Cleveland – Gareth McDowell believes it is time to move ahead with rotational molding – towards multi-layer molds, new materials, more sophisticated high-tech parts and closed process control based on precise temperature control in the mold.
Rotomolding, in simple terms, is too inefficient, McDowell told attendees at the Society of Plastics Engineers’ rotational molding conference. “I think we need to get there to take rotational molding to the next level,” he said in a keynote address on June 7th at the Cleveland conference.
McDowell said the rotational molding industry needs to make coordinated efforts to advance the technology, possibly through the establishment of a special committee.
He asked: Why can’t parts for cars be rotomoulded? “I dream of more than tanks and kayaks,” said McDowell.
McDowell is the director of a UK company, 493K Ltd., which developed temperature and pressure monitoring technology in the rotational molding industry.
He identified several reasons for rotational molding being withheld.
One is the over-reliance on single-layer parts. Some casters work with multiple layers, using methods like drop boxing, but McDowell said you don’t know if the box will actually open or not until you open the mold to remove the finished part. The Dropbox also increases cycle time and costs, and requires people with years of experience to operate it properly, he added.
The key, McDowell said, is being able to measure the temperature accurately. His idea is a technology to get to the mold in the oven unhindered by the high heat of rotational molding.
On the materials side, rotomoulders have long complained about the industry’s over-reliance on polyethylene, which McDowell described as a forgiving, “safe” material with a wide processing window. He said he didn’t knock on PE, “but it wasn’t really advertised as an engineering plastic.”
Why not a wider range of materials such as polypropylene, nylon, polycarbonate, fluorocarbons, polyester and polystyrene? This results in the need for improved process control within the tool. “If you plan to use anything other than polyethylene, please keep the temperature accurate,” he said.
Rotational molds must also have double sided molds to bring heat and cooling to both sides of the plastic part. More use of internal air cooling would help.
Vent pipes are also a problem. “We have to step it up and get to the point where we can open and close our vents during the cycle,” he said.
Another item on McDowell’s wish list: rotational molding machines with no arms in the oven, just the mold. Right now, the process is heating and cooling steel on the arms, an inefficient way of doing it, he said.
“At least insulate your arms,” he said.
McDowell also said current rotational molding machine technology does not control wall thickness accurately enough. I would love to see encoders on all arms, as well as software that allows you to set speed, temperature and other parameters so that the machine runs in a closed loop and fully automatically.