Machine Design

Plastics Keep Electronics Out of the Weather

Meese Orbitron Dunne Co.,

Equipment for utilities such as telecommunications, electric power, and water and wastewater management must withstand permanent, outdoor installation in demanding weather conditions. This has traditionally meant keeping sensitive electronics in metal boxes. However, exposure to solar radiation, heat, cold, and precipitation weakens metal. The enclosures eventually develop leaks and fail to protect the electronics inside.

To solve this problem, a telecom company talked with rotational molding and design firm Meese Orbitron Dunne Co. (MOD), Ashtabula, Ohio, about rotationally molding a plastic telecom cabinet. The company hoped to cut costs with enclosures that would need less maintenance and less-frequent replacement.

Traditional metal boxes use several parts joined with metal fasteners in several assembly steps. Metal exposed to the elements requires an extra step — labor-intensive application of a corrosion-resistant powder coat. The boxes last three to five years before they need a major overhaul.

MOD engineers used Solid- Works 3D solid-modeling package to test several design concepts. The final design, consisting of two rotationally molded polyethylene parts, measures 36 × 36 × 52 in., and weighs 300 lb less than metal cabinets.

To fabricate the boxes, technicians load plastic resin into several cast or fabricated molds in a rotomolding machine. The molds move into the machine’s oven where melting resin sticks to the molds’ hot walls. The rotomolder slowly spins the molds vertically and horizontally as the resin melts. The rotation ensures all mold surfaces get coated. After cooling, parts are removed from their molds.

The resulting double-wall parts are rigid and keep out water. Injection molding doesn’t permit double- wall construction, and its tooling is often more expensive because it must accommodate significant pressure while holding tight part tolerances.

Another common technique, blow molding, calls for additional manufacturing to create an internal core and adds the expense of matched-mold tolerances.

Skipping matched-mold tooling let MOD engineers quickly build molds for a prototype cabinet. Designers can quickly make minor changes to molds to accommodate material swaps, and some changes do not call for new tooling.

The engineers tested a prototype to ensure it could withstand exposure to the elements. To that end, they built a hurricane simulation chamber in MOD’s Madison, Ind., plant. The chamber holds products measuring up to 4 × 4 × 5 ft and replicates intense rain and the driving, 70-mph winds of hurricane that could spawn an F1 tornado.

Inside the chamber, a pump generates 5-in.-H2O of vacuum on the telecom box in an attempt to pull water through seals and gaskets. Meanwhile, rain simulated by 70-psi fresh water driven through spray nozzles pelts the boxes.

Although this product did not call for it, MOD engineers say objects of varying sizes could be placed in the hurricane chamber to simulate flying debris common in high winds. Engineers monitored the 5-min test with pressure and vacuum gages.

Switching to plastic enclosures let the telecom company accelerate production of boxes and cut lead times and manufacturing costs. Preassembling boxes is another advantage. MOD ships them to customers with electronics already installed.

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