By Shell Polymers on Mar 12, 2021
Unlike converting processes such as blow molding, injection molding, pipe extrusion, and blown film, rotomolding requires a unique preparation process to effectively complete a cycle. Key differentiators includes:
- The use of resin as a powder versus a pellet
- The biaxial rotation of the molds on the X and Y axis
These unique characteristics require rotational molders to take in a few other considerations before beginning.
According to the National Institutes of Health, more than 95% of the rotational molding materials market is polyethylene. Specifically, high-density (HDPE), low-density (LDPE), and linear low-density polyethylene (LLDPE).1 In an academic paper published from the Department of Mechanical Engineering at Birla Institute of Science, the researchers explain that among the different polyethylenes, linear low-density polyethylene (LLDPE) is often used because of its unique melt flow properties and less shear sensitivity.2
Understand Rotomolding’s Unique Characteristics
Rotomolding is a low pressure conversion process that gives the ability to create hollow plastic parts of all sizes. Due to the capabilities of mold design, rotomolding is ideal for creating complex shapes, such as playground equipment or toy cars, which requires both soft curves and a sturdy structure to protect its users.
Explore more about Polyethylene in Everyday Life: LLDPE Plastic Product
While the rotomolding process provides brands with many benefits, like flexible design geometry, it poses challenges converters need to address. Some of these include how rotomolding is:
- An operator-dependent, manual process, which makes it harder to automate
- Relatively slow, producing only one part per mold, per cycle
- A low-volume process, with demand for certain rotomolded parts ranging from as low as five to 20,000+ per year.
Converters need to operate at peak efficiencies to overcome the idiosyncrasies of rotomolding and maximize the impact of each cycle. To help improve LLDPE rotational molding operations, consider these best practices.
Three Best Practices for LLDPE Rotational Molding
1. Ensure LLDPE Resin Powder is Uniform
In its pellet form, polyethylene cannot be used for rotational molding. To appropriately transfer the heat from the mold to the resin, the resin size must be reduced to a powder. Our experts recommend that you start with resin powder that has been uniformly pulverized. Take the extra step to measure the bulk density of the powder before inserting it into the mold.
Polyethylenes, such as LLDPE, can be ground to 35 mesh at high rates, which is why they're one of the most widely used powders for rotational molding.
2. Run Thickness Experiments with New Molds
In rotomolding, a mold is assembled with a specific amount of resin powder inside, heated, and then rotated biaxially in an oven. The slow rotation of the mold inside the oven helps ensure that the powder evenly coats the whole interior surface before melting and fusing together into one seamless part. Since rotomolding doesn’t use force to push the plastic into the mold, this tends to produce parts with fairly even wall thickness in comparison to other processing types.
Prior to conducting the first cycle with a new mold, our experts advise that rotational molders run a series of thickness experiments. Despite the extra step, these critical tests will help determine the optimum major and minor axis speeds and ratios.
3. Monitor Equipment Settings
One of the main rotational molding defects, warpage, can be the cause of over or under baking the final part. Throughout the baking process, rotomolders should look at the bubbles formed on the LLDPE to help determine the doneness of a cooked part. Bubbles form when the powder particles melt and bond, creating air pockets. Lots of bubbles indicates that the part is undercooked. Taking the part out of the oven at this stage will likely lead to failures or breaks. A monitoring system must be in place to reduce bubbles and ensure parts are baked to completion.
As rotomolders get more familiar with the equipment, its settings, and the resin they are using, they’ll be able to better predict baking times and improve overall product quality and cycle times.
There is a strong need for educational materials across the plastics sector. Especially as many highly skilled and experienced operators prepare to retire and the next-generation of plastics professionals play a more active role in the industry. To help ensure that the skill level of the workforce remains high, Shell Polymers is committed to creating informational resources across injection molding, blow molding, polyethylene film, pipe, and rotomolding to help transfer knowledge and best practices.