What is Rotational Molding? | An In-Depth Guide

What is Rotational Molding?

Rotational molding, sometimes also known rotomolding thermoplastic manufacturing technique suitable for one-piece hollow components. Unlike other manufacturing methods, rotomolding doesn’t involve pressure. Even the molds are also very inexpensive because they don’t need to withstand pressure.

It’s a cost-effective production method for short production cycles(somewhere around 2500-3000 pieces a year), making it perfect for small businesses, start-ups, and inventors.

Rotational Molding is compatible with making a diverse range of products. It gives designers and manufacturers extraordinary freedom. Just about any shape can be produced as there are no limitations with the size of the part produced.

Rotational Molding Process – 

• It all begins with plastic material generally in powder from poured into a mold usually made from car aluminum.
• The mold is tightly closed and rotated slowly on two axes.
• The mold will be rotated and heated simultaneously, and the material inside the mold will spread.
• Once the material melts down completely, the mold will be brought to a Cooling station and cooled with air(sometimes a spray of water is also used).
• As the part solidifies and cooled enough for safe removal, the process is stopped, and the product is taken out manually.

The overall rotomolding process might look effortless on the surface (and it is), but sometimes things can go out of hand.

As it is a casting process and there is no pressure involved, controlling the material within the process is quite complicated compared to high-pressure processes such as injection molding. Constant pressure on material makes things quite straightforward.

The factors crucial in affecting the production quality are surrounding temperature and humidity, types of mold, and powder quality.

Undercuts, ribs, inserts, kiss-offs, and foam reinforcements are integrated as secondary processing.

Types of Rotational Molding Machines – 

• Rock and Roll
• Clamshell
• Shuttle
• Swing Arm 
• Vertical or Up & Over
• Carousel

Rock and Roll:

Rock and roll machines are specially utilized for producing long and narrow parts. It consists of two arms, each arm rotating and rolling the mold 360 degrees in one direction and at the same time tipping and rocking the mold 45 degrees above or below horizontal in the other direction. The latest machines utilize hot air to heat the mold. These machines are a great option for producing large parts with a high length-to-width ratio.

Rock and roll machines are also light n your pocket as smaller heating chambers inside the machine makes it consume less heat resulting in significant cost savings.

Clamshell:

Clamshell is a single-arm rotational molding machine. The arm is generally supported and coordinated by other arms on both ends. The machine heats and cools the mold in the same chamber, thus consuming less space compared to other bulky rotational molding machine variants. It is available in smaller scopes for schools and micro-businesses looking for prototyping high quality and low quantity models.

Shuttle:

Shuttle machines consist of two arms that move the molds back and forth between the heating and cooling chamber. The arms are free from each other and they move the mold biaxially. Exceptionally, but in some cases, a shuttle machine can have only one operating arm. The machine is cost-effective compared to the capability of product size and it is also available in smaller scales for utilization in schools and prototyping.

Swing Arm:

Swing arm machine can have up to 4 arms, all independent from each other and not necessarily operating at the same time. Each arm is attached to a corner of the oven and swings in and out of the oven. On some swing arm machines, a pair of arms are located at the same corner, so that a four arms machine can achieve two pivot points. The swing arm machine is more suitable for companies with extended cooling cycles compared to the cook time.

The four arms provide the swing machine with an integral advantage of not halting production while running a new mold or regular maintenance.

Vertical or Up & Over:

Vertical or up and over machines are known for the compactness of their heating and cooling chambers. The loading and unloading area is present in the middle of the heating and cooling chamber. The machine’s energy efficiency and small size make it a prime choice for replacing carousel machines that have similar capabilities in terms of production but consumes more space.

Carousel:

Carousel is a biaxial type machine, available in a variety of sizes depending on the application and can have up to four arms and six stations. The machine has two different types – Fixed and Independent. A fixed arm carousel machine comes with three fixed mounted arms that just move in sync. One arm is for the cheating chamber, the second one is for the cooling chamber and the third arm is for the loading or unloading area. The fixed-arm carousel is suitable when identical times are used for each arm.

The independent arm carousel machine comes with three or four arms moving independently. The machine works well with different sized molds, different cycle times and thickness requirements.

Which is the Best Material for Rotational Molding? – 

The most suitable thermoplastic for rotomolding is polyethylene(PE). Almost 97% of rotational molding products are manufactured with polyethylene.

PE has excellent mechanical properties and chemical resistance making it highly compatible with rotational molding. It has completely dominated the industry without any major competition from other polymers.

As rotomolding is a casting technique, powder from the material is used in most scenarios rather than granules. That means the polymer must be ground, and that’s where PE takes the cake as it is easy to grind it in surrounding temperatures.

Having said that, there is a recent series of development in applying polymers like polyamide, PVC, Polypropylene, etc.

These polymers have impressive properties and processability but are difficult to grind. Grinding them would require additional machinery and expertise, which can lead to increased costs.

Interesting Read – Compression Molding Process – A Detailed Guide 

Advantages – 

• The process makes it easy to manufacture complicated geometries. Rotomolding easily assists additional production intricacies such as ribs, inserts, surface textures, etc.
• Low Production costs are no less than a blessing for someone with a new business or a bright new idea. No pressure involvement makes the process very affordable for someone with a low production volume.
• Uniform wall thickness is another advantage that makes rotomolding stand apart. Consistent wall thickness with straight and thick corners is achievable mostly all the time. That results in increased strength and uprightness of the product.
• Low upfront costs for setting up machinery.
• There is no limit to the size of products, and several different parts can be processed at the same time.

Disadvantages – 

• High Cycle time can become a big drawback. At eight rotations per minute, rotomolding can take up to 3 hours to complete one part.
• Labor cost can become a drag as automation and robots are not yet introduced in the mainstream for rotational molding.
• As mentioned earlier, the process needs polymer in powder form to work which limits its compatibility to most thermoplastics.
• The soft metal utilized in the process must be replaced after 3000 cycles making repeatability impossible.

