Rotational Molding vs. Thermoforming: Which Plastic Manufacturing Process Is Better?

How tooling cost, part geometry, wall construction, durability, production volume and product size influence the right manufacturing process.

When comparing rotational molding vs thermoforming, the best manufacturing process depends heavily on the size, geometry, durability requirements, production volume and intended use of the finished plastic product.

Both processes can manufacture relatively large plastic components without the extremely expensive tooling commonly associated with large injection molded parts. However, rotational molding and thermoforming create products in fundamentally different ways and are generally suited to different types of applications.

Thermoforming is often an excellent choice for panels, covers, housings, liners and other components that can be formed from a heated plastic sheet.

Rotational molding, commonly called rotomolding, is particularly well suited for durable hollow products, complex geometries, large components and products requiring seamless construction.

Understanding these differences can help product designers and manufacturers determine which process is best suited for their application.

What Is Rotational Molding?

Rotational molding is a low-pressure plastic manufacturing process used to produce hollow plastic products.

A measured amount of powdered resin is placed inside a hollow mold. The mold is then heated while rotating simultaneously around multiple axes.

As the mold rotates, the plastic material gradually melts and coats the interior surface of the mold.

After the material has been properly processed, the mold enters a cooling cycle. The finished plastic part is then removed from the mold.

Unlike many manufacturing processes, rotational molding does not force molten plastic into a mold under high pressure.

The low-pressure nature of the process allows rotational molding tooling to be significantly less massive than tooling used in many high-pressure molding processes.

Rotational molding can also manufacture extremely large products that may be difficult or prohibitively expensive to manufacture using other plastic processing technologies.

Common rotationally molded products include plastic tanks, material handling containers, refuse products, marine products, recreational products, industrial housings, aerospace containers, agricultural products and many other large plastic components.

What Is Thermoforming?

Thermoforming begins with a plastic sheet rather than powdered resin.

The plastic sheet is heated until it becomes pliable and is then formed against a mold using vacuum, pressure or mechanical assistance.

After the material cools, the formed component is typically trimmed to its final dimensions.

One of the most common forms of thermoforming is vacuum forming, where vacuum pressure pulls the heated plastic sheet against the surface of the mold.

Thermoforming is frequently used for items such as equipment covers, machine housings, vehicle interior components, trays, liners, panels and packaging.

The process can be very efficient when the required component can be produced from a formed sheet.

Rotational Molding vs. Thermoforming

Although both processes manufacture plastic products, they approach part construction very differently.

Neither manufacturing process is universally better.

The correct choice depends on what the product needs to accomplish.

Hollow Plastic Products

One of the most important differences between thermoforming and rotational molding involves hollow products.

Rotational molding naturally produces a hollow component.

Material coats the internal surface of an enclosed mold, allowing the entire product to be manufactured as a single molded component.

This can eliminate seams, welds, fasteners and secondary assembly operations.

Consider a large plastic tank.

Rotational molding allows the tank body to be produced as one continuous molded structure.

Producing a comparable enclosed structure using conventional thermoforming may require multiple formed components that must subsequently be joined together.

Every additional joint can introduce additional manufacturing operations and potentially create another area requiring structural or leak-performance consideration.

For this reason, large seamless hollow products are one of the strongest applications for rotational molding.

Panels, Covers and Open Components

Thermoforming becomes particularly attractive when the component does not need to be enclosed.

A machine guard, equipment cover, interior panel or large plastic liner can often be efficiently manufactured by heating a sheet and forming it over a mold.

There may be little reason to rotationally mold a hollow structure if only one molded surface is actually required.

This is an important part of manufacturing-process selection.

The goal should not be to make every product fit a particular process.

The goal is to select the process that naturally produces the geometry the product requires.

Wall Thickness and Material Distribution

Wall construction is another important difference between rotational molding and thermoforming.

During rotational molding, resin travels throughout the interior of the rotating mold as it melts.

Proper mold design, processing conditions and resin selection allow manufacturers to develop the desired material distribution throughout the molded product.

Additional material can also be designed into strategic regions of many rotationally molded parts.

Thermoforming starts with a sheet having a predetermined thickness.

As the heated sheet is stretched over or into the mold, material is redistributed. Deep draws and complex geometries can cause certain areas of the sheet to become thinner.

Experienced thermoformers can compensate for many of these effects through tooling, material selection and process design, but the basic mechanics of stretching a sheet remain different from building a hollow product through rotational molding.

Double-Wall Plastic Products

Rotational molding also provides significant design opportunities for double-wall products.

A properly designed mold can create an outer wall and an inner wall as part of the same molding operation.

Double-wall construction can provide rigidity, impact resistance, insulation opportunities and substantial durability without requiring the mass associated with a completely solid plastic component.

This capability is used throughout the rotational molding industry for products ranging from material handling containers to marine and industrial applications.

