Structural Foam Molding vs. Vacuum Forming

When making plastic parts, structural foam molding and vacuum forming have special advantages. Both methods are versatile and can produce parts up to 10 feet long (18 feet in thermoforming) for both indoor and outdoor applications. The details provided here were developed to assist teams from engineering, purchasing, and program management in deciding which manufacturing method is most appropriate for their mechanical plastic product.


Structural foam injection molding

Produces small to large-sized and full family assembly parts in a single mold using hard plastic with a solid outer surface and a ridged plastic foam core.

The components are highly robust, weatherproof, impact-, chemical-, and corrosion-resistant.

Agricultural equipment, industrial panels & housings, utility cabinets, irrigation, and other underground systems are products that are frequently made with low-pressure injection molding.

Thermoforming / Vacuum forming

Produces small to large sized solid parts from a heated sheet of plastic. The color, texture, & material type is determined before forming based on the sheet material selected.

Parts are also durable, impervious to weather, & resistant to impact, chemical, & corrosion based on the sheet material selected.

Thermoformed parts are commonly found in outdoor equipment like golf carts & gas stations. They can be found in automotive, medical, heavy truck / aftermarket, & other equipment like food & beverage.

Processes In Brief

Structural foam injection molding

Low-pressure structural foam injection molding is also referred to as structural foam injection molding. In contrast to conventional high-pressure injection molding, the plastic melt is mixed with inert gases. This helps fill and pack out the mold by acting as a filling agent.

The method produces components with a solid outer surface and a cellular “foam” center.
Large, sturdy, lasting objects with excellent impact resistance and weather resistance are molded using structural foam molding. The structural foam molding method is often designed for applications with higher volume requirements.
With tighter tolerances than thermoforming and extremely high part-to-part uniformity throughout the production, structural foam molding can create products with complicated geometry.

The big platen presses are even more versatile thanks to gas-assist and multi-nozzle technology, which enable the simultaneous molding of various tools or multiple parts.

Consumer, agricultural, and commercial vehicles, office and medical equipment, maritime, garden, and architectural products, as well as many other products, all utilize structural foam.

Thermoforming / Vacuum forming

Vacuum forming, also known as thermoforming, is a plastic forming technique used to create plastic parts across various industries. The process has two similar yet different processing techniques, one is called vacuum/thermoforming & the other is pressure forming.

Vacuum forming starts with straight thermoplastic sheets typically .030” to .500” thick being heated to make them pliable and then placed over a female or male mold with extremely small holes throughout the mold.

Once over the mold, a vacuum on the other side of the mold is turned on & the pressure from the vacuum draws the part into the mold. After the part is formed, the warm plastic is cooled quickly to maintain its new shape.

Upon completion, the vacuum pressure is shut off, the part is released from the mold, & then taken to have excess material trimmed, sanded, & smoothed, typically by a CNC machine.

The geometries produced by thermoforming are less complex than that of structural foam but offers more material options like LDPE, HMWP, PVC, ABS, HIPS, & soft touch materials. Unlike structural foam molding, the thickness of vacuum formed parts can change even after tooling has been made based on the preferred material thickness.

Vacuum formed parts can be found in similar industries like housings, consumer products, packaging, lawn & garden, heavy trucking, & medical products.

Part Design And Characteristics

Structural foam injection molding

Ideal for big three-dimensional plastic products like shelf systems, manufacturing pallets, water chamber or drain systems, dunnage, housings, and enclosures.

The structural foam plastic injection method, like injection molding, does not allow die lock or undercuts in component geometry. The straight open and closed mold with little side motions is the best and most cost-effective strategy.

Multi-nozzle injection or gas-assist molding, which allows for the simultaneous molding of several component parts or a single large part, might be advantageous for product designs.

Production of thicker wall sections with lengthy material flow requirements, such as telecommunications or tall outdoor utility enclosures, can also be aided by multi-nozzle or gas-assist molding. For more details, please check out our design considerations page or plastic structural foam injection molding design guide here.

Thermoforming / Vacuum forming

Thermoforming is ideal for small or large 3-dimensional products like covers, trays, packaging, & dunnage.

Making undercuts or die locks are difficult to make & typically parts are made with simple open & close molds. Mechanical features like holes or bosses will need to be machined into the part after it is formed. Mechanical supports like ribs need to be added to the part after it is formed.

For higher volume projects like food containers or trays, multi-cavity tools can be made to achieve the demand requirements.

Part Dimensions

Structural foam injection molding

Single part sizes or a family of assembled parts up to 72” x 72” x 24” are feasible using structural foam as a injection molding alternative.

Material flow & finished part cosmetics can both be improved by using gas-assist and/or multi-nozzle injection molding.

Thermoforming / Vacuum forming

Single parts as large as 120”x216” are achievable in vacuum forming; however, machines large enough to form parts this size are limited.

Wall Thickness

Structural Foam Molding 

Wall sections are best to not be thicker than .250”/6.35mm. Wall sections can be as small as .180”/4.5mm if the part still wants to have structural foam. Once less than .180”, the part can still be molded but will not have a foamed core.

The wall section thickness is determined by how the tool is built so the wall sections will be consistent to the part design & not vary from shot-to-shot.

