Ask Dr. Nick: A Basic Review of Foam in Rotational Molding

ARM often receives questions about the foaming process as it relates to rotomolded parts and we thought a basic review might be useful.

 A number of different foam products have been used in conjunction with rotomolded articles, in order to impart enhanced properties.  These include:

1. Polyurethane (PU) Foam, injected into a cavity in the final rotomolded part, with the aim of completely filling it.  Typically, a fully cooled part is contained inside a foaming fixture and the foam components (polyol and isocyanate) are mixed and injected through a special nozzle.  The creation of the PU Foam is extremely rapid, once the components are fully mixed.  PU Foams have very low density (typically 0.050 g/cm³), which gives them excellent heat insulation properties.  They are also used to add buoyancy to marine components.  There is no bond between the PU and the PE part and de-lamination of the foam is instantaneous.  The PU Foam does not impart any additional stiffness to the product.
2. Expanded Polystyrene (EPS) Foam, created in a cavity in the final rotomolded part, with the aim of completely filling it.  Typically, a fully cooled part is contained inside a foaming fixture and pre-expanded EPS beads are poured inside the part.  Steam lances are then inserted into the part and the associated heat further expands the beads.  This process takes time (typically tens of minutes) to complete.  EPS Foams have low density (typically 0.150 g/cm³), which give them moderate heat insulation properties.  However, they are mainly used to add buoyancy to marine components.  There is no bond between the EPS and the PE part.  The EPS Foam does not impart any additional stiffness to the product.
3. “Syntactic” Foam, created in a cavity in the final rotomolded part, with the aim of completely filling it.  These are composite materials; for rotomolded applications they usually consist of an epoxy-based polymer matrix with hollow glass spheres suspended in it.  This structure provides low density and very high stiffness / crush resistance.  The density can be adjusted over a wide range, but when used in rotomolded products, it is typically in the range 0.400-0.500 g/cm³.  The main application is for subsea flotation devices (eg flotation collars around undersea pipelines), where they impart high resistance to crushing by water pressure.  There is no bond between the PU and the PE part and de-lamination of the foam is instantaneous.
4. PE Foam, which differs significantly from other types.  PE Foam is generally added as a second charge during the molding process, when an outside skin of standard solid PE has already been formed.  In this case, the aim is not normally to completely fill the cavity; rather, the aim is to produce a second layer of even thickness around the inside of the rotomolded part.  This imparts a degree of extra stiffness and a degree of heat and sound insulation (although significantly less that Options 1&2).  There is a full bond between the PE Foam and the outside PE skin.  The density of PE Foam can be adjusted over a limited range, the minimum practical density that can be achieved is approx. 0.200 g/cm³ and the maximum density is, theoretically, the density of the PE used in its formulation (i.e. zero foaming).

For more information, ARM’s website includes a free webinar for members on In-Process Rotational Foam Molding, conducted by Dru Laws. Late this summer and throughout 2019, ARM will conduct a series of webinars on Finishing that will go into more detail on foams.

Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 25 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.