An extruder is a heated steel barrel, a screw or ram that pushes plastic along it, and a die that shapes the melt, and the barrel’s size is set by the force you can supply: a ram in a 50 mm bore at an assumed 10 MPa needs about 19.6 kN. A hand-driven screw with a 5 mm pitch and 200 mm lever can supply that for roughly 156-391 N at the handle once friction is allowed for, so a batch ram extruder is a realistic first machine.
Part of Plastics and Polymers
Extruder design covers the barrel, the pushing element and the die of a machine that melts thermoplastic and forces it through a profile.
Ram Extruders Are Simpler; Screw Extruders Run Continuously
Plastics Manufacturing describes the barrel as heavy steel tube, 30-60 cm long with a 5-10 cm inner diameter, and says a screw is ideal because it feeds and pressurises continuously, while a ram works for batch production.
- Ram (piston) extruder. Load a charge, push it out, stop, reload. Easy to build; each stroke yields a limited length of product, and you must stop to refill.
- Screw extruder. A rotating screw conveys, melts by friction and heat, and pressurises. It runs without stopping but needs a precisely machined screw fitting close inside the barrel, a drive with real torque, and thrust bearings behind it.
Industrial screws change their depth along the length so that the plastic is squeezed and air is driven back out of the feed. This is from general knowledge, not from the corpus, and a hand-built first machine can leave it out.
The Die Sets Back-Pressure And Shape
A die is a plate bolted to the barrel end. A solid rod uses a round hole. A tube uses a hole with a centre pin (mandrel) held on spider legs, and the gap between pin and hole is the wall thickness. After the die the melt swells slightly and then shrinks as it cools, so the die opening is not the final size; tune the opening on test pieces.
Worked Example: Force And Volume For A Ram Extruder
All inputs are assumptions you should replace with measurement.
- Bore 50 mm, so the radius is 0.025 m and the area is π × 0.025² = 1.9635 × 10⁻³ m².
- Assume a melt pressure of 10 MPa (the same assumption that Rubber Extrusion makes; real pressure depends on the die and plastic). Force = 10 × 10⁶ Pa × 1.9635 × 10⁻³ m² = 19,635 N, about 19.6 kN.
- Drive it with a screw of 5 mm pitch and 200 mm lever radius. Screw Mechanisms gives an ideal advantage of 2π × 200 / 5 = 251.
- Ideal handle force = 19,635 / 251 = 78 N. With 20-50% efficiency the handle force is 78 / 0.5 = 156 N to 78 / 0.2 = 391 N.
- Barrel volume for a 30 cm stroke: 19.63 cm² × 30 cm = 589 cm³.
- Pipe of 25 mm outer diameter and 2.5 mm wall has an area of π/4 × (25² - 20²) = 176.7 mm², which is 1.767 cm². A full barrel of melt gives at most 589 / 1.767 = 333 cm, about 3.3 m. Shredded feed contains air gaps, so the real length is shorter.
If the die is a long, thin passage, pressure rises well above 10 MPa. Build the barrel wall and the die mounting with margin, and fit a way to relieve pressure.
Heating And Temperature Measurement
Heat the barrel evenly from outside and measure the temperature at the barrel wall near the die. The processing ranges in the carrier (polyethylene 120-180 °C, polypropylene 160-220 °C) are wide, so find the lowest temperature at which the melt flows smoothly. The carrier warns that degradation begins 30-50 °C above the working temperature. Hot spots at the barrel are the usual cause.
Common Mistakes
- Barrel wall too thin for the pressure — it swells or splits at the die end.
- Unsupported mandrel — a pin that moves off-centre makes uneven pipe wall.
- Dead spots in the barrel — plastic that sits too long overheats and chars, then breaks off into the product.
Pressure, heat and blowback
A blocked die raises the pressure at once. Never stand in line with the feed port or die when a run starts, never open a die while the barrel is hot and loaded, and fit a relief path. Molten plastic sticks to skin and burns deeply. See Fume Safety for what overheated plastic gives off.
What’s Next
- Learn next: Extrusion Products — what the die can make
- Combine with: Remelting — feeding the extruder with sorted salvage