Thermoforming / Vacuum forming

Optimal wall sections for vacuum forming are anywhere from .020” to .500” depending on the application & requirements.

Wall sections are controlled by the original sheet gauge.

Surface Aesthetics

Structural foam injection molding

The outside surfaces of finished structural foam molded parts are smooth. On the surface, there is the part has a silvering, or swirled utilitarian look.

If the silvered look of structural foam isn’t ideal for a given application, alternatives include adding texture to the aluminum mold, utilize the gas-assist process, and/or paint parts after they are molded.

Often, a gas-assist part can compare to the surface finish of a tradition high-pressure injection molded part.

Thermoforming / Vacuum forming

The surface aesthetics of thermoformed parts are representative of the plastic sheet it is formed from. Parts can have a high-gloss, texture, matte, soft-touch, & more.

After forming, parts can be painted, hot stamped, silk screened, & EMI/RFI shielded.

Part Weight

Structural foam injection molding

When compared to high-pressure molded products, the low-density core with the rigid high-density outer layer can reduce part weight by 10% to 15%.

This weight reduction is especially beneficial for larger plastic parts.

Part weight uniformity from shot-to-shot is also consistent.

Thermoforming / Vacuum forming

The part weight of a vacuum formed part will be the same as the sheet it was formed from less the material that is removed via routing after the part is formed.

Material Options

Structural foam injection molding

Depending on the project budget and the part requirements, a wide variety of resins, usually commodity resins, can be utilised.

The most frequently used plastics are PP, HDPE, ABS, and HDPE, including reprocessed & recycled or regrind material.

Additives can be used with the material including additives for flammability ratings, ROHS requirements, UV resistance, & impact modifiers.

Thermoforming / Vacuum forming

Thermoforming allows for increased material options. Common materials used in thermoforming include: ABS, Acrylic, PETG, PV, Polyethylene, PVC, LDPE, HMWP, HIPS, & TPO.


Structural foam injection molding

While part size, features, & materials affect part tolerancing, more tight tolerances typically are achievable in structural foam.

Thermoforming / Vacuum forming

Formed tolerances of +/- .020” are achievable with an additional .001” Per Inch Beyond 12”. Material, size, & design will impact what tolerancing can be achieved.


Structural foam injection molding

Since the process uses less machine pressure, aluminum molds are used for production parts offering a long life for the mold. Large parts require large molds & the capital needed for these tools can reach hundreds of thousands of dollars. Using aluminum helps reduce these costs & still provides exceptional mold life.

Depending on the intricacy of the part and work load at the tool manufacturer, structural foam molds might take anywhere from 12 to 24 weeks to complete.

Thermoforming / Vacuum forming

Since molds are less complex than structural foam molds, wood, 3D printed, epoxy/polyurethane, & aluminum molds can be used. The part design, material selected, & number of pieces needed will play a large role in what material is used for the tooling.

Depending on the size & material, tooling for vacuum forming can take 2-12 weeks.

Cycle Time

Structural foam injection molding

 Common cycle times are less than 6 minutes, typically 2-3 minutes.

Thermoforming / Vacuum forming

Cycle times of 1 to 10 minutes are required for parts.

Multiple Part Molding

Structural foam injection molding

As an alternative to injection molding, structural foam molding allows for molds of a single large part, multiple parts in one mold, or multiple molds on a single large press platen. Often, this is a benefit to the assembly costs of products, size of machines needed to make a single part, or use of multiple machines to make a full assembly. Learn more here about the benefits of multi-nozzle injection molding.

Thermoforming / Vacuum forming

Dependent on the part design & size, multiple parts can be formed from a tool & then machined after to separate the parts from each other.

Production Volumes

Structural foam injection molding

Production volumes as high as 130,000 can be met using a single tool with one cavity. To reach higher demand, more tools and/or cavities can be used.

Due to the capital required to build molds, EAU’s of 1,000 or less are typically difficult to justify tooling costs/investment.

Thermoforming / Vacuum forming

While vacuumforming can certainly produce more parts per year based on cavitation and capacity, most thermoformed projects are less than 15,000 pieces per year.


Structural foam injection molding

Hardware like reverse threaded or press-in inserts work well with structural foam injection molding once the part has been molded as a secondary operation. Due to the size of the machines & cycle time, insert or overmolding is not recommended.

Thermoforming / Vacuum forming

Hardware like fasteners can be used to fasten parts or you can also bond them with heated plastic much like a glue gun.

Secondary Operations

Structural foam injection molding

Common decorating processes include hot stamping, labeling, or pad printing. Secondary machining or drilling can be used; however, since parts are typically molded to their net shape it is not common.

Thermoforming / Vacuum forming

Decorating parts using hot stamping, labeling, painting, & silk screening are not uncommon.Parts are also machined/routed after forming so it is not uncommon to use the machines to create additional holes or design features.


Structural foam injection molding

Structural foam is typically used in projects where costmetics are not critical and/or the utilitarian silvering or swirl is seen as an advantage to engineering, purchasing, & marketing or branding teams since the impact and strength resistance is critical.

If needed, parts can be painted.

Thermoforming / Vacuum forming

Since materials can come in different glosses, cosmetic finishes, & features, secondary decorating is not always needed.

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