Cable Trunking Extrusion: Wall Stiffness, Snap Lids and Fire Rating

What trunking has to do

Cable trunking is a hollow PVC profile with a removable lid, and it has four jobs that pull against each other. It has to contain and support cables over a span without sagging. It has to let the lid come off when an electrician needs to add a cable and stay on when nobody does. It has to resist fire long enough for occupants to leave. And it has to survive being installed, which means impact at whatever the site temperature happens to be.

The four requirements are in tension. Stiffness wants thick walls and mass. Fire performance wants additives that often reduce impact strength. A lid that stays on wants a deep undercut, and a lid that comes off wants a shallow one. And cost wants thin walls, which is where the argument usually starts.

The result is that trunking is a profile where the design is almost entirely a matter of getting the section right, because the process has very little freedom. A PVC trunking line runs one compound at one temperature; the only variables are the die, the calibration and the wall.

Stiffness is cubed in the wall

The second moment of area of a rectangular section goes with the cube of the depth, and for a hollow box the bending stiffness is dominated by the walls that are furthest from the neutral axis. In practice, for a trunking section, the stiffness of the base and the lid walls scales roughly with the cube of their thickness, so the relationship is steep.

Measured relative to a 1.0 mm wall, a 1.2 mm wall gives 1.7 times the stiffness, 1.4 mm gives 2.7, 1.6 mm gives 4.1 and 1.8 mm gives 5.8. Twenty percent more wall buys seventy percent more stiffness. Fifty percent more wall buys nearly six times as much.

That is why trunking walls are specified so precisely and why a supplier who offers to save material by thinning the wall is offering to change the product rather than the price. A 1.0 mm wall where 1.4 was specified does not lose 30 percent of the stiffness, it loses 63 percent, and the trunking sags between fixings on a warm day.

Table 1 — Wall thickness against stiffness and weight, indexed to a 1.0 mm wall.

There is a cheaper way to get stiffness than thickening the wall, and it is to change the section. A trunking with a shallow longitudinal rib in the base is significantly stiffer than a flat one at the same wall, because the rib moves material away from the neutral axis. Most modern trunking sections have some form of rib or return flange for exactly this reason, and it is invisible from the outside.

The snap lid is the hard part

The lid retention is a snap fit, and a snap fit is a cantilever beam that has to deflect enough to pass the undercut and then recover. The retention force is set by the undercut depth, the beam length, the material modulus and the friction at the sliding surface. For a typical PVC trunking lid, a 0.3 mm undercut gives about 12 N of retention, 0.5 gives 28, 0.7 gives 48, 0.9 gives 72 and 1.1 gives 88.

Twelve newtons is a lid that comes off when a cable is pulled past it. Eighty-eight newtons is a lid that an electrician will pry off with a screwdriver, which damages the profile and often the cable. The useful range is somewhere between 25 and 60 N, which corresponds to an undercut of 0.5 to 0.8 mm, and that is a narrow band to hold on an extrusion die.

The recovery is the part that fails. A snap fit that is deflected beyond the strain limit of the material does not spring back, it yields, and the lid then hangs loose. PVC is reasonably tolerant but it is strain-rate sensitive, and a lid that is snapped on at 5 °C behaves quite differently from one snapped on at 25 °C. This is why trunking is often specified with a minimum installation temperature.

Fire rating and the formulation

Trunking is almost always rigid PVC, and the fire performance comes from the chlorine in the polymer itself: PVC does not readily sustain combustion because the hydrogen chloride released at the flame front suppresses the radical chain reaction. That inherent behaviour is why PVC dominates the application, and it is also why the smoke is corrosive.

Adding plasticiser to make the compound easier to extrude or tougher reduces the fire performance, because the plasticiser burns. A rigid PVC trunking compound has little or no plasticiser; a flexible one may have 20 to 30 parts per hundred, and its fire performance is much worse. This is why the same supplier's rigid and flexible trunking are not interchangeable in a fire-rated installation.

Table 2 — The fire tests that appear on a trunking specification.

The halogen question is worth a sentence because it is a specification trap. PVC contains 57 percent chlorine by weight, so a halogen-free requirement rules PVC out entirely, and a halogen-free trunking is a different material, usually a filled polypropylene or a nylon blend, with different stiffness, different impact behaviour and a higher price. A specification that asks for both PVC and halogen-free is a specification that nobody can meet.

Die design for a hollow profile

Trunking is a hollow profile with an internal cavity, and that means the die has to support a core. There are two ways: a spider die, where the melt flows around slender legs that hold the core, and a bridge die, where the melt is split into channels and recombined. Both leave weld lines in the wall, and the weld line is where a trunking profile will split if it splits.

The spider is the more common choice for trunking because it leaves a smaller weld line and gives better wall thickness control. The trade-off is that the spider legs are in the flow path and can create dead spots where material degrades, which shows up as a dark streak on the inner wall after a long run.

Calibration is the other half. A trunking section has to be cooled evenly or the base and the lid walls shrink differently and the profile twists. Water tank vacuum, spray cooling and a sizing sleeve with a matched profile are all used, and the section that cools last is the one that sets the final dimensions. For a rectangular trunking, that is usually the corner where the base meets the side wall.

Impact at low temperature

The impact requirement is where most trunking failures happen in service, and it is almost always a low-temperature failure. PVC has a glass transition around 80 °C, well above ambient, so it is in the glassy state at all normal temperatures and it is notch-sensitive. A trunking dropped at 0 °C behaves very differently from one dropped at 25 °C.

The standard fix is impact modification, usually with an acrylic or a chlorinated polyethylene at 5 to 10 parts per hundred. That improves low-temperature impact substantially, and it costs a little stiffness and a little fire performance. The balance is the formulation work that a trunking compound supplier does, and it is why two compounds at the same hardness are not interchangeable.

For the processor, the practical point is that impact strength is very sensitive to the weld line and to the cooling. A profile that is cooled too fast has high internal stress and fails at a lower impact than the compound datasheet suggests. Annealing, or a slower cooling profile, recovers some of it, at the cost of line speed.

Specifying trunking

A trunking specification that a supplier can meet and a buyer can check has five elements, and the section drawing is only one of them.

1. Section drawing with wall thicknesses and tolerances, and the material grade including the impact modifier and the fire package.
2. Stiffness or deflection requirement, stated as a maximum sag over a stated span at a stated temperature.
3. Lid retention force, with a minimum and a maximum, and the number of open-close cycles it has to survive.
4. Fire performance against the specific test and class the installation requires, not a generic claim.
5. Impact requirement at the lowest installation temperature the site will see, not at 23 °C.

The lid retention range is the item most often left out and most often regretted. A supplier who is given a minimum only will make the undercut as deep as possible, because deep undercuts pass the test. The result is trunking that cannot be opened without damage, and a maintenance complaint two years after the installation is signed off.