Polymer Film & Flexible Packaging, Tubular Blown-Film Extrusion BUR, Layflat & DDR
Blown-Film Extrusion BUR, Layflat & Draw-Down Formula
Calculate Transverse Blow-Up Ratio BUR = D_bubble/D_die, Collapsed Nip Layflat Width (mm), Machine-Direction Draw-Down Ratio DDR, and Forming Orientation Ratio DDR/BUR.
Stretch a blown-film bubble too hard in the haul-off nip without enough Blow-Up Ratio and the bag splits lengthwise like string cheese!
Tubular **Blown-Film Extrusion** inflates molten LLDPE / LDPE / HDPE exiting an Annular Die of Diameter `D_{die}` (`mm`) and Lip Gap `t_{die}` (`mm`) with internal bubble air up to Frost-Line Bubble Diameter `D_{bubble}` (`mm`) while Overhead Nip Rolls pull the collapsed tube upward down to Target Film Gauge `t_{film}` (`um`). Biaxial polymer chain orientation—and therefore dart-drop impact toughness (`ASTM D1709`) vs Elmendorf tear resistance—is governed by TWO orthogonal stretch ratios: **Transverse Hoop Blow-Up Ratio `BUR = D_{bubble} / D_{die}`** (`typically 2.2–3.2` for LLDPE, setting **Collapsed Layflat Width `LFW = (PI() / 2) * D_{bubble}`** mm!) and **Machine-Direction Draw-Down Ratio `DDR = (t_{die} * 1,000) / (t_{film} * BUR)`**! Keeping **Forming Balance Ratio `DDR / BUR`** modest prevents hyper-aligned axial chains that tear effortlessly along the machine direction!
Verify on a `200` mm Annular LLDPE Stretch-Hood Blown-Film Tower—Annular_Die_Diameter `D_{die}` = 200 mm (B2), Die_Lip_Gap `t_{die}` = 1.80 mm (B3), Inflated_Bubble_Diameter `D_{bubble}` = 560 mm (B4), and Target_Single-Wall_Film_Gauge `t_{film}` = 35.0 um (B5): Blow-Up Ratio `BUR` equals `2.80` (E2), Collapsed Layflat Width equals `879.6` mm (E3), Draw-Down Ratio `DDR` equals `18.37` (E4), and Forming Orientation Ratio `DDR / BUR` balances at `6.56` (E5)!
| TAPPI Blown-Film Tower KPI (Die D=200 mm, Gap=1.80 mm: Balanced 560mm/35um vs Narrow 320mm/20um) | Formula | Verified Output |
|---|---|---|
| Transverse Blow-Up Ratio BUR | Bubble_Diam_mm / Die_Diam_mm | 2.80 (1.60 under-blown @ 320 mm!) |
| Collapsed Layflat Width LFW (mm) | (PI() / 2) * Bubble_Diam_mm | 879.6 mm (502.7 mm @ 320 mm) |
| Machine Draw-Down Ratio DDR | (Die_Gap_mm * 1000) / (Film_um * BUR) | 18.37 (56.25 excessive axial draw!) |
| Orientation Balance (DDR / BUR) | Draw_Down_Ratio_DDR / Blow_Up_Ratio_BUR | 6.56 (35.16 splitty MD tear failure!) |
Tubular Blown-Film Biaxial Stretch Ratios (BUR, Layflat & DDR) Formula
Validate `Die_Diam D_{die} > 20` mm, `Lip_Gap t_{die}` in `[0.3, 5.0]` mm, `Bubble_Diam D_{bubble} >= D_{die}`, and `Film_Gauge t_{film}` in `[5, 500]` um!
=LET(dd_m, B2, tg_m, B3, db_m, B4, tf_u, B5, bu_r, db_m / dd_m, dd_r, (tg_m * 1000) / (tf_u * bu_r), IF(OR(dd_m <= 20, tg_m < 0.3, tg_m > 5, db_m < dd_m, tf_u < 5, tf_u > 500), "Invalid Blown-Film Input", ROUND(dd_r / bu_r, 2)))Under-Inflated 1.6 BUR Thin-Gauge (320 mm Bubble, 20 um Film) MD Splitting Check
Run a narrow `320` mm bubble (`BUR = 1.60` in E2) on the wide `1.80` mm LLDPE die gap while drawing down to `20` um film (`B4 = 320`, `B5 = 20`): `DDR` E4 rockets to `56.25` and `DDR / BUR` E5 jumps `5.4x` to **`35.16`**—over-aligning polyethylene lamellae axially so bags zipper-split down the machine direction!
=IF(ISTEXT(E5), E5, LET(bu_r, B4 / B2, ROUND((B3 * 1000) / (B5 * bu_r), 2)))Try the example
Use the original values or change them to test your understanding.