Structural foam product parts and components are produced by using low-pressure injection molding. Rather than relying solely on machine pressure like high-pressure/traditional injection molding, low-pressure structural foam molding introduces a chemical blowing agent into the plastic melt which helps fill a mold. This blowing agent, along with possible use of gas-assist, aids in the fill and pack out of the mold. The process produces a cellular ‘foam’ core with solid outer surfaces known as the skin. The unique properties of finished structural foam molded material make it more rigid because of the solid outer skin. The cellular core adds to the strength and impact resistance. Plus, the density reduction of the core means parts are 10% to 15% lighter in weight compared to solid injection molded plastic. 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. Since structural foam molding is injection molding much of the process, technology, terminology and design considerations are the same as with conventional injection molding. But there are also important differences between the two. Here are some of the main differences: 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. 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: More details on the advantages of structural foam are listed below. How large parts are molded using low-pressure comes down to several important differences between structural foam and conventional molding. Other than their very large size and gas injection apparatus, low-pressure structural foam molding presses are similar to high-pressure injection molding machines. The thermoplastic resin material is fed via a hopper into a barrel and screw heating unit before going into the injection stage. This is where the low-pressure structural foam molding process begins to differ. An inert gas, usually nitrogen, is introduced to the plastic melt prior to injection into the mold. The inert gas expands within the mold cavity. This aids the plastic flow. The gas and the action it produces is so efficient that only a partial short-shot is required. The foaming action and low-pressure work together to push the resin flow to the furthest extremes of the mold. The plastic cells collapse or freeze as they come in contact with the walls of the mold. This forms a rigid and smooth outer skin. Once the cavity is completely packed-out and the part begins to cool, the material in the core of the walls develops a cellular or honeycomb structure. The cellular structure, together with the solid outer skin gives the part its integrity and superior strength to stiffness property.
Structural foam molding is a low-pressure plastic injection molding process that offers unique qualities and opportunities compared to conventional injection molding and other forms of custom molding.
Modern structural foam molding technology is very advanced. However, some product designers and engineers are less familiar with the capabilities of structural foam than conventional plastic injection molding.
We have created this page to help demystify and explain the process which produces very robust ridged parts and not a soft foam product as the name could suggest. The benefits of structural foam are also outlined to help companies decide if structural foam molding is a good fit for their products.
Medium to large-sized durable parts ideal for many applications.
What are the differences between high-pressure vs low-pressure molding?
Similar but very different
Ideal for large durable parts
Other key differences
Process and Technology
Structural foam product parts and components are produced by using low-pressure injection molding. Rather than relying solely on machine pressure like high-pressure/traditional injection molding, low-pressure structural foam molding introduces a chemical blowing agent into the plastic melt which helps fill a mold. This blowing agent, along with possible use of gas-assist, aids in the fill and pack out of the mold.
The process produces a cellular ‘foam’ core with solid outer surfaces known as the skin. The unique properties of finished structural foam molded material make it more rigid because of the solid outer skin. The cellular core adds to the strength and impact resistance. Plus, the density reduction of the core means parts are 10% to 15% lighter in weight compared to solid injection molded plastic.
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.
Since structural foam molding is injection molding much of the process, technology, terminology and design considerations are the same as with conventional injection molding.
But there are also important differences between the two. Here are some of the main differences:
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.
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:
More details on the advantages of structural foam are listed below.
How large parts are molded using low-pressure comes down to several important differences between structural foam and conventional molding.
Other than their very large size and gas injection apparatus, low-pressure structural foam molding presses are similar to high-pressure injection molding machines.
The thermoplastic resin material is fed via a hopper into a barrel and screw heating unit before going into the injection stage.
This is where the low-pressure structural foam molding process begins to differ.
An inert gas, usually nitrogen, is introduced to the plastic melt prior to injection into the mold.
The inert gas expands within the mold cavity. This aids the plastic flow.
The gas and the action it produces is so efficient that only a partial short-shot is required. The foaming action and low-pressure work together to push the resin flow to the furthest extremes of the mold.
The plastic cells collapse or freeze as they come in contact with the walls of the mold. This forms a rigid and smooth outer skin.
Once the cavity is completely packed-out and the part begins to cool, the material in the core of the walls develops a cellular or honeycomb structure. The cellular structure, together with the solid outer skin gives the part its integrity and superior strength to stiffness property.
Structural foam low-pressure molding process
The first stages of the low-pressure process are similar to high-pressure molding. The resin material is fed from a hopper into a barrel and reciprocating screw heating unit. The plastic melt then goes into the injection unit.
Before injection into the tool, inert gas is introduced into the melt.
