Rotomolders learning the language of automation

Rotational molding is one of the fastest-growing plastics processing methods, with annual growth estimates ranging from 8 to 20 percent through 2005. Still, competition-particularly from blowmolding and twin-sheet thermoforming-remains tough.

One rotomolder, French toymaker Smoby (Lavans les Saint-Claude), over the last two years has acquired six TA 2000 accumulator-head blowmolding machines from Meico (Monza, Italy) for the processing of large toys. Advantages cited with blowmolding over rotomolding include the ability to better automate the process and the subsequent higher outputs achieved. Compared with other plastics processes that are largely automated, rotomolding's labor costs generally are higher since employees often are responsible for pouring materials into molds, opening and shutting molds, and parts removal and finishing.

Automating the process is high on most rotomolders' wish lists. "Go to any ARM (Association of Rotational Molders) meeting and the topic that is always discussed is robotics," says Mick Shorter, principle at consultant ECMS (Tienen, Belgium). Automation must consider molding and secondary operations, adds Bob Dunne, GM at rotomolder Meese Orbitron Dunne (MOD; Saddle Brook, NJ). For secondary operations, he says his firm and others have acquired 5-axis CNC machinery to assist in finishing of parts. CNC machining gained greater exposure to rotomolders when Ferry Industries (Stow, OH), the leading U.S. rotomolding machine maker, acquired CNC machine-center supplier Quintex CNC Routers in 1998. As CNC machines have become more user-friendly in the last decade, Dunne says it is often possible to "elevate the level of your people," by retraining employees who previously did manual finishing to program and run CNC machinery.

Also citing CNC machining for secondary finishing as the most significant step his firm has made in reducing personnel costs is Klaus-Dieter Liehr, managing director at Bonar Plastics (Hockenheim, Germany). Bonar's Hockenheim site now has four CNC machines, up from zero just three years ago. In this system, as products finish rotating an operator removes parts from molds and moves them to measuring units to check critical measurements. Once ascertained, this data is shared with a CNC machine, which accounts for it when completing finishing steps such as boring, trimming, and other more complex finishing operations, says Liehr. Previously, these steps were performed manually.

One innovation being developed with support from ARM is quick-release tooling, says Dunne. Such tooling enables a tool to be released from the rotomolding carousel without stopping processing on other arms. "We're just at the tip of the iceberg with this," he says, but predicts developments will help rotomolding become a more flexible process. To date, such tooling has been feasible only for processors making identical parts on each arm of a machine, but the goal is to make it suitable for processing a variety of parts on the same machine. The difficulty is ensuring that such tooling does not dramatically increase tool costs, says Dunne, since low costs-often a rotomolding tool costs just 10 percent of a blowmold-are one of rotomolding's distinct advantages over other processes.

Manufacturer Persico SpA (Nembro, Italy) has tackled rotomolders' need to cut labor costs with its Leonardo rotational molding machine. "The line requires no manpower," says Sergio Zilioli, sales manager. An insulated box surrounds an aluminum mold and, in the space between the two, electrical resistance heats and air convection cools the mold. On the box's exterior, pneumatics control opening and closing of the mold and multiple mold sections. A robot is used to place inserts such as in-mold graphics and threaded caps, dispense material for processing and, at cycle's end, for parts removal. Molds open and close automatically, robots service the molds and parts, and alarms signal deviations from process settings. "With this system, rotomolders can reach much higher outputs, enabling them to better compete against blowmolding and [large-part] thermoforming," Zilioli claims. Machines are in commercial use at five sites in Europe, including a French processor making PVC bumpers for recreational vehicles. Liehr says his firm is strongly considering purchase of a Leonardo unit.

Trimming energy costs high on many lists

Since only 1 to 2 percent heat efficiency is achieved on standard rotomolding equipment, cutting energy costs in the process is a top priority, notes Mark Kearns, molding research manager at Queen's University of Belfast (QUB).

One machine said to do this is the Logica 1850-3C from Polivinil Rotomachinery SpA (Cerano, Italy). The machine employs a heat exchanger, rather than an open flame, to heat molds-a change that can save processors up to 20 percent in energy, according to Chiara Sacchi, marketing manager. "The heat exchanger allows us to control the mix of gas and air very well, thereby saving energy as well as heating the mold more quickly," she explains. The design also allows for a reduction in floor space vs. machines with similarly sized molds and output capacity, enabling processors to realize more production per square meter. Since the prototype was first displayed K 2001, Polivinil has improved machine controls and strengthened arms so that heavier molds can be carried, Sacchi says. Eight have been sold.

Not yet commercial is the MECH (Mold Efficient Cooling & Heating) system being codeveloped by PPA Teo (Galway, Ireland) and QUB. P.J. Feerick, managing director at PPA Teo, says his firm is in talks with machine manufacturers regarding use of the system on new machines. MECH does away with the ovens and cooling stations used on standard rotomolding machinery by using direct, electrical mold heating at the mold surface. It is said to work with existing molds following some adjustments.

Kearns says MECH can increase heating efficiency to at least 20 to 25 percent, with MECH heating times comparable to those of standard units. He predicts broad commercial use may be a few years away, but expects the unit to appeal to custom rotomolders of complex parts as well as plastics processors who are not yet rotomolding but are eager to offer customers another option.

Automating materials handling is another cost cutter. Ferry Industries markets a semi-automatic powder dosing unit, called GraviMold, for conveying materials from storage and automatically dispensing them into molds. Operators indcate which molds are in use and the system dispenses the correct amount and type of material for each mold. A similar system using bar codes on molds, where operators read the bar codes and the information is transferred to dosing units, is marketed by Polivinil.

Bar codes also see use in a machine developed by Bonar Plastics and machine maker Rheinhart (Villingen-Schwenningen, Germany; also see November 1999 MP/MPI). The machine is now fully integrated into production, says Liehr, and is used for a broad mix of products. Bar codes on molds contain all necessary processing information such as temperature, time, and degree of rotation required, with information from the bar codes passed to heating and cooling chambers. Inline processing, rather than the use of rotating arms, saves space and integrates different-sized parts into a single processing stream.

QUB is widely considered the premier rotomolding research institute, and Kearns says the University has a number of projects nearing commercial status including K-Kontrol, a closed loop method to better control mold temperatures. A Canadian firm is now testing another development for commercialization. Rotospray is a polyethylene powder that is aerated, heated, and then sprayed onto rotomolded parts. The spray can be an alternative to flame spray welding; used to concentrate a material-such as an anti-microbial-on a specific section of a part; or used to improve a part's appearance. Also nearing commercialization is an internal mold cooling system codeveloped with Beko Technologies (Neuss, Germany). The system uses compressed cooling air.

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