PVC Conduit and Trunking Extrusion: Impact Strength and Fusion

The most under-designed profiles in the plant

Conduit, trunking, cable duct and skirting duct are the highest-volume PVC profiles made and the ones that get the least design attention. They are thin-walled, run fast, and are sold on price per metre. That combination produces a category of product where the difference between a good and a bad profile is invisible at the point of sale and obvious two winters later when a length shatters under a hammer blow.

The governing requirement is not dimensional tolerance — conduit tolerances are loose. It is impact strength at low temperature, and that is set almost entirely by formulation and fusion, not by the extrusion line settings.

The product family

Table 1 — The electrical PVC profile family. Impact at low temperature is the common thread.

All of these are made from the same base — rigid PVC, calcium-zinc stabilised, with an impact modifier — but the modifier loading varies enormously. A conduit for indoor use might carry 4 phr of CPE; a conduit for outdoor or cold-store use might carry 8 – 10 phr. The difference is invisible and it doubles the cost of the additive.

Impact: the property that decides everything

PVC is notch-sensitive and its impact strength falls sharply as temperature drops. The glass transition of rigid PVC is around 78 °C, so at room temperature the material is well below Tg and behaves in a brittle manner under impact. Impact modifiers — chlorinated polyethylene (CPE), acrylic (ACR) or MBS — work by dispersing rubber particles that cavitate and absorb energy.

Table 2 — Impact modifiers. MBS is the best performer and the worst for outdoor use.

MBS gives the best impact per phr of any modifier, which is why it dominates indoor applications. It is also a butadiene-styrene copolymer, and the butadiene double bonds oxidise in UV — so an MBS-modified profile used outdoors will embrittle within a few years and start failing the very test it was formulated to pass. For anything that sees sunlight, ACR or a CPE/ACR blend is the correct choice.

The second factor in impact strength is fusion. PVC is not a true melt in the extruder — it is a particulate gel, and the quality of that gel determines the mechanical properties. Under-fused PVC has visible particle boundaries, and those boundaries are where an impact crack starts. A profile with 10 phr of impact modifier and poor fusion will fail a low-temperature impact test that a profile with 5 phr and good fusion passes.

Fusion: how to measure it and how to control it

Table 3 — Fusion tests. The DCM test is the standard shop-floor method and takes 30 minutes.

Fusion is controlled by three things: melt temperature, residence time and the lubricant balance. A higher melt temperature fuses better but risks degradation; a longer residence time fuses better but limits output. The lubricant balance is where most formulations are tuned — external lubricants delay fusion, internal lubricants promote it, and the ratio determines where in the screw the gel forms.

The practical consequence for a high-output conduit line is a real tension: you want to run fast (short residence time, less fusion) and you want to pass the impact test (needs fusion). The resolution is usually a higher melt temperature with a very well-balanced stabiliser, plus a screw designed for PVC — which is why conduit lines often run 195 – 205 °C melt rather than the 185 – 190 °C used for window profiles.

Processing a thin-walled electrical profile

Table 4 — Processing windows. Conduit runs at speeds that window profiles never see.

The draw-down on conduit is modest but important. The die is cut oversize and the profile is pulled down to size, which orients the polymer along the axis. Some axial orientation is beneficial — it raises the hoop strength and the impact resistance — but too much makes the profile split longitudinally when it is bent. A draw-down of 1.05 – 1.20 is the usual band.

Cooling is the constraint on speed. A 2 mm conduit wall cools quickly, but at 15 m/min the profile is in the tank for only 12 – 20 seconds. That is enough for a 2 mm wall but not for a 3 mm one, which is why heavy-wall conduit runs slower and why the tank length is matched to the product range rather than to the fastest item.

Flame, glow wire and smoke

Table 5 — Flame requirements. Halogen-free is the one that usually forces a change of base material.

Standard rigid PVC is inherently self-extinguishing because of its 57 % chlorine content — it has an oxygen index of about 45 %, which is why it does not need much help. That is one of the reasons PVC dominates conduit. But "self-extinguishing" is not the same as "passes glow wire at 960 °C", and the FR package needed for the higher ratings costs 8 – 20 % more and usually reduces impact strength, which brings you back to the impact modifier balance.

Defects and causes

Table 6 — Electrical profile defects. "Brittle after storage" is the MBS weathering failure in slow motion.

What to actually inspect

Table 7 — Inspection plan. The impact and fusion tests are the two that predict field failure.

The impact test is the one worth insisting on, and it is cheap: a length of conduit conditioned at −5 °C for two hours and struck with a specified weight. It takes a freezer, a weight and ten minutes. Suppliers who do not do it are relying on the formulation being right, which is exactly the assumption that fails when a modifier supplier changes a grade.

Frequently asked questions