Metal or Plastic? The One Support Leg Joint That Decides How an Awning Ages

An awning has one connection that works every time it opens or closes: the joint where the support leg meets the front bar. Nothing else on the awning rotates as often, or carries load while it rotates.

Before giving the answer, it is worth setting out the questions. Every rotating joint has to have a gap in it, or it would not turn, so what happens to that gap over the years? Powder coating is what protects aluminium, so what happens when it is put on a surface that turns against another surface? Nylon is the stronger material on a datasheet, so why is it the wrong one in this position? And why would a part that no customer will ever see carry the name of a Japanese material brand?

This is the first of a series on the parts an awning is actually judged by.

Where the Joint Sits and What It Does

The part is the rotating connection between the support leg and the front bar, and two things about its position matter. When the awning is closed, it is fully enclosed inside the housing, out of sight and out of the sun — the state the vehicle spends most of its life in. When the awning is open, it sits directly beneath the fabric, with the aluminium housing of the front bar in front of it.

Very little direct sunlight reaches it in either state. That single fact rules out most of what people assume about plastic parts on a vehicle, and it is the reason the discussion below is about wear, heat and moisture rather than about weathering.

Why This Joint Is Plastic and Not Metal

A rotating joint has to have clearance, because without clearance it does not rotate. That is mechanics rather than a question of machining accuracy, so the real question is not how tight the fit is but what material sits on each side of that gap.

If this part were made from coated aluminium, two things would follow. The coating would wear through, because powder coating is a corrosion and appearance finish rather than a wear surface; on a face that rotates against another face it is removed a little at a time until the aluminium underneath is exposed. The thickness of that coating is itself part of the fit, so once it is gone the clearance grows and the joint loosens.

The second consequence is noise. Metal against metal, across a gap that has to exist, means two metal faces knocking together every time the vehicle meets a bump. This is the most common complaint about rotating mechanisms on vehicles, and it is the one an end user notices first.

An engineering plastic in that position removes both. Its wear against the mating steel part is close to zero, its friction coefficient stays stable over time, and it has damping, so it absorbs road vibration rather than transmitting it, the same principle that puts plastic in car door hinges and seat adjusters. In this position plastic is not a downgrade from metal; it is the more reliable of the two. Which plastic, though, is the whole question — and it is really two questions: which one works, and which one is still working in five years.

The Difference Between Plastic and Engineering Plastic

What most people mean by “plastic” is ABS and materials like it, the family used for housings, panels and trim. POM, also called polyoxymethylene or acetal, belongs to a different class. Its principal industrial use is exactly this kind of work: gears, bearings, slides and rotating parts that carry load while they move.

ABS is not designed for a loaded part that rotates repeatedly, and in this position wear and deformation would appear quickly. POM costs considerably more than ABS as a raw material, and that cost is what the function of this part requires.

What Ageing Actually Does to This Material

Plastic parts have a reputation for going brittle with age, and it is a fair thing to ask about rather than dismiss. The most common route to brittleness, however, does not exist here. Plastics that harden and crack with age usually do so because they lose plasticiser — many materials get their flexibility from an additive that evaporates or migrates out over time, leaving the material hard and prone to cracking, which is what has happened to the perished seals and brittle PVC pipework familiar from older vehicles.

POM is inherently tough and contains no plasticiser. Its toughness comes from its molecular structure rather than from an additive that can leave, so that ageing route simply does not apply. Three further properties matter over the years: it is a high-crystallinity engineering plastic, so its mechanical properties stay stable without drift; its water absorption is very low, so nothing about it cycles with the weather; and it is self-lubricating, so there is no applied lubricant whose loss would cause performance to decline.

The automotive industry has been testing this for decades: POM sits in seat-belt retractors, window regulators and seat adjustment mechanisms, where the service life is expected to match the vehicle’s. If this class of material went brittle in a few years it would not be there. Salt-laden coastal air does not attack it either, and acetal copolymer resists the agents an awning is washed with.

Why Not Nylon, Which Is Stronger on Paper

That leaves one serious alternative, and it is the one most often proposed. Nylon is an engineering plastic too, and on a dry datasheet it beats acetal on tensile strength, on melting point and on cost. The word carrying the weight there is dry. Every one of those figures is measured on a bar that has just left the mould, and no part in service stays in that condition.

Nylon is hygroscopic: once it is fitted it draws moisture out of the air continuously and reaches equilibrium over weeks to months. Acetal absorbs almost none, so its published numbers are also its working numbers.

