Screw Diameter and Output: How to Calculate Extrusion Capacity

Throughput is the parameter that defines the commercial viability of a sheet extrusion line. screw diameter extrusion capacity involves specific considerations that differ from general extrusion processes. Too little, and the downstream equipment runs below capacity, eroding margin. Too much, and the cooling rolls cannot solidify the sheet fast enough, forcing a line speed reduction. At the center of this calculation is the extruder screw diameter—the single variable that most directly limits how much polymer the machine can process per hour. Screw diameter extrusion capacity is not a fixed number; it depends on screw speed, polymer properties, screw geometry, and the operating pressure. The relationship between these variables is described by the fundamental flow equations of single-screw extrusion, which separate total output into drag flow, pressure flow, and leakage flow components. This article explains the theory behind extrusion capacity calculations, provides practical formulas, and offers reference throughput data for common sheet extrusion polymers. For the broader extrusion process context, see the guide on sheet extrusion technology.

The Physics of Extruder Output: Drag Flow, Pressure Flow, and Leakage Flow

The output of a single-screw extruder is the net result of three flow mechanisms. Drag flow is the forward conveying action produced by the relative motion between the screw and the barrel. It is proportional to screw speed, channel depth, and screw diameter, and represents the maximum theoretical output.

Pressure flow acts in the opposite direction. The die at the end of the extruder creates resistance to flow, generating a pressure gradient that drives backward flow from the high-pressure die end toward the low-pressure feed end. Pressure flow is proportional to the pressure gradient and the cube of the channel depth. Leakage flow occurs through the small clearance between screw flight tips and the barrel wall—typically 0.1 to 0.15 percent of the screw diameter—and normally accounts for less than 5 percent of total output.

The total volumetric output is the drag flow minus the pressure flow and leakage flow. In simplified form: Q = Q_drag - Q_pressure - Q_leakage. The practical implication is that extruder output is not a single number but a function of operating conditions. At a given screw speed, output decreases as die pressure increases. A shallow-channel screw has a flatter characteristic curve, meaning its output is less sensitive to die pressure changes; a deep-channel screw has higher output at low pressure but drops more sharply as pressure increases.

Screw Diameter and the Throughput Scaling Law

Screw diameter affects output through the drag flow term, proportional to the square of the diameter. Doubling the screw diameter increases the theoretical output by a factor of four. A 150mm screw can process roughly 2.25 times the output of a 100mm screw at the same screw speed. The scaling is not perfectly quadratic because channel depth, L/D ratio, and allowable screw speed also change with diameter. Larger screws typically run at lower RPM to maintain the same tip speed, limited by the polymer's shear sensitivity.

The practical throughput across the common diameter range provides a rough sizing guide. A 65mm single-screw extruder processing PP can deliver 150-250 kg/hr. A 90mm machine: 350-500 kg/hr. A 120mm machine: 800-1,200 kg/hr. A 150mm machine: 1,200-1,800 kg/hr. The extrusion screw design LD ratio influences the practical throughput ceiling: a longer L/D provides more melting capacity, allowing higher screw speeds before melting becomes incomplete.

Practical Throughput Tables for Common Sheet Polymers

The following throughput ranges represent typical performance for well-designed single-screw extruders processing common sheet polymers at 28:1 to 33:1 L/D. The values assume a balanced screw design with appropriate compression ratio and mixing section for the polymer.

Polypropylene (PP) Sheet:

• 65mm: 150-220 kg/hr

• 90mm: 320-480 kg/hr

• 120mm: 750-1,100 kg/hr

• 150mm: 1,100-1,700 kg/hr

Polyethylene (HDPE/LDPE) Sheet:

• 65mm: 130-200 kg/hr

• 90mm: 300-450 kg/hr

• 120mm: 700-1,050 kg/hr

• 150mm: 1,050-1,600 kg/hr

PET Sheet:

• 65mm: 120-180 kg/hr

• 90mm: 280-400 kg/hr

• 120mm: 650-950 kg/hr

• 150mm: 950-1,450 kg/hr

PET throughput is lower than PP at equivalent screw diameter due to the narrower processing window and the two-stage vented screw design.

