A new respect for rotomolding
"Rotational molding is getting to the point that it's starting to infringe on [injection molding and blow molding] markets," says Doug Cunningham, president of Rescraft Plastic Products (Paris, ON). "...
“Rotational molding is getting to the point that it’s starting to infringe on [injection molding and blow molding] markets,” says Doug Cunningham, president of Rescraft Plastic Products (Paris, ON). “We’ve now developed process control that takes us out of the “black art” realm. Even automotive designers have started to embrace us a little more.”
As an example, Cunningham cites a sled molded by Rescraft. Both rotomolding and blow molding were considered for the project, but in the end 82,000 units were rotomolded.
Although large, single-piece parts remain the trademark of rotomolding, the process is capable of more highly engineered applications. Speaking at the Association of Rotational Molders annual fall meeting in Tororoto, Carl Van Gilst, of Behlen Engineered Plastics (Goshen, IN) noted that the single-shot skin-foam process has worked well for engine covers. In that case, rotomolding replaced a layered combination of vacuum formed ABS, FRP and an insulated liner for two customers.
Better input, better output
Parallel developments in materials and process technology are spurring rotomolding to new heights. The emphasis in materials is on better physical properties, and better physical conditions, i.e. easier handling and less dust.
Borealis, for example, offers Borcene Compact, a metallocene polyethylene in powder form that offers improved flow to fill tight areas, and provides a better surface finish with less pinholing. The material is not ground, and thus is dust-free. It is particularly suitable for complex molds with narrow spaces and threads, such as automotive parts and tanks.
A. Schulman has improved its SuperLinear resins to provide higher stiffness, higher HDT, better impact properties and better flow. New generation SuperLinear materials are 30% stiffer than the original formulations. A major kayak manufacturer switched to SuperLinear XL 0370 and was able to reduce part weight by an average of 10% with no loss of stiffness. The lighter parts required slightly shorter cycles, tight areas filled better and costs were reduced.
Drawing on an innovative technology for producing flexible vinyl in the form of tiny spherical pellets, Teknor Apex has developed free-flowing rotational molding compounds that increase manufacturing efficiency and improve end-use performance for rotomolding. Tek-Spheres micropellets are available in diameters from 0.020 to 0.060 in. The micropellets are produced by melt compounding, which provides an opportunity for incorporating additives and other ingredients.
The micropellet technology will permit rotomolders to improve the performance of flexible vinyl and pioneer new applications, says Charles E. Gates, automotive industry manager for Teknor Apex. Unlike drysol formulations, there is no problem achieving durometers below 60 Shore A with micropellets, he notes. Improved physical properties, such as low-temperature flexibility, are possible, and the capability to incorporate a range of solid ingredients promises to yield new combinations of PVC and other polymers.
New design freedom
An innovative product that could help rotomolders fill small spaces is RMC3 rotational molding compound from Mold In Graphic Systems.
RMC3 is a pliable form of polyethylene (like putty) that can be formed and inserted into the mold to create critical solid areas or design features that are incorporated into the final part. The putty-like compound cures during the rotomolding cycle.
Matt Stevenson of Mold In Graphics reports that one molder is using the new compound to form the threaded neck and mounting flanges for a motorcycle fuel tank.
The material may also allow designers to incorporate solid handles or external nipples.
Precolored compounds have visual appeal
To improve aesthetics and part consistency rotomolders are turning to precolored compounds, according to suppliers at the Rotoplas 2002 show in Toronto.
Dave Cobb of General Polymers reports that sales of precolored ground compounds have grown substantially. “General Polymers pursued this market heavily this year, and we’ve seen a lot of growth. At first, nobody was doing it; now everybody’s doing it.”
Doug Cunningham echoes that conclusion. In addition to its molding business, Rescraft sells compounded materials for rotomolding. And that business, Cunningham says, is “taking off”.
Other coloring options for rotomolders are high intensity blending performed in-house, or the more traditional dry blending process.
Technology pushes past previous limits
As researchers are providing insights to what’s happening inside the mold, machinery manufactures are translating that knowledge into more productive, more accurate equipment.
The triennial Rotoplas trade show provided an opportunity for this tight-knit community to show off the latest developments.
Ferry Industries, a manufacturer of complete rotational molding systems, introduced automated mold balancing software. “Because rotomolding is generally suited to short runs, mold changes are a frequent occurrence,” explains Terry Gillian, vice-president Ferry sales. The new software uses feedback from an axial drive to calculate how much weight is needed, and where to place it. It replaces the trail-and-error method, and can reduce mold change time to minutes instead of hours, says Gillian. The technology can be retrofit to older machines.
Ferry also offers IRT, a closed-loop process control system based on infrared thermometry. Bonar Plastics (Lindsay, ON) recently purchased a Ferry RS-330 rotomolding machine equipped with IRT for monitoring the temperature of the plastic as it is heating and cooling.
Also introduced at the show was a totally automated rotational molding machine called Leonardo. Developed by the Italian manufacturer Persico, Leonardo is said to reduce cycle time by directly heating the mold using electric resistance heating instead of the traditional hot air heating in an oven. Internal mold temperature is monitored, and a PC interface provides production monitoring.
Because of the automation, productivity on a Leonardo unit can be up to four times that of a standard machine. The downside, however, is that these machines are not well suited to low volume production.
A computer simulation software package for rotational molding is now available to assist with product development. RotoSim, from The Plastics Development Centre, predicts wall thickness distributions, mold and internal air temperatures, and heating and cooling conditions.
As rotomolding becomes more high-tech, it also seems to be attracting new participants. “We’re seeing a lot of consolidation in the industry,” says Gillian, “but we’re also seeing a lot of companies bringing work in-house.” Now that process variables are measureable, simulation is possible, and more materials options are available, it seems rotomolding has more appeal.