When buyers ask me “What’s the output of this blown film line?” they usually want one number—kg/hr. But in a real factory, output is only meaningful if you also state the film width, thickness, resin density, and stable line speed used in the calculation.
This article is a calculation guide. I’ll show you:
- the fast formulas buyers use on a call
- the unit-safe step-by-step method to avoid mistakes
- and how to validate output during a trial run so the number is real, not marketing
Quick answer formulas for kg per hour and line speed
At its core, output is mass per time:
Output (kg/hr) = Film area per time × Thickness × Density
In blown film, film area per time is based on how much film you make every minute:
- Layflat width tells you film width in the flattened state
- Line speed tells you how fast film is being produced
A very practical buyer-friendly form is:
Output (kg/hr) = Layflat width (m) × Line speed (m/min) × Thickness (m) × Density (kg/m³) × 60
If you prefer thickness in microns:
- Thickness (m) = Thickness (µm) × 10⁻⁶
So:
Output (kg/hr) = Layflat (m) × Speed (m/min) × Thickness (µm) × 10⁻⁶ × Density (kg/m³) × 60
If you only remember one warning, remember this:
Most wrong output quotes come from wrong units. Micron vs mm, g/cm³ vs kg/m³, and layflat vs circumference are the common traps.
What output means in blown film production
You’ll hear “output” used in two different ways:
- Mass output: kg/hr or lb/hr
This is what buyers use to estimate resin consumption and capacity. - Line speed: m/min or ft/min
This matters for productivity, but speed alone is meaningless without width and thickness.
A supplier can honestly claim “high speed,” but if they only run that speed at a thick gauge or narrow width, your real output might not match your target.
Inputs you need before doing any calculation
Before we calculate anything, I ask for these inputs. If you don’t have them, the result will be guesswork.
- Layflat width of the product you want (not the bubble circumference)
- Target thickness (micron, mil, or mm—just be consistent)
- Stable line speed at that product (m/min or ft/min)
- Resin density (use your resin TDS whenever possible)
- Optional but very useful for cross-checks:
- Die diameter
- Die gap
- Bubble diameter (or BUR)
If you want a quote that survives a factory trial, those are the minimum.
Unit conversions that avoid bad quotes
This section saves more time than any “advanced” formula.
Thickness
- 1 micron (µm) = 0.001 mm = 10⁻⁶ m
- 1 mil = 0.001 inch = 25.4 µm
Density
- 1 g/cm³ = 1000 kg/m³
Example: 0.92 g/cm³ = 920 kg/m³
Width
- Layflat width is the width of the flattened tube.
If someone gives you bubble circumference instead, convert it before using the formula.
When I see an output number that looks too good to be true, it’s usually because someone mixed µm and mm, or used g/cm³ directly as kg/m³ without multiplying by 1000.
Blow up ratio calculation from layflat and die diameter
You don’t need BUR to calculate kg/hr if you already know layflat width. But BUR is useful for sanity checks and for understanding whether the operating point is realistic.
BUR (Blow-Up Ratio) = Bubble diameter ÷ Die diameter
A practical shortcut if you only know layflat width:
- Bubble diameter ≈ (2 × layflat width) ÷ π
- So BUR ≈ [(2 × layflat) ÷ π] ÷ die diameter
If BUR is extremely high for your product, the line may be operating in a narrow stability window, which can limit real output even if the math says you “can” run faster.
Drawdown ratio and what it tells buyers
DDR is the machine-direction draw relative to the melt curtain leaving the die. Different references define DDR slightly differently, but the buyer takeaway is consistent:
- Higher DDR means more MD stretch
- Very high DDR can make the process more sensitive and can limit stable output
I rarely ask buyers to compute DDR perfectly during an RFQ. I ask for enough information that we can estimate whether the requested output is realistic for the target gauge, width, resin, and cooling capability.
