Injection molding of crosslinking polymers using strain data
Non-time dependent calculated variables based on measured strain are used to effectively determine an optimal hold profile for an expanding crosslinking polymer part in a mold cavity. A system and/or approach may first inject molten expanding crosslinking polymer into a mold cavity, then measure strain at the mold cavity or at another location within the injection molding system, and then calculate at least one non-time dependent variable during an injection molding cycle. Next, the system and/or method commences a hold profile for the part, and upon completing the hold profile, the part is ejected from the mold cavity, whereupon a cure profile is commenced.
1. A method for determining whether an expanding crosslinking polymer part is ready to be ejected from a mold cavity during an injection molding cycle of an injection molding system, the method comprising:
injecting molten expanding crosslinking polymer into a mold cavity at a temperature that is below an activation temperature of the expanding crosslinking polymer;
heating the mold cavity to a temperature that is higher than the activation temperature of the expanding crosslinking polymer;
measuring strain and temperature at an external surface of a mold side forming the mold cavity during the injection molding cycle;
calculating from the measured strain and the measured temperature at least one non-time dependent variable during the injection mold cycle;
commencing a hold profile for the expanding crosslinking polymer part;
upon completing the hold profile, ejecting the molded part from the mold cavity and then commencing a cure profile for the molded part to complete expansion of the expanding crosslinking polymer, and
wherein commencing the hold profile comprises:
upon the calculated at least one non-time dependent variables reaching a first threshold value, commencing the hold profile;
restricting additional molten expanding crosslinking polymer from being injected into the mold cavity; and
upon the calculated at least one non-time dependent variables reaching a second threshold value, terminating the hold profile,
wherein the first threshold value is indicative of the mold cavity being substantially full of molten expanding cross linking polymer, and
wherein the second threshold value is indicative of the part being structurally sound to commence expansion and crosslinking of the expanding crosslinking polymer outside the mold cavity.
2. The method of claim 1 , wherein the calculated at least one non-time dependent variable additionally comprises a cavity pressure value.
3. The method of claim 1 , wherein the hold profile commences at a substantially constant pressure.
4. The method of claim 1 , wherein commencing the cure profile comprises:
further calculating from the measured strain a different non-time dependent variable; and
upon the calculated different non-time dependent variables reaching a third threshold value, ejecting the molded part from the injection molding system.
5. The method of claim 4 , wherein the third threshold value is indicative of the expanding crosslinking polymer of the part being fully formed.
6. The method of claim 4 , wherein the calculated different non-time dependent variable comprises a pressure value.
7. The method of claim 1 , wherein commencing the cure profile comprises allowing the part to cool for a predetermined amount of time.