Structural Foam Molding vs. Rotational Molding
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.
Compare the two plastic manufacturing processes to determine which is best for your product.
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.
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.
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.
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.
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.
Part Dimensions
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 high-density polyethylene (HDPE) and low-density polyethylene (LLDPE). 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.
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.
Volumes
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.
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