Roto 101

New to rotational molding? Start here. This page explains what the process is, how it works, what it makes, and why designers and buyers choose it. If you want the full technical treatment, the ARM Design Guide goes deeper.

What it is

Rotational molding, or rotomolding, makes hollow, seamless plastic parts in one piece. It runs at low pressure and needs no internal cores, which sets it apart from injection and blow molding. Because the plastic is never forced or sheared, parts carry little built-in stress. That makes it a strong fit for large, durable, hollow products like tanks.

How it works

Every part goes through four stages:

1. Load. An operator weighs plastic powder into the mold and closes it. The powder weight sets the wall thickness.
2. Heat and rotate. The mold turns slowly on two axes in an oven. The powder coats every inside surface and melts into an even layer.
3. Cool. Fans, and sometimes water spray, cool the mold as it keeps turning. The plastic sets, shrinking slightly as it solidifies.
4. Unload. The operator removes the part while it is still warm, then refills the mold and repeats.

What it makes

Tanks and containers, materials handling bins and drums, kayaks and playground equipment, road barriers and buoys, planters and furniture, signage, and recycling bins. Sizes run from handheld parts to 400-gallon tanks.

Why choose it

• Low tooling cost, which suits low to medium volumes
• Seamless one-piece parts with no weld lines or seams
• Even walls, with corners that can be made thicker for strength
• Design freedom, from double walls to molded-in inserts and graphics
• Any size, small parts and very large ones on the same process

Materials

Polyethylene (PE) is by far the most common, thanks to its easy processing and strong chemical and impact resistance. Other options serve specific needs: XLPE for extra toughness, PVC for flexible parts, nylon for heat and chemical resistance, polycarbonate for clarity, and polypropylene for stiffness. See Common Plastic Materials for the full list.

Go deeper with ARM

Roto 101 is the starting point. When you are ready for more:

• ARM Design Guide — the complete technical reference on the process, materials, machines, molds, and part design.
• ARM Training Academy — self-paced certificate courses in Operator Training, Health and Safety (English and Spanish), Wall Thickness Control, and XLPE.
• Find a Rotomolder and Find a Supplier — connect with ARM member companies.
• Join ARM — train your team, improve your process, and connect with the people who move the industry forward.