In palm oil mills, the screw press is more than a machine—it is a yield determinant. Well‑engineered, well‑maintained presses consistently push crude palm oil (CPO) yields toward ~23–24% of fresh fruit bunch (FFB) weight; poor setups and neglect forfeit several percentage points and invite downtime.
Modern palm oil mills lean on continuous screw (expeller) presses: a perforated cylindrical cage and one or more internal augers that compress digested fruit mash so oil escapes through the cage holes as back‑pressure rises toward a tapered outlet with one or more constricting cones (FAO). Double‑ or twin‑screw presses—two parallel screws—dominate high‑capacity mills because they handle more throughput in less time than single‑screw designs (industry survey).
Crucially, the screw press kneads. That action breaks a significant fraction of oil‑bearing cells left intact after digestion. FAO guidelines describe the screw press as “an additional digester,” releasing oil that hydraulic or centrifuge extraction would otherwise miss (FAO). With good operations, large mills routinely achieve roughly 23–24% oil yield on FFB weight (FAO).
Continuous press architecture and geometry
The press cage is perforated to drain oil while retaining fibrous cake. A well‑designed unit uses progressively tighter cones or pressure zones and a screw with carefully chosen pitch and diameter. Constant‑width (cylindrical) screws build pressure faster than variable‑pitch ones, reaching peak pressure in fewer revolutions—at the cost of greater shaft stress—whereas tapered or stepped shafts can extend machine life (design study) (design study).
Multi‑zone designs (sequential cones) and double‑screw heads temper the pressure profile while increasing throughput. Across layouts, the screw’s kneading and the outlet restriction work together to drive oil out through the cage holes (FAO).
Pressure profile, speed, and oil recovery
Extraction rate—fraction of mesocarp oil recovered—hinges on the pressure profile and screw geometry. “A high pressure causes a better recovery of the oil,” one engineering study notes (design study). Mills raise screw pressure by tightening outlet cones or slowing screw speed until diminishing returns set in. Excessive pressure increases kernel breakage (smashing the nut) and can degrade oil quality—bleachability and oxidation stability suffer at very high pressures (FAO) (design study).
Typical operating screw speeds sit around 10 rpm (revolutions per minute). The driveshaft is very low‑speed—an input of ~200 rpm reduced by gearing—and that setting repeatedly appears as optimal across studies. In one fabricated press, the maximum crude yield was achieved at 10 rpm: an oil extraction ratio of ~17.9% of FFB, equivalent to 79.6% of available oil (development study). In that trial, lower speeds (4 rpm) or higher speeds (16 rpm) tended to jam or under‑press the fruit. Industry guidelines (PORIM/MPOB, 1985) recommend this order of magnitude for screw speed.
Screw configuration and back‑pressure targets
Constant‑pitch screws build pressure quickly, with pressure rising roughly linearly as the screw advances. Variable‑pitch or tapered threads spread compression over more of the screw length, extending press life but often needing more rotations for the same squeeze (design study). Empirically, well‑engineered presses push FFB oil yields into the low‑20s percent (23–24% by weight) (FAO). Suboptimal pressure or configuration easily leaves several percent of oil in the cake.
A rough industry rule of thumb cited in the literature: a few bar (several MPa) of back‑pressure is needed. Small presses have found optimal hydraulic pressures on the order of 3–4 MPa (megapascal; a pressure unit), equivalent to 30–40 bar, for best yields. In short, screw geometry and the outlet restriction must be tuned so that nearly all mesocarp oil is expelled; too loose a press leaves oil in the cake, too tight stresses the machine.
Wear, overhaul intervals, and maintenance discipline
The mechanical wear is heavy. Flights, cones, and bearings meet abrasive fibers, sand, and nuts. In practice, a typical palm screw press needs major servicing every few hundred hours. One design study found existing machines usually last only 500–900 operational hours before requiring overhaul (design study). Field surveys agree that fatigue and corrosion are expected failure modes, but the primary reported causes of breakdown are irregular maintenance schedules and improper handling (industry survey).
Poor maintenance directly lowers extraction efficiency. An “ineffective” press—loose tolerances, blocked holes, worn threads—has been explicitly linked to “high oil loss and maintenance cost” (design study). A case study at an Indonesian mill used fault‑tree analysis to trace pressing‑station oil losses to inadequate upkeep and failure to maintain the designed pressure (Engineering Proceedings).
Figurative outcomes and yield math
When presses are optimized and maintained, mills routinely hit ~23% CPO (crude palm oil) yields (FAO). Mills reporting chronic press issues typically see yields 2–5 points lower. The revenue stakes are clear: dropping from 24% to 20% FFB yield because the press is not adjusted or serviced leaves an extra 4 tons of oil in the fiber per 100 ton of bunches.
Downtime compounds the damage. Press repairs can halt production, delaying FFB processing; over‑fermentation then raises free fatty acids. The through‑line across the engineering guides and field surveys is consistent: press design/configuration and diligent maintenance are paramount to oil extraction efficiency. Operators balance pressure against kernel breakage—often via ~10 rpm screw speed and staged cones—to maximize OER (oil extraction rate) and rely on rigorous upkeep to sustain those gains.
Sources and references (embedded)
Authoritative engineering guides and field studies support these conclusions: FAO; design study; development study; industry survey; design study (maintenance note); Engineering Proceedings.
References (as cited in the source material): FAO (Food and Agriculture Organization). Palm Oil Processing. FAO Agricultural Services Bull. No. (1988) (FAO) (FAO). Olufemi Adetola, J. O. Olajide & A. P. Olalusi. “Development of a Screw Press for Palm Oil Extraction” (2014): 1416–22 (link). M. Firdaus et al. “Preliminary Design on Screw Press Model of Palm Oil Extraction Machine” (2017) (link) (link). M. A. Che Yunus et al. “An Industry Survey on Screw Press Systems in Palm Oil Mills: Operational Data and Malfunction Issues” (survey of 105 Malaysian mills) (link). Nismah Panjaitan et al. “Root Cause Analysis of Oil Losses in Press Machines Using Fault Tree Analysis” (Engineering Proceedings 84, 2025: 96) (link). Irvin Barizi Muhammad et al. “Development and Performance Evaluation of a Variable‑Pitch Tapered‑Shaft Palm Oil Press” (Constraint‑ready 2021); see also engineering bulletins (e.g., MPOB Eng. Bull.) for maintenance guidelines—these sources underscore that an “ineffective press” yields high oil loss and costs (link).
