Rotomolding: Tough skin harbors foamed interior

  　　　　　　　　　　　　　　　
The developing company, Shiina Kasei Co. (Okano, Japan), is working with rotomolding consultant Paul Nugent to help find potential licensees. In answer to questions from MPW, Nugent says foam-filled parts made via the process differ from rotomolded parts with a foamed interior realized with foaming agents. The pellets developed by Shiina Kasei are cored with foam so that as they expand, they form foam balls with outer shells. This is a one-step process; the powder for the shell of a product and the foam pellets are loaded at the same time before the mold is closed.

The outer shells on the foam create a network within the part which reinforces the structure, while the ultralow density foam inside the shells keeps the overall density of the part low. Typical overall expansion rates are around 13 times. The material can be used with any mold but the venting of escaping gases as the foam expands must be taken into consideration, notes Nugent. Pressure build-up inside the mold can reach 2 kg/cm².

The process is said to be a good choice for shapes that require complete or near-complete foaming as the foam can expand to fill the entire part. Parts can be completely recycled, with the recyclate suitable for use as foaming pellets. 

The base materials need to be specially formulated because standard rotomolding materials typically do not have the necessary rheology profile for the process, he explains. Shiina already has licensed an Icelandic company, IFoam, to work with processors and license the process, and Nugent says that company already is in talks with rotomolders regarding potential commercial applications. Perhaps not coincidentally, the world’s largest rotomolder, Promes, has its headquarters in Reykjavík. Shiina says patents are pending in the U.S., Canada, Europe, Australia, India, and China.

According to Shiina, additional flexural strength can be achieved by inserting a non-foaming strengthening member in the foamed core. Shiina offers two types of foaming pellets. The specific gravity for the Type I single-layer, high-expansion pellets ranges from 0.05-0.1, with expansion rates of from six-40 times’ pellet size; for the double-layer (structural) type II pellets, it is 0.1-0.2, with expansion rates of five-13 times’ pellet size. Processing is done at 300ºC. —Matt Defosse