Extrusion makes continuous plastic products such as pipe, tube, rod, sheet and film by pushing molten thermoplastic through a shaped opening called a die and cooling it as it emerges. The most valuable product for a settlement is water pipe, but the sensible first target is solid rod, because pipe needs a centred mandrel and rod lets you master temperature, feed rate and cooling first.
Part of Plastics and Polymers
Extrusion is the forming of thermoplastics by forcing a melt through a die of constant cross-section to produce a continuous profile.
The Die Decides The Shape; Everything Else Serves It
A basic extruder has three parts: a heated barrel in which the plastic melts, a screw or piston that pushes it forward, and a die at the end whose opening gives the product its cross-section. A round hole makes rod, a slit makes sheet or film, and a ring around a centre pin makes tube. Detail is in Extruder Design.
Only thermoplastics can be extruded, because only they melt and set again. A thermoset cures and cannot be pushed through a die.
The Run Follows Six Steps
- Prepare the feed. Shred or granulate the plastic to small pieces; Plastics Manufacturing gives 5-10 mm.
- Feed the barrel.
- Heat to the plastic’s working range. The carrier lists polyethylene at 120-180 °C, polypropylene at 160-220 °C, PVC at 160-200 °C and polystyrene at 180-260 °C. Treat these as processing ranges, start at the low end and raise only as needed.
- Push. The melt leaves through the die.
- Cool. Air or a water bath fixes the shape.
- Cut to length.
What You Need
- A plastic identified by type — see Sorting and Identification; a mixed feed gives weak, streaky product.
- A barrel and screw or ram that can withstand the push; Screw Mechanisms gives the mechanical advantage.
- A temperature measurement. Without one you are guessing at the melt.
- A cooling bath and a puller that draws the product away at a steady rate.
Start With Rod, Then Move To Pipe
A solid rod needs only a round hole. Practise until temperature, feed rate and cooling stay steady and the rod comes out uniform. Pipe adds a centre pin held in the die; the gap between pin and die wall sets the wall thickness, and a pin that drifts off-centre gives a wall thick on one side and thin on the other. For uses of the finished pipe see Extrusion Products and Pipes and Containers.
Common Mistakes
- Starting with pipe — an off-centre pin ruins every metre.
- Overheating to make it flow — degradation begins 30-50 °C above the working temperature, according to the carrier, and the fume is the most toxic then.
- Pulling unevenly — the wall thickness varies with the pull rate.
Hot melt and pressure
Molten plastic sticks to skin and keeps burning it, and the die end of an extruder is under pressure. Never stand in line with the die when starting, never clear a blockage with the heat on, and keep the feed port covered. Do not extrude an unidentified plastic: some give off corrosive or poisonous gas when heated. See Fume Safety.
What’s Next
- Learn next: Extruder Design — barrel, screw and die in detail
- Combine with: Injection Molding — the process for complex shapes