Rotomolding is still considered the simplest of the primary plastics processing methods, but it has made steady gains in sophistication over the past few years. These advances include the advent of process simulation, use of CAD/CAM technology, real-time process monitoring, pressurized molding, quick-change tooling, automated mold-filling, and “ovenless” internally heated systems, to name a few. With these new capabilities, rotomolders are pursuing more ambitious applications, from dump-truck bodies to aircraft ducts.
The latest developments in machinery, materials, and applications were showcased at the recent 25th Annual Spring Conference of the Association of Rotational Molders (ARM) in San Diego, Calif. Molders were introduced to two new ovenless molding technologies that offer reduced machine footprint, energy consumption, and materials usage. Another new mold technology can impart features to a hollow product that were nearly impossible with conventional tooling—solid ribs, flanges, mounting tabs, and handles, as well as living hinges, internal reservoirs, undercuts, or overlapping sections. New modular machines are designed on a simple build-it-yourself principle. Two other lines of machines were unveiled, including one from a brand-new supplier.
On the materials side, the ARM show unveiled the first semiconductive polyethylene resins for rotomolding and a new one-shot foam system. Rotomolding experts at the show also showed part samples and test data illustrating the possibilities of rotomolding high-performance materials, such as long-glass reinforced PE, fluoropolymers (ECTFE, PTFE, PVDF, PFA), polycarbonate, cyclic olefin copolymers, and metallocene resins.
‘Oven inside the tool’
Mold-Efficient Cooling and Heating, or MECH, is the new direct mold-heating and cooling method for rotomolding developed by Plastic Processing Alternatives (PPA) of Ireland. “Typically, the rotomolding process wastes over 90% of its energy input for heating and cooling, while the manufactured product typically requires 20% more material [to avoid thin corners in parts]. And rotomolders still experience high reject rates,” says P.J. Feerick, managing director of PPA. The MECH system replaces conventional oven and cooling chambers by incorporating those functions in the mold itself. “Our system applies electrical heating directly to the location on the mold where it is required, while cooling channels constructed on top of the heating system allow a high volume of conditioned air to be blown over the mold for rapid cooling,” says Feerick.
This ovenless configuration reportedly produces a host of benefits, including energy cost reductions of as much as 80% and ability to use real-time process-monitoring without fear of overheating sophisticated electronics. The MECH system is offered as a turnkey package of machinery and molds, or it can be retrofitted on existing equipment.
The basic mold is a thin shell (0.0015 to 0.004 in.) of sheet steel, cast aluminum, or electroformed nickel. Customized electric heating panels are cemented to the outside of the shell with a heat-transfer paste. Then an outer insulation panel with air-cooling channels is fitted over the heating panel to cover the mold. An external temperature controller monitors the electrical panels. The user can vary the output of each heater panel separately in order to optimize the heating profile of the tool. “This provides energy efficiency that is always better than a conventional system,” says Feerick. “That results in at least 50-60% energy savings.” What’s more, the zoned heating capability allows a molder to vary wall thickness repeatably and accurately to make more efficient use of materials.
The mold is cooled by a compressed-air system connected to a series of venturi air movers. The latter blow a high volume of conditioned air over the entire outer surface of the mold.
Heating and cooling inside the tool is featured in a new machine from Konan Tokushu Sango (KTX) Co., Ltd. in Japan. It eliminates the need for an oven yet maintains process temperatures high enough to mold resins such as polycarbonate and polypropylene. KTX’s new Gyro Space Dual-Axis rotomolding unit uses hot and cool oil supplied to a jacketed tool from a mold-temperature controller. It can supply oil at 104 F to 608 F. A PLC touchscreen panel regulates oil flow and temperature. One oil unit can supply two clamshell molds, one being heated while the other is being cooled.
KTX developed a method by which the oil piping is plated rather than welded on the back of the nickel-shell mold. Welding would cause temperature stresses, warpage, and shrinkage in the nickel shell, explains North American sales agent Jeffrey Shimizu. The plating method reportedly improves thermal conductivity, strength, and thermal distribution.
The Gyro Space unit rotates on two axes using two electric motors of 2.9 and 2 hp. Rotation speeds up to 70 rpm are possible, depending on mold weight. Swing-arm diameter is 1200 mm. Without an oven or cooling chamber, the machine occupies only 25 sq meters.
The largest mold yet built for the KTX system was for a 23-in.-diam. octagonal polycarbonate terrarium. Tools cost about $50,000 apiece, while the machine with oil unit and controls is priced around $350,000.
Articulated molds debut
Spring-loaded mold sections now enable rotomolders to add ribs, hinges, flanges and other part features normally associated with injection molding. The new Transfer Rotational Injection Process (TRIP) from SJS Industries adds articulating sections to a machined or cast aluminum mold. When actuated manually with a trip rod, these moving mold sections push or compress melted material inside the tool. TRIP technology reportedly can eliminate metal inserts or foam used for stiffening and can create parts with 20% greater usable volume. It produces smooth part interiors without hang-up or bridging inside the tool. It can be used with conventional machines and does not lengthen part cycle time, says SJS president Don Schraegle.
