Why the quotation depends on this arithmetic
Every extrusion quotation is built on a throughput number: kilograms per hour at a given size, times the machine rate, divided by the yield. If the throughput is wrong, the price is wrong — and the error is usually discovered after the order is placed.
The arithmetic is not difficult. It is four relationships that most people in the industry know vaguely and few apply precisely:
Output = specific output × screw speed. Specific output is kilograms per hour per rpm, and it depends on screw diameter, channel depth and the resin.
Line speed = output ÷ (cross-section × density). This is what connects the extruder to the haul-off.
Draw-down = die opening ÷ finished dimension. This sets the relationship between the die and the product.
Yield = good product ÷ total output. Start-up, off-spec and trim all come out of the same barrel.
The number most often overestimated: specific output. Sales literature quotes the maximum for the screw at the resin's easiest grade. A 60 mm screw rated at 350 kg/h in HDPE will run 240–260 kg/h in rigid PVC and less in a highly filled compound. Quote on the material you are actually running.
Specific output and what sets it
Specific output is the throughput per revolution of the screw. It is governed by the metering section: the channel depth, the flight width and the length over which the channel is full.
| Screw diameter | Typical metering depth | Specific output (PE) | Specific output (PVC) |
|---|---|---|---|
| 45 mm | 2.5 – 3.5 mm | 1.6 – 2.2 kg/h/rpm | 1.0 – 1.5 kg/h/rpm |
| 60 mm | 3.5 – 5.0 mm | 3.0 – 4.2 kg/h/rpm | 2.0 – 2.8 kg/h/rpm |
| 75 mm | 4.5 – 6.5 mm | 5.0 – 6.8 kg/h/rpm | 3.3 – 4.5 kg/h/rpm |
| 90 mm | 5.5 – 8.0 mm | 7.5 – 10.0 kg/h/rpm | 5.0 – 6.5 kg/h/rpm |
| 120 mm | 7.0 – 10.0 mm | 13 – 18 kg/h/rpm | 9 – 12 kg/h/rpm |
Table 1 — Specific output by screw size and resin. The PVC column is lower because the compound is more viscous and must run cooler, not because the screw is different.
The relationship with screw speed is not perfectly linear. As speed rises, pressure flow back over the flights increases, and at very high speed the output curve flattens. Most screws are rated for a maximum speed that keeps them in the linear region, typically 60–120 rpm for a 60 mm screw.
Output and the die
The die affects output through its resistance. A restrictive die raises head pressure, which increases backflow, which reduces net output. That is why the same extruder produces different output on different dies.
| Die type | Relative resistance | Effect on output |
|---|---|---|
| Large pipe die | Düşük | High output, low head pressure |
| Sheet die, wide | Orta | Orta düzeyde |
| Profile die, multi-cavity | Medium-high | Reduced |
| Thin-wall tube die | Yüksek | Reduced significantly |
| Fine fibre die | Very high | Much reduced; often needs a melt pump |
Table 2 — Die resistance and its effect on output. A restrictive die on an oversized extruder wastes both capital and energy.
The practical rule is that the die should consume 60–80% of the extruder's pressure capability at the design output. Below that, the extruder is oversized and running slowly, which causes long residence time and degradation. Above it, there is no headroom for a formulation change or a screen pack loading up.
From output to line speed
This is where the arithmetic becomes concrete. Line speed is output divided by the mass per metre of product.
Line speed (m/min) = output (kg/h) ÷ (cross-section area (m²) × density (kg/m³)) ÷ 60
For a solid rectangular profile 40 mm × 6 mm in rigid PVC at 1,400 kg/m³:
Cross-section = 0.040 × 0.006 = 0.00024 m²
Mass per metre = 0.00024 × 1,400 = 0.336 kg/m
At 120 kg/h output: 120 ÷ 0.336 = 357 m/h = 5.95 m/min
| Product | Cross-section | Mass/m (PVC) | Line speed at 120 kg/h |
|---|---|---|---|
| Rod, 20 mm dia | 314 mm² | 0.440 kg/m | 4.5 m/min |
| Profile, 40 × 6 solid | 240 mm² | 0.336 kg/m | 6.0 m/min |
| Tube, 32 mm OD × 2 mm wall | 188 mm² | 0.264 kg/m | 7.6 m/min |
| Profile, hollow 60 × 30, 2 mm wall | 340 mm² | 0.476 kg/m | 4.2 m/min |
| Sheet, 400 × 2 mm | 800 mm² | 1.120 kg/m | 1.8 m/min |
Table 3 — Line speed from output for common PVC products. Note how quickly line speed falls as cross-section grows; a sheet line running 1.8 m/min is normal, not slow.
The line speed then sets the cooling length requirement. A profile needs roughly 3–6 m of cooling per metre per minute of line speed, depending on wall thickness. At 6 m/min that is 18–36 m of tank — which is why profile lines are long and why line speed is often limited by cooling rather than by the extruder.