Thermoforming can also produce hollow or double-wall structures through processes such as twin-sheet thermoforming, but it requires a different tooling and manufacturing approach.

For products designed from the beginning around a robust enclosed hollow structure, rotational molding frequently deserves serious consideration.

Product Size

Both thermoforming and rotational molding can manufacture large plastic products.

However, extremely large hollow products are one of rotational molding's most recognized strengths.

The relatively low molding pressures involved allow manufacturers to build molds and molding equipment capable of producing components that would require enormous machinery and tooling in some competing processes.

Large tanks, bulk containers, marine products and industrial structures are common examples.

Thermoforming is also capable of producing very large parts, particularly large covers, panels, liners and housings.

Once again, geometry matters as much as overall dimensions.

A large open equipment cover may be an excellent thermoforming application.

A large enclosed tank may be an excellent rotational molding application.

Tooling Costs

Tooling cost is often one of the reasons companies investigate both rotational molding and thermoforming.

Neither process necessarily requires the massive high-pressure molds associated with large injection molded parts.

Rotational molds are commonly manufactured from fabricated metal or cast aluminum, depending on the product geometry, expected production requirements and desired surface characteristics.

Because rotational molding is a low-pressure process, molds generally do not need to withstand the enormous clamping and injection pressures associated with injection molding.

Thermoforming tooling can also be economically attractive because the mold is used to shape a heated sheet rather than contain extremely high injection pressures.

Actual tooling costs for either process vary considerably according to size, geometry, material, surface requirements, complexity and production expectations.

This is why broad internet claims that one process always has cheaper tooling can be misleading.

The actual product design must be evaluated.

For additional information about rotomolding tooling, see How Much Does a Mold for Rotational Molding Cost?

Production Volume

Thermoforming generally offers faster molding cycles than rotational molding.

That can make thermoforming particularly attractive when a product's geometry is appropriate for the process and higher annual production quantities are expected.

Rotational molding typically has longer heating and cooling cycles.

However, cycle time alone does not determine the lowest total manufactured cost.

If rotational molding eliminates multiple components, welding, assembly, hardware or secondary fabrication, the overall economics can change substantially.

Tooling investment must also be considered.

A product requiring modest annual quantities may benefit from a manufacturing process that provides reasonable tooling costs even if the molding cycle itself is longer.

Instead of asking only:

"Which process has the faster cycle?"

Product developers should ask:

"Which manufacturing process produces the complete product at the best total cost?"

Design Complexity

Rotational molding provides substantial design freedom for large hollow plastic products.

Features such as molded-in inserts, ribs, bosses, handles, lettering, mounting areas and other functional details can often be incorporated directly into the mold design.

The process can also create unusual shapes that would be difficult to manufacture from flat sheet stock.

Thermoforming provides excellent design freedom across many open geometries but must account for how a heated sheet stretches across the mold.

Very deep draws, severe undercuts and complicated enclosed geometries can create manufacturing challenges.

The intended geometry should therefore be considered very early in the design process.

Durability and Impact Resistance

Rotational molding is widely used for products expected to survive harsh environments.

Polyethylene is the most commonly used family of rotational molding materials and can provide excellent impact resistance, corrosion resistance and outdoor durability when the correct resin is selected.

Because many rotationally molded products are manufactured as seamless structures, there may also be fewer assembled joints or seams exposed to impact and environmental stresses.

This makes rotational molding attractive for products such as industrial containers, marine equipment, outdoor products, material handling equipment and other demanding applications.

Thermoformed products can also be extremely durable. Performance depends heavily on material selection, sheet thickness, geometry and application requirements.

The important distinction is not simply which process produces "stronger plastic."

The important question is what type of loading, impact, environment and structural design the finished product must withstand.

Material Selection

Polyethylene is by far the most common material used in rotational molding.

Grades of linear low-density polyethylene and medium-density polyethylene are widely used because of their processing characteristics, impact performance, chemical resistance and durability.

Other polymers can also be rotationally molded for specialized applications.

Thermoforming offers a broad selection of sheet materials because numerous polymers can be extruded into sheet stock and subsequently formed.

Material selection should ultimately be based on the finished product's requirements rather than simply the manufacturing process.

Chemical exposure, operating temperature, UV exposure, stiffness, impact requirements, regulatory requirements and expected product life should all be considered.

For a deeper look at polyethylene selection, see Rotational Molding Polyethylene — LLDPE vs. HDPE Resin Selection.

When Is Rotational Molding the Better Choice?

Rotational molding should receive strong consideration when a product requires a large or complex hollow geometry, seamless construction, double-wall construction, significant durability, molded-in features or relatively economical tooling compared with high-pressure molding alternatives.

Typical applications include tanks, bulk containers, material handling products, refuse containers, marine products, underground structures, aerospace containers, recreational products and industrial components.