Only a partial shot is used because along with the low machine pressure, the gas creates a foaming action that helps the flow, fill and pack of the mold.
The plastic cells collapse as they come into contact with the wall of the mold creating the solid plastic skin. The foaming action continues to push the leading edges of the melt into the furthest extremes of the mold.
Once the mold is completely packed out and begins to cool the ‘frozen’ outer skin becomes solid and rigid. The inner core develops a cellular or honeycomb structure when cooled.
Advanced structural foam molding production techniques.
Added technologies that improve production and product.
Gas-Assist / Structural Web
Gas-assisted molding technology, also known as structural web, compliments structural foam molding. Specialized nozzles, inlets and channels are incorporated into the part and tool design.
During the injection stage, inert pressurized gas, usually nitrogen, is injected into the tool and resin using a specialized nozzle or gas pin. The pin design and gas work together to prevent plastic backflow.
With the resin shot completed, the pressurized gas continues. This keeps the flow front moving by forming a bubble inside the piece stretching the skin and material to the furthest extremes of the tool.
After the tool has been completely filled the gas bubble continues to be pressurized creating an internal cushion to compensate for the resin shrinkage and fully form the outer skins of the part.
Finally, once the part has cooled adequately and the skin has its strength, the gas pressure is vented before the mold is opened the part is ejected.
Hollow sections within a part can be formed using gas-assist.
Gas-Assisted Molding is Ideal for:
- Strong parts with hollow sections such as handles
- Large molded plastic parts that require cosmetic outer surfaces
- Large plastic panels
- Supporting ribs & reducing visible sink on the surfaces
Benefits of Gas-Assisted Molding:
Large molded parts with attractive outer surface cosmetics can be produced with gas-assist.
Swirl is eliminated and secondary painting operations are not needed with gas-assisted molding.
Gas-assisted molding gives designers more latitude and options for introducing various component features such as bosses and ribs.
Gas-assist can help reduce cycle time.
Other Design and Product Benefits Include:
- Taller and thicker bosses, ribs and wall sections can be molded.
- Sink is significantly reduced or even eliminated.
- Surface cosmetics are greatly improved by reducing swirling and flow lines.
- Hollow thicker sections can be molded. This allows for features such as handles to be incorporated in the part.
- Can eliminate finishing work such as coring out or thick sections.
- Creates the look of a traditional injection molded part while still getting the benefits of structural foam molding
Production and Cost Benefits Include:
- Saves on costs by allowing the use of a smaller press with larger platens. The pressurized gas assists the low-pressure molding machine.
- Reduces the cavity pressure required for fill and pack.
- Lower pressure allows the use of aluminum molds which save on costs.
- Saves on tooling costs by eliminating the need for undercuts.
- Parts previously requiring metal can be molded using gas-assist.
- Helps produce larger sized plastic parts, or a family of molds can be run simultaneously.
- Improves cycle time of structural foam molded parts.
Multi-nozzle molding is used for more complex designs and parts that can’t be filled from a single nozzle. The large press platens contain hundreds of inlet holes where multiple injection nozzles can be precisely located so the plastic will fill the hard to reach extremes of the tool.
Multi-nozzle injection offers a key advantage when producing very large or complex parts. The technique is used because it can greatly improve the cosmetic finish of a part.
Multi-nozzles eliminates the need for a hot runner system which saves on costs.
Multiple tools with multiple cavities can be run at the same time using multi-nozzles and the large platen presses. This can be done on the same machine that would be used to mold a single large plastic part.
Multiple materials or colors can be molded simultaneously.
Multi-Nozzles Give Complete Control
Instead of a single nozzle, a structural foam molding press has a manifold and multiple nozzles.
Shot size, sequence, open & close time, gas-assist and more are digitally controlled at each individual nozzle. This saves material and time and produces a higher quality product.
The nozzle inlet holes are spaced 6 inches apart. This allows precise positioning of every nozzle that is being used.
When to build a multi-cavity tool instead of multiple tools for a multi-component assembly.
Assume your program is an outdoor garden bin. Your product has 2 sides, a front, back, base & lid for a total of 6 parts. Once molded, the cabinet will be placed in a single package for shipping for the end-user to assemble.
This program could use 5 separate tools: 1 tool for the sides, 1 tool for the front, 1 for the back, 1 tool for the base, and another for the lid. Each tool would need to be running on a separate machine and would require a separate cell for final assembly and/or packaging. Since each piece will be running on a different machine, the color could have some variation between the 6 separate pieces.
This tooling strategy could be used in a multi-nozzle process; however, a larger machine would be needed to make up for the size in multiple tools. Unless the tool for the sides was a 2 cavity mold, it would also require more tool cycles to make a complete unit since the production ratio would be off. The material color would be uniform and the product could be packaged after molding.