Comparison: Blow Molding and Rotational Molding – 

Blow molding is often compared with rotomolding as a better method to produce hollow parts. We can’t decide what best works for you but can show you a brief comparison between the two.

Common Products – 

The technology is known for its versatility to literally thousands of products that can be made using rotational molding. Here are some of them.

The products manufactured from this method are unmatched in terms of quality compared to those produced by other methods.

• Material handling products like pallets, cooler boxes, containers, crates, etc.
• Tanks for storing waters and chemicals – upto 50,000 litres.
• Toys, playground equipment, bowling balls, and equipment.
• Automobile products such as ducting, diesel fuel tanks, tractor dashboards, etc.
• Outdoor products such as water butts, garden furniture, and planters.
• Environmental and cleaning-related products include road cones, bollards, litter bins, traffic dividers, road signs, etc.
• Interior, exterior lighting-related decorative products.

Interesting Read – Extrusion Molding Process – Detailed Guide | Applications of Extrusion Molding | Advantages of Extrusion Molding | Disadvantages of Extrusion Molding

History – When and What Happened?

The first patent related to rotational molding was taken out by R. Peters in the year 1855 in Britain documented the first use of a rotating mechanism producing “two centrifugal motions at right angles to each other.” The prime purpose of creating this process was to maintain density and consistency. It was utilized to make artillery shells and other hollow vessels.

A U.S patent was filed in the year 1905 with a slight variation with the actual process including a polymer in the method for production of article utilizing paraffin wax. In the year 1910 things became more interesting when more development was made to produce hollow chocolate easter eggs. Further enhancements were made in the rotational molding technique when R.J Powell mentioned the more practical ratio of 4:1 between major and minor axes of rotation at slow rotation speeds.

The world of plastics got introduced to rotational moulding in the early 1950s and the first product to be manufactured utilizing the process was doll heads. The first plastic material used for it was liquid PVC plastisol. The cooling method included pouring the mold into cold water. As the demand for the process increased, it gave a boost to the stagnant plastic toy industry and led to the creation of many new toys.

Consistent developments were made in the process which further increased its applicability. Manufacturers realize the process’ potential and used it to manufacture products like car armrests, road cones, and marine buoys.

Fast forward, in the year 1976, the Association of Rotational Moulders (ARM) was commenced as a worldwide trade association. The main motto of this group was to spread awareness of the rotational molding process and technology.

The Future of Rotational Molding – 

According to a study conducted by Transparency Research, The rotomolding market is expected to reach an incremental revenue of USD 7.7 Billion by 2030.

Rotational molding provides some crucial benefits which are difficult to avoid for manufacturers looking to create innovative products with demanding geometries.

In recent years, the fundamental understanding of polymer chemistry has seen significant growth leading to the creation of new monomers with suitable properties.

Companies working in the rotomolding segment are increasing their exposure in medical & hygiene, traffic management, sewage treatment, etc. These companies are becoming more competitive and taking new landscapes in Several high-tech services like 3D (dimensional) CAD (Computer-Aided Design) design and tooling project management.

Looking at everything from a broader perspective, things in the future look good for rotomolded products. The road is tough, but the target is achievable.

FAQs – 

1. What is the difference between rotational molding and Injection Molding?

Ans. The most notable difference between rotomolding and injection molding is the expanse associated with the mold. As rotational molding uses a low-pressure key and naturally needs less pressure, less expensive materials are used to make its molds. On the other hand, injection molding requires significantly higher pressure than rotomolding resulting in more expensive and intricate molds.

Another difference the high low pressure used by both the techniques creates is the time taken to produce a single article. As injection molding uses more pressure it has a high production speed compared to rotational molding, which is very slow.

2. Why is rotational molding process is slow?

Ans. The rotomolding process is slow because it doesn’t rely on centrifugal forces to throw the plastic against the mold wall. That makes the rotation speed slow, and the powder undergoes a regular tumbling and mixing action. Effectively powder stays in the bottom of the mold, and different points on the surface of the mold come down into the powder pool.

3. What is the speed of the mold in rotational molding?

Ans. The mold rotation speed is slow but varies from 2 to 20 RPM.

4. What is biaxial rotation in rotational molding?

Ans. Biaxial rotation, aka “rock and roll,” happens when the ratio of rotation speeds between the two axes varies in accordance with production requirements. Here, large hollow molds contain air vents to prevent contraction of the air inside on cooling, which can cause a bend in the molding.

5. What is the most common rotational molding defect?

Ans. The most common defect found in the mold can have severe parting lines that may damage the mold so that resin starts leaking from inside during molding that can leave gaps and voids in the finished product.

Suggested Read – 

• What is Blow Molding? | Guide to Blow Molding Process | Blow Molding Advantages | Blow Molding Disadvantages
• What is Injection Molding? | The Complete Analysis
• What is Ultrasonic Welding? | Ultrasonic Welding for Injection Molded Parts | Common Welding Problems and Solutions | Ultrasonic Welding Advantages
• What is PEEK Material? | A Simple and Detailed Guide
• What is UHMW Plastic Material? | The Definitive Guide
• What are Modulus of Elasticity Units | The Ultimate Guide
• What is Liquid Plastic? | Liquid Plastic Vs. Resin | An In-Depth Guide

Final Words – 

That was my view and knowledge on rotational molding. Rotational molding is a must for someone who has just started and wants to achieve quality over scale in the short terIn addition; its inexpensiveness gives a massive advantage to small businesses looking for cost-effectiveness.

Kindly share your reviews and thoughts in the comment box.

Have a fantastic day.