The process becomes particularly compelling when several fabricated or assembled components can be redesigned as one rotationally molded plastic product.

Part consolidation can reduce assembly operations, hardware, welds, corrosion concerns and inventory while simplifying the finished product.

When Is Thermoforming the Better Choice?

Thermoforming may be the better manufacturing process when the product is fundamentally a panel, shell, cover, tray, liner or enclosure that can efficiently be created from formed sheet.

It can also be particularly attractive when production quantities favor its faster cycle times or when a specific thermoformable sheet material provides the required appearance or performance.

A product should not be converted to rotational molding simply because rotomolding can manufacture it.

Manufacturing-process selection should begin with the product requirements.

Rotational Molding vs. Vacuum Forming

Vacuum forming is a form of thermoforming, so the same fundamental differences generally apply when comparing rotational molding vs. vacuum forming.

Vacuum forming heats a plastic sheet and uses vacuum to pull it against a mold.

Rotational molding places resin inside an enclosed mold and distributes that material around the internal mold surface through controlled biaxial rotation and heat.

Vacuum forming is extremely useful for large open components.

Rotational molding is particularly useful for large enclosed hollow components.

For manufacturers comparing vacuum forming vs. rotomolding, the first question should therefore be whether the finished product naturally resembles a formed sheet or an enclosed hollow structure.

That distinction can quickly point a project toward the appropriate manufacturing technology.

Designing a Product for the Right Manufacturing Process

One of the most expensive mistakes in product development is completing a design before considering how the product will actually be manufactured.

A design optimized for machining may not be optimized for molding.

A design optimized for thermoforming may not take advantage of rotational molding.

And a design originally intended to be fabricated from numerous individual components may be dramatically simplified through rotational molding.

Manufacturing input early in the design process can identify opportunities to reduce components, improve durability and simplify production before substantial money has been invested in tooling.

The Granger Plastics Company has worked with customers across a wide range of industries to convert product concepts, fabricated assemblies and existing products into rotationally molded plastic components.

Which Process Is Right for Your Product?

There is no universal winner in the rotational molding vs. thermoforming comparison.

Thermoforming is an excellent process for many sheet-formed panels, covers, housings and liners.

Rotational molding provides significant advantages for many large, durable, seamless and complex hollow products.

The best manufacturing process depends on part geometry, annual volume, required performance, tooling investment, material requirements and the total cost of producing the finished product.

If you are evaluating rotational molding for a new or existing plastic product, The Granger Plastics Company can review your application and help determine whether rotational molding is an appropriate manufacturing solution.

Contact The Granger Plastics Company to discuss your custom rotational molding application or request a quotation.

Frequently Asked Questions About Rotational Molding vs. Thermoforming

Is rotational molding cheaper than thermoforming?

It depends on the product. Both processes can offer relatively economical tooling compared with some high-pressure molding processes. Thermoforming can provide shorter production cycles for appropriate geometries, while rotational molding may reduce total manufacturing costs by producing complex hollow structures as a single component. Tooling, production volume, assembly requirements and product geometry should all be evaluated.

What is the biggest difference between rotational molding and thermoforming?

Thermoforming shapes a heated plastic sheet against a mold. Rotational molding places plastic resin inside a hollow mold and rotates the mold while heating it, creating a hollow molded product. This fundamental difference makes thermoforming particularly effective for open panels and shells and rotational molding particularly effective for seamless hollow products.

Is vacuum forming the same as thermoforming?

Vacuum forming is one type of thermoforming. The heated plastic sheet is pulled against the mold using vacuum pressure.

Can thermoforming make hollow plastic products?

Hollow structures can be produced using specialized techniques such as twin-sheet thermoforming or by joining multiple formed components. Conventional rotational molding naturally produces hollow products within an enclosed mold.

Can rotational molding produce large plastic parts?

Yes. Manufacturing large plastic products is one of the primary advantages of rotational molding. The low-pressure process allows very large hollow components to be manufactured without the high clamping pressures required by some other plastic molding technologies.

Which process is better for plastic tanks?

Rotational molding is widely used for manufacturing plastic tanks because tanks are naturally hollow products and can be molded without seams in the tank body. The ideal process still depends on tank size, geometry, material, volume, operating conditions and other application requirements.

Which process is better for plastic equipment covers?

Thermoforming can be an excellent choice for open equipment covers and housings that can be formed from sheet material. If the component requires an enclosed hollow structure, double walls or other complex molded features, rotational molding may offer advantages.

Can an existing fabricated product be converted to rotational molding?

Often, yes. Products assembled from sheet metal, fabricated plastic or multiple individual components may sometimes be redesigned as a single rotationally molded product. Successful conversion depends on geometry, structural requirements, production volume, material requirements and tooling feasibility.