Rotational Molding

Rotational Molding Process

Often described as a black art, the rotational molding process is a culmination of science, chemistry, some black art and lots of experience. A multi-step plastics molding process commonly utilizing powder polymer materials, rotomolding is best defined as a manufacturing process used to create hollow, one-piece plastic products. While the synonyms rotomolding, rotational molding and rotocasting are virtually interchangeable, the process does not use pressue unlike most polymer molding procedures. Rotomolding uses the bi-axial rotation of the arm, which has a mold mounted to the end of the arm, to distribute the materials uniformly through the mold. The mold is placed in a PLC controlled oven where the molds are heated as the arm continues to rotate.

Watch as Alli from Granger Plastics explains the rotomolding process and explains each step in further detail!

Rotomolding Process

Rotational Moulding is a 4-stage process, used to mold durable, polymer based products for a wide facet of industries. Lets review the process in detail.

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Rotational Molding: A Four-Stage Process

Stage One: Loading the mold

The mold is opened up, vent tubes checked to be in place and any other necessary components are installed, such as threaded molded in inserts, mold-in graphics or any other molded in features necesary as specified by the customer for a succesful, high quality custom rotomolded product. Pulverized resin, typically polyethylene, as it the most frequently demanded material for the rotomolding process, is loaded into the mold. The mold is then closed up using clamps or mechanical fastners installed on to the mold. the mold is then rotated outside of the oven to ensure there is no leaks or spillage.

Loaded mold for Rotational Molding, Loaded Mold for Rotomolding
Molded loaded up and about to be closed to begin rotomolding process

Stage Two: Oven Cycle

With the mold closed and secure, the arm is indexed into the oven where the mold will rotate bi-axially at a controlled speed pre-set into the machines computer, sending signal to the arm thru a PLC controller. The molds are heated in the oven as the arm continues to rotate for a length of time, again pre-set into the machines computer. Most rotomolders operate their ovens cook cycle in the 500 to 650 temperature range. Some may cook at higher temps in effort to reduce cycle times. Cycle times are critical to rotomolding, as overcooked and undercooked products are generally scrap production as they do not have the physical characteristics of a quality rotomolded product.

Rotomolding Arm going into the oven, Rotational Molding Arm Going Into Oven

Stage Three: Pre-Cooling & Cooling

The pre-cooling and cooling cycles are an integral step to any rotationally molded products. It is the cooling process that is critical to final product dimensions, warpage and numerous quality aspects in rotational molding or custom rotomolding. The mold begins the cooling cycle, usually ambient air, fans or a mixture of both as the mold continues to rotate. After the pre-determined duration of the pre-cool cycle, the mold moves into the cooling chamber where it is cooled by air, water spray or a combination of both. The mold continues to rotate as the cooling cycle takes place. The cooling cycle is typically some comparable duration to the cook cycle, which is why production volumes of the rotomolding process are not as significant as other processes such as injection molding or blow molding, as the cook cycles and cool cycles can range from just 15 minutes to nearly an hour for some heavy wall, complex rotomolded products.

Rotational Molding Molds Cooling, Rotomolding Molds in the Cooling Chamber
An arm of spidered molds cooling in cooling chamber

Stage Four: Unloading the Molded Product

Once the cooling cycle is complete, the arm is indexed out of the cooling chamber, where the molds will be manually opened up by the machine's operators. With the molds at optimum temperature for product removal, the mods are opened up and the molded product is removed from the mold. The mold is then prepared again to begin the rotomolding process all over again and the product is sent for quality inspection, secondary assembly and/or packaging contingent upon the customers specifications and requirements.

Removing molded product

Additional Rotomolding Information

Granger Plastics Rotational Molding Information

For more information on Granger Plastics Rotational Molding: Click Below

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Granger Plastics Company Rotomoulding Information






  • Rotomolding Tooling being built, Rotational Molding Tooling

    Rotational Molding Tooling

    Rotationally molded items require a mold or a tool to produce. While prototypes can be easily mapped out using a variety of processes, production projects must have tooling designed and produced to materialize. Tooling varies greatly in cost and time to produce before manufacturing can begin. Contact Granger Plastics for a complete quote including tooling costs.

  • Rotomolding Design, Rotational Molding Design

    Rotomolding Design

    Design for rotational molding requires knowledge of the process and its many variables. The nature of this application requires certain design elements to be implemented in order to produce the most reliable parts. Granger Plastics Company leads the industry in groundbreaking innovation in rotomolding design.

Rotational Molding Info

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