Draw-down and the numbers it changes
Draw-down ratio is the die opening divided by the finished dimension. It affects output indirectly: a high draw-down lets a smaller die produce a bigger product, but it also thins the wall, which changes the mass per metre.
| Product | Typical draw-down | Consequence |
|---|---|---|
| Pipe | 1.0 – 1.2 | Almost none; die sets the size |
| Profile | 1.05 – 1.3 | Moderate; die slightly oversize |
| Sheet | 1.0 – 1.05 | Minimal; roll stack sets thickness |
| Tube (thin wall) | 2.0 – 5.0 | High; die gap much larger than wall |
| Film (blown) | 20 – 60 | Very high; orientation is the point |
| Monofilament | 5 – 15 | High; orientation for strength |
Table 4 — Typical draw-down ratios. High draw-down amplifies any die error, which is why thin-wall tube needs a better die than pipe.
Draw-down also affects the effective output per unit of product length. If you double the draw-down, the wall halves, the mass per metre halves, and the line speed doubles for the same output. That is the mechanism behind high-speed thin-wall tube lines.
Yield: the number that decides profitability
Output is what comes out of the die. Yield is what gets sold. The gap between them is start-up scrap, dimensional reject, surface reject and trim.
| Loss category | Typical range | Where it comes from |
|---|---|---|
| Start-up | 0.5 – 3% | Until dimensions and surface stabilise |
| Dimensional reject | 1 – 5% | Drift, die wear, calibration problems |
| Surface reject | 0.5 – 3% | Die lines, gels, marks |
| Trim and cut waste | 1 – 8% | Edge trim on sheet, cut length allowance |
| Changeover | 0.5 – 2% | Purge and re-stabilisation |
Table 5 — Where output is lost before it becomes saleable product. Total yield of 88–95% is normal for a well-run profile line.
The cost calculation has to use yield, not output. A line running 200 kg/h at 90% yield produces 180 kg/h of saleable product, and the machine rate has to be recovered over that 180 kg. Quoting on 200 kg/h understates the price by 10%.
A complete worked example
A 60 mm single screw, rigid PVC, 2.4 kg/h/rpm specific output, running at 55 rpm on a 40 × 6 mm solid profile.
Output = 2.4 × 55 = 132 kg/h
Mass per metre = 0.00024 m² × 1,400 kg/m³ = 0.336 kg/m
Line speed = 132 ÷ 0.336 ÷ 60 = 6.55 m/min
Cooling length needed = 6.55 × 5 = 33 m (at 5 m per m/min)
Saleable output at 92% yield = 121 kg/h
Machine rate at $45/h → $0.372/kg of saleable product
| Adım | Value | Comment |
|---|---|---|
| Specific output | 2.4 kg/h/rpm | From Table 1, PVC at 60 mm |
| Vida hızı | 55 rpm | Mid-range, leaves headroom |
| Gross output | 132 kg/h | Product of the two |
| Mass per metre | 0.336 kg/m | Section × density |
| Line speed | 6.55 m/min | Output ÷ mass per metre |
| Cooling length | 33 m | Often the limiting factor |
| Yield | 92% | Typical for profile |
| Saleable output | 121 kg/h | Gross × yield |
Table 6 — Worked example, step by step. The two numbers people most often get wrong are specific output (using a sales figure) and yield (forgetting it entirely).
If the cooling tank on this line is only 20 m, the line cannot run at 6.55 m/min — it is limited to about 4 m/min, which means 80 kg/h, which means the machine rate per kilogram rises by 65%. That is why cooling length is a commercial constraint and not just an engineering detail.
How do I calculate extrusion output?
Output = specific output × screw speed. Specific output (kg/h per rpm) comes from the screw geometry and the resin — roughly 3–4 kg/h/rpm for a 60 mm screw in PE, 2–2.8 in rigid PVC. For a first estimate, use the manufacturer's figure and reduce it 20–30% for the actual material.
Why is my line not reaching the rated output?
Usually one of three things: the die is more restrictive than the rating assumed, the cooling section is too short to run at that speed, or the material is more viscous than the one the rating was based on. Check head pressure first — if it is near the machine limit, the die is the constraint.
How do I convert output to line speed?
Divide output by mass per metre, then by 60 to get m/min. Mass per metre is cross-section area times density. A 40 × 6 mm PVC profile at 1,400 kg/m³ is 0.336 kg/m, so 120 kg/h gives 5.95 m/min.
What is a normal yield for extrusion?
88–95% for a well-run profile line. Sheet lines are lower because of edge trim, sometimes 85–90%. Anything below 85% is worth investigating — the loss is usually in dimensional drift rather than in visible defects.
Does screw speed affect product quality?
Yes. Higher speed means higher shear, higher melt temperature and shorter residence time. Some resins benefit (better mixing), others degrade. Most screws are designed to run at 60–80% of maximum for the best balance of output and melt quality.
Why does draw-down matter for costing?
Because it changes mass per metre. Higher draw-down means a thinner wall, which means less material per metre, which means higher line speed for the same output. It also amplifies die errors, so there is a quality cost alongside the throughput gain.
How long does the cooling tank need to be?
Roughly 3–6 m of tank per m/min of line speed, depending on wall thickness and resin. Thicker walls need the upper end. If the tank is too short, line speed is limited by cooling and the extruder runs below its capacity.
Should I size the extruder for the die or the die for the extruder?
Size the extruder so the die consumes 60–80% of its pressure capability at the design output. An oversized extruder running slowly gives long residence time and degradation; an undersized one cannot reach the output and runs at its pressure limit.
Need a throughput or cost estimate for a profile?
Send us the drawing, the resin and the annual volume. We will come back with the screw size, the line speed, the cooling length required and a price per kilogram at your yield.
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