Cleveland — Rotational molding happens inside big molds spinning through hot ovens, then through a cooling cycle. But what goes on in the mold doesn't have to be a mystery, according to speakers at the Society of Plastics Engineers' Rotational Molding Conference in June.
The industry officials discussed how to control and analyze the rotomolding process as it's running as well as the importance of proper cooling.
Denis Rodrigue, professor at the Université Laval in Quebec City, Canada, explained how to analyze internal temperature curves for the process control of rotomolded parts.
"The idea is to make an analogy between the rotational molding and cooling in terms of control and [material] characterization," Rodrigue said. Studying internal curing data can help determine if molded parts are good or bad, he said.
Impact strength, bubble content and surface quality are all important features. A rotomolder can do impact tests, cut out sections of a part to show bubble formation and do other physical measurements, but Rodrigue said that has limitations.
"It's always a compromise between too much time or too short time in the mold," he said.
Instead, he said, molders can understand and control mold temperature, pressure and things like heat transfer and the rate of cooling to get a fingerprint of the process.
"You have the oven temperature. You have the outside tool temperature. And you have the inside temperature. So now you have a lot of information to measure and control the heat transfer from the oven to the material inside the mold," Rodrigue said.
He said particle size and shape of the rotomolded power is a key to controlling sintering and bubble removal, but he added that this area needs more study.
Rodrigue also updated Rotational Molding Conference attendees on the university's research with the university of Guadalajara, Mexico, on using natural fibers — derived from agave, a plant used to make tequila. In Canada, researchers mainly use hemp and cellulose materials.
He said that traditional thinking says if you add more fiber to the rotomolding resin, the fiber acts like an insulator and slows down the heat transfer inside the mold, giving longer cycle times.
But the cycle times are actually shorter, Rodrigue said.
"Actually, that's what we have seen, because the more fiber that you put into the material, the less polymer you have to melt. So, the melting is sooner and that's how so we're able to reduce the cycle time. Less material to heat up and cool down," he said.
Rodrigue also talked about the university's research with the University of Guadalajara into cellular foamed plastics for the rooftop water tanks that are so popular in Mexico. The foaming reduces weight and adds thermal insulation.
He said the team has found a promising sandwich structure for the tanks that combines both foam and natural fibers: an inner layer of virgin polyethylene, a middle layer of foamed PE and an outer layer that is both foamed and reinforced with agave fibers.
Researchers are now analyzing the life span of tanks made of that three-layer construction, Rodrigue said.