Rubber extrusion forms tubing, cord and strip by forcing warm, uncured compound through a die with a ram or screw, after which the soft profile is supported and cured. The ram force is the pressure in the barrel times the barrel’s cross-section, so a barrel with a 50 mm bore at an assumed 10 MPa needs about 20 kN, which is a job for a hydraulic ram or a screw press rather than an arm. Volume is conserved, so a 392 cm³ load of compound makes about 2.4 m of a tube with a cross-section of 1.6 cm².
Part of Rubber Processing
Pushing rubber compound through shaped dies for tubing and profiles, and what the push demands.
Extrusion Trades A Strong Push For A Continuous Length
The carrier names extrusion for tubing and profiles. The corpus gives no extruder design, no barrel temperature, no pressure and no die dimensions, so this article states the mechanics that do not depend on them and marks every number that is assumed.
Where an extruder cannot be built, the corpus’s Hoses and Tubing and Rubber Products articles give two substitutes: build a hose up from dipped coats on a mandrel, or wrap rubber strips round a mandrel and bond them with rubber cement.
What You Need
- A barrel, a strong tube closed at one end by a die plate and open at the other for a piston.
- A piston (ram) and a means of driving it: a screw press, a lever with a large mechanical advantage, or a hydraulic ram; see Hydraulic Systems.
- A die whose hole is the outer profile, and for a hollow tube a centre pin held in the stream.
- Warm compound, from Compound Mixing, softened enough to flow but not enough to cure.
- A way to support and cure the profile: Curing Temperature and Time.
Method: Extruding And Curing A Tube
Step 1: Warm the compound by milling; keep it below cure temperature.
Step 2: Load the barrel and close the ram.
Step 3: Push steadily. The profile emerges through the die.
Step 4: Support the profile on a talc-dusted bed or over a mandrel, since it is soft.
Step 5: Cure it. The rubber-products article boils a wrapped tube for 2-4 hours; the hoses article cures at 140-150 °C for 45-60 minutes with 5-8% sulfur. Wrap tightly in wet cloth to hold the bore during cure.
Worked Example
Force. A barrel with a 50 mm bore has area π ÷ 4 × 5² = 19.6 cm² = 1.96 × 10⁻³ m². The corpus gives no barrel pressure; take 10 MPa as an assumed value for the arithmetic only. Force = 10,000,000 Pa × 0.00196 m² = 19,600 N, about 19.6 kN. That is about 19,600 ÷ 9.81 ≈ 2,000 kgf, a little over 2 tonnes of force, so a person cannot push it by hand and the drive must be mechanical or hydraulic.
Length. A 200 mm long barrel holds 19.6 cm² × 20 cm = 392 cm³. Take a tube of 20 mm outer and 14 mm inner diameter (illustrative): cross-section π ÷ 4 × (20² − 14²) = 0.7854 × 204 = 160 mm² = 1.6 cm². Length = 392 ÷ 1.6 = 245 cm, about 2.4 m of tube per load.
Common Mistakes
- Barrel too hot, so the compound starts curing in the barrel and blocks the die.
- Barrel too weak. The assumed 10 MPa is a large load on the barrel wall.
- Unsupported tube, which collapses before the cure.
Safety
Pressurised barrel and the ram you cannot stop by hand
A ram delivering about 20 kN does not stop when the operator lets go if it is driven by a screw or hydraulic ram; name the stop before starting. A release valve or a locking pin that withdraws the drive must be reachable from where the operator stands at the die. A barrel burst or a die ejected under load throws parts and hot compound; use a barrel designed with a large margin and stand to the side. Hot dies and warm compound burn.
What’s Next
- Learn next: Hoses and Tubing — what tubing is for
- Combine with: Molding and Forming — choosing between methods