How to Calculate Extruder Size and Die Diameter for Blown Film Lines

In a film blowing machine, output capacity and film width are related but governed by different hardware limits. The extruder size (screw diameter) determines the volume of melt you can generate, while the die head diameter and Blow-Up Ratio (BUR) determine the final layflat width.

1. Sizing the Extruder (Screw Diameter)

Your output requirements (kg/hr) dictate the screw size. For standard LDPE processing, use these proven benchmarks:

• 55mm Screw: Delivers 80–120 kg/hr

• 65mm Screw: Delivers 150–200 kg/hr

• 90mm Screw: Delivers 300–400 kg/hr

2. Sizing the Die Head (Layflat Width)

Once you have the melt capacity, you must size the die to achieve the desired width. The calculation follows this formula:

Layflat Width = (Die Diameter × 3.14 × BUR) / 2

For example, a 200mm die head with a 2.5 BUR will result in a 785mm layflat width.

Note: While the formula gives the theoretical width, the final Layflat Width can be slightly affected by the cooling rate and the neck-in effect of different resins (e.g., HDPE vs. LDPE). A BUR of 2:1 to 3:1 is the most stable operating window for most polyolefin materials.

• Example A: A 200mm die at a 2.5:1 BUR produces approx. 785mm layflat.

• Example B: To hit 1,200mm layflat at a 2.5:1 BUR, you need a die diameter of roughly 300mm.

3. The Engineering Approach: Working Backward

Don't guess your machine size. Follow this professional spec sequence:

1. Define the Product: Determine your maximum target layflat width and minimum film thickness.

2. Calculate Required Output: Based on your desired haul-off speed (m/min), calculate the kg/hr needed to maintain that speed at your target thickness.

3. Select Hardware: Choose an extruder screw that can comfortably hit that output at 70-80% load, then pair it with a die head optimized for your BUR range (typically 2:1 to 3:1).