Seamless, weather-resistant plastic housings: which process is best?

Outdoor equipment has a hard life. Control boxes, pump and blower housings, battery and charger enclosures, water-treatment casings and roadside cabinets all sit in rain, frost and sun for years, get knocked by vehicles and tools, and still have to keep water out of whatever is inside. Choosing the right manufacturing process is the single biggest decision in getting that right. Here's how the main options compare, and what to look for in a manufacturing partner.

Why rotational moulding suits outdoor housings

In rotational moulding, polyethylene powder is loaded into a hollow mould, heated to between 220 and 350°C while the mould rotates on two axes, then slowly cooled. The plastic coats the inside of the mould evenly and sets as a single, hollow part. For a housing that has to survive outdoors, that brings some real advantages:

• No seams to fail. The part is one continuous skin of plastic. There is no weld line, bonded joint or clipped-together pair of halves for water to find. (The mould opens to release the part, which can leave a faint cosmetic line, but it isn't a join; the wall underneath is continuous.)
• Stress-free walls. Because the plastic isn't forced in under pressure, the finished part has very little moulded-in stress, so it is far less prone to cracking with age, cold or impact.
• Thick, tough corners. Rotomoulding naturally builds material in outside corners, exactly where housings take the knocks.
• Built for the weather. UV-stabilised polyethylene doesn't rust, rot or corrode, stays tough in freezing conditions, and is coloured all the way through, so a scuff won't reveal a different colour underneath.
• Affordable tooling. Rotomoulding tools are typically 3 to 5 times cheaper than injection moulding tools, which makes large housings viable at volumes of tens, hundreds or a few thousand.

Rotomoulding vs thermoforming vs injection moulding

Rotomoulding isn't the only way to make a plastic housing. Here's an honest comparison of the three processes buyers most often weigh up:

When another process may suit you better

If you need hundreds of thousands of small, highly detailed housings, injection moulding will usually win on unit cost once the expensive tooling is paid for. If you only need a thin cover or a shallow tray, thermoforming can be cheaper still. But for a tough, sealed, one-piece enclosure that lives outdoors, especially anything bigger than a shoebox, rotational moulding is very hard to beat. If you're not sure, ask us: we'll tell you honestly if another process would serve you better.

Designing a housing that stays weatherproof

A seamless body is the starting point. These details make the difference between a good housing and a great one (our guide for product designers goes deeper):

• Generous radii and draft. Rounded corners and slight draft angles mould more evenly and release cleanly.
• Inserts where you need fixings. Threaded metal inserts can be moulded in or fitted afterwards, giving strong, repeatable mounting points without drilling into the wall on site.
• Openings cut after moulding. Cable entries, vents and access panels are CNC trimmed into the finished part, keeping the body seamless and every opening accurate.
• Proper sealing faces. Where a lid or door is unavoidable, design in a flat land for a gasket so the joint can be properly weather-sealed.
• Foam where it helps. Polyurethane foam filling adds insulation, flotation or extra stiffness to double-walled designs.
• The right material grade. Specify a UV-stabilised polyethylene, and choose the colour, density and melt-flow grade to match how and where the product will be used.

Moulded, fitted and assembled before it ships

A bare moulding is rarely the finished product. Most housings need hinges, latches, seals, brackets, inserts, valves, pumps, castors or electrical components before they can do their job. Many rotomoulders stop at the moulding and leave you to arrange that work elsewhere, which means extra suppliers, extra handling and extra risk.

At Revolution Rotational Mouldings (RevRoto) we handle the whole job under one roof at our facility in Kelly Bray, Cornwall:

• Tooling: new moulds designed and made for you, or your existing tools collected from your current supplier
• Moulding: multi-arm rotational moulding in your chosen colour and grade
• CNC trimming: precise cut-outs, openings and edges
• Foam filling: for insulation, flotation or structural reinforcement
• Assembly: fitting hardware and inserts, plus multi-component builds
• Finishing: hand finishing, painting, decals and branding
• Delivery: packed ready to install, and shipped direct to your customers if you need it

One supplier means one point of contact, one quality check on the finished product and fewer chances for parts to be damaged between workshops. It's why “assembly experts” is as much a part of what we do as moulding.

Precision-moulded jerry can housings for Icon Lifesaver's water purification systems.

Housings we've made for demanding environments

What to ask a rotomoulder before you commit

Whoever you choose, these questions will quickly show whether they can deliver a finished housing or just a moulding:

• Do you make tooling in-house, and can you take over my existing tools?
• Which UV-stabilised grades do you recommend for my application?
• Can you mould in or fit threaded inserts, and CNC trim openings after moulding?
• Will you fit hardware, seals and components, and test the assembled product?
• Can you pack and ship finished products direct to my customers?
• What are your lead times for tooling (typically 6 to 12 weeks) and for repeat production?

Frequently asked questions

Have a housing or enclosure in mind? Get in touch with a sketch, a CAD file or your existing tooling, and we'll help you find the best way to make it.