Process Start-Up (Optimizing Fill, Packing and Other Parameters)
The following procedure works well for any velocity-controlled injection molding machine. The following conditions, although generally applicable, are not necessarily optimum for a specific application.
1. Set pack and hold timers to zero.
2. Set pack and hold pressures to zero.
3. Set injection pressure to the maximum setting.
4. Set transfer position to 6 to 7 mm (0.25 to 0.30").
5. Set injection time long enough to reach transfer position.
6. Set injection speed to achieve a fill time of 0.5 to 1.5 seconds.
7. Set shot size to achieve a short shot.
8. Increase shot size until part is 95% full.
9. Note the injection pressure required to maintain 95% fill of the part.
10. Set injection pressure to 21 to 28 bar (300 to 400 psi) above the injection pressure determined in the previous step; this insures a velocity-controlled filling.
11. Set pack-hold pressure to 50% of the injection pressure setting.
12. Set pack-hold time to 2 to 3 seconds to ensure that the remaining 5% of the part is filled and that there is enough additional material to compensate for shrinkage.
13. Fine-tune the pack-hold time by running a series of test molding cycles. Weigh each part after each test cycle. Continue to increase the pack-hold time as required until the part weight does not increase. This ensures that gate freeze-off has occurred and that parts have a repeatable, correct weight.
14. Set overall injection forward timer for 3 to 5 times more than boost time.
15. Take additional shot to verify that forward travel of screw has stopped and a cushion of 3.18 to 6.35 mm (0.125 to 0.250") remains.
16. Add cooling time depending on part thickness.
17. Use of mold release is not recommended under any circumstances.