The study concerns a polycarbonate (PC) motorcycle helmet, fed through a single injection point by a long runner. The goal was to define, before the mold was built, the process parameters for the first sampling: filling time, temperatures and flow rates of the cooling circuits, packing profile and cycle time, while also checking warpage, weld lines and air vents.
1. Choice of the filling time
Four filling simulations were compared, with times of 1.5 – 2 – 2.5 – 3 seconds. These times require very different flow rates at the runner inlet: from about 120 cm³/s for 1.5 seconds to about 60 cm³/s for 3 seconds.
- Temperature at flow front: in all cases the temperature tends to rise during filling, so there are no low-temperature weld lines.
- Bulk temperature: with 1.5 seconds the high velocities bring the material to excessive temperatures, with a risk of degradation.
- Shear stress at wall: shear stress increases as the filling time decreases; with 1.5 seconds it approaches the limits recommended by the material supplier.
On the other hand, 3 seconds is unfavourable from an economic point of view and requires slightly higher pressures, and therefore clamp force. The recommended range was between 2 and 2.5 seconds; 2.5 seconds was used for the following analyses.
2. Mold cooling
16 simulations were run with different temperatures in the cooling circuits, inside and outside the helmet. The goal was to make the temperature at the polymer–mold interface as uniform as possible, above all between the inner and outer side of the part.
With the same temperature in all circuits the thermal imbalance is evident and would lead to differential shrinkage and warpage. Using different temperatures, higher in the circuits near the runner, the situation clearly improves. The simulation also provided the minimum flow rates to ensure turbulent flow in each circuit (from about 1.4 to 1.8 litres/min).
3. Packing profile and cycle time
The packing pressure was set at 850 bar, at the low end of the range recommended for PC (800–1400 bar), to limit the required clamp force. The frozen layer fraction shows when each area of the helmet is no longer connected to the injection point:
From this comes the packing profile: constant pressure for about 12–14 seconds, then a decreasing ramp down to zero in about 7 seconds.
The time to freeze result shows that the cycle time (about 70 + 7 seconds) is driven by the runner: the helmet would be ready for ejection after 35–40 seconds. The runner section cannot be reduced without increasing velocities and stresses in the material too much; by shortening it, down to direct injection on the helmet, the cycle time would be almost halved.
4. Warpage
With the chosen parameters the helmet deforms fairly uniformly, reducing its size. Only the visor attachment area deforms a little more: about 1 mm per side.
Separating the causes of warpage shows that this millimetre depends almost entirely on the thickness difference of the attachment area compared to the rest of the part; cooling accounts for about 0.08 mm. To reduce warpage the injection point should be placed in the thicker areas.
5. Weld lines and air vents
Weld lines form at every opening; the longest is near the side attachments. Given the temperatures at which they form, they should be of good quality. The simulation also shows where air is trapped: besides the last area to fill, air vents on both sides of the helmet are recommended.
The result: parameters for the first sampling
- Injection time: about 2.5 seconds;
- Melt temperature: 300 °C; target average mold temperature: 100 °C;
- Temperatures and flow rates of each cooling circuit;
- Packing: 850 bar for about 14 seconds, then a gradual decrease to zero in about 7 seconds;
- Velocity/pressure switch-over: 96 % of volume;
- Cycle time: at least 70 + 7 seconds, linked to the runner size.
Approximate and interdependent values: the simulation provides a reliable starting point for sampling at the press, reducing trials and changes to the mold.