Every plastic has a permissible maximum circumferential speed; when this limit is exceeded, shear heat causes thermal damage to the material. The permissible speed depends on the screw diameter:
| Screw Diameter | 12 m/min (0.2 m/s) | 18 m/min (0.3 m/s) | 36 m/min (0.6 m/s) | 60 m/min (1.0 m/s) |
|---|---|---|---|---|
| 20 mm | 191 rpm | 286 rpm | 573 rpm | 955 rpm |
| 25 mm | 153 rpm | 229 rpm | 458 rpm | 764 rpm |
| 30 mm | 127 rpm | 191 rpm | 382 rpm | 637 rpm |
| 35 mm | 109 rpm | 164 rpm | 327 rpm | 546 rpm |
| 40 mm | 95 rpm | 143 rpm | 286 rpm | 477 rpm |
| 50 mm | 76 rpm | 115 rpm | 229 rpm | 382 rpm |
| 60 mm | 64 rpm | 95 rpm | 191 rpm | 318 rpm |
| 70 mm | 55 rpm | 82 rpm | 164 rpm | 273 rpm |
| 80 mm | 48 rpm | 72 rpm | 143 rpm | 239 rpm |
| 90 mm | 42 rpm | 64 rpm | 127 rpm | 212 rpm |
| 100 mm | 38 rpm | 57 rpm | 115 rpm | 191 rpm |
| 120 mm | 32 rpm | 48 rpm | 95 rpm | 159 rpm |
The screw advance speed set on the machine (mm/s) means a different volume flow rate (cm³/s) depending on the screw diameter. To achieve the same flow rate, a small screw needs a much higher mm/s:
| Screw Diameter | 20 cm³/s | 50 cm³/s | 100 cm³/s | 200 cm³/s | 400 cm³/s |
|---|---|---|---|---|---|
| 20 mm | 64 mm/s | 159 mm/s | 318 mm/s | 637 mm/s | — |
| 25 mm | 41 mm/s | 102 mm/s | 204 mm/s | 407 mm/s | 815 mm/s |
| 30 mm | 28 mm/s | 71 mm/s | 141 mm/s | 283 mm/s | 566 mm/s |
| 35 mm | 21 mm/s | 52 mm/s | 104 mm/s | 208 mm/s | 416 mm/s |
| 40 mm | 16 mm/s | 40 mm/s | 80 mm/s | 159 mm/s | 318 mm/s |
| 50 mm | 10 mm/s | 25 mm/s | 51 mm/s | 102 mm/s | 204 mm/s |
| 60 mm | 7 mm/s | 18 mm/s | 35 mm/s | 71 mm/s | 141 mm/s |
| 70 mm | 5 mm/s | 13 mm/s | 26 mm/s | 52 mm/s | 104 mm/s |
| 80 mm | 4 mm/s | 10 mm/s | 20 mm/s | 40 mm/s | 80 mm/s |
| 90 mm | 3 mm/s | 8 mm/s | 16 mm/s | 31 mm/s | 63 mm/s |
| 100 mm | 3 mm/s | 6 mm/s | 13 mm/s | 25 mm/s | 51 mm/s |
| 120 mm | 2 mm/s | 4 mm/s | 9 mm/s | 18 mm/s | 35 mm/s |
The required volume flow rate rises as the part weight increases and falls as the flow path gets longer (on a long path the filling time is longer anyway). The values below are guide magnitudes:
| Part Weight | 100 mm flow path | 200 mm | 300 mm | 400 mm |
|---|---|---|---|---|
| 25 g | ~38 cm³/s | ~20 cm³/s | — | — |
| 50 g | ~62 cm³/s | ~34 cm³/s | ~22 cm³/s | — |
| 100 g | ~100 cm³/s | ~55 cm³/s | ~38 cm³/s | ~28 cm³/s |
| 200 g | — | ~95 cm³/s | ~68 cm³/s | ~50 cm³/s |
| 500 g | ~500 cm³/s | ~250 cm³/s | ~165 cm³/s | ~120 cm³/s |
| 1000 g | — | ~490 cm³/s | ~340 cm³/s | ~250 cm³/s |
| 2500 g | — | — | ~750 cm³/s | ~600 cm³/s |