A captive plastics processor is willing to license its new technology for rotomolding polyethylene parts with foam and solid layers. The method is said to be simple, inexpensive, fully automatic, and able to foam thin sections, and it does not leave a blemish on the part. No other method of foaming in rotomolding can boast all of these advantages, says the developer of the process, R. Dru Laws, engineering director at Mity Fence Systems, Orem, Utah.
Laws developed what he calls “fusible membrane technology” while working on his Master of Science degree at Queens University in Northern Ireland, a major center for rotomolding R&D. Laws was looking for a foaming method that avoided the disadvantages of existing techniques. A popular one is using a “drop box,” or insulated funnel attached to the outside of the mold. According to Laws, drop boxes can cost upwards of $6000 and require a source of compressed air to actuate the open/close cylinder. The boxes are usually actuated manually based on time from the start of the molding cycle, so part-to-part consistency is far from guaranteed. Also, because the drop box typically has a large opening, it leaves a blemish on the part.
A more automated method is to introduce the foamable resin at the beginning of the cycle—either in the form of a foamable pellet dispersion that melts more slowly than the powdered resin or using foaming agent inside a sealed plastic bag that melts during the process. Both of these approaches add to raw-materials cost, and pellets can leave blemishes on the part surface, while the bags are usually made of an incompatible material that becomes incorporated in the part and could compromise physical properties. Laws also says these two approaches cannot foam thin sections very easily.
Why a fusible membrane?
Laws’ answer was a smaller, simpler alternative to a drop box. It uses a small PTFE container to hold the blowing agent. It is sealed with a plastic film membrane and is mounted over one of the standard PTFE mold vents with the film acting as a “window” covering the vent hole. When the internal mold temperature reaches a certain point, the membrane window softens and shrinks and then pulls away from the edges of the container opening, thereby releasing the blowing agent. (In his trials Laws used an exothermic type, Celogen OT from Chemtura Corp., Middlebury, Conn.).
Laws found that EVA film is a suitable membrane for molding polyethylene parts. He experimented with molding HDPE but says any PE should work with this method. Melt won’t stick to the membrane, and because its behavior is temperature dependent, the membrane functions automatically and consistently, Laws says. The release temperature can be tailored by changing the vinyl acetate content and/or thickness of the EVA membrane.
In his experiments, Laws found that blowing-agent powder alone could be added via the fusible membrane—no resin was needed. He found that adequate distribution of the foaming agent was achieved by the mold’s rotation.
Finally, he says the fusible membrane method can foam any size part, thick or thin. It also can be used for rotomolding any second layer of a different material from the first.