July 14, 2022

Rotational Molding Roof for Buildings

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I was deeply involved with the rotationally molded roof for the world-famous JCB backhoe loader. About roofing for buildings, there is no reason why rotational molding cannot be used. The hollow construction would provide impressive stiffness, especially if the modules were designed with regular “kiss-offs”. Great attention would need to be given to detail design of fixing points, to allow expansion/contraction with temperature changes and in specifying the material compound, including adequate UV protection for the required duration and geographic location.

Implicit in above opinions about expansion/contraction was the need to factor in fixing details that take account of differences in CLTE of plastics and other materials to which they are affixed. Specifically, one can consider one reasonably fixed anchor point, but any others need must provide clearance and low-clamping-force fixing. In this respect, look at STEMS fasteners, with captive, shouldered washers. There are many examples in the “real world” of both bad and good practice in fixing plastics to other materials. I’ve seen lots of both and even (literally!) picked up the pieces where designers got it wrong.

Technically, there’s no doubt that roof structures — large, small and modular (small units to cover larger areas — CAN benefit from a twin-wall construction afforded by rotational molding, as the hugely increased flexural and torsional stiffness is a very welcome property for this application. However, as with any application, the designers, developers and funders must satisfy themselves that the financial figures also stack up when considering process, material(s), numbers, operating environment, building regulations (not least fire performance), affordability by the purchaser, cost of ownership over its lifetime and design options.

Rotational molding process is unlimited in end product applications, whatever the roof size or shape make sure you study the environment in detail in which you plan to place it and design it accordingly based on all the controlling elements and the materials available to you in the rotational molding process, limited materials are available vs. the other plastic processes. Always know all of your options, cost, and market demands before choosing you end product process. Keep in mind if the rotational molding process does not fit for the long haul sometimes it will fit for the short run, i.e. (R/D phase, minimum capital on the front end, and allows for initial market studies).

Material choice is limited in rotational molding. Also, although tooling is relatively inexpensive, part costs are relatively high. Blow-molding also gives the twin-wall structural benefits, with a greater choice of materials. Tooling costs higher; part costs lower; size range smaller. For instance, because of market size differences, roof structures on children’s play houses tend to be rotationally molded in the UK companies and blow molded in the USA companies. There is obviously a world of difference between the requirements for a children’s toy roof, a roof for an off-road construction vehicle, a domestic roof and a factory roof.

Twin-wall thermoforming is yet another option for achieving a hollow panel structure that could be part or all of a roof structure. All the hollow panel options can be internally foamed, to further improve stiffness, strength, insulation properties — but with further cost penalties.

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