Injection molding machines and methods for accounting for changes in material properties during injection molding runs
A method and a machine that account for changes in material properties of molten plastic material during an injection run. If viscosity of the molten plastic material changes during an injection run, a controller alters an injection pressure to ensure that molten plastic material fills a mold cavity within a correct amount of time to prevent part flaws such as short shots or flashing.
1. A method of automatically adjusting pressure in an injection molding cycle of an injection molding run to adapt for variations in the flowability of a molten plastic material, the method comprising:
(a) obtaining a predetermined target time for a shot of molten plastic material, the predetermined target time being based, at least in part, on a time for a shot of molten plastic material, having known flowability, to reach a flow front sensor that is configured to sense the presence of a flow front of the molten plastic material at a particular location in a mold cavity;
(b) during a first injection molding cycle of an injection molding run, injecting a first shot of molten plastic material into the mold cavity at a first injection pressure;
(c) during the first injection molding cycle, measuring a first time for the molten plastic material to reach the flow front sensor;
(d) a first comparing of the first time to the predetermined target time to determine a first comparison result;
(e) determining a second injection pressure, based, at least in part, on the first comparison result; and
(f) during the first injection molding cycle of the injection molding run, subsequent to the measuring of the first time, applying the second injection pressure to the first shot of molten plastic material.
2. The method of claim 1 , wherein the second injection pressure is applied during a fill portion of the first injection molding cycle.
3. The method of claim 1 , wherein the second injection pressure is applied during a pack and hold portion of the first injection molding cycle.
4. The method of claim 1 , wherein the second injection pressure is determined by modifying a target injection pressure curve.
5. The method of claim 4 , wherein the modification includes adjusting the entire target injection pressure curve.
6. The method of claim 4 , wherein the modification includes adjusting a portion of the target injection pressure curve.
7. The method of claim 1 , wherein the first time is measured during a filling portion of the first injection molding cycle.
8. The method of claim 7 , wherein the second injection pressure is applied during the filling portion of the first injection molding cycle.
9. The method of claim 7 , wherein the second injection pressure is applied during a pack and hold portion of the first injection molding cycle.
10. The method of claim 1 , wherein the measuring includes measuring the first time from a start of the first injection cycle until the flow front sensor detects arrival of the flow front of the molten plastic material.
11. The method of claim 1 , wherein the first comparing includes comparing the first time to the predetermined target time to determine whether the first time is greater than or less than the predetermined target time.
12. The method of claim 11 , wherein the first comparing includes comparing the first time to the predetermined target time to determine the first comparison result, which is a time difference between the first time and the predetermined target time.
13. The method of claim 12 , wherein the second injection pressure is proportional to the time difference between the first time and the predetermined target.
14. The method of claim 1 , wherein:
the first comparing includes determining that the first time is greater than the predetermined target time; and
the second injection pressure is greater than the first injection pressure.
15. The method of claim 1 , wherein:
the first comparing includes determining that the first time is less than the predetermined target time; and
the second injection pressure is less than the first injection pressure.
16. The method of claim 1 , wherein during the first injection molding cycle, the measuring includes measuring the first time for the molten plastic material to reach the flow front sensor, which is a transducer.
17. The method of claim 1 , wherein during the first injection molding cycle, the measuring includes measuring the first time for the molten plastic material to reach the flow front sensor, which is located within 30% of an end of the mold cavity.
18. The method of claim 1 , wherein during the first injection molding cycle, the measuring includes measuring the first time for the molten plastic material to reach the flow front sensor, which is located within 20% of an end of the mold cavity.
19. The method of claim 1 , wherein during the first injection molding cycle, the measuring includes measuring the first time for the molten plastic material to reach the flow front sensor, which is located within 10% of an end of the mold cavity.
20. The method of claim 1 , wherein during the first injection molding cycle, the measuring includes measuring the first time for the molten plastic material to reach the flow front sensor, which is a sensor configured to directly sense a presence of a flow front of molten plastic material within the mold cavity.