The Rotomolding Process Overview

Rotational molding, or rotomolding, is a manufacturing process used to produce strong, seamless hollow plastic parts. The process begins when a measured amount of powdered resin is placed into a hollow mold. This mold is then bi-axially rotated in an oven, allowing the resin to evenly coat the interior surfaces as it heats and melts. Once the resin fuses and cures, the mold is cooled and the finished part is removed.

This method offers several advantages over other plastics processing techniques. One of the most notable is scale. Rotomolding equipment is capable of producing parts ranging from small  components to massive items such as storage tanks, playground equipment, or even boat hulls — all in a single piece without seams or welds. The ability to produce extremely large parts in one cycle sets rotational molding apart from processes like injection or blow molding.

Design flexibility is another key benefit. Complex geometries, undercuts, and multi-wall designs can be achieved without the need for multiple components or costly assembly. Because the tooling is relatively lightweight and stress-free, molds are less expensive to produce and can be modified with greater ease. This makes rotomolding a cost-effective option for prototyping as well as full-scale production.

rotational molding process

The process also naturally reinforces high-stress areas. Unlike injection molding, where corners may thin, rotomolded parts tend to build extra material in edges, ribs, and corners, enhancing durability. Combined with uniform wall thickness, this results in products that are both tough and reliable, making the process a preferred choice across industries such as automotive, marine, industrial, and consumer goods.

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Need Parts on Short Notice?

Let us know your target production date and we’ll outline a realistic schedule using our streamlined process.

Let us know your target production date and we’ll outline a realistic schedule using our streamlined process.

Advantages of Rotationally Molded Parts

Rotational molding uses low-pressure processes, allowing molds to be made from less expensive materials such as aluminum or sheet steel. This makes tooling more affordable, especially for short-run or prototype parts.

The process supports intricate shapes, undercuts, and complex contours—including stiffening ribs, molded-in inserts, and multilayer structures—without the need for additional assembly.

Rotational molding handles a vast size range, from tiny items to massive tanks measuring thousands of gallons, often within a single cycle.

The continuous rotation promotes even material distribution, with corners and ribs naturally becoming thicker—leading to enhanced structural integrity.

Parts are formed without weld lines or internal stresses, enhancing durability and reducing weak points.

Designers can incorporate double-wall or multi-layered designs—including foam cores—for insulation, rigidity, or structural enhancements.

Cast or machined aluminum molds allows for detailed textures, graphics, and logos to be molded directly into the part. Surface finishes can be high gloss, matte, or customizable. Mold-in graphics are permanently fused into the molded part during the manufacturing process.

Metal inserts, threads, and functional elements can be integrated directly into each part during molding, improving assembly efficiency.

With no runners, sprues, or excessive trimming required, this process produces very little scrap, which can usually be recycled—promoting sustainability.

Given the lower tooling costs and flexible setup, rotational molding is economical for smaller production volumes, prototypes, and custom parts.

Rotomolded parts often outperform alternatives in terms of durability, chemical resistance, and impact resistance, ideal for harsh or demanding environments.

In-mold foam filling involves introducing foam directly into the mold cavity during the molding process. This technique creates a “sandwich” structure with an outer plastic skin and an inner foam core, resulting in a lightweight yet strong part.

Post-mold foam filling is a secondary process where foam is injected into a pre-molded part. This method allows for the addition of foam to parts that were not originally designed with a foam core.

The Rotational Molding Process With Integrity

At Integrity, we believe that the best part design begins with knowledge. By educating customers on the true capabilities of rotational molding and proactively addressing potential challenges, we help prevent costly revisions and unnecessary tooling expenses. Our commitment to transparency and collaboration ensures every project is designed for manufacturability from the start—delivering durable, efficient, and cost-effective solutions that stand the test of time.