Canadian Plastics

Giving rotational molding a spin

M easured by resin volumes alone, I/M and extrusion are the major processes in the Canadian market -- but they're far from the only way to mold resin parts.

January 1, 2008   Canadian Plastics



Measured by resin volumes alone, I/M and extrusion are the major processes in the Canadian market — but they’re far from the only way to mold resin parts.

Consider one of the oldest and most versatile processing techniques: rotational molding. “Rotomolding” isn’t a new technology, but what it lacks in glamour it makes up in flexibility. The principle is simple: charge a closed mold with resin, heat the mold, spin and tumble to coat the mold’s inside surface evenly, then cool. Compared to injection molding, it’s stunningly simple, but for many applications rotomolding is the only game in town. It’s great for large, hollow parts, such as tanks or bins, and delivers consistent wall thickness and excellent strength at corners and edge discontinuities in the mold. Big hollow parts are the most common application, but multi cavity molds for solid parts are also possible.

Other advantages? Molds don’t need to withstand high pressure, and complex ejection mechanisms aren’t neces- sary, so tooling costs can be low enough to justify small production runs. Equipment is relatively simple, reliable and maintainable with less training and experience than I/M processes molding similar parts. Resins, additives and mold release agents are readily available, and as a mature technology, there’s a large knowledge base to draw from.

But rotomolding also has its limitations. The equipment is large and cycle times are slow by I/M standards. The centrifugal forces that spin the resin into the mold cavity generate limited pressure, so intricate shapes aren’t always possible. The large swing and floor area requires significant safety infrastructure and the large mold surface area takes plenty of energy to heat.

So why get into rotomolding? Because for an I/M shop, it’s a great way to diversify and add value to your operation, offering the ability to grab smaller jobs and run them profitably, plus use a mold technology that’s not easy for your customer to “shop around.”

Check out www.rotomolding.orgfor more about rotational molding.

———Jim’s Tip of the Month:

Air filtration for automation equipment

Most shop floor automation is still pneumatic, and every manufacturer states what level of air cleanliness their equipment requires. The filtration needed to get that clean air is specified by the size of particles it traps, usually expressed in microns. While standard airline filters trap particulates, they allow gases to pass, including potentially corrosive or harmful gases. Filtration doesn’t replace condensation or desiccation as drying strategies…if your air is wet and you can’t dry it at the compressor, consider a desiccant unit on a dedicated “dry line” providing high-purity air for delicate machines. Don’t manifold the line excessively or provide extra outlets or else someone will plug a blow gun into this source of “expensive” air. I like to place a coarse filter just downstream of desiccant units; I’ve seen the drying pellets break up and blow through an entire circuit. The coarse filter gives you a window on that possibility with minimal pressure drop.

When do you change the filter? You can use your “book” or maintenance software, but I like to steal a “dry erase” marker from the front office and write the change date on the clear filter housing. And remember: mark the next change date, not the change date just passed.


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