Types of Molds Used in Rotational Molding

The success of any rotationally molded part begins with the right rotational mold. Because the process relies on resin evenly coating the interior surface during rotation, mold design and material selection are critical for part quality, durability, and production efficiency.

There are three primary types of molds commonly used in rotational molding:

rotational molding tooling

Cast Aluminum Molds

These are the most widely used molds in the industry, known for their ability to capture fine detail and produce consistent parts. Cast aluminum offers excellent heat transfer, relatively low cost compared to other tooling, and the flexibility to form complex shapes. Most rotationally molded products on the market today are made using cast aluminum molds.

CNC Machined Aluminum Molds

Machined molds are milled from solid blocks of aluminum using CNC technology. They provide exceptional accuracy, tight tolerances, and smooth finishes. While more expensive to produce than cast aluminum molds, CNC machined molds are ideal when precision and repeatability are top priorities.

Fabricated Sheet Metal Molds

These molds are produced through electroforming or fabricated from steel or aluminum sheet metal. They are typically used for specific applications where durability and cost balance are critical. Fabricated molds may not capture intricate detail as well as cast or CNC aluminum, but they are effective for certain large or simple parts.

Each mold type has unique advantages, but cast aluminum remains the standard for most projects due to its balance of cost, flexibility, and performance. From small medical devices to tanks measuring hundreds of gallons, cast molds provide the versatility needed for today’s complex designs.

Cast Aluminum Molds

Cast aluminum molds are the backbone of the rotational molding industry and account for the majority of tools in use today. Their popularity stems from the balance they provide between cost, durability, and design flexibility, making them a practical choice for a wide range of products.

One of the key strengths of cast aluminum is its ability to capture fine detail from the master pattern. This allows even small text, intricate textures, or precise curves to be faithfully reproduced in the final part — an essential feature for applications where product appearance matters. Foundries can also produce multiple molds from the same pattern, ensuring consistency from part to part and across production runs.

Performance is another area where cast molds stand out. The most commonly used alloy, 356 aluminum, offers excellent heat transfer, which reduces cycle times and promotes even wall thickness throughout the molded part. Its malleability also makes it easy to texture, polish, or modify as product designs evolve. This flexibility is especially valuable when small adjustments are needed without the expense of creating an entirely new tool.

Typical wall thickness for cast molds is around ¼” to ⅜”, providing strength while keeping the mold lightweight enough for efficient handling. Tolerances generally fall around ±0.005 in/in, though small dimensions may vary slightly. Lead times are relatively short, with many patterns completed in 1–2 weeks and molds ready in about 6 weeks for small to medium-sized parts.

From tiny medical devices to large-scale storage tanks, cast aluminum molds are adaptable to almost any product size or geometry. Compared to CNC machined or electroformed molds, they are often the most economical option, delivering repeatable, high-quality results without excessive upfront costs. For manufacturers looking to balance performance with affordability, cast aluminum remains the go-to choice in rotational molding.

Production of Cast Aluminum Molds

Creating a cast aluminum mold is a multi-stage process that combines engineering precision with skilled craftsmanship. Each step is carefully controlled to ensure the mold delivers consistent, high-quality parts with minimal maintenance.

The process begins with detailed product drawings or electronic CAD files (2D or 3D). These designs are reviewed to estimate tooling costs and to identify opportunities to improve moldability and reduce long-term maintenance.

A high-tolerance model of the part is produced, typically slightly oversized to account for aluminum and plastic shrinkage. Models may be CNC machined from wood, plastic, or composite materials. Every dimension is inspected and documented to confirm accuracy before moving forward.

Engineers analyze the design for potential molding issues, checking oven fit, airflow, undercuts, deep core areas, and wear points. This stage ensures the mold will function efficiently and safely during production.

Urethane rubber tooling is often used to create the mold pattern. Urethane provides reduced mismatch on parting lines, allows undercuts, and supports quick design changes when necessary.

Molten aluminum is cast around the pattern to form the mold. Each casting step is closely monitored for consistency, ensuring strong and accurate mold shells.

Once cast, the mold undergoes finishing and surface treatments. Textures, from high gloss to matte, can be applied depending on the product’s appearance requirements. This step directly impacts how the final part looks and feels.

