Structural Foam vs Rotomolding

Structural Foam Molding vs Rotational Molding

Compare the two plastic manufacturing processes to determine which is best for your product.

Structural foam molding and rotational molding offer many benefits unique to each process for the production of plastic parts and systems.
The two processes share some similarities but the outcome is vastly different. Both are capable of producing parts from a modest toaster sized part to parts over 10’ in length. The products produced from each are excellent for indoor and outdoor applications and can come in a wide variety of colors.
Both are used in consumer products, medical products, transportation and industrial applications.
They are also quite different in a number of ways and require totally different design criteria.
Below are descriptions and comparisons of the two methods.

PROCESSES IN BRIEF

Structural Foam Molding

Structural foam molding is a low-pressure injection molding process. Unlike standard high-pressure injection molding, inert gases are introduced into the plastic melt. This acts as a blowing agent and aids the fill and pack-out of the tool.

The process produces parts that have a cellular ‘foam’ core with solid outer surfaces.

Structural foam molding is used to mold large, solid durable parts and products that have high impact resistance and are impervious to the weather. The structural foam molding process is generally geared to higher volume applications.

Structural foam molding is capable of producing parts with complex geometries, tighter tolerances than rotomolding and very high part to part consistency throughout the production.

Gas-assist and multi-nozzle technologies bring even more flexibility to the large platen presses, such as the capability to run multiple tools simultaneously.

Structural foam is used for many products including – consumer, agricultural and commercial vehicles, business and medical equipment, marine, garden and architectural products and in many other industries.

Diagram of a structural foam molding tool.
Structural foam molding tools are made with multiple plates and a core. The final molded plastic part fills the entire cavity area.

Rotational Molding

Rotational molding, also know as rotomolding or rotomold, is a plastic manufacturing process that produces hollow parts.

Resin material, usually in pellet or powder form, is added to the tool at the first of five process stages – Loading, Oven (heating), Pre-Cool or Intermediate, Cooling and Unloading.

The tool is rotated on two axes throughout the process. This distributes and fuses the resin within the mold. There is no pressure involved. Gravity does all the work.

Rotomolding offers economical tooling options which makes it ideal for lower volume production applications.

Properly designed rotational molded parts can be used to manufacture highly engineered components such as housings for business and medical equipment.

Modern advancements such as in-mold temperature monitoring allow complex geometries to be molded along with double walls, thicker walls at the corners of the part or other features that produce very strong, versatile, rigid and durable components.

Parts can be filled with urethane foam to give them excellent strength, rigidity, insulation or buoyancy characteristics.

Diagram of a rotational molding tool.
Resin material in powder or pellet form is added to the opened tool at the beginning of the rotational molding process. A hollow part is then formed within the cavity.

PART DESIGN AND CHARACTERISTICS

Structural Foam Molding

Ideal for large 3-dimensional products. These might include panels, housings and enclosures, factory pallets, dunnage and carts and shelving systems.

Similar to injection molding the Structural Foam process is not conducive to die lock or undercuts in part geometry. The best most economical case is for a straight open and closed mold with limited side actions.

Product designs can benefit from multi-nozzle or gas-assist molding where multiple part numbers can be molded simultaneously.

Multi-nozzle or gas-assist molding can also assist in the production of larger wall sections with long material flow requirements such as telecommunications or tall outdoor utility enclosures.

Structural Foam Molded Cart
Product using structural foam molded parts.

Rotational Molding

Designers have the flexibility to create complex outside geometries because the parts are hollow and made without pressure. Unlike Injection molding or structural foam molding, roto-molding can easily accommodate part designs with undercuts and features that are difficult or even impossible to produce with structural foam molding.

However, where structural foam molding can easily mold ribs and bosses, those types of features are not possible with roto-molding. The roto-molding wall section is variable and not able to feature perpendicular ribs or thin wall sections or transitions.

Parts are hollow and can incorporate double walls in the design.

Rotomolded Truckbed Case
Finished product that uses rotomolded parts.

PART DIMENSIONS

The press or machine size will determine the maximum part dimensions that can be molded with any process. The specifications below are approximate and are assuming a large machine is being used.

Structural Foam Molding

Single part dimensions up to 72”x72”x24” are achievable.

Gas-assist and multi-nozzle technologies can be used to aid and improve material flow and finished surface cosmetics of large-sized parts.

Rotational Molding

Parts up to 10 feet in width or larger can be molded.

Small parts, down to several inches in size, can benefit from a rotational molding program.

WALL THICKNESS

Structural Foam Molding

Optimum nominal wall section for structural foam molding is .250” (6.35mm). Walls down to 0.180” (4.5mm) and up to 0.500” (12.7mm) in thickness can be molded effectively.

Wall sections are controlled by the tooling and are therefore very consistent with structural foam molding.

Rotational Molding

Wall thicknesses can range from 0.100” (2.54mm) to 0.320” (8.128mm)

The wall thickness can vary throughout the part. Thicker walls can be designed and molded at the outer corners of the part for added strength.

Consistent wall sections are difficult to achieve with roto-molding due to process limitations. The wall sections are controlled by the process and not tooling.

SURFACE AESTHETICS

Structural Foam Molding

Finished molded parts have smooth outer surfaces. Some swirling on the surface can be seen. However, gas-assist can greatly reduce or even eliminate visible swirling.

