Ismert describes Ripple Engineering's pressure-cooled rotomolding process as a hybrid of rotational molding, injection molding and blow molding.
"You get a rotomolded part, which is a biaxial rotation, a slow rotation. It's a very uniform wall thickness, high-impact strength part," he said. "But with our machine, it's like an injection molding machine because you set it up for one mold at a time, so scheduling becomes much simpler."
The process is similar to blow molding, he added, because you are using pressure inside the part to help form and cool it.
But, there are a few differences. On a Ripple Engineering rotomolding machine, the oven and the mold rotate together.
"Everything happens at one 'station,' meaning we don't move from a heating chamber to a cooling chamber to a service area, etc.," Ismert said. "It also makes it possible to get all of our sensors inside the hot areas; while keeping the heat-sensitive electronics on the outside of the oven."
And instead of passive vent tubes stuffed with steel wool, for example, Ripple Engineering's machines use what Ismert calls "active venting technology."
The machine automatically opens and closes the vent during the heating stage to equalize pressure between the inside of the part and the surrounding atmosphere, eliminating the potential for blow holes and thin spots at tool parting lines and removable inserts, according to a company brochure on the technology. The active vent can also be programmed to hold a specific pressure inside the part during the heating cycle after a layer of material forms on the inside surface of the mold.
"This could be helpful in improving the part's surface aesthetics and potentially improve the part's strength characteristics," Ismert explained.
But, above all, "temperature is what matters — not time — but temperature," he said.
"During the traditional process, the inside temperature of a part continues to climb long after it is removed from the oven to the cooling chamber. We refer to that as heat momentum," he explained. "It happens in our oven as well, but we can easily see it and control it. So, during heating, our cycle is set to look for a specific internal air temperature."
When that temperature is achieved, the machine switches to the cooling stage and large volumes of air begin to circulate inside the part. The air is at a low pressure, which holds the part against the mold wall, improving cooling times, Ismert said.
"The internal air temperature and mold surface temperature are kept within a customer-specified range to control warpage and shrinkage," he explained. "The cooling cycling ends at a specified internal temperature or mold surface temperature."
The controlled cooling cycle boasts shorter cycle times, improved impact strength and reduced warping when compared with traditional rotomolding. The result is "predictable and controllable shrinkage," Ismert said.
A mold featured in the company's brochure posted a 10-minute savings during the cooling cycle, but Ismert said Ripple Engineering is experimenting with chilled internal air and better air circulation in the oven to reduce the cooling cycle time even further.
Ismert said he is also "extremely" interested in exposing the process to material types outside of polyethylene — "key to executing our company mission," he added — as the processing window becomes more transparent and controllable with the addition of pressure and temperature monitoring inside the part.
"I think we can double the number of products that are molded with this process," he said. "I think we can easily bring in 10, 20, 30 different kinds of materials, and I'm not talking grades of polyethylene. I'm talking about the stuff that injection molders get to play with."