Speakers: Molders can gain process control in rotomolding

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.

Ben Ismert, founder of a new rotomolding machinery company, Ripple Engineering of Kansas City, Mo., said the machine uses what he calls "active venting" on the mold, instead of passive vent tubes stuffed with steel wool, to give greater process control as well as pressurized cooling, to cut cycle times.

"Active venting opens and closes automatically, and it lets the pressure equalize automatically during the process, during the heating," Ismert said. A thermocouple constantly measures internal mold air temperature and pressure. "And you can add pressure if you want at certain times. That's during heating. And then during cooling, the same thing opens and it lets high volumes of air come in, in a controlled fashion," he said.

Ripple machines have one station, so the mold is heated and cooled in the same area of the machine, instead of moving through the steps like a carousel machine. Electric heating, driven by fans, is done by elements surrounding the oven that are controlled by specific heating zones.

"The goal is to make a perfect part every time," Ismert said. "And that's defined by the customer. In order to do that, you have to have these temperatures and pressures that are happening inside of it."

Faster cooling can do more than reduce cycle time. Ismert said the faster you cool a semicrystalline structure, the less time the crystals have to re-form, and they remain in an amorphous state, making the finished part less brittle and giving it more impact strength.

A rotomolder can swap out Ripple's active vent for use in different molds, Ismert said. "So if you have customers that want a very high-end acetal or ABS or polypropylene part, and they just want one per day, you can make them one per day. Then take that mold and make 16 of those other things," he said. "It's however you want to do it. You can make different colors."

He said machines from Ripple Engineering, by actively controlling the rotomolding process, can run a wide range of materials, opening up the industry beyond polyethylene.

"That's the main reason I invented it," Ismert said. "I'd like to improve parts made from polyethylene, but my passion is doubling the size of the industry by introducing more materials. And when you have more materials, you have more applications."

The largest rotomolding machinery manufacturer, Ferry Industries Inc. in Stow, Ohio, has long offered two systems that give a lot of information about the process. InfraRed Thermometry, known as IRT technology, uses infrared sensors to monitor temperatures on the surface of the mold and automatically make adjustments to the oven and cooling time of the molding cycle. RotoLog uses sensors inside the mold to measure internal air temperature.

"Without the data, you're trying to guess, as an operator," said Patrick Calderone, Ferry's vice president of sales.

Calderone said the temperature levels inside the mold determine when the powder starts to stick to the hot mold surface and form layers. RotoLog lets molders figure the optimum melt temperature, he said.

IRT can improve startup times and help the user get consistent part dimensions, he said.

"IRT provides monitoring and control of the machine. It adds improved startup and control," Calderone said.