Process FAQs

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

Along with the Q&As below, we’ve created a Structural Foam Molding Design Guide to help you get the most out of this valuable process.

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 here (link to Machine List page)

See a complete machine list here.

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?

The names of some of the advanced technologies covered above and several other related terms are sometimes used to refer to structural foam molding.

  • Structural Web
  • Low-Pressure Molding
  • Cellular Plastics
  • Gas-Assisted Molding
  • Multi-Nozzle Molding

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

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