The simple output calculation method buyers actually use
Here is the clean, most practical version again:
Output (kg/hr) = Layflat (m) × Speed (m/min) × Thickness (m) × Density (kg/m³) × 60
You can use solid density from the resin datasheet for a good estimate. For most PE films, that’s usually enough for buyer-level planning. For detailed engineering, melt density and slip effects can matter, but buyers rarely need that level unless they are optimizing to the limit.
A step by step example in metric units
Let’s calculate output for a common scenario.
Given
- Layflat width = 1.0 m
- Thickness = 30 µm
- Line speed = 80 m/min
- Density = 0.92 g/cm³ = 920 kg/m³
Step 1: Convert thickness to meters
- 30 µm = 30 × 10⁻⁶ m = 0.00003 m
Step 2: Calculate area per minute
- Area/min = layflat × speed
- Area/min = 1.0 m × 80 m/min = 80 m²/min
Step 3: Convert to volume per minute
- Volume/min = area/min × thickness
- Volume/min = 80 m²/min × 0.00003 m = 0.0024 m³/min
Step 4: Convert to mass per minute
- Mass/min = volume/min × density
- Mass/min = 0.0024 m³/min × 920 kg/m³ = 2.208 kg/min
Step 5: Convert to kg/hr
- Output = 2.208 × 60 = 132.48 kg/hr
Result
- Output ≈ 132.5 kg/hr
This is the “clean math” output. Real output may be lower once you include trim waste, startup scrap, and speed limits.
A step by step example in imperial units
Now the same idea in imperial units, because many buyers compare specs this way.
Given
- Layflat width = 40 inches
- Thickness = 1.2 mil
- Line speed = 250 ft/min
- Density ≈ 0.92 g/cm³ (we’ll use metric density for conversion, or you can use lb/ft³ tables)
A practical way is to convert everything to metric and reuse the same method (this avoids the most imperial mistakes). Convert:
- 40 in = 1.016 m
- 250 ft/min = 76.2 m/min
- 1.2 mil = 30.48 µm = 0.00003048 m
- density = 920 kg/m³
Now compute quickly:
- area/min = 1.016 × 76.2 ≈ 77.4 m²/min
- volume/min = 77.4 × 0.00003048 ≈ 0.00236 m³/min
- mass/min = 0.00236 × 920 ≈ 2.17 kg/min
- kg/hr ≈ 2.17 × 60 ≈ 130 kg/hr
If you need lb/hr:
- 130 kg/hr × 2.2046 ≈ 287 lb/hr
The method is the same. The key is to stay consistent and convert cleanly.
Film yield calculation area per kilogram
Procurement teams often care about yield because it connects directly to resin cost.
Yield (m²/kg) = 1 ÷ [Thickness (m) × Density (kg/m³)]
If thickness is 30 µm (0.00003 m) and density is 920 kg/m³:
- Thickness × Density = 0.00003 × 920 = 0.0276 kg/m²
- Yield = 1 ÷ 0.0276 ≈ 36.2 m²/kg
This is a powerful planning number:
- If you buy 1000 kg resin, you can estimate total film area produced at that gauge (before scrap).
Why calculated output and real output don’t match
This is the part buyers should always ask about. A perfect calculation is not the same as a stable, sellable run.
Common reasons real output is lower:
- Trim waste and edge quality requirements
- Gauge variation forcing you to run thicker than target to avoid thin spots
- Startup and changeover scrap
- Cooling limits that cap stable speed at thin gauges
- Winder limits that cap line speed even if extrusion can push more
- Material changes (regrind ratio, resin batch viscosity drift) that reduce stable window
If a supplier quotes output without stating gauge and width, I consider it incomplete. If they quote output without a stable run demonstration, I consider it unverified.
How buyers validate output during a factory trial
If you want output to be real, validate it with a simple trial protocol. You don’t need fancy instruments to do the basics.
During a trial run, I recommend recording:
- Stable run window (for example, 20–30 minutes after stabilization)
- Actual roll weight gain over time (kg in 10 minutes is enough)
- Layflat width and average thickness readings across the web
- Line speed, haul-off setting, and any speed drift
- Melt pressure trend and any signs of instability
Then compute:
- Measured output = (mass gained) ÷ (time)
and compare it to the theoretical value from the formula.