Critical features such as inserts, pull pins, or custom cavities are machined into the mold. CNC machining ensures precision where fine detail is required.

The mold is mounted to a spider or frame, which protects the mold, maintains dimensional stability, and provides secure clamping for production.

Finally, the mold is treated with a permanent coating to aid release and control surface finish. Coatings can provide glossy, matte, or textured effects while also improving heating efficiency and extending mold life.

Through this detailed process, cast aluminum molds are engineered to deliver repeatable results, withstand production demands, and provide design flexibility for a wide range of parts.

Production of CNC Machined Aluminum Molds

CNC machined aluminum molds offer exceptional precision, tight tolerances, and smooth finishes, making them an ideal choice for applications requiring high accuracy and consistency. The production of CNC machined aluminum molds combines advanced technology with skilled craftsmanship, resulting in high-quality, reliable tools that produce parts with excellent detail.

Process Overview

The production of CNC machined aluminum molds begins with the creation of detailed product designs, often communicated through 2D or 3D CAD files. These designs are reviewed by engineers to assess any potential issues that might arise during the molding process. Once the design is finalized, it is sent to the CNC machining process.

Steps in CNC Machined Aluminum Mold Production:

The process begins with precise product designs created in CAD software. Engineers ensure that all aspects of the mold, including dimensions, tolerances, and material properties, are optimized for manufacturing. The design is then checked for moldability, airflow, and heat distribution.

High-grade aluminum alloys are selected for their strength, malleability, and heat transfer capabilities. The choice of alloy will depend on the intended application and the properties required, such as heat resistance or durability.

The mold is cut and shaped using Computer Numerical Control (CNC) machines. These machines are programmed to follow the exact design specifications, ensuring that every detail is captured with high accuracy. CNC machining offers flexibility for intricate features and precise tolerances, which is crucial for complex parts.

After machining, the mold is thoroughly inspected using coordinate measuring machines (CMM) to ensure all dimensions and tolerances meet specifications. Any adjustments required are made at this stage to ensure the mold’s precision.

Once the mold has been machined, the final finishing touches are applied. This may include polishing the mold surface, applying textures, or adding features like inserts and pull pins. Polishing is essential for parts that require smooth finishes or fine details.

After finishing, the mold components are assembled into a complete mold system. This includes adding any necessary inserts, pull pins, or other features required for the final product. The mold is then mounted on a spider or frame to ensure stability during the molding process.

Before full-scale production, the mold is tested with initial trial runs to check for any issues such as uneven heating or parting line problems. This testing phase ensures that the mold will function as intended in a production environment.

Advantages of CNC Machined Aluminum Molds:

  • Precision: CNC machining provides unparalleled accuracy and tight tolerances, making it ideal for products requiring exact specifications.
  • Smooth Finishes: The process results in molds with smooth surfaces, which are essential for creating high-quality, visually appealing parts.
  • Consistency: CNC machined molds offer excellent repeatability, ensuring each part produced is identical in appearance and performance.
  • Design Flexibility: CNC machining allows for the creation of complex geometries, intricate details, and undercuts that may not be achievable with other mold-making methods.

Applications:

  • Automotive parts
  • Medical devices
  • Consumer electronics
  • Aerospace components

Production of Sheet Metal Fabricated Molds

Sheet metal fabricated molds are a versatile and cost-effective solution for rotational molding applications that require durability and precision but are less complex than those made from aluminum or CNC-machined materials. These molds are commonly used for producing large parts, simple designs, and parts with low to medium-volume production runs. The process of creating sheet metal fabricated molds combines skilled craftsmanship with advanced manufacturing techniques, ensuring the molds are ready for high-quality part production.

Process Overview

The production of sheet metal fabricated molds involves several key steps, from initial design and material selection to assembly and testing. While not as intricate as CNC-machined molds, sheet metal molds are strong, reliable, and often the most affordable option for parts with simpler geometries.

Steps in the Production of Sheet Metal Fabricated Molds:

The process begins with the creation of detailed product designs, typically communicated through 2D or 3D CAD drawings. These designs are reviewed by engineers to ensure that the mold can be fabricated effectively and that any challenges related to material flow or wall thickness are addressed early.

