Method and apparatus for substantially constant pressure injection molding of thinwall parts
A substantially constant pressure injection molding method and machine that forms molded parts by injecting molten thermoplastic material into a mold cavity at a substantially constant pressure. As a result, the mold cavity is filled with molten thermoplastic material by advancing a continuous flow front of thermoplastic material from a gate to an end of the mold cavity.
1. A method of injection molding a thinwall part at a substantially constant pressure, the method comprising:
providing an injection molding machine having an injection system and a mold with a mold cavity having a L/T ratio of more than 100;
operating the injection system to advance a shot of a molten thermoplastic material into the mold cavity such that a flow rate of the injection system increases until a predetermined injection pressure is reached; and
adjusting a filling pressure profile to cause the flow rate of the injection system to peak within a first 30% of the mold cavity being filled; and
reducing the flow rate of the injection system to maintain a substantially constant injection pressure from when the predetermined injection pressure is reached until the mold cavity is at least substantially filled with thermoplastic material.
2. The method of claim 1 , wherein the adjusting includes adjusting the filling pressure profile to cause the flow rate of the injection system to peak within a first 20% of the mold cavity being filled.
3. The method of claim 1 , wherein the adjusting includes adjusting the filling pressure profile to cause the flow rate of the injection system to peak within a first 10% of the mold cavity being filled.
4. The method of claim 1 , wherein the operating includes operating the injection system to maintain a substantially continuous and moving flow front of the molten thermoplastic material that moves without hesitation as the shot advances through the mold cavity.
5. The method of claim 1 , wherein the operating includes operating the injection system to fill the mold cavity at least 99% full with the molten plastic material, before the thermoplastic material begins to freeze at a flow front thereof.
6. The method of claim 1 , wherein the providing includes providing the injection molding machine having the mold wherein the mold cavity has an L/T ratio of 200 or greater.
7. The method of claim 1 , wherein the reducing includes reducing the power of the injection system to maintain the substantially constant injection pressure, which fluctuates no more than 30%.
8. The method of claim 1 , wherein the reducing includes reducing the power of the injection system to maintain the substantially constant injection pressure, which fluctuates no more than 20%.
9. The method of claim 1 , wherein the reducing includes reducing the power of the injection system to maintain the substantially constant injection pressure, which is less than 10,000 psi.
10. The method of claim 1 , wherein the reducing includes reducing the power of the injection system to maintain the substantially constant injection pressure, which is less than 8,000 psi.
11. The method of claim 1 , wherein the reducing includes reducing the power of the injection system to maintain the substantially constant injection pressure, which is less than 7,000 psi.
12. The method of claim 1 , wherein the reducing includes reducing the power of the injection system to maintain the substantially constant injection pressure, which is less than 6,000 psi.
13. The method of claim 1 , wherein the reducing includes reducing the power of the injection system to maintain the substantially constant injection pressure from when the predetermined injection pressure is reached until the mold cavity is completely filled with the thermoplastic material.