If the measured output is significantly lower, ask why. The explanation usually reveals whether the limitation is cooling, winding, stability, or scrap rate.
Are you looking for a reliable film blowing machine manufacturer
If you share your target layflat width, thickness range, resin type, layer structure, and target output, we can calculate a realistic output window and recommend a line configuration that can actually run it stably.
At Wilson Machines, we don’t treat output as a single number. We treat it as a matched system: extruder capacity, die head, cooling, haul-off, winding, and controls—so the output you calculate is the output you can ship.
FAQ
1) How to calculate BUR in blown film
In a real factory, the fastest way to calculate BUR is to measure bubble diameter and die diameter, then divide:
- BUR = Bubble Diameter ÷ Die Diameter
If you don’t have a way to measure bubble diameter directly, you can estimate it from layflat width:
- Bubble circumference ≈ 2 × layflat width
- Bubble diameter ≈ (2 × layflat width) ÷ π
- Then divide by die diameter to get BUR.
Field tip: Don’t measure the bubble where it’s still “soft and moving.” Measure at a stable zone where the bubble is steady (usually around a consistent frost line region), otherwise your BUR calculation will jump around and mislead you.
2) What is the blow up ratio
Blow-up ratio (BUR) tells you how much the bubble is expanded in the transverse direction compared with the die size. Practically, BUR affects:
- Layflat width range you can achieve on a given die
- TD orientation (tensile/tear direction balance changes as BUR changes)
- Bubble stability window (very high BUR often becomes more sensitive to air ring balance, drafts, and haul-off disturbances)
What procurement should listen for: If a supplier claims a very wide layflat on a relatively small die, ask what BUR they’re assuming and whether they can run it stably at your target thickness and output. High BUR can be possible, but it often narrows the stable operating window.
3) What is the draw down ratio in blown film
Draw-down ratio (DDR) is a way to express how much the film is stretched in the machine direction after the melt exits the die. Higher DDR generally means more MD stretching.
Why DDR matters in troubleshooting and quoting:
- Higher DDR at thin gauge often demands better cooling and more stable haul-off/winding
- If DDR is pushed too high, small speed or tension fluctuations can show up as thickness drift or unstable film handling
- DDR is also a clue when output numbers look unrealistic: if the implied DDR is extreme, the line may not run that condition stably for long
Important note (so you don’t get confused): Different references and regions may define DDR slightly differently (you’ll see “European style” and “North American style” variants). As a buyer, the key is not memorizing one exact formula—it’s using DDR as a sanity check: “Is the stretching level reasonable for this resin, gauge, and speed?”
4) How do you measure output
In practice, the most credible output measurement is mass over time—because it doesn’t depend on assumptions.
The simple factory method (works anywhere):
- Run at a stable condition (same width, thickness target, speed) for a defined window, like 10–20 minutes
- Weigh the roll (or record roll weight increase) over that window
- Output = (weight gained) ÷ (time)
Example:
- If your roll gains 25 kg in 10 minutes, output is
25 ÷ (10/60) = 150 kg/hr
Field tip: Don’t measure during warm-up, after a screen change, or during speed ramping. Measure when the process is stable. If a supplier shows “peak output” during unstable conditions, treat it as marketing, not production capability.
5) What is the formula for output
For buyers, the most useful output formula is the one that connects directly to what you can verify on a trial run:
- Output depends on how much film area you produce per minute, multiplied by film mass per area (which comes from thickness and density), then converted to per hour.
That’s the clean concept.
What I recommend in real quoting work:
- Use the formula to get an expected number
- Then validate output by weighing the roll over time (the method in the previous FAQ)
If the calculated number and measured number don’t match, the reason is usually one of these:
- trim waste and scrap rate
- gauge variation forcing thicker-than-target average
- cooling limit preventing stable speed at thin gauge
- winding/haul-off constraints (tension or speed ceiling)
Procurement tip: Always ask suppliers to state output with the conditions: width, thickness, resin, and stable speed. “Output” without conditions is not a usable specification.
