Extruder Head Pressure
Estimate an extruder's head pressure, ΔP = (Q·μ) ÷ K, from the volumetric flow Q (m³/s), the melt viscosity μ (Pa·s) and the die conductance constant K (m³, summarizing the head+die flow-resistance geometry). Head pressure is the pressure the melt reaches at the screw end, before being forced through the die that gives the product its final shape. It results from the balance between the screw's pumping capacity (drag flow) and the die's resistance: more restrictive dies (smaller orifices, longer narrower channels) need higher pressure for the same flow. Extrusion pressures are very high — typically 100-400 bar (10-40 MPa) — and measuring and controlling them is essential: pressure indicates process state (blockages, viscosity changes from temperature, screw wear), governs flow and product uniformity, and has safety limits (excessive pressure can rupture the head or trigger burst disks). The screw-die balance, shown in the extruder's characteristic curve, is the heart of process control. Enter the flow, viscosity and die constant.
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Extruder head pressure
The head pressure of an extruder can be estimated as ΔP = (Q·μ) ÷ K, from the volumetric flow rate Q, the melt viscosity μ and the die conductance constant K (which summarises the flow resistance geometry of the head and die assembly). It is the pressure the melt reaches at the end of the screw, just before being forced through the die that gives the product its final shape. It follows from the balance between the pumping capacity of the screw (drag flow) and the resistance offered by the die: more restrictive dies (smaller openings, longer and narrower channels) demand higher pressures at the same throughput. Extrusion pressures are very high — typically 100 to 400 bar (10 to 40 MPa) — and measuring and controlling them is essential: pressure reveals the state of the process (blockages, viscosity swings driven by temperature, screw wear), governs throughput and product uniformity, and carries safety limits (excessive pressure can burst the head or trigger rupture discs). The screw-die balance, captured in the extruder characteristic curve, is the heart of process control. Enter the flow rate, the viscosity and the die constant.
Related Tools
Extruder Drag Flow
Calculate the drag flow of a single-screw extruder, Q_d = ½·π²·D²·N·H·sin(φ)·cos(φ), from the barrel diameter D (m), screw speed N (rev/s), metering-zone channel depth H (m) and helix angle φ (degrees). Drag flow is an extruder's main pumping mechanism: the melt is dragged forward by the relative motion between the rotating screw and the fixed barrel, like a screw pushing a nut that cannot turn. This viscous drag is proportional to screw speed and channel geometry, and would be the maximum theoretical flow with no back-pressure. In practice the net flow is the drag flow MINUS the pressure flow (the backflow from die/head resistance). The balance between drag and pressure sets the extruder's operating point on its characteristic curve. Drag flow is the basis of extrusion screw design, the process that makes pipes, profiles, films, sheets, wire and the pellets of nearly all transformed plastic. Enter the diameter, speed, channel depth and helix angle.
Screw Channel Shear Rate
Calculate the average shear rate in an extrusion screw channel, γ̇ = (π·D·N) ÷ H, from the barrel diameter D (m), screw speed N (rev/s) and channel depth H (m). Shear rate is the velocity gradient the molten polymer experiences between the moving screw surface and the fixed barrel, and it is central to plastics processing for a key reason: molten polymers are NON-Newtonian pseudoplastic fluids whose viscosity DECREASES as shear rate rises (shear thinning). Knowing the shear rate lets you estimate the material's real viscosity in the machine (via the power law) and thus pressure, power and viscous heating. Very high shear can degrade the polymer (chain scission by shear and heat); too low leaves melting incomplete. Each polymer has a suitable range. This screw-channel shear rate differs from the (much higher) die shear rate at the exit restriction. It is a basic processing-rheology calculation. Enter the diameter, speed and channel depth.
Extrusion Haul-Off Speed
Calculate the haul-off speed of an extrudate by mass conservation, v = Q ÷ A, from the extruder volumetric flow Q (m³/s) and the final product cross-sectional area A (m²). After the die, the extrudate is pulled by a haul-off (belts, rollers, winder) at a speed that must be SYNCHRONIZED with the extruder flow: by mass conservation, in steady state, the volume leaving the extruder per second must equal the volume the haul-off removes per second (product area times line speed). If haul-off is too fast for the flow, the product thins below size or breaks; if too slow, material accumulates and deforms. This speed sets the line's PRODUCTIVITY (metres per minute) and, with die swell and draw-down ratio, sets the final dimensions. Controlling the extrusion-haul-off synchrony — often with dimension sensors and closed loop — is essential for dimensional uniformity of pipes, profiles, wire and sheet. This gives the theoretical line speed from flow and desired section. Enter the flow and the product section area.
Screw Compression Ratio
Calculate an extrusion screw's compression ratio, CR = H_feed ÷ H_metering, from the channel depth in the feed zone H_feed and the metering zone H_metering. An extrusion screw has three zones: feed (deep channel, receiving solid pellets), compression (transition, channel tapering) and metering (shallow channel, homogenizing and pumping the melt). The compression ratio is how much the channel narrows from inlet to outlet — typically 2:1 to 4:1. This compression is essential: by reducing channel volume it compacts the pellets, expels trapped air (which must vent back through the feed, not go forward) and generates the shear and pressure that melt the polymer by viscous heating (plus barrel heat). The right ratio depends on the polymer: materials melting with large volume reduction and amorphous ones need different ratios from semicrystalline. A wrong ratio causes incomplete melting, air pumping, flow instability (surging) or degradation. It is one of the parameters that define whether a screw suits a given material. Enter the feed and metering channel depths.
Extrusion Pressure by Temperature
Estimates linear FDM extrusion pressure from temperature and filament type.
Die Swell Ratio
Calculate the die swell ratio, B = D_extrudate ÷ D_die, from the extrudate diameter once it stabilizes D_extrudate and the die orifice diameter D_die. Die swell is one of extrusion's most characteristic and challenging phenomena: on leaving the die, the molten polymer EXPANDS, ending up larger than the orifice that shaped it (swells of 1.2-2× are common). The cause is the VISCOELASTIC nature of polymers: inside the die, the long molecular chains are compressed and oriented (stretched) by the flow; on exiting and losing confinement, they relax and elastically recoil, like a spring, swelling the material. Swell is greater the more elastic the polymer, the higher the shear rate and the shorter the die (less time to relax inside). It is critical in die design: to make a pipe or profile of the exact target size, the die must be designed SMALLER, anticipating the swell — and since it varies with temperature and speed, controlling swell is essential for dimensional accuracy. Enter the extrudate diameter and the die diameter.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.