Rotational Molding: Definition, Process, and Advantages
It’s not for nothing that this manufacturing method has been around for a century
Although it’s more than 100 years old, rotational molding is still popular with manufacturers who want to make high-quality and uniform products. It’s quite a fascinating process and can make a wide range of products, from hollow water tanks to kayaks. If you want to know how to use this process in your business, this article explains all the ins and outs of rotational molding, as well as how Xometry can help lessen your company’s workload with this process.
What is Rotational Molding?
Also known as roto molding, roto casting, or roto mold, rotational molding features a large machine with molds in the right shapes and specifications needed for the final product—whether it’s a container for holding chemicals or a children’s plastic slide. It’s often compared to the blow molding process, but rotational molding costs much less and is more versatile.
The steps that commonly take place when it comes to rotational molding, though they’ll vary slightly depending on the product and material, involve heating and slowly rotating a hollow mold with powdered plastic (usually polyethylene) inside in various directions. The heat melts the plastic powder, and it sticks to the inside of the mold. The rotating trick helps it do so evenly. It’s then cooled and allowed to harden before the solid part can be taken out of the mold.
The process is quite slow compared to other super speedy high-pressure processes. Many machines spin molds around eight rotations per minute, and it can take up to three hours to complete. Even still, it’s a remarkably effective method for creating hollow, one-piece construction products.
There are specific materials that are better suited to this type of molding, and you’ll find that polyethylene is the most common because of its low cost, thermal stability, and the structural integrity it has after cooling. A very tiny percentage of companies use other materials, like silicone, ABS, and acrylic, but that’s extremely rare for most applications.
To help you visualize what this looks like, you can see several important steps from the rotational molding process in the image below, including filling, heating, cooling, and removing the product from the molds.
What Is the Other Term for Rotational Molding?
Rotational molding is also known as roto molding, roto casting, or roto mold. In all cases, the process is the same: plastic powder is put into a mold designed to rotate on multiple axes. When heated, the plastic melts and coats the interior surface of the mold to form the desired shape. Rotomolding is renowned for its affordability and capacity to create sizable plastic components with uniform wall thickness.
What Is the History of Rotational Molding?
The history of rotational molding, or roto molding, can be traced back to 1855. British manufacturer R. Peters utilized biaxial rotation and heat to produce hollow metal artillery shells and other vessels. The process was later adapted for plastics around 1950 for manufacturing doll heads. Since then, roto molding has evolved and expanded to become a common and versatile method for producing plastic products.
What Is the Primary Purpose of Rotational Molding?
The primary purpose of rotational molding is to produce parts with high-quality finishes, uniform wall thicknesses, and excellent stability. This manufacturing process allows inserts and spin weld attachments to be directly added to the rotomolded product.
What Is the Process of Rotational Molding?
The process of rotational molding can be described as follows:
- Fill the hollow mold with powdered plastic resin.
- Initiate bi-axial rotation of the mold.
- Transfer the rotating mold into an oven.
- Maintain the rotation while the resin melts and evenly coats the inner walls of the mold.
- Cool the rotating mold to allow the resin to harden and take the desired shape.
- Stop the rotation and open the mold.
- Carefully remove the finished part from the mold.
What Are the Types of Equipment Used for Rotational Molding?
There are several types of rotational molding machines to consider, and the best one for your operation will depend on the type, size, and use of the products you’re interested in making.
Turret Machines
This type of roto molding machine has between three and six rotating arms, which makes it look similar to an amusement park ride. It’s built in such a way so that each mold on all of the arms gets the same number of rotations, level of heat, and cooling time.
Rock and Roll Machines
No, this machine isn’t musically inclined. In fact, its name speaks to the motion it moves in when shaping products. It has a tilting oven and automatic cart movement, which makes this large piece of equipment ideal for bigger roto mold products, like tanks and vessels.
Clamshell Machines
These smaller machines have a lid that opens and closes similar to a clamshell, and they’re a good fit for businesses after low-quantity production orders. Normally, they’ll have one arm and an oven, meaning they’re more compact and great for smaller facilities, too.
Open-Flame Machines
An open-flame machine has exactly what its name suggests, rows of open flames that get as close as possible to the molds. If you want to make cylindrical parts, it’s worth looking into because it’s efficient and cost-effective.