The mold material is usually sheet steel or aluminum, chosen for its strength, resistance to wear, and ease of fabrication. The choice of material depends on the desired performance characteristics of the part and the production volume.

Sheet metal is cut, bent, and shaped according to the design specifications. This step often involves laser cutting, waterjet cutting, or stamping to achieve the required shape and size for the mold. The material is then formed using presses or brakes to ensure proper curvature and alignment.

After the components are cut and shaped, they are welded or mechanically joined together. This could involve spot welding, seam welding, or bolting the sections together. For molds that need to be split into two parts for easy removal of the molded item, the joining process must be done carefully to maintain mold integrity.

Once the mold structure is welded and assembled, finishing processes such as grinding, sanding, and polishing are applied to remove rough edges and smooth out the mold surfaces. For molds with complex patterns or texture, additional techniques like sandblasting or texturing are employed to create the desired finish.

The fabricated mold is mounted on a spider or other support frame, ensuring that it can be clamped securely during the rotational molding process. During this stage, inserts, pull pins, and other necessary features are added to the mold to ensure proper part release and functionality.

A crucial step in the process, the mold is subjected to trial runs and testing to check for any potential issues with part quality, mold alignment, or thermal efficiency. Any necessary adjustments are made to improve performance before full-scale production begins.

Advantages of Sheet Metal Fabricated Molds:

  • Cost-Effective: Sheet metal molds are generally more affordable to produce than their CNC machined or cast aluminum counterparts, making them ideal for lower-volume production or simpler designs.
  • Speed of Production: The fabrication process is quicker than other methods, which can be especially beneficial for tight production timelines.
  • Durability: While not as resistant to wear as some other mold types, sheet metal molds are still durable enough to withstand medium-volume production and are excellent for products with low to moderate complexity.
  • Ease of Modification: Modifying sheet metal molds is relatively simple compared to more complex mold types, allowing for quick design changes.

Applications:

  • Large, simple rotationally molded parts such as tanks, bins, and containers.
  • Products requiring medium-volume production with less complex shapes.
  • Applications where cost is a primary factor, such as for testing new designs or prototypes.

Permanent Coatings

Permanent coatings are an important step in preparing a mold for long-term use. These coatings not only provide consistent part release but also influence the appearance and surface finish of the molded product. By selecting the right coating, manufacturers can reduce maintenance, extend mold life, and achieve the desired aesthetic — from high-gloss shine to a soft matte texture.

In rotational molding, permanent coatings are typically applied after the mold has been cast, machined, and finished. They help control heating efficiency, protect the mold surface, and minimize operator-applied release sprays, which can add variability to the process.

Some of the most widely used coatings in the industry include:

  • High-Gloss Finishes: Provide fast release and a smooth, polished appearance. Often used when the final product requires a premium look.
  • Matte Finishes: Deliver a more subdued surface while still ensuring reliable part release. These are common in applications where glare or shine is undesirable.
  • Semi-Gloss Finishes: Offer a balance between gloss and matte, giving parts a uniform and professional appearance.
  • Exterior Mold Coatings: Applied to the outside of molds and spiders to improve heating performance and reduce corrosion.

Industry examples of permanent coatings include the Stonekote series, such as:

  • Stonekote 7000: Bright green, high gloss, fast release.
  • Stonekote 1000: Olive green, matte, slower release.
  • Stonekote 5000: Black, semi-gloss, medium release.
  • Stonekote 3000: Brown, matte, medium release.
  • Stonekote 85: Black exterior coating to improve heating and reduce spider corrosion.

By matching coating type to the part’s functional and aesthetic needs, rotational molders can optimize both performance and appearance while reducing downtime for mold maintenance.

At Integrity Rotational, we understand that selecting the right tooling is a critical step in the manufacturing process. Whether you’re deciding between cast aluminum, CNC machined, or sheet metal fabricated molds, our team is here to guide you through every decision. We take the time to understand your design, production needs, and budget to ensure that you get the most cost-effective, durable, and efficient tooling solution. With our extensive expertise in rotational molding, we’ll help you make an informed choice that maximizes your production efficiency and product quality.

Reach out today to consult with us on your tooling requirements and let us help bring your designs to life with the perfect tools for the job.