Extrusion

1
Corrugated pipes extrusion process

– Schematic illustrates shows the movement of moulding jaws in the corrugator

In the extrusion process for corrugated pipes, plastic granulate is heated in an extruder, melted through multiple heating stages, and finally forced under pressure through a forming die. The shaping and cooling are handled by the corrugator.

This consists of a chain of driven mold halves that form a closed loop and define the external shape of the pipe. The speed of the molds guarantees that the plastic strand is pulled through the system. The corrugated pipe is formed and cooled simultaneously inside the central channel of the corrugator, where the mold halves move in a closed formation. Forming is achieved using positive or negative pressure.

The process is designed for continuous material flow. Standardised mold halves move along guide tracks via one or more drives.

2
Smooth pipe extrusion process

– Schematic shows the smooth pipe production process using a vacuum tank

In smooth pipe extrusion, a vacuum tank replaces the corrugator. Directly after the extruder, there is a longitudinal tank. This is usually filled with water and divided into two chambers, both under adjustable vacuum pressure. The front, shorter chamber houses the calibration unit, which determines the final shape and diameter of the smooth pipe. Calibration uses draw-down plates, calibration discs or sleeves, depending on the application.

This unit, along with the die head, constitutes the shaping section of the process. The vacuum pressure pulls the soft plastic melt onto the calibration surfaces, shaping it into a round pipe. A special cooling inlet prevents the material from sticking to the calibration unit, and additional spray pipes around the unit ensure further cooling and prevent gas bubble formation.

3
Positive pressure extrusion

– with a sealing rod and plug

The positive pressure method involves pressing the molten plastic into the mold cavity using defined air pressure via a sealing rod. The rod is hollow and has cross-drilled holes through which air enters the pipe interior. It is screwed directly into the inner nozzle of the die head just before production starts. A plug or plug assembly is inserted at the end of the sealing rod to prevent air from escaping the system. This plug maintains internal pressure and ensures a tight seal. This allows the plastic melt to be pressed against the mold wall, forming the final shape of the pipe.

4
Negative pressure extrusion

– vacuum pump for negative pressure extrusion

The negative pressure method does not use a sealing rod. The mold halves have fine holes and slots. A dedicated vacuum pump creates a defined negative pressure, which acts on the plastic melt through these openings. The vacuum draws the melt directly into the mold contour and shapes it accordingly.

This method’s key advantage is that no additional sealing (such as plug assemblies) is required. This allows for the efficient production of pipes with varying diameters or shape transitions. It is clear that this results in more uniform wall thickness distribution from valley to crest, improving product quality.

5
Complete extrusion process

– with die centring (positive/negative pressure)

In the complete extrusion process with centering die tooling – as shown in the video – the homogeneous, molten plastic is conveyed from the extruder via a screw to the die head. The mandrel holder is where the melt is split into two streams to form a hollow body. The inner die is mounted on the mandrel holder and surrounded by the outer die. These components can be precisely aligned with each other using screws.

It is vital to centre the die to achieve uniform wall thickness in the final product, regardless of whether the forming process uses positive or negative pressure. Precise centering improves dimensional accuracy and product quality.