Shuttle Machines
This style of roto mold machine is great for larger quantities and bigger products because it contains two carts and two stations as well as an oven. Operators can choose to use one station at a time or both, which cranks up efficiency and productivity and keeps the process running smoothly since multiple steps are happening at once.
What Are the Materials Used for Rotational Molding Process?
Some of the materials used for rotational molding include:
1. Aluminum
Aluminum is a lightweight, durable material known for its excellent thermal conductivity and strength-to-weight ratio. Aluminum is less common in rotational molding than polymers due to its cost and the limited availability of suitable aluminum powders.
2. Polyethylene
Polyethylene is a versatile thermoplastic used extensively in rotational molding. It provides excellent impact strength, chemical resistance, and ease of processing. On the downside, it is not as stiff or heat-resistant as many other materials.
3. Polyester
Polyester is a synthetic polymer known for its dimensional stability. It is simultaneously strong and resistant to heat and chemicals. However, it typically has a higher cost and may require additional processing steps.
4. Polypropylene
Polypropylene, a thermoplastic renowned for its toughness and chemical resistance, delivers notable impact resistance, stiffness, and dimensional stability. Unfortunately, it has trouble with heat and impacts, especially compared to materials like ABS.
5. Nylon
Nylon is a synthetic polymer recognized for its strength and toughness. It offers excellent wear resistance, impact strength, and dimensional stability. Unfortunately, it is characterized by relatively high prices and may necessitate specialized processing equipment.
6. Silicone
Silicone is a versatile elastomer, renowned for its flexibility and temperature resistance. It’s durable and resists both heat and adverse weather conditions. Those traits make it more expensive than natural rubber, though.
7. Polycarbonate
Polycarbonate is a transparent thermoplastic that is widely recognized for its impact resistance. It is often used as a substitute for glass due to its exceptional strength, clarity, and dimensional stability. However, polycarbonate may require additional precautions, such as UV stabilizers or coatings, to prevent yellowing or degradation when exposed to sunlight.
How Long Does it Take for the Rotational Molding Process to Complete?
A single rotationally molded part can take up to three hours to complete when the mold is rotated at a speed of 8 rotations per minute. This duration allows the polymer material inside the mold to evenly coat all surfaces and cool enough to maintain its final form and wall thickness.
What Are the Factors That May Affect the Rotational Molding Process?
There are a few unique details to be mindful of when you design a product to be made with rotational molding. These might not come up when you’ve worked with other manufacturing methods:
- Draft angles: By factoring a draft angle into your part’s design, it’ll be easier for operators to remove the components from the molds after the rotating, heating, and cooling process.
- Wall thickness: Think about how thick you intend the walls of your product to be as this can affect the heat transfer rate and how long it takes for the part to cool down. If you’re working with the ever-popular polyethylene, a good rule of thumb is 0.125 and 0.25 inches of thickness.
- Large flat surfaces: To prevent the material from distorting, it’s best to add steps or recesses to these types of surfaces. Parallel walls are also prone to the same issues, but making sure they’re evenly spaced will prevent them from getting too thin.
- Material flow around corners: Rotational molding isn’t the right process if you’re looking for extremely sharp corners and edges. As the material melts, it can get caught up in these points, leading to uneven coatings and bridging. So, it’s better to opt for smoother corners and less severe radii.
- Aesthetics and extra details: If there are small details and flourishes you can work into the overall design, it’s worth testing out so the final product is attractive when it reaches your customer's hands.
What Are the Applications of Rotational Molding Products?
While it might seem niche, roto casting can be a helpful process for making all kinds of interesting and useful products like:
- Watercraft like kayaks, canoes, and pontoons
- Containers and vessels for storing chemicals, water, and other loose goods
- Kids toys, such as tricycles, slides, and playground equipment
- Trash cans and recycling bins
- Coolers, usually the portable kind for food and beverages
- Tanks for processing and agricultural needs
- Carts and carriers
- Barriers for roads and highways
What Are the Common Defects of Rotational Molding Products?
Common defects in rotational molding products include:
- Shrinkage and Deformation: Residual stress during cooling can cause the plastic to shrink and deform. Proper cooling methods and structural design can mitigate these defects.