Using the multi-nozzle process, you can fit each one of these parts in a single tool with 6 cavities. Or 6 separate tools all running at the same time in the same machine with the same mold cycle. The material color would also be uniform across all parts.
Gas Counter Pressure
Gas counter pressure is another molding technique used to improve the quality of single-nozzle structural foam molded parts.
Inert gas, usually nitrogen, is used to charge the cavity end of the mold. The pressure slows the flow velocity and controls the foaming expansion during the low-pressure injection process.
This improves the formation of the outer skin and reduces the swirling effect on the surface of the part. The counter pressure controls the foaming rate, shape, size and density of the leading edge of the fill.
The benefits of gas counter pressure
- Eliminates swirl.
- Eliminates sink marks.
- Reduces part shrinkage.
- Produces a smoother outer surface.
- Can be used to create a textured outer surface.
- Reduces molded-in part stress.
- Allows the use of a smaller tonnage press.
- Reduces cycle time.
Surface Appearance and Finishing
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.
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.
Left. Surface showing normal swirling of structural foam surface. Right. Glossy and textured painted finishes on structural foam part.
Other names for
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
Other processes not to be confused with structural foam molding.
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.
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.
The Benefits of Structural Foam Molding
Structural foam molding offers designers and manufacturers many benefits when planning and producing a new product.
The process delivers advantages from design through production.
Part Size and Specifications
- Parts, up to 156”x72”x24” in dimension, and larger, are achievable.
- Complex parts with features such as bosses and ribs can be molded with no sink marks.
- Taller ribs and bosses can be used in the design without sink on the opposite side.
- Wall thicknesses from 0.180” (4.5 mm) up to 0.500” (12.7 mm) can be produced.
- Consistent part to part geometry throughout the production run.
- Molded-in detail comparable to standard injection molding.
- Process offers a wide range of design flexibility.
- Well-suited for parts requiring features such as ribs and bosses.
- Gates can be located at multiple points on the part to optimize filling.
- Two different colors or materials can be run at the same time.
- Walls of varying thicknesses can be molded.
- Low-pressure molding produces less part stress and warpage.
- Can replace components made of other materials such as sheet metal, fiberglass, wood and concrete.
- 15% to 20% part weight reduction compared to solid molded plastic. Even greater weight savings if replacing other non-plastic materials.
- High strength-to-weight ratio compared to solid plastics or other materials.
Production and Cost
- Large platen presses can run multiple molds simultaneously.
- Capable of shot sizes up to 200 pounds.
- Larger parts can be produced with smaller tonnage presses.
- Low pressures allow for the use of aluminum molds which cost less than steel.
- Aluminum molds allow faster cycle times.
- Improved dimensional stability over entire production run.
- Recycled post-consumer plastics can be used for many products.
- Parts that have reached their lifecycle can be recycled.
- Can deliver savings from tool making through multiple high-volume runs.
Structural Foam Molding vs Rotational Molding
Rotational molding, also known as rotomolding, uses rotation and gravity to distribute plastic material and fill the mold. Like structural foam molding, rotational molding is capable of molding large-sized plastic parts.
Manufacturers that are developing a new product should look at and compare these two processes to determine which would be the best fit for their large part production.
Below is a brief comparison of the structural foam molding vs rotational molding.
Structural Foam Molding. Produces small to large-sized solid parts with cellular core and rigid solid outer surfaces or skins.
The parts are extremely durable, impervious to the weather and resistant to impact, chemicals and corrosion.
Products commonly produced with structural foam molding include agricultural equipment and industrial panels & housings, utility cabinets, irrigation and other underground systems.
Rotational Molding. Capable of producing small to large-sized hollow parts that can incorporate double walls and thicker walls at the part corners.
The finished product is strong and durable and when required the hollow parts can be filled with urethane foam to give them excellent strength, rigidity, insulation or buoyancy.
Rotomolding is used for a wide range of parts, from retail displays to durable components with complex geometries, such as enclosures for business, consumer and medical equipment.
Structural Foam Molding. Tight tolerances of +/- 10% are achievable with high part to part consistency even on very large parts.
Rotational Molding. Close tolerances are more difficult to form and can be +/- 20% or more. Part to part consistency is not as reliable.
Structural Foam Molding. The outer surfaces of parts may have some visible swirling in the finish. When required, gas-assisted molding can reduce and eliminate swirling.
Rotational Molding. Finished products have attractive outer surfaces. Molded-in graphics, colored material and surface textures are often used to improve outer cosmetics.
