Rotational Molding

Quick Glance

Unit Cost
Surface Finish​
Tolerance
Cost of capital investment
Production volumes
Possible geometries
Also known as Rotomolding and Rotocasting, used on large hollow parts that are not conducive to blow molding or injection molding. 

Basic Rotomolding Process:
The process begins with fine plastic powder of a much smaller size than the pellets used in injection molding. Though less common, self-curing resins may also be used which eliminate the need for heating. 

The plastic is measured out to determine wall thickness and added inside the mold. The mold is moved into a large furnace where the and continuously rotated to ensure the plastic is evenly melted and coated on the inside wall of the mold. After the plastic has sufficiently melted the mold is brought out and cooled down typically with misted water while still being rotated so the plastic leaves a uniform wall thickness. 

Mold Design:
Molds are typically cast from aluminum and finished with the desired surface texture for the part. A lower clamping force is needed compared to Injection Molding or Blow Molding allowing for a relatively thin mold that better allows faster heating and cooling cycles. The mold needs to be spun or rotated on two axes during the heating and cooling phases which can limit the size of the product. Similar to injection molding and blow molding, draft angles and overhangs need to be considered. 

Additional features:
Due to the nature of the process, inside corners of the design tend to have additional material built up which can help strengthen them. Inserts, such as male and female threads as well as stickers, can be added to the mold to add extra functionality. Multilayer walls and foam fills can be added as well to add additional material properties. 

 Applicable Material:
Due to the nature of the process plastics used must have a low viscosity, temporal stability, and resistant to oxidation. 

Polyethylenes (most common), LLDPE, LDPE, HDPE, PVC, Nylon, Polypropylene, Themoplastic Polyester Elastomers (TPE)

Advantages:
Able to mold some of the largest parts
Uniform wall thicknesses with little internal stress
Naturally reinforces inside corners
Able to add inserts and decals into the mold

Could be more cost effective than blow molding and small volumes
Can change colors without purging equipment of the previous color
Very little waste plastic and any waste can be recycled into new products

Disadvantages:
Longer cycle times than Blow Molding
High energy costs from having the heat and cool the entire mold rather than just the part

Thin walls and details such as threads are difficult to achieve due to low pressure of the process

Often needs a large floor space and large oven

Example Products:
Most large liquid storage tanks as well as more everyday products like Kayaks and large trash bins.

Looking to learn more about injection molding or planning on using injection molding for your product?

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