systems

Hot runner systems are often not required with structural foam molding.

Multiple Gates

Multi-nozzle molding is a beneficial technology that is often used with structural foam molding. Multiple gates and the ideal gate location need to be included in the tool design.

Standard Tool Making

Just as with high-pressure molding, a top-quality tool-maker will serve the manufacturing company well when designing, building and testing your molds.

Important considerations when designing your parts and tools.

Critical considerations when designing a part and tool for any plastic injection molding and special factors for structural foam include the following:

  1. Material options and consequences
  2. Critical tolerances
  3. Minimizing sink marks
  4. Steel safe areas
  5. Gate locations
  6. Shut-off angles
  7. Draft angle orientation
  8. Texturing and draft
  9. Scheduling of critical start-up phases
  10. Secondary operations and fixtures

What are the benefits of aluminum tooling for structural foam molding?

Aluminum tools cost less, and aluminum tools cool faster, allowing for a faster cycle time.

How long do structural foam mold aluminum molds last?

With proper care aluminum tools can last much longer than steel molds used for traditional injection molding. Molds have been known to last 25 years or longer.

Purchasing / Commercial Questions

How much does a tool cost?

Impossible to answer without seeing your part design but suffice it to say that tools of this size and complexity are usually many 10s of thousands of dollars. Schedule a consultation with our team to get a quote!

How much does a part cost?

Anywhere between 2x and 5x the material cost.

Do you build tools in China?

No, we are unable to build the type of tools required for structural foam in China.

Who builds the tools?

We will request quotes from several highly qualified tool makers. From those bids we select the best candidate to build your tools. The price of the tools are not marked up. We send them as a straight pass through.

What industries and applications use structural foam molding?

Consumer Outdoor and Marine Products

Structural foam molding allows for high-quality mass production of many products that used to require bulky materials and high amounts of hand assembly work.

  • Drainage Systems – Downspouts, Grates, Underground Reservoirs
  • Deck Floor Boards, Railings, Posts, Baluster
  • Exterior Building Features – Shutters, Fencing, Gates
  • Stair Components
  • Indoor and Outdoor Electrical Enclosures, Protective Barriers, Guards
  • Mailboxes
  • Building and Construction Products
  • Boating and Dock Components

Outdoor Garden Products

  • Decorative Arbors, Lattices, Gates
  • Irrigation System Components
  • Barrels, Pots, Rain Barrels
  • Outdoor Furniture
  • Storage Sheds and Storage Boxes
  • Wheelbarrows, Garden Carts
  • Tractor Carts

Pools, Play and Sports Products

  • Pool Panels, Enclosures, Stairs, Slides, Ladders
  • Playsets
  • Sports and Recreation Equipment

Consumer Indoor and Architectural

Techniques such as gas-assisted molding produce plastic parts with smooth, good looking outer surfaces. This allows the manufacture of many products that previously required other more expensive processes.

  • Baby Furniture
  • Shelving
  • Adult Furniture Including Frames and Legs
  • Shower Safety Equipment
  • Tubs
  • Billiard Table Legs and Ball Dispensing Frame
  • Laundry and Storage Bins
  • Architectural Features and Products, Cornices, Covings, etc.

Ready to start your project?

Fill out a quote form or schedule a free call with our experts for smart solutions and American-made large structural foam and gas assist plastics

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