Structural Foam Molding. High volumes of EAU from 500 to 130,000 parts per year can be run with one tool.
Rotational Molding. Best for products with an EAU from 1,000 to 25,000. More tools can be added to increase output.
Of course, there are many other factors, including cost, that should be considered when choosing the process.
Applications and Industries
Some of the industries and applications that structural foam molding is used for:
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
- 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
- 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
- Adult Furniture Including Frames and Legs
- Shower Safety Equipment
- Billiard Table Legs and Ball Dispensing Frame
- Laundry and Storage Bins
- Architectural Features and Products, Cornices, Covings, etc.
Industrial and Warehousing Products
Versatile and durable structural foam molded products for storage, transport and enclosures within factories and other commercial settings, including outdoors, are changing today’s work environments.
- Retail Display Racks and Bins
- Kiosk and Point-of-Purchase Displays
- Caskets and Crypts
- Cargo Systems
- Material Handling Systems
- Carts and Dollies
- Floor Panels
- Gates and Doors
- Utility Storage Chests and Cabinets
- Tool Storage and Workbenches
- Industrial Sinks
- Industrial Equipment Lids
Medical Equipment and Furnishings
With the increased use of advanced equipment and machinery within medical settings, new and practical products are needed to hold, house and organize. Structural foam components fit the bill.
- Display Housings
- Medical Storage
- Equipment Covers and Bases
- Patient Handless
- Robot Covers
- Side rails for hospital beds
Large Consumer & Commercial Trucks
The strength and durability of structural foam part together with the lighter weights make it ideal for the vehicle and transportation industries.
- Protective Walls & Panels
- Storage & Luggage Compartments
- Floor Panels
- Protective Edge Trim
- Electrical Component Housings
- Wheel Well Liners
Pet and Farm Products
Structural foam molding is ideal for sturdy, easy to clean products with smooth outer surface and no sharp corners.
- Animal Houses, Barriers and Enclosures
Facilities and Utility Companies
New electronics, drainage and storage systems are used more and more in outdoor public spaces, shopping, entertainment and living communities. Structural foam offers a more economical way of producing new utilitarian products and solutions.
- Drainage, Irrigation, and Sprinkler Housing
- In-Ground Enclosures and Reservoirs
- Telecommunication Housings
- Trash and Composting Bins and Containers
- Storage Containers, Totes, and Bins
- Tool and Equipment Housings and Boxes
The four resins commonly used most in structural foam molding are:
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-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.
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.
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.
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.
Structural Foam Molding Machines
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)
Platen Size 98”x89” (2489 mm x 2261 mm)
Maximum Shot Size: 150 lbs. (68 kg)
Platen Size 103”x186” (2616 mm x 4724 mm)
Maximum Shot Size: 200 lbs. (91 kg)
Platen Size 110”x200” (2794 mm x 5080 mm)
Maximum Shot Size: 300 lbs. (136 kg)
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.
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.
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:
Material options and consequences
Minimizing sink marks
Steel safe areas
Draft angle orientation
Texturing and draft
Scheduling of critical start-up phases
Secondary operations and fixtures
What are the differences and benefits of aluminum tooling vs steel tooling?
- Aluminum tools cost less
- Aluminum tools cool faster allowing for a faster cycle time
How long can structural foam tools last? How does this compare to high-pressure/injection molding tools?
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.
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.
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.
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.
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.
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.
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
Download our free Structural Foam Molding Design Guide.
We’ve compiled this reference PDF guide that covers the structural foam molding process and includes tips on design considerations and more.
A brief history of the technology
Structural foam molding is a relatively new technology compared to conventional injection molding.
Adventurous molders in the 1930s were the first to experiment with introducing additives to the melt in an attempt to expand the material during the injection cycle. Their efforts did not yield a lot of success.
Breakthroughs in methods to create foamed parts didn’t come until the early 1960s. Researchers at Union Carbide are credited with being the first to inject nitrogen gas into the resin and produce a rigid part with a cellular core.
Many advancements to the technology, materials and equipment followed during the next decades once GE Plastics and others began developing new materials. Also, Springfield Cast Products (now Uniloy Milacron) began specializing in designing and building structural foam molding machines.
Technologies including multi-nozzle injection took the basic process and helped turn it into the advanced manufacturing process capable of molding the complex parts with attractive cosmetics that are made today.
Computer-aided design means advancements continue at a great pace today. And the experts at specialist molders, from the engineers to the production technicians, are constantly working together to improve the craft.
Dedicated experts in structural foam molding.
Two USA based facilities with experience and capabilities that are second to none.
Engineering Tooling Quality Assurance Finishing