What a screw oil press is
A screw press is the workhorse of mechanical oil extraction. Unlike a hydraulic press, which squeezes a fixed batch between plates, a screw press works continuously: a hardened worm (screw) shaft turns inside a cylindrical cage and conveys the seed forward while progressively compressing it. The same machine is used two ways — as a pre-press ahead of solvent extraction (taking the cake down to roughly 16–20% residual oil), or as a full / single press that finishes the job mechanically for small and medium oil mills. It can run cold or, more commonly, on seed that has been heat-conditioned first.
Three things make the press work: the geometry of the worm-and-cage assembly that builds pressure, the plasticity of the conditioned meal that lets it release oil cleanly, and the residence time the material spends under load. Get those three right and the cake comes out dry; get them wrong and oil flows back toward the inlet or the cake crumbles.
Video: how a single-screw oil press extracts oil (third-party demonstration).
The three pressing stages
Inside the barrel the charge passes through three distinct zones. Most of the oil — well over half — is actually expressed in the first half of the barrel, in the feed and main-press zones.
The incoming meal is compacted, trapped air and a little moisture are driven off, and it consolidates into a loose “soft cake”. Oil begins to flow here already, especially with high-oil seeds. Because loose particles can simply spin with the shaft or push backward (back-flow, known as huiliao in Chinese practice), a good press uses forced feeding and pre-compression to grip the charge early.
This is where the bulk of the oil leaves. The free volume drops rapidly and regularly, the particles bond into a continuous porous mass, and peak pressure is reached. Interruptions in the worm flights, cage scraper knives and the sharp edges of the cage bars constantly shear the mass and re-open drainage channels so oil keeps finding a way out.
By now the material is a compact, tile-like cake that moves almost as a single body. Compressibility is nearly exhausted, but high pressure must be maintained so the last oil drains rather than being re-absorbed. As the cake exits past the choke it expands slightly from elastic recovery.
How pressure is built inside the barrel
Screw-press pressure is not applied from outside — it is generated by steadily shrinking the space available to the seed. The space between the worm and the cage, called the free volume, falls along the axis for three reasons working together:
A second lever is the choke (cone) at the discharge: tightening the cake-outlet gap raises back-pressure and gives a drier cake; opening it does the opposite. Barrel pressure rises along the axis as an exponential-type curve, with the peak in the main-press zone. The gap between the worm crest and the cage bars matters too — practice shows the most workable cage gap is about 1.25–1.5 mm: wider and oil-bearing fines wash back through it, tighter and the meal overheats as it squeezes past.
Key components
The worm shaft is the heart of the machine and wears fastest. Small presses use a one-piece machined shaft that must be replaced whole; almost all production presses use an assembled (segmented) shaft — individual worm segments and spacer rings keyed onto a central shaft, so only worn segments are swapped. The decreasing pitch and increasing root diameter of those segments form the stepped, pressure-building profile. Worm and spacer segments are typically a carbon steel, surface-hardened (carburised ~1.5–2 mm, around HRC 55–62) to survive the abrasion.
The cage is built from cage bars set with precise gaps: wide enough to drain oil, narrow enough to hold back the solids. The bars also give the inner surface “grip” so the charge does not just rotate with the shaft, and internal scraper knives keep oil paths open.
Key operating parameters
The figures below are typical industry values that vary with machine model, seed type and whether the press is run as a pre-press or a full press — use them as a starting reference, not fixed specifications.
| Parameter | Pre-press (typical) | Full / single press (typical) |
|---|---|---|
| Worm shaft speed | ~15 r/min | ~8 r/min |
| Pressing (residence) time | ~60–70 s | ~150 s |
| Cake thickness | ~12–15 mm | ~5–8 mm |
| Actual compression ratio | ~2.7–10+ (higher for high-oil seeds) | |
| Cage gap (worm crest to bars) | ~1.25–1.5 mm | |
| Cooling-oil temperature on cage | ~60–70°C | |
| Residual oil in cake | ~16–20% | typically higher than solvent extraction |
Two parameters dominate: shaft speed and compression ratio. Lower speed lengthens the time the seed spends under pressure, which recovers more oil but cuts throughput — so pre-pressing (speed up, oil out fast, finish later in solvent) runs faster than single full pressing. A higher actual compression ratio — the volume reduction the meal really undergoes — leaves less oil in the cake; high-oil seeds need and tolerate higher ratios.
Conditioning: why moisture & temperature decide yield
A screw press can only express oil well if the meal arrives with the right plasticity, and plasticity is set by the combination of moisture and temperature reached during cooking/conditioning before the press. This is the single biggest lever an operator has.
Too dry / under-cooked: particles don’t bond — the press throws hard, oily fines or crumbs, motor load spikes and oil yield drops.
Too wet / over-cooked: the meal turns into a soft, formless paste that squeezes out through the cage gaps, the cake won’t form, oil flows back toward the feed and motor load falls.
Rising or falling motor load is, in fact, a direct readout of barrel pressure: it climbs when the meal resists compression and falls when an over-plasticised meal flows too easily.
Friction heat & cooling
The intense friction between meal, worm and cage generates heat — usually far more than the press body can shed on its own — so the oil and cake can overheat without control. Common cooling methods are spraying cooled press oil (~60–70°C) over the outside of the cage, circulating cooling water through a hollow-bored worm shaft, or a water-jacketed cage. Shaft cooling is effective but adds machining cost and, if mishandled, thermal shock can crack the shaft — which is why cage-side cooling is the most common choice.
Common problems & what causes them
- Oil flowing back toward the feed / poor cake — usually an over-wet, over-plasticised meal; correct the conditioning moisture and temperature.
- Hard, crumbly, oily cake + high motor load — meal too dry or under-cooked; raise conditioning moisture/heat.
- Falling capacity and rising residual oil over time — worn worm flights and cage bars increase back-flow and lengthen pressing time; rotate or replace segments.
- Fines washing into the oil — cage gap opened up beyond ~1.5 mm or worn bars; re-shim or replace.
Need help spec’ing a press? Tell us your seed, daily capacity and whether you want a single full press or a pre-press line, and we’ll recommend the right screw-press configuration. Try the plant capacity calculator, or compare screw vs hydraulic presses and cold vs hot pressing.