- Reduced Impact Strength: Factors like heating temperature and time, cooling speed, particle size, mold, and material influence impact strength. A recise mold layout is required to maintain the parts' impact resistance and prevent other design flaws. Sharp corners, inadequate draft angles, issues with material flow, and flat walls are all design flaws that can compromise the usability and quality of rotomolded products. Designers should include rounded corners, suitable draft angles, enough room for material flow, and structural elements for flat surfaces to prevent these problems.
- Bubbles and Perforations: Bubbles that form in the plastic during the melting or bonding phases will impact both the part’s appearance and its mechanical properties.
What Is the Lifespan of Rotational Molding Products?
Rotational molding products' lifespans can vary depending on variables like application, material, and environmental conditions. However, rotational molding products generally last for a very long time with proper use and maintenance. Storage tanks, playground equipment, automotive parts, watercraft, agricultural tanks, and numerous other custom-made goods last for many years.
Is Rotational Molding Sustainable?
In the strictest sense, no, rotational molding is generally not sustainable because its raw materials are non-renewable plastics made from petroleum. Compared to many plastic forming techniques, though, it is less environmentally hazardous. The process does not release harmful chemicals, creates minimal material waste, and requires less fuel compared to other methods. The plastic that goes into roto-molded parts is also often recycled and/or recyclable, thus reducing the need for virgin raw materials and limiting the carbon footprint.
How Thick is the Rotational Molding Product?
The thickness of rotational molding products can vary depending on their specific applications and design requirements. Generally, wall thicknesses range from 1/32 to 1 inch (0.8-25 mm). The resins used in rotational molding are typically supplied in powder form, ground to 35 mesh, with particle diameters ranging from 74-2000 microns. Overall part thickness depends upon the function, material characteristics, and structural requirements that went into the design.
What Are the Advantages of Rotational Molding?
Here are some of the reasons that this manufacturing method is so loved (and hasn’t gone extinct even after a century):
- Products come out with even sides and thicknesses all around thanks to the consistent rotation of the machine.
- Its low-pressure operation means you can use metals like aluminum for the mold (something that can’t be said of many other types of molding).
- Creating double-walled containers is a cinch and doesn’t have arduous post-processing needs.
- No need for joining with this method—roto molding makes a one-piece construction, saving time, money, and manual labor.
- During the process, the material stays stable, meaning fewer defects, flaws, and deformations on the finished product.
What Are the Disadvantages of Rotational Molding?
And now for the downsides:
- It can only really work well with thermally stable materials.
- The machinery and tools needed are pricey, and the price only goes higher when it comes to larger sizes of molds and those that are custom made.
- It takes longer than some other molding processes, so your timeline may be drawn out.
Frequently Asked Questions About Rotational Molding
Is Rotational Molding Expensive?
At face value, rotational molding—and all the equipment and tools that are necessary for this process—may seem expensive. But from a long-term perspective, this investment will usually pay for itself. In comparison to other molding techniques, it’s actually pretty affordable as far as start-up costs go. Pulling information from one case study, you’ll find it’s much cheaper than blow molding. For 50-gallon trash cans, the tooling cost was $20,000 and the maintenance cost was $1,000 a year for rotational molding, whereas blow molding the same product would cost $210,000 for tooling and $5,000 a year for maintenance.
Is Rotomolding Cheaper Than Injection Molding?
Yes, roto molding is generally cheaper than injection molding. The low-pressure process of roto molding means the molds can be made of less expensive materials. There is also no need for high-pressure pumps or heavy-duty hydraulics. This cost advantage makes roto molding the best option in terms of up-front manufacturing costs.
What Is the Difference Between Rotational Molding and Compression Molding?
Compression molding and rotational molding are two separate processes intended for different materials. The materials that go into rotational molds must be capable of melting and reforming when they’re reheated. Thermoplastics and metals fit this bill, but thermoset polymers do not. Compression molding can produce thermoset items that are similar in appearance, but which are put under higher pressure and cannot be reshaped once the process is completed.
How Xometry Can Help
Our team’s biggest goal is to help you with manufacturing and processing needs and we have a long list of solutions to do so. We offer various molding options, including quick-turn molding, prototype molding, bridge molding, insert molding, overmolding, and assembly.
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