Corrugated Plastic Tubing Extrusion: Forming the Ribs Without Losing the Wall

An extrusion line with a forming machine bolted on

Corrugated plastic tubing — conduit, drainage pipe, cable duct, flexible hose — is often described as an extrusion product. It is more accurate to call it a hybrid: an extruder makes a tube, and a separate machine reshapes it before it cools.

The machine is a pair of travelling mould chains. Each chain carries a sequence of half-moulds with the corrugation profile cut into them. The chains run forward at the same speed as the tube, close around it, and a vacuum or air pressure pulls the still-molten tube out into the corrugation before the mould opens and the chains return.

The process step by step

Table 1 — The corrugated tube process. The forming stage is the one that distinguishes it from plain tube extrusion, and it is where most of the tooling cost sits.

Some lines run downward and some horizontally. Downward is more common for large diameter because gravity helps the tube stay round before it enters the moulds; horizontal is used for small diameter and for lines where floor height is limited.

Wall distribution: the defining problem

Blowing a tube into a corrugation stretches the wall. The crest of each corrugation is stretched more than the valley, so the wall is thinner at the crest. On a deep corrugation the crest wall can be 40–50% thinner than the valley wall.

Table 2 — Wall thinning at the corrugation crest. The nominal wall on the drawing is usually the valley wall; the crest is what fails.

This is why corrugated tube specifications often state a minimum wall at the crest rather than a nominal wall. A 1.0 mm nominal tube with a deep corrugation may have only 0.55 mm at the crest, and that is the number that determines crush resistance.

The wall distribution can be improved by controlling where the tube is hottest when it enters the mould. A slightly hotter tube forms more easily and distributes better, but too hot and it bursts or sticks to the mould. The window is typically 10–20 °C wide.

Vacuum forming versus pressure forming

Table 3 — Forming methods. Internal pressure is what makes deep corrugations and large diameters possible, and it is what makes the line harder to run.

Internal pressure forming requires a plug or a seal at the downstream end of the forming section, so the tube can be pressurised without the air escaping. The seal is a wear item and a common source of downtime. When it leaks, the corrugation depth becomes inconsistent and the operator usually blames the vacuum.

Mould blocks: the tooling that wears

Table 4 — Mould block components and their wear. The inserts are the consumable; the blocks themselves usually last the life of the machine.

Block alignment is critical and it degrades slowly. When the two chains drift out of register, the corrugation halves do not meet cleanly and a flash line appears along the tube. Operators compensate by increasing pressure, which accelerates wear. Checking alignment monthly is cheaper than replacing inserts.

Materials and what each one needs

Table 5 — Materials for corrugated tube. PVC's narrow processing window is the reason it is used less than PP for conduit despite being cheaper.

Rigid PVC is the traditional conduit material and it is the most difficult. The window between "too cold to form" and "hot enough to degrade" is only about 15 °C, and the material releases HCl if it overshoots. Most modern conduit is PP for exactly this reason.

Defects and causes

Table 6 — Corrugated tube defects. Incomplete corrugation and burst tube are opposite failures of the same window, which is why the temperature setting is so sensitive.

Pitch inconsistency is worth a note because it is often mistaken for a mould problem. The corrugation pitch is set by the mould block spacing and the chain speed. If the haul-off pulls faster than the chain runs, the tube is stretched between blocks and the pitch increases. The fix is to match the haul-off to the chain, not to change the moulds.