Modern technologies and resin materials produce structural foam molded surface finishes that rival high-pressure injection molding.

Parts can be painted. Mold textures are often used to improve appearance.

Rotational Molding

Produces very attractive surface cosmetics and molded details. Mold textures are often used to improve appearance.

Molded-in graphics and custom plastic material colors are commonly used.

Cast molds allow special surface textures to be used to enhance the product design.

PART WEIGHT

Structural Foam Molding

The low-density core with a harder high-density outer skin can reduce part weight by 10% to 15% compared to high-pressure molded parts.

Larger parts can especially benefit from this weight reduction.

Part to part weight consistency is very good.

Rotational Molding

Hollow parts that can incorporate double walls. This means the parts have a very low weight-to-size ratio compared to solid parts.

Part weight is controlled by the amount of material that is added into the mold prior to heating.

MATERIAL OPTIONS

Structural Foam Molding

Many different resins can be used depending on product specifications and budget requirements.

The most common plastics used are ABS, HDPE, HIPS or PP.

Post-consumer regrind material is often used.

UL registered flame retardant material is available.

Rotational Molding

Resin material in a pellet or powder form is used.

Resins used include HDPE, LLDPE, and high-density polyethylene. Low-density or high-density polyethylene resins work well for many products and offer cost savings when used.

UL registered flame retardant material is available.

TOLERANCES

Structural Foam Molding

Tight tolerances of +/- 10% are achievable. The part size and material will affect the tolerance.

Rotational Molding

Close tolerances are more difficult to form. The commercially accepted tolerance for flatness is +/- 20%.

However, certain part variables can increase the tolerance to +/- 50% inch-per-inch or more. Engineers can advise on tolerances.

TOOLING

Structural Foam Molding

Structural foam molding can allow the use of machined aluminum molds that have a long tooling life. Aluminum costs less than steel and reduces tooling costs, however investment in tooling for a large structural foam product can easily run into the hundreds of thousands of dollars.

Rotational Molding

Cast aluminum molds are often used. This, together with molds that are generally simpler in complexity when compared to injection molding tooling allows for reduced tooling costs and lead times.

CNC machined aluminum or electro formed and fabricated sheet steel tools can be used when required.

CYCLE TIME

Structural Foam Molding

Cycle times of 2 to 6 minutes are required for parts, including very large parts.

Rotational Molding

Cycle times of 30 minutes up to 60 minutes per part are required. Additional tools can be introduced to increase throughput.

MULTIPLE PART MOLDING

Structural Foam Molding

Large platen presses and multi-nozzle capabilities allow the use of multiple tools and multiple parts to be molded simultaneously. This can save costs. Learn more here.

Multi-Nozzles = Multi-Options Illustration
Multi-nozzle structural foam molding brings great versatility to the process.

Rotational Molding

Rotational molding machines can hold and run several tools at a time. Families of parts can be molded during the same run and saves on costs.

Multiple molds on rotational molding machine.
Multiple molds being run at the same time on a rotational molding machine.

VOLUME

Structural Foam Molding

High volumes and EAU from 500 to 130,000 parts per year are easily achievable with one tool.

Rotational Molding

Best for products with an EAU from 1,000 to 25,000. This is due to longer processing times required.

More tools can be added to increase output. One roto-molding tool can produce a range of 5,000-10,000 parts per year.

ASSEMBLY

Structural Foam Molding

Self-tapping fasteners and threaded press-in inserts, when needed, are usually added after the part has been molded. The cellular core with a hard outer surface is ideal for these fasteners.

Rotational Molding

Simple riveting to complex assemblies of up to 50 parts or more can be done.

Molded-in threaded inserts and fasteners are simple to add to a roto-molding design.

SECONDARY OPERATIONS

Structural Foam Molding

Decorating techniques that can be used include hot stamping, labeling, and pad printing.

Parts can also be machined or drilled to accommodate design features.

Rotational Molding

High-pressure foaming is a beneficial value-add operation that can be used. Hollow parts are filled with a polyurethane foam material. This adds extra strength and rigidity without excess weight.

Foaming is used to give buoyancy or insulation to products such as refrigerator doors, flotation devices and ridged EMT back-boards.

Parts are often routed or machined to accommodate design features and tolerances that are not possible within the process.

FINISHING

Structural Foam Molding

Attractive outer cosmetics and material characteristics such as excellent strength and impact resistance mean the process is often used for parts and components that don’t need finishing work.

When required, parts can be painted.

Rotational Molding

CNC Routers are often used to trim edges and produce parts with the required attractive finish.

Structural Foam Molding vs Rotational Molding

Contact us at 216-533-1410 to discuss which process is best for your products.

We hope you’ve found the comparisons of the two plastic molding processes useful. Please give us a call or send an email if you have any questions or would like to discuss your program.

Find much more information on structural foam molding on this website beginning with the structural foam process overview.
Visit the main Miles Products website to learn more about rotational molding.

Miles Products offer a range of plastic molding services including:

Injection Molding, Insert Molding, Structural Foam Molding and Rotational Molding

Visit the Miles Products website to learn more about all the molding processes and how we help manufacturers produce American-made products.

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