Rotational Molding Powder Shot Weight & Bulk Fill Formula

Pulverized rotomolding polyethylene takes up 2.56x MORE volume than solid plastic—overfilling a hollow mold past 55% chokes powder tumbling!

**Biaxial Rotational Molding (`Rotomolding`)** tumbles `35`-mesh pulverized LLDPE powder (`Apparent Bulk Density rho_{bulk} ≈ 340–400` kg/m³ per `ASTM D1895`) inside a zero-pressure cast-aluminum or sheet-steel shell of **Internal Surface Area `A_{mold}`** (`m²`) and **Enclosed Cavity Volume `V_{mold}`** (`L`) inside a `280 °C` convection oven until Peak Internal Air Temperature (`PIAT ≈ 200 °C`) sinters a stress-free hollow tank or kayak of **Target Wall Thickness `t`** (`mm`) and **Solid Polymer Density `rho_{solid}`** (`~934–940` kg/m³)! Charge Weigh-Out follows **`M_{shot} = A_{mold} * (t / 1,000) * rho_{solid}`** (`kg`), occupying **Cold Loose Powder Volume `V_{loose} = (M_{shot} / rho_{bulk}) * 1,000`** (`L`). Crucially, **Cold Cavity Powder Fill `Fill% = (V_{loose} / V_{mold}) * 100%`** MUST stay below **`~55%–60%`** so the powder pool cascades freely across every internal rib instead of locking solid (requiring split two-drop `"drop-box"` charging when `Fill% > 60%`)!

Verify on a `420`-Liter Agricultural Sprayer Tank Mold—Internal_Surface_Area `A_{mold}` = 3.85 m² (B2), Enclosed_Cavity_Volume `V_{mold}` = 420.0 L (B3), Target_Wall_Gauge `t` = 5.20 mm (B4), Solid_LLDPE_Density `rho_{solid}` = 936 kg/m³ (B5), and `35`-Mesh_Powder_Bulk_Density `rho_{bulk}` = 365 kg/m³ (B6): Required Powder Shot Weight equals `18.74` kg (E2), Cold Loose Volume equals `51.3` L (E3), Cold Mold Fill sits comfortably at `12.22%` (E4), and Solid-to-Bulk Compaction Ratio equals `2.56` (E5)!

Rotomolding Shell Mass Balance & Cold-Powder Free-Cascade Fill Formula

Validate `Mold_Area A > 0.05` m², `Mold_Volume V > 1` L, `Wall_Gauge t` in `[1.0, 25.0]` mm, `Solid_Density` in `[850, 1,500]` kg/m³, and `0.25 * rho_{solid} <= rho_{bulk} <= 0.65 * rho_{solid}`!

=LET(am_s, B2, vm_l, B3, tw_m, B4, rs_k, B5, rb_k, B6, IF(OR(am_s <= 0.05, vm_l <= 1, tw_m < 1, tw_m > 25, rs_k < 850, rs_k > 1500, rb_k < 0.25 * rs_k, rb_k > 0.65 * rs_k), "Invalid Rotomold Input", ROUND(rs_k / rb_k, 2)))

Narrow Double-Wall Dock Float (V = 110 L, Wall = 8.5 mm) Drop-Box Choking Check

Mold a flat, high-surface-area `110` L marine float with heavy `8.5` mm armor walls (`B3 = 110`, `B4 = 8.5`): Shot Weight E2 rises to `30.63` kg (`83.9` L loose powder in E3), driving Cold Cavity Fill E4 to **`76.29%`**—starving corner tumbling unless `40%` of the resin is held inside an insulated pneumatic drop-box and released mid-oven cycle!

=IF(ISTEXT(E5), E5, LET(ms_k, B2 * (B4 / 1000) * B5, vl_l, (ms_k / B6) * 1000, ROUND((vl_l / B3) * 100, 2)))

Further reading

• Arte Sheets Lesson — Extrusion Blow Molding Parison Swell & Blow-Up Ratio Formula
• Arte Sheets Lesson — Vacuum Thermoforming Areal Draw Ratio & Wall Thinning Formula