Typical published values for unfilled grades; individual grades vary.

Three things follow, and they are worth knowing. A conditioned nylon part has given up more than half of the modulus it was specified on, and the bracket at this joint carries bending load — it is ribbed for exactly that reason. Its friction against steel drifts with humidity, which is the difference between an awning that opens the same way every time and one that is heavier in August than it was in March. And its famous toughness turns out to be borrowed: nylon is tougher than acetal only while the part is new, has taken up moisture and is at moderate temperature, which is never the condition a complaint arrives in.

None of those three is why we ruled nylon out. Every one of them reverses: dry the part and it comes back. Two things follow that do not reverse so easily, and the first of them is a problem before the awning is ever used.

The Part Is Finished by Wherever It Ends Up

Nylon has to be dried before it can be moulded, so a nylon part leaves the factory dry. Everything measured at final inspection is measured in that state — the one state the part will never be in again.

From the moment it is packed it takes up moisture from whatever air it is in, and that air is not ours. The same part number goes to a heated flat through a northern winter, to a coastal summer in Japan or Queensland, and to everything between — and each brings it to a different equilibrium, and so to a different stiffness, toughness and feel. Months after it left us, with nothing having gone wrong.

For a manufacturer that is the uncomfortable part: a nylon component cannot be fully validated by the factory that makes it, because half of its working condition is set by the customer’s climate. Test it here and you have tested one case out of many, and for a distributor the complaint rate acquires a geography. Acetal takes up almost nothing, so the part that passes final inspection is the part the customer has, in every market we ship to.

All of that is still consistency, and consistency can be designed around. What comes next cannot.

The One Ageing Route That Does Not Reverse

This is the reason, and the rest of the comparison is secondary to it.

Nylon’s molecular backbone is held together by amide bonds, and amide bonds are broken by heat and water acting together. This is not the take-up of moisture described above. That is physical, and it reverses. This is chemical: the chains are cut, the molecular weight falls, and nothing puts them back. A part that has been through it is permanently less tough than the part that was fitted, whatever the weather does afterwards and however dry the season.

A closed housing in summer supplies every condition the reaction needs, and supplies them together — heat accumulating in a sealed cavity, condensation forming as it cools overnight, and very little ventilation to carry the moisture away. It is close to a description of the environment you would build on purpose if your intention were to age a polyamide.

Acetal’s backbone is —CH₂—O—. There is no hydrolysable bond anywhere in it, so the route does not exist for the material — not slowly, not eventually, not in a hot climate.

It is worth being plain about how much weight this one carries. Stiffness, friction and seasonal toughness are arguments about how a material performs, and a competent designer can work around any of them: add a rib, open a clearance, specify a different grade. Hydrolysis is not an argument about performance. It is an argument about whether the part you approved is still that part in five years, and there is no way to design around a bond that is not there to begin with.

A Part Nobody Sees, Specified as if They Did

Within acetal there was still a choice to make. It comes in two forms, homopolymer and copolymer, and the homopolymer is roughly ten percent stronger. It is also weaker against alkalis, against hot water, under sustained heat — and against exactly the hydrolysis described above.

An awning is washed with water and cleaning agents, and the inside of the housing accumulates heat in summer. Those are precisely the conditions under which the homopolymer gives trouble and the copolymer does not, so we accepted a small concession in strength for the stability that matters over years of use.

The grade itself is DURACON®, an acetal copolymer made by Daicel Corporation of Japan — in production since 1964, and one of the longest-established acetal brands in the world, in the same tier as Celanese and BASF. It sits considerably above general-purpose domestic POM in cost, and far above ABS.

This is an internal part. A customer will never see it, and no specification sheet in this industry lists it. Using a first-tier Japanese engineering plastic in a position like that is the clearest statement we can make about where we do not save money.

Material Choices Must Serve the Position

A rotating joint, a load-bearing profile and a fabric exposed to weather each answer a different question, and each deserves to be decided on its own terms.

That principle cuts both ways, and it is worth saying where it goes against acetal. In direct sun the ranking reverses: acetal degrades by an unzipping mechanism that a stabiliser can delay but cannot interrupt once it has started, while nylon’s photo-oxidation can be intercepted effectively. For the hardware that sits out in the open — tie-down buckles, ground pegs, fabric clamps — the right material is black UV-stabilised polyamide, and acetal is the wrong one. The same product line uses both, on purpose.

The joint described here is the part of the awning that works hardest and is seen least, which is exactly why it is worth explaining.