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.
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Razão de compressão da rosca
A razão de compressão de uma rosca de extrusão é RC = H_alimentação ÷ H_dosagem, a razão entre a profundidade do canal na zona de alimentação (que recebe os grânulos sólidos) e na zona de dosagem (que homogeneíza e bombeia o fundido). Uma rosca tem três zonas: alimentação (canal profundo), compressão (transição, canal afilando) e dosagem (canal raso). A razão de compressão — quanto o canal afina da entrada à saída, tipicamente entre 2:1 e 4:1 — é essencial: ao reduzir o volume do canal, ela compacta os grânulos, expulsa o ar aprisionado entre eles (que precisa retornar pela alimentação, não seguir adiante) e gera o cisalhamento e a pressão que fundem o polímero por aquecimento viscoso, complementando o calor do barril. A razão correta depende do polímero: materiais que fundem com grande redução de volume e os amorfos pedem razões diferentes dos semicristalinos. Uma razão inadequada causa fusão incompleta, bombeamento de ar, instabilidade de vazão (surging) ou degradação térmica — por isso a razão de compressão é um dos parâmetros que definem se uma rosca serve para um dado material. Informe as profundidades do canal na alimentação e na dosagem.
Related Tools
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.
Draw-Down Ratio
Calculate an extrudate's draw-down ratio (DDR), DDR = (D_die ÷ D_product)², from the die orifice diameter D_die and the final product diameter D_product, as the ratio of cross-sectional areas. After leaving the die, the extrudate (a wire, tube, filament) is often PULLED and stretched by a haul-off at a speed higher than the exit speed, reducing its cross-section to the final size. The draw-down ratio is how much the section area is reduced. Drawing not only sets the final size but ORIENTS the molecular chains in the pull direction, which can greatly increase the product's mechanical strength in that direction (used in oriented fibers, tapes and films, far stronger than unoriented material). The draw-down ratio, combined with die swell, sets the relation between orifice size and final product. There are limits: excessive drawing can break the extrudate or cause defects. It is a key parameter in making fibers, monofilaments, small-diameter tubes and wire coating. Enter the die diameter and the final product diameter.
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.
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.