The Molding Co., a custom rotomolder in Farmington, Mo., last month was awarded the first license to use the TRIP technology.
Modular sizes and more
Molders can build their own shuttle, swing, or carousel-style machine in sizes from 43 to 236 in. with a new modular construction concept from Caccia Engineering SpA of Italy, represented here by E.A.P. Machinery Systems. The Modulo series offers lower cost and faster delivery, owing to standard components designed for a broad range of machine types and sizes. Caccia is applying this modular concept to an independent-carriage rotary unit with 118-in. swing diameter. The entire system is expected to take four to six weeks to build, about the same time as for a new mold.
The Modulo series uses tubular steel, manufactured in a range of lengths that can be joined together to form the frame of the heating and the cooling chamber in any size desired. Mold carriages are also modular and built of common sectional parts. Carriages come in a few sizes, but all use the same new power-transmission system. One combustion chamber is designed for use throughout the machine product line. The entire machine, including the combustion chamber, is said to be easily assembled by the molder. Self-assembly can drastically cut start-up costs, says Caccia v.p. Mario Monti. Caccia keeps a stock of the standard tubes, panels, joints, carriages, and arms.
Meanwhile, a new generation of shuttle machines adaptable to a range of rotomolding techniques is in development at STP Equipment. The new line will have a maximum swing diameter range of 80 to 220 in. One new feature will be forced-air convection in place of radiant heating elements, a change that reportedly will allow greater flexibility in machine sizing and could reduce cycle times by 25% due to higher heat-transmission efficiency, faster response, and greater reliability. The unit will also boast simpler controls, although more advanced process monitoring will be optional.
A supplier of custom-built ovens for rotomolders recently unveiled its own line of custom biaxial shuttle and rock-and-roll molding machines. NW Rotofab offers independent, turret, and shuttle-arm machines with five process stations (oven, pre-cool, water-cooling, load/unload, and staging). Standard features include air-equipped arms on all models, forward and reverse indexing of all arms, and adjustable AC frequency drives for major and minor spin axes. The latter feature permits molding split-colored or marbled-parts. Rock-and-roll ovens have hydraulic door actuation and separate hydraulics for mold clamping and rocking the oven.Other features are a gas-burner system and a cooling booth with high-volume fans and water mist. The machine-mounted or stand-alone PLC control allows presetting of all process parameters and directs the operator through the process sequence.
News in materials
Wedco/ICO Polymers rolled out three of what are said to be the first semiconductive polyethylenes intended specifically for rotomolding. Icorene C501, C503, and C517 are all black, 500-micron powders said to exhibit permanent conductivity, low warpage, and a high level of uv stability. Intended uses include antistatic bins, industrial containers, and high-velocity air nozzles. Their surface resisitivity runs 300-500 ohm/sq.
C501 is an ethylene-octene copolymer with 0.935 g/cc density and 4.0 MI. C503 is an ethylene-butene copolymer with 0.924 density and 5.0 MI. C517 is also a butene copolymer and has 0.934 density and 6.0 MI.
Wedco/ICO also rolled out Icorene KCEL100, a new one-shot linear MDPE foam compound with a 0.940 base density and 0.20 expanded density. Its MI is 3.5. The uv-stabilized material produces open cells with average size of 2 mm (at 0.20 density). The product is intended to be used with an appropriate skin material.
Officials of Chroma Corp.’s Rotational Molding Div. encourage molders to consider non-traditional, higher-performance material options. For example, custom molders can manufacture parts having reduced shrinkage, lower stresses, and improved strength and stiffness by using long-glass-filled PE resins, says Stuart Lipsteuer, director of sales and marketing. He presented data illustrating the higher performance properties of LLDPE filled with 30% by weight of 1.5-in. glass rovings. Shrinkage dropped from 0.015-0.022 in./in. for natural LLDPE to 0.002-0.006 in./in. Tensile strength rose from 1200-4000 psi to 7500-9000 psi. HDT improved from 104-122 F to 260-265 F at 66 psi. On the other hand, elongation dropped from 100-955% for natural to 1.5-2.5% with glass.Lipsteuer says Chroma is willing to enter into concurrent-engineering projects involving further study of glass-filled materials for specialty rotomolding applications requiring enhanced structural properties and dimensional stability. Chroma Corp. chairman Bob Swain proposed that rotomolders evaluate the potential of a brand-new engineering material family, cyclic olefin copolymers (COCs). These materials are water-clear, very stiff, and highly heat resistant (176-356 F HDT). Chroma has molded sample parts using Topas COCs from Ticona.Although melt-processable fluoropolymers have long been rotocast into linings for chemical-process piping and valves, Ravi Mehra of Norstar Aluminum Molds suggests that new applications are waiting in the wings. In particular, he said that packaging for electronic components could benefit from the high purity of materials such as PFA and ECTFE and could justify their high cost ($22-30/lb).