Vacuum molding of thermoset sheet articles
View Patent ↗A process of compression molding an article from a curable resin is provided that includes opening a mold to an open position by displacing an upper die having an upper mold surface from a lower die having a lower mold surface. Resin is placed a mold surface and the dies are closed to an intermediate start position at a rate of from 1 to 10 cm/sec. The dies are then closed to a start position at a rate of between 25 and 60% of the rate of closure to the intermediate position. A shroud volume is evacuated for at least 3 seconds to a reduced pressure of less than 0.16 atmospheres while continuing to press toward a mold cavity volume at a rate of between 12 and 30% of the rate of closure to the intermediate start. The reduced pressure is removed as the full compression is applied.
1. A process of compression molding an automotive body part article from a sheet molding composition (SMC), comprising:
opening a mold to an open position, said mold having an upper die having an upper mold surface and a lower die having a lower mold surface that upon full compression define a mold cavity complementary to the article;
placing the SMC as a multiple ply charge on one of the lower mold surface or the upper mold surface;
rapidly closing the upper die and the lower die from the open position to an intermediate start position of 5 to 10 cm at a rate of from 1 to 5 cm/sec;
slowly closing the upper die and the lower die from the intermediate start position to a start position at a rate of between 25 and 50% of the rate of closure to the intermediate position and simultaneously initiating a vacuum to evacuate the mold cavity to a reduced pressure of less than 0.16 atmospheres (16.2 kPa);
maintaining the vacuum on the mold cavity for at least a total of 3 seconds while continuing to press the upper die and the lower die toward a volume of the mold cavity at a rate of between 12 and 30% of the rate of closure to the intermediate start;
removing the reduced pressure when the upper die and the lower die are within 1 centimeter of the volume of the mold cavity;
allowing the curable resin to cure in the mold cavity; and
opening the mold cavity and removing the article.
2. The process of claim 1 wherein the SMC has reinforcing fibers of glass, polyaramide, carbon, or a combination thereof.
3. The process of claim 1 wherein the mold cavity is evacuated for from 3 to 10 seconds.
4. The process of claim 1 further comprising returning the upper die to and the lower die to the open position with a cycle time of 50 to 80 seconds.
5. The process of claim 4 further comprising placing a new charge of the SMC on one of the lower mold surface or the upper mold surface.
6. The process of claim 1 wherein the open position is a separation of between 15 and 30 centimeters.
7. The process of claim 1 further comprising extending a pin from the lower mold surface to remove the article from contact with the lower mold surface.