Polystyrene (PS) Sheet:

• 65mm: 140-210 kg/hr

• 90mm: 310-460 kg/hr

• 120mm: 720-1,080 kg/hr

• 150mm: 1,080-1,650 kg/hr

These ranges provide a starting point for line sizing. Actual throughput should be verified through extrusion trials or detailed engineering calculations.

The Role of Melt Pumps in Extrusion Capacity

A melt pump—a positive-displacement gear pump installed between the extruder and the die—decouples the extruder's output from the die pressure. The extruder feeds the melt pump at a slightly higher rate than the pump's set point, maintaining a constant inlet pressure. The pump meters a precise volumetric output to the die, regardless of die pressure fluctuations or screen pack loading.

With a melt pump, the extruder operates against a lower, constant back pressure, and the pump assumes the pressure-generation function. This allows the screw to be designed for melting efficiency rather than pressure generation. The pump's output is nearly independent of pressure, so variations in die resistance do not affect the mass flow rate to the die. The result is more consistent sheet thickness, particularly during startup and process transitions. For a line with a melt pump, the extruder is typically sized to deliver 10-15 percent more output than the pump's rated capacity. The servo motor standard motor extrusion drive selection for the extruder and melt pump affects both the precision of speed control and the energy efficiency of the line.

Matching Screw Diameter to Downstream Equipment

The downstream equipment—calendering stack, thickness gauge, and winding or cutting station—must be sized to handle the extruder's output at the target line speed. For a given sheet width, thickness, and polymer density, the required line speed is the extruder output divided by the cross-sectional area of the sheet and the melt density. If the calculated line speed exceeds the maximum speed of the calender rolls or winder, the extruder is oversized. If the line speed is below the minimum stable speed of the downstream equipment, the extruder is undersized.

The calender roll cooling capacity is another constraint. The heat that must be removed from the sheet is the product of the throughput, the polymer's specific heat, and the temperature drop from the melt to the solid state. If the roll cooling system cannot remove this heat at the required rate, the sheet will exit the calender above its softening point, causing sticking or deformation.

Selecting the correct screw diameter for a given throughput target avoids the common pitfall of undersizing or oversizing an extrusion line. JWELL provides detailed throughput calculations and capacity projections during the line specification phase, helping customers select screw diameters from 65mm to 200mm based on polymer type, target output, and downstream processing requirements —a consultative approach that ensures the selected equipment meets both current production needs and future growth targets.

FAQ

How is extrusion capacity calculated from screw diameter?

The theoretical output of a single-screw extruder is calculated from the drag flow equation: Q = (1/2) * pi^2 * D^2 * N * H * sin(phi) * cos(phi), where D is the screw diameter, N is the screw speed in revolutions per second, H is the channel depth in the metering zone, and phi is the helix angle of the screw flight. The actual output rate is the theoretical drag flow minus the pressure flow and leakage flow, which depend on the die pressure, melt viscosity, and flight clearance.

What is the relationship between screw speed and output?

In theory, extruder output is directly proportional to screw speed. In practice, the relationship is approximately linear but not perfectly proportional. At very low screw speeds, the pressure flow component is a larger fraction of the drag flow, and the output falls below the linear projection. At very high screw speeds, the feeding capacity of the screw may limit the amount of material entering the barrel, and the output may plateau or even decrease. Within the normal operating range of 30-80 percent of maximum RPM, the output is roughly linear with screw speed.

Does a larger screw diameter always mean higher output?

Yes, output increases with screw diameter, approximately as the square of the diameter. However, the maximum practical throughput is also limited by the melting capacity of the screw, the cooling capacity of the downstream equipment, and the maximum line speed of the calender and winder. A larger screw diameter only translates to higher output if the entire line is sized to handle the increased throughput.

How does melt density affect the output calculation?

The volumetric output calculated from the drag flow equation must be converted to mass output using the melt density of the polymer at the processing temperature. Melt density is lower than solid density due to thermal expansion. For PP, the melt density at 230 degrees Celsius is approximately 0.75 g/cm3, compared to a solid density of 0.90 g/cm3. Using the solid density instead of the melt density overestimates the mass output by about 20 percent. Accurate melt density values at the processing temperature are